draft-ietf-tcpm-rfc793bis-14.txt   draft-ietf-tcpm-rfc793bis-15.txt 
Internet Engineering Task Force W. Eddy, Ed. Internet Engineering Task Force W. Eddy, Ed.
Internet-Draft MTI Systems Internet-Draft MTI Systems
Obsoletes: 793, 879, 2873, 6093, 6429, July 30, 2019 Obsoletes: 793, 879, 2873, 6093, 6429, December 20, 2019
6528, 6691 (if approved) 6528, 6691 (if approved)
Updates: 5961, 1122 (if approved) Updates: 5961, 1122 (if approved)
Intended status: Standards Track Intended status: Standards Track
Expires: January 31, 2020 Expires: June 22, 2020
Transmission Control Protocol Specification Transmission Control Protocol Specification
draft-ietf-tcpm-rfc793bis-14 draft-ietf-tcpm-rfc793bis-15
Abstract Abstract
This document specifies the Internet's Transmission Control Protocol This document specifies the Internet's Transmission Control Protocol
(TCP). TCP is an important transport layer protocol in the Internet (TCP). TCP is an important transport layer protocol in the Internet
stack, and has continuously evolved over decades of use and growth of stack, and has continuously evolved over decades of use and growth of
the Internet. Over this time, a number of changes have been made to the Internet. Over this time, a number of changes have been made to
TCP as it was specified in RFC 793, though these have only been TCP as it was specified in RFC 793, though these have only been
documented in a piecemeal fashion. This document collects and brings documented in a piecemeal fashion. This document collects and brings
those changes together with the protocol specification from RFC 793. those changes together with the protocol specification from RFC 793.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 31, 2020. This Internet-Draft will expire on June 22, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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3. Functional Specification . . . . . . . . . . . . . . . . . . 6 3. Functional Specification . . . . . . . . . . . . . . . . . . 6
3.1. Header Format . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Header Format . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Terminology Overview . . . . . . . . . . . . . . . . . . 11 3.2. Terminology Overview . . . . . . . . . . . . . . . . . . 11
3.2.1. Key Connection State Variables . . . . . . . . . . . 11 3.2.1. Key Connection State Variables . . . . . . . . . . . 11
3.2.2. State Machine Overview . . . . . . . . . . . . . . . 13 3.2.2. State Machine Overview . . . . . . . . . . . . . . . 13
3.3. Sequence Numbers . . . . . . . . . . . . . . . . . . . . 16 3.3. Sequence Numbers . . . . . . . . . . . . . . . . . . . . 16
3.4. Establishing a connection . . . . . . . . . . . . . . . . 22 3.4. Establishing a connection . . . . . . . . . . . . . . . . 22
3.5. Closing a Connection . . . . . . . . . . . . . . . . . . 29 3.5. Closing a Connection . . . . . . . . . . . . . . . . . . 29
3.5.1. Half-Closed Connections . . . . . . . . . . . . . . . 31 3.5.1. Half-Closed Connections . . . . . . . . . . . . . . . 31
3.6. Precedence and Security . . . . . . . . . . . . . . . . . 32 3.6. Segmentation . . . . . . . . . . . . . . . . . . . . . . 32
3.7. Segmentation . . . . . . . . . . . . . . . . . . . . . . 33 3.6.1. Maximum Segment Size Option . . . . . . . . . . . . . 33
3.7.1. Maximum Segment Size Option . . . . . . . . . . . . . 34 3.6.2. Path MTU Discovery . . . . . . . . . . . . . . . . . 35
3.7.2. Path MTU Discovery . . . . . . . . . . . . . . . . . 35 3.6.3. Interfaces with Variable MTU Values . . . . . . . . . 35
3.7.3. Interfaces with Variable MTU Values . . . . . . . . . 36 3.6.4. Nagle Algorithm . . . . . . . . . . . . . . . . . . . 36
3.7.4. Nagle Algorithm . . . . . . . . . . . . . . . . . . . 37 3.6.5. IPv6 Jumbograms . . . . . . . . . . . . . . . . . . . 36
3.7.5. IPv6 Jumbograms . . . . . . . . . . . . . . . . . . . 37 3.7. Data Communication . . . . . . . . . . . . . . . . . . . 36
3.8. Data Communication . . . . . . . . . . . . . . . . . . . 37 3.7.1. Retransmission Timeout . . . . . . . . . . . . . . . 37
3.8.1. Retransmission Timeout . . . . . . . . . . . . . . . 38 3.7.2. TCP Congestion Control . . . . . . . . . . . . . . . 37
3.8.2. TCP Congestion Control . . . . . . . . . . . . . . . 38 3.7.3. TCP Connection Failures . . . . . . . . . . . . . . . 38
3.8.3. TCP Connection Failures . . . . . . . . . . . . . . . 39 3.7.4. TCP Keep-Alives . . . . . . . . . . . . . . . . . . . 39
3.8.4. TCP Keep-Alives . . . . . . . . . . . . . . . . . . . 40 3.7.5. The Communication of Urgent Information . . . . . . . 39
3.8.5. The Communication of Urgent Information . . . . . . . 40 3.7.6. Managing the Window . . . . . . . . . . . . . . . . . 40
3.8.6. Managing the Window . . . . . . . . . . . . . . . . . 41 3.8. Interfaces . . . . . . . . . . . . . . . . . . . . . . . 45
3.9. Interfaces . . . . . . . . . . . . . . . . . . . . . . . 46 3.8.1. User/TCP Interface . . . . . . . . . . . . . . . . . 45
3.9.1. User/TCP Interface . . . . . . . . . . . . . . . . . 46 3.8.2. TCP/Lower-Level Interface . . . . . . . . . . . . . . 54
3.9.2. TCP/Lower-Level Interface . . . . . . . . . . . . . . 55 3.9. Event Processing . . . . . . . . . . . . . . . . . . . . 57
3.10. Event Processing . . . . . . . . . . . . . . . . . . . . 57 3.10. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 82
3.11. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 82
4. Changes from RFC 793 . . . . . . . . . . . . . . . . . . . . 87 4. Changes from RFC 793 . . . . . . . . . . . . . . . . . . . . 87
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 91 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 92
6. Security and Privacy Considerations . . . . . . . . . . . . . 92 6. Security and Privacy Considerations . . . . . . . . . . . . . 92
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 93 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 93
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 94 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 94
8.1. Normative References . . . . . . . . . . . . . . . . . . 94 8.1. Normative References . . . . . . . . . . . . . . . . . . 94
8.2. Informative References . . . . . . . . . . . . . . . . . 95 8.2. Informative References . . . . . . . . . . . . . . . . . 95
Appendix A. Other Implementation Notes . . . . . . . . . . . . . 98 Appendix A. Other Implementation Notes . . . . . . . . . . . . . 99
A.1. IP Security Compartment and Precedence . . . . . . . . . 99 A.1. IP Security Compartment and Precedence . . . . . . . . . 99
A.2. Sequence Number Validation . . . . . . . . . . . . . . . 99 A.1.1. Precedence . . . . . . . . . . . . . . . . . . . . . 100
A.3. Nagle Modification . . . . . . . . . . . . . . . . . . . 99 A.1.2. MLS Systems . . . . . . . . . . . . . . . . . . . . . 100
A.4. Low Water Mark . . . . . . . . . . . . . . . . . . . . . 100 A.2. Sequence Number Validation . . . . . . . . . . . . . . . 101
Appendix B. TCP Requirement Summary . . . . . . . . . . . . . . 100 A.3. Nagle Modification . . . . . . . . . . . . . . . . . . . 101
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 104 A.4. Low Water Mark Settings . . . . . . . . . . . . . . . . . 101
Appendix B. TCP Requirement Summary . . . . . . . . . . . . . . 102
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 106
1. Purpose and Scope 1. Purpose and Scope
In 1981, RFC 793 [12] was released, documenting the Transmission In 1981, RFC 793 [12] was released, documenting the Transmission
Control Protocol (TCP), and replacing earlier specifications for TCP Control Protocol (TCP), and replacing earlier specifications for TCP
that had been published in the past. that had been published in the past.
Since then, TCP has been implemented many times, and has been used as Since then, TCP has been implemented many times, and has been used as
a transport protocol for numerous applications on the Internet. a transport protocol for numerous applications on the Internet.
For several decades, RFC 793 plus a number of other documents have For several decades, RFC 793 plus a number of other documents have
combined to serve as the specification for TCP [37]. Over time, a combined to serve as the specification for TCP [39]. Over time, a
number of errata have been identified on RFC 793, as well as number of errata have been identified on RFC 793, as well as
deficiencies in security, performance, and other aspects. A number deficiencies in security, performance, and other aspects. The number
of enhancements has grown and been documented separately. These were of enhancements has grown over time across many separate documents.
never accumulated together into an update to the base specification. These were never accumulated together into an update to the base
specification.
The purpose of this document is to bring together all of the IETF The purpose of this document is to bring together all of the IETF
Standards Track changes that have been made to the basic TCP Standards Track changes that have been made to the basic TCP
functional specification and unify them into an update of the RFC 793 functional specification and unify them into an update of the RFC 793
protocol specification. Some companion documents are referenced for protocol specification. Some companion documents are referenced for
important algorithms that TCP uses (e.g. for congestion control), but important algorithms that TCP uses (e.g. for congestion control), but
have not been attempted to include in this document. This is a have not been attempted to include in this document. This is a
conscious choice, as this base specification can be used with conscious choice, as this base specification can be used with
multiple additional algorithms that are developed and incorporated multiple additional algorithms that are developed and incorporated
separately, but all TCP implementations need to implement this separately, but all TCP implementations need to implement this
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This document describes the basic functionality expected in modern This document describes the basic functionality expected in modern
implementations of TCP, and replaces the protocol specification in implementations of TCP, and replaces the protocol specification in
RFC 793. It does not replicate or attempt to update the introduction RFC 793. It does not replicate or attempt to update the introduction
and philosophy content in RFC 793 (sections 1 and 2 of that and philosophy content in RFC 793 (sections 1 and 2 of that
document). Other documents are referenced to provide explanation of document). Other documents are referenced to provide explanation of
the theory of operation, rationale, and detailed discussion of design the theory of operation, rationale, and detailed discussion of design
decisions. This document only focuses on the normative behavior of decisions. This document only focuses on the normative behavior of
the protocol. the protocol.
The "TCP Roadmap" [37] provides a more extensive guide to the RFCs The "TCP Roadmap" [39] provides a more extensive guide to the RFCs
that define TCP and describe various important algorithms. The TCP that define TCP and describe various important algorithms. The TCP
Roadmap contains sections on strongly encouraged enhancements that Roadmap contains sections on strongly encouraged enhancements that
improve performance and other aspects of TCP beyond the basic improve performance and other aspects of TCP beyond the basic
operation specified in this document. As one example, implementing operation specified in this document. As one example, implementing
congestion control (e.g. [25]) is a TCP requirement, but is a complex congestion control (e.g. [27]) is a TCP requirement, but is a complex
topic on its own, and not described in detail in this document, as topic on its own, and not described in detail in this document, as
there are many options and possibilities that do not impact basic there are many options and possibilities that do not impact basic
interoperability. Similarly, most common TCP implementations today interoperability. Similarly, most common TCP implementations today
include the high-performance extensions in [35], but these are not include the high-performance extensions in [37], but these are not
strictly required or discussed in this document. strictly required or discussed in this document.
A list of changes from RFC 793 is contained in Section 4. A list of changes from RFC 793 is contained in Section 4.
Each use of RFC 2119 keywords in the document is individually labeled Each use of RFC 2119 keywords in the document is individually labeled
and referenced in Appendix B that summarizes implementation and referenced in Appendix B that summarizes implementation
requirements. Sentences using "MUST" are labeled as "MUST-X" with X requirements. Sentences using "MUST" are labeled as "MUST-X" with X
being a numeric identifier enabling the requirement to be located being a numeric identifier enabling the requirement to be located
easily when referenced from Appendix B. Similarly, sentences using easily when referenced from Appendix B. Similarly, sentences using
"SHOULD" are labeled with "SHLD-X", "MAY" with "MAY-X", and "SHOULD" are labeled with "SHLD-X", "MAY" with "MAY-X", and
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TCP provides a reliable, in-order, byte-stream service to TCP provides a reliable, in-order, byte-stream service to
applications. applications.
The application byte-stream is conveyed over the network via TCP The application byte-stream is conveyed over the network via TCP
segments, with each TCP segment sent as an Internet Protocol (IP) segments, with each TCP segment sent as an Internet Protocol (IP)
datagram. datagram.
TCP reliability consists of detecting packet losses (via sequence TCP reliability consists of detecting packet losses (via sequence
numbers) and errors (via per-segment checksums), as well as numbers) and errors (via per-segment checksums), as well as
correction of losses and errors via retransmission. correction via retransmission.
TCP supports unicast delivery of data. Anycast applications exist TCP supports unicast delivery of data. Anycast applications exist
that successfully use TCP without modifications, though there is some that successfully use TCP without modifications, though there is some
risk of instability due to rerouting. risk of instability due to changes of lower-layer forwarding
behavior.
TCP is connection-oriented, though does not inherently include a TCP is connection-oriented, though does not inherently include a
liveness detection capability. liveness detection capability.
Data flow is supported bidirectionally over TCP connections, though Data flow is supported bidirectionally over TCP connections, though
applications are free to flow data only unidirectionally, if they so applications are free to send data only unidirectionally, if they so
choose. choose.
TCP uses port numbers to identify application services and to TCP uses port numbers to identify application services and to
multiplex multiple flows between hosts. multiplex multiple flows between hosts.
A more detailed description of TCP's features compared to other A more detailed description of TCP's features compared to other
transport protocols can be found in Section 3.1 of [40]. Further transport protocols can be found in Section 3.1 of [42]. Further
description of the motivations for developing TCP and its role in the description of the motivations for developing TCP and its role in the
Internet stack can be found in Section 2 of [12] and earlier versions Internet stack can be found in Section 2 of [12] and earlier versions
of the TCP specification. of the TCP specification.
3. Functional Specification 3. Functional Specification
3.1. Header Format 3.1. Header Format
TCP segments are sent as internet datagrams. The Internet Protocol TCP segments are sent as internet datagrams. The Internet Protocol
(IP) header carries several information fields, including the source (IP) header carries several information fields, including the source
and destination host addresses [1] [11]. A TCP header follows the and destination host addresses [1] [11]. A TCP header follows the
Internet header, supplying information specific to the TCP protocol. Internet header, supplying information specific to the TCP protocol.
This division allows for the existence of host level protocols other This division allows for the existence of host level protocols other
than TCP. In early development of the Internet suite of protocols, than TCP. In early development of the Internet suite of protocols,
the IP header fields had been a part of TCP. the IP header fields had been a part of TCP.
TCP Header Format
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port | | Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number | | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number | | Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data | |C|E|U|A|P|R|S|F| | | Data | |C|E|U|A|P|R|S|F| |
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The destination port number. The destination port number.
Sequence Number: 32 bits Sequence Number: 32 bits
The sequence number of the first data octet in this segment (except The sequence number of the first data octet in this segment (except
when SYN is present). If SYN is present the sequence number is the when SYN is present). If SYN is present the sequence number is the
initial sequence number (ISN) and the first data octet is ISN+1. initial sequence number (ISN) and the first data octet is ISN+1.
Acknowledgment Number: 32 bits Acknowledgment Number: 32 bits
If the ACK control bit is set this field contains the value of the If the ACK control bit is set, this field contains the value of the
next sequence number the sender of the segment is expecting to next sequence number the sender of the segment is expecting to
receive. Once a connection is established this is always sent. receive. Once a connection is established, this is always sent.
Data Offset: 4 bits Data Offset: 4 bits
The number of 32 bit words in the TCP Header. This indicates where The number of 32 bit words in the TCP Header. This indicates where
the data begins. The TCP header (even one including options) is an the data begins. The TCP header (even one including options) is an
integral number of 32 bits long. integral number of 32 bits long.
Rsrvd - Reserved: 4 bits Rsrvd - Reserved: 4 bits
Reserved for future use. Must be zero in generated segments and Reserved for future use. Must be zero in generated segments and
must be ignored in received segments, if corresponding future must be ignored in received segments, if corresponding future
features are unimplemented by the sending or receiving host. features are unimplemented by the sending or receiving host.
Control Bits: 8 bits (from left to right): Control Bits: 8 bits (from left to right):
CWR: Congestion Window Reduced (see [8]) CWR: Congestion Window Reduced (see [8])
ECE: ECN-Echo (see [8]) ECE: ECN-Echo (see [8])
URG: Urgent Pointer field significant URG: Urgent Pointer field significant
ACK: Acknowledgment field significant ACK: Acknowledgment field significant
PSH: Push Function (see the Send Call description in PSH: Push Function (see the Send Call description in
Section 3.9.1) Section 3.8.1)
RST: Reset the connection RST: Reset the connection
SYN: Synchronize sequence numbers SYN: Synchronize sequence numbers
FIN: No more data from sender FIN: No more data from sender
The control bits are also know as "flags". Assignment is managed The control bits are also know as "flags". Assignment is managed
by IANA from the "TCP Header Flags" registry [42]. by IANA from the "TCP Header Flags" registry [46].
Window: 16 bits Window: 16 bits
The number of data octets beginning with the one indicated in the The number of data octets beginning with the one indicated in the
acknowledgment field which the sender of this segment is willing to acknowledgment field that the sender of this segment is willing to
accept. accept.
The window size MUST be treated as an unsigned number, or else The window size MUST be treated as an unsigned number, or else
large window sizes will appear like negative windows and TCP will large window sizes will appear like negative windows and TCP will
now work (MUST-1). It is RECOMMENDED that implementations will now work (MUST-1). It is RECOMMENDED that implementations will
reserve 32-bit fields for the send and receive window sizes in the reserve 32-bit fields for the send and receive window sizes in the
connection record and do all window computations with 32 bits (REC- connection record and do all window computations with 32 bits (REC-
1). 1).
Checksum: 16 bits Checksum: 16 bits
The checksum field is the 16 bit one's complement of the one's The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header and text. If a complement sum of all 16 bit words in the header and text. The
segment contains an odd number of header and text octets to be checksum computation needs to ensure the 16-bit alignment of the
checksummed, the last octet is padded on the right with zeros to data being summed. If a segment contains an odd number of header
form a 16 bit word for checksum purposes. The pad is not and text octets, alignment can be achieved by padding the last
transmitted as part of the segment. While computing the checksum, octet with zeros on its right to form a 16 bit word for checksum
the checksum field itself is replaced with zeros. purposes. The pad is not transmitted as part of the segment.
While computing the checksum, the checksum field itself is replaced
with zeros.
The checksum also covers a pseudo header conceptually prefixed to The checksum also covers a pseudo header conceptually prefixed to
the TCP header. The pseudo header is 96 bits for IPv4 and 320 bits the TCP header. The pseudo header is 96 bits for IPv4 and 320 bits
for IPv6. For IPv4, this pseudo header contains the Source for IPv6. For IPv4, this pseudo header contains the Source
Address, the Destination Address, the Protocol (PTCL), and TCP Address, the Destination Address, the Protocol (PTCL), and TCP
length. This gives the TCP protection against misrouted segments. length. This gives the TCP connection protection against misrouted
This information is carried in IPv4 and is transferred across the segments. This information is carried in IP headers and is
TCP/Network interface in the arguments or results of calls by the transferred across the TCP/Network interface in the arguments or
TCP on the IP. results of calls by the TCP implementation on the IP layer.
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Source Address | | Source Address |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Destination Address | | Destination Address |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| zero | PTCL | TCP Length | | zero | PTCL | TCP Length |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
Psuedo header components: Psuedo header components:
Source Address: the IPv4 source address in network byte order Source Address: the IPv4 source address in network byte order
Destination Address: the IPv4 destination address in network Destination Address: the IPv4 destination address in network
byte order byte order
zero: bits set to zero zero: bits set to zero
PTCL: the protocol number from the IP header
PTCL: the protocol number from the IP header
TCP Length: the TCP header length plus the data length in octets TCP Length: the TCP header length plus the data length in octets
(this is not an explicitly transmitted quantity, but is (this is not an explicitly transmitted quantity, but is
computed), and it does not count the 12 octets of the pseudo computed), and it does not count the 12 octets of the pseudo
header. header.
For IPv6, the pseudo header is contained in section 8.1 of RFC 8200 For IPv6, the pseudo header is contained in section 8.1 of RFC 8200
[11], and contains the IPv6 Source Address and Destination Address, [11], and contains the IPv6 Source Address and Destination Address,
an Upper Layer Packet Length (a 32-bit value otherwise equivalent an Upper Layer Packet Length (a 32-bit value otherwise equivalent
to TCP Length in the IPv4 pseudo header), three bytes of zero- to TCP Length in the IPv4 pseudo header), three bytes of zero-
padding, and a Next Header value (differing from the IPv6 header padding, and a Next Header value (differing from the IPv6 header
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Case 2: An octet of option-kind, an octet of option-length, and Case 2: An octet of option-kind, an octet of option-length, and
the actual option-data octets. the actual option-data octets.
The option-length counts the two octets of option-kind and option- The option-length counts the two octets of option-kind and option-
length as well as the option-data octets. length as well as the option-data octets.
Note that the list of options may be shorter than the data offset Note that the list of options may be shorter than the data offset
field might imply. The content of the header beyond the End-of- field might imply. The content of the header beyond the End-of-
Option option must be header padding (i.e., zero). Option option must be header padding (i.e., zero).
The list of all currently defined options is managed by IANA [41], The list of all currently defined options is managed by IANA [45],
and each option is defined in other RFCs, as indicated there. That and each option is defined in other RFCs, as indicated there. That
set includes experimental options that can be extended to support set includes experimental options that can be extended to support
multiple concurrent uses [34]. multiple concurrent usages [36].
A given TCP implementation can support any currently defined A given TCP implementation can support any currently defined
options, but the following options MUST be supported (MUST-4) (kind options, but the following options MUST be supported (MUST-4) (kind
indicated in octal): indicated in octal):
Kind Length Meaning Kind Length Meaning
---- ------ ------- ---- ------ -------
0 - End of option list. 0 - End of option list.
1 - No-Operation. 1 - No-Operation.
2 4 Maximum Segment Size. 2 4 Maximum Segment Size.
A TCP MUST be able to receive a TCP option in any segment (MUST-5). A TCP implementation MUST be able to receive a TCP option in any
A TCP MUST (MUST-6) ignore without error any TCP option it does not segment (MUST-5).
implement, assuming that the option has a length field (all TCP A TCP implementation MUST (MUST-6) ignore without error any TCP
options except End of option list and No-Operation have length option it does not implement, assuming that the option has a length
fields). TCP MUST be prepared to handle an illegal option length field (all TCP options except End of option list and No-Operation
(e.g., zero) without crashing; a suggested procedure is to reset have length fields). TCP implementations MUST be prepared to
the connection and log the reason (MUST-7). handle an illegal option length (e.g., zero); a suggested procedure
is to reset the connection and log the reason (MUST-7).
Specific Option Definitions Specific Option Definitions
End of Option List End of Option List
+--------+ +--------+
|00000000| |00000000|
+--------+ +--------+
Kind=0 Kind=0
skipping to change at page 10, line 48 skipping to change at page 10, line 47
the options would not otherwise coincide with the end of the TCP the options would not otherwise coincide with the end of the TCP
header. header.
No-Operation No-Operation
+--------+ +--------+
|00000001| |00000001|
+--------+ +--------+
Kind=1 Kind=1
This option code may be used between options, for example, to This option code can be used between options, for example, to
align the beginning of a subsequent option on a word boundary. align the beginning of a subsequent option on a word boundary.
There is no guarantee that senders will use this option, so There is no guarantee that senders will use this option, so
receivers must be prepared to process options even if they do receivers MUST be prepared to process options even if they do
not begin on a word boundary. not begin on a word boundary (MUST-64).
Maximum Segment Size (MSS) Maximum Segment Size (MSS)
+--------+--------+---------+--------+ +--------+--------+---------+--------+
|00000010|00000100| max seg size | |00000010|00000100| max seg size |
+--------+--------+---------+--------+ +--------+--------+---------+--------+
Kind=2 Length=4 Kind=2 Length=4
Maximum Segment Size Option Data: 16 bits Maximum Segment Size Option Data: 16 bits
If this option is present, then it communicates the maximum If this option is present, then it communicates the maximum
receive segment size at the TCP which sends this segment. This receive segment size at the TCP endpoint that sends this
value is limited by the IP reassembly limit. This field may be segment. This value is limited by the IP reassembly limit.
sent in the initial connection request (i.e., in segments with This field may be sent in the initial connection request (i.e.,
the SYN control bit set) and must not be sent in other segments. in segments with the SYN control bit set) and MUST NOT be sent
If this option is not used, any segment size is allowed. A more in other segments (MUST-65). If this option is not used, any
complete description of this option is in Section 3.7.1. segment size is allowed. A more complete description of this
option is in Section 3.6.1.
Experimental TCP option values are defined in [20], and [36]
describes the current recommended usage for these experimental
values.
Note: There is ongoing work to extend the space available for
TCP options, such as [50].
Padding: variable Padding: variable
The TCP header padding is used to ensure that the TCP header ends The TCP header padding is used to ensure that the TCP header ends
and data begins on a 32 bit boundary. The padding is composed of and data begins on a 32 bit boundary. The padding is composed of
zeros. zeros.
3.2. Terminology Overview 3.2. Terminology Overview
This section includes an overview of terminology needed to understand This section includes an overview of key terms needed to understand
the detailed protocol operation in the rest of the document. the detailed protocol operation in the rest of the document. There
is a traditional glossary of terms in Section 3.10.
3.2.1. Key Connection State Variables 3.2.1. Key Connection State Variables
Before we can discuss very much about the operation of the TCP we Before we can discuss very much about the operation of the TCP
need to introduce some detailed terminology. The maintenance of a implementation we need to introduce some detailed terminology. The
TCP connection requires the remembering of several variables. We maintenance of a TCP connection requires the remembering of several
conceive of these variables being stored in a connection record variables. We conceive of these variables being stored in a
called a Transmission Control Block or TCB. Among the variables connection record called a Transmission Control Block or TCB. Among
stored in the TCB are the local and remote socket numbers, the IP the variables stored in the TCB are the local and remote IP addresses
security level and compartment of the connection (see Section 3.6 and and port numbers, the IP security level and compartment of the
Appendix A.1), pointers to the user's send and receive buffers, connection (see Appendix A.1), pointers to the user's send and
pointers to the retransmit queue and to the current segment. In receive buffers, pointers to the retransmit queue and to the current
addition several variables relating to the send and receive sequence segment. In addition several variables relating to the send and
numbers are stored in the TCB. receive sequence numbers are stored in the TCB.
Send Sequence Variables Send Sequence Variables
SND.UNA - send unacknowledged SND.UNA - send unacknowledged
SND.NXT - send next SND.NXT - send next
SND.WND - send window SND.WND - send window
SND.UP - send urgent pointer SND.UP - send urgent pointer
SND.WL1 - segment sequence number used for last window update SND.WL1 - segment sequence number used for last window update
SND.WL2 - segment acknowledgment number used for last window SND.WL2 - segment acknowledgment number used for last window
update update
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Receive Sequence Variables Receive Sequence Variables
RCV.NXT - receive next RCV.NXT - receive next
RCV.WND - receive window RCV.WND - receive window
RCV.UP - receive urgent pointer RCV.UP - receive urgent pointer
IRS - initial receive sequence number IRS - initial receive sequence number
The following diagrams may help to relate some of these variables to The following diagrams may help to relate some of these variables to
the sequence space. the sequence space.
Send Sequence Space
1 2 3 4 1 2 3 4
----------|----------|----------|---------- ----------|----------|----------|----------
SND.UNA SND.NXT SND.UNA SND.UNA SND.NXT SND.UNA
+SND.WND +SND.WND
1 - old sequence numbers which have been acknowledged 1 - old sequence numbers that have been acknowledged
2 - sequence numbers of unacknowledged data 2 - sequence numbers of unacknowledged data
3 - sequence numbers allowed for new data transmission 3 - sequence numbers allowed for new data transmission
4 - future sequence numbers which are not yet allowed 4 - future sequence numbers that are not yet allowed
Figure 2: Send Sequence Space Figure 2: Send Sequence Space
The send window is the portion of the sequence space labeled 3 in The send window is the portion of the sequence space labeled 3 in
Figure 2. Figure 2.
Receive Sequence Space
1 2 3 1 2 3
----------|----------|---------- ----------|----------|----------
RCV.NXT RCV.NXT RCV.NXT RCV.NXT
+RCV.WND +RCV.WND
1 - old sequence numbers which have been acknowledged 1 - old sequence numbers that have been acknowledged
2 - sequence numbers allowed for new reception 2 - sequence numbers allowed for new reception
3 - future sequence numbers which are not yet allowed 3 - future sequence numbers that are not yet allowed
Figure 3: Receive Sequence Space Figure 3: Receive Sequence Space
The receive window is the portion of the sequence space labeled 2 in The receive window is the portion of the sequence space labeled 2 in
Figure 3. Figure 3.
There are also some variables used frequently in the discussion that There are also some variables used frequently in the discussion that
take their values from the fields of the current segment. take their values from the fields of the current segment.
Current Segment Variables Current Segment Variables
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3.2.2. State Machine Overview 3.2.2. State Machine Overview
A connection progresses through a series of states during its A connection progresses through a series of states during its
lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED, lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED,
ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK,
TIME-WAIT, and the fictional state CLOSED. CLOSED is fictional TIME-WAIT, and the fictional state CLOSED. CLOSED is fictional
because it represents the state when there is no TCB, and therefore, because it represents the state when there is no TCB, and therefore,
no connection. Briefly the meanings of the states are: no connection. Briefly the meanings of the states are:
LISTEN - represents waiting for a connection request from any LISTEN - represents waiting for a connection request from any
remote TCP and port. remote TCP peer and port.
SYN-SENT - represents waiting for a matching connection request SYN-SENT - represents waiting for a matching connection request
after having sent a connection request. after having sent a connection request.
SYN-RECEIVED - represents waiting for a confirming connection SYN-RECEIVED - represents waiting for a confirming connection
request acknowledgment after having both received and sent a request acknowledgment after having both received and sent a
connection request. connection request.
ESTABLISHED - represents an open connection, data received can be ESTABLISHED - represents an open connection, data received can be
delivered to the user. The normal state for the data transfer delivered to the user. The normal state for the data transfer
phase of the connection. phase of the connection.
FIN-WAIT-1 - represents waiting for a connection termination FIN-WAIT-1 - represents waiting for a connection termination
request from the remote TCP, or an acknowledgment of the request from the remote TCP peer, or an acknowledgment of the
connection termination request previously sent. connection termination request previously sent.
FIN-WAIT-2 - represents waiting for a connection termination FIN-WAIT-2 - represents waiting for a connection termination
request from the remote TCP. request from the remote TCP peer.
CLOSE-WAIT - represents waiting for a connection termination CLOSE-WAIT - represents waiting for a connection termination
request from the local user. request from the local user.
CLOSING - represents waiting for a connection termination request CLOSING - represents waiting for a connection termination request
acknowledgment from the remote TCP. acknowledgment from the remote TCP peer.
LAST-ACK - represents waiting for an acknowledgment of the LAST-ACK - represents waiting for an acknowledgment of the
connection termination request previously sent to the remote TCP connection termination request previously sent to the remote TCP
(this termination request sent to the remote TCP already included peer (this termination request sent to the remote TCP peer already
an acknowledgment of the termination request sent from the remote included an acknowledgment of the termination request sent from
TCP). the remote TCP peer).
TIME-WAIT - represents waiting for enough time to pass to be sure TIME-WAIT - represents waiting for enough time to pass to be sure
the remote TCP received the acknowledgment of its connection the remote TCP peer received the acknowledgment of its connection
termination request. termination request.
CLOSED - represents no connection state at all. CLOSED - represents no connection state at all.
A TCP connection progresses from one state to another in response to A TCP connection progresses from one state to another in response to
events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE, events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE,
ABORT, and STATUS; the incoming segments, particularly those ABORT, and STATUS; the incoming segments, particularly those
containing the SYN, ACK, RST and FIN flags; and timeouts. containing the SYN, ACK, RST and FIN flags; and timeouts.
The state diagram in Figure 4 illustrates only state changes, The state diagram in Figure 4 illustrates only state changes,
together with the causing events and resulting actions, but addresses together with the causing events and resulting actions, but addresses
neither error conditions nor actions which are not connected with neither error conditions nor actions that are not connected with
state changes. In a later section, more detail is offered with state changes. In a later section, more detail is offered with
respect to the reaction of the TCP to events. Some state names are respect to the reaction of the TCP implementation to events. Some
abbreviated or hyphenated differently in the diagram from how they state names are abbreviated or hyphenated differently in the diagram
appear elsewhere in the document. from how they appear elsewhere in the document.
NOTA BENE: This diagram is only a summary and must not be taken as NOTA BENE: This diagram is only a summary and must not be taken as
the total specification. Many details are not included. the total specification. Many details are not included.
+---------+ ---------\ active OPEN +---------+ ---------\ active OPEN
| CLOSED | \ ----------- | CLOSED | \ -----------
+---------+<---------\ \ create TCB +---------+<---------\ \ create TCB
| ^ \ \ snd SYN | ^ \ \ snd SYN
passive OPEN | | CLOSE \ \ passive OPEN | | CLOSE \ \
------------ | | ---------- \ \ ------------ | | ---------- \ \
skipping to change at page 16, line 30 skipping to change at page 16, line 28
It is essential to remember that the actual sequence number space is It is essential to remember that the actual sequence number space is
finite, though very large. This space ranges from 0 to 2**32 - 1. finite, though very large. This space ranges from 0 to 2**32 - 1.
Since the space is finite, all arithmetic dealing with sequence Since the space is finite, all arithmetic dealing with sequence
numbers must be performed modulo 2**32. This unsigned arithmetic numbers must be performed modulo 2**32. This unsigned arithmetic
preserves the relationship of sequence numbers as they cycle from preserves the relationship of sequence numbers as they cycle from
2**32 - 1 to 0 again. There are some subtleties to computer modulo 2**32 - 1 to 0 again. There are some subtleties to computer modulo
arithmetic, so great care should be taken in programming the arithmetic, so great care should be taken in programming the
comparison of such values. The symbol "=<" means "less than or comparison of such values. The symbol "=<" means "less than or
equal" (modulo 2**32). equal" (modulo 2**32).
The typical kinds of sequence number comparisons which the TCP must The typical kinds of sequence number comparisons that the TCP
perform include: implementation must perform include:
(a) Determining that an acknowledgment refers to some sequence (a) Determining that an acknowledgment refers to some sequence
number sent but not yet acknowledged. number sent but not yet acknowledged.
(b) Determining that all sequence numbers occupied by a segment (b) Determining that all sequence numbers occupied by a segment
have been acknowledged (e.g., to remove the segment from a have been acknowledged (e.g., to remove the segment from a
retransmission queue). retransmission queue).
(c) Determining that an incoming segment contains sequence numbers (c) Determining that an incoming segment contains sequence numbers
which are expected (i.e., that the segment "overlaps" the receive that are expected (i.e., that the segment "overlaps" the receive
window). window).
In response to sending data the TCP will receive acknowledgments. In response to sending data the TCP endpoint will receive
The following comparisons are needed to process the acknowledgments. acknowledgments. The following comparisons are needed to process the
acknowledgments.
SND.UNA = oldest unacknowledged sequence number SND.UNA = oldest unacknowledged sequence number
SND.NXT = next sequence number to be sent SND.NXT = next sequence number to be sent
SEG.ACK = acknowledgment from the receiving TCP (next sequence SEG.ACK = acknowledgment from the receiving TCP peer (next
number expected by the receiving TCP) sequence number expected by the receiving TCP peer)
SEG.SEQ = first sequence number of a segment SEG.SEQ = first sequence number of a segment
SEG.LEN = the number of octets occupied by the data in the segment SEG.LEN = the number of octets occupied by the data in the segment
(counting SYN and FIN) (counting SYN and FIN)
SEG.SEQ+SEG.LEN-1 = last sequence number of a segment SEG.SEQ+SEG.LEN-1 = last sequence number of a segment
A new acknowledgment (called an "acceptable ack"), is one for which A new acknowledgment (called an "acceptable ack"), is one for which
the inequality below holds: the inequality below holds:
skipping to change at page 18, line 19 skipping to change at page 18, line 19
0 0 SEG.SEQ = RCV.NXT 0 0 SEG.SEQ = RCV.NXT
0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND 0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
>0 0 not acceptable >0 0 not acceptable
>0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND >0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
Note that when the receive window is zero no segments should be Note that when the receive window is zero no segments should be
acceptable except ACK segments. Thus, it is be possible for a TCP to acceptable except ACK segments. Thus, it is be possible for a TCP
maintain a zero receive window while transmitting data and receiving implementation to maintain a zero receive window while transmitting
ACKs. However, even when the receive window is zero, a TCP must data and receiving ACKs. A TCP receiver MUST process the RST and URG
process the RST and URG fields of all incoming segments. fields of all incoming segments, even when the receive window is zero
(MUST-66).
We have taken advantage of the numbering scheme to protect certain We have taken advantage of the numbering scheme to protect certain
control information as well. This is achieved by implicitly control information as well. This is achieved by implicitly
including some control flags in the sequence space so they can be including some control flags in the sequence space so they can be
retransmitted and acknowledged without confusion (i.e., one and only retransmitted and acknowledged without confusion (i.e., one and only
one copy of the control will be acted upon). Control information is one copy of the control will be acted upon). Control information is
not physically carried in the segment data space. Consequently, we not physically carried in the segment data space. Consequently, we
must adopt rules for implicitly assigning sequence numbers to must adopt rules for implicitly assigning sequence numbers to
control. The SYN and FIN are the only controls requiring this control. The SYN and FIN are the only controls requiring this
protection, and these controls are used only at connection opening protection, and these controls are used only at connection opening
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data octet in a segment in which it occurs. The segment length data octet in a segment in which it occurs. The segment length
(SEG.LEN) includes both data and sequence space occupying controls. (SEG.LEN) includes both data and sequence space occupying controls.
When a SYN is present then SEG.SEQ is the sequence number of the SYN. When a SYN is present then SEG.SEQ is the sequence number of the SYN.
Initial Sequence Number Selection Initial Sequence Number Selection
The protocol places no restriction on a particular connection being The protocol places no restriction on a particular connection being
used over and over again. A connection is defined by a pair of used over and over again. A connection is defined by a pair of
sockets. New instances of a connection will be referred to as sockets. New instances of a connection will be referred to as
incarnations of the connection. The problem that arises from this is incarnations of the connection. The problem that arises from this is
-- "how does the TCP identify duplicate segments from previous -- "how does the TCP implementation identify duplicate segments from
incarnations of the connection?" This problem becomes apparent if previous incarnations of the connection?" This problem becomes
the connection is being opened and closed in quick succession, or if apparent if the connection is being opened and closed in quick
the connection breaks with loss of memory and is then reestablished. succession, or if the connection breaks with loss of memory and is
then reestablished.
To avoid confusion we must prevent segments from one incarnation of a To avoid confusion we must prevent segments from one incarnation of a
connection from being used while the same sequence numbers may still connection from being used while the same sequence numbers may still
be present in the network from an earlier incarnation. We want to be present in the network from an earlier incarnation. We want to
assure this, even if a TCP crashes and loses all knowledge of the assure this, even if a TCP endpoint loses all knowledge of the
sequence numbers it has been using. When new connections are sequence numbers it has been using. When new connections are
created, an initial sequence number (ISN) generator is employed which created, an initial sequence number (ISN) generator is employed that
selects a new 32 bit ISN. There are security issues that result if selects a new 32 bit ISN. There are security issues that result if
an off-path attacker is able to predict or guess ISN values. an off-path attacker is able to predict or guess ISN values.
The recommended ISN generator is based on the combination of a The recommended ISN generator is based on the combination of a
(possibly fictitious) 32 bit clock whose low order bit is incremented (possibly fictitious) 32 bit clock whose low order bit is incremented
roughly every 4 microseconds, and a pseudorandom hash function (PRF). roughly every 4 microseconds, and a pseudorandom hash function (PRF).
The clock component is intended to insure that with a Maximum Segment The clock component is intended to insure that with a Maximum Segment
Lifetime (MSL), generated ISNs will be unique, since it cycles Lifetime (MSL), generated ISNs will be unique, since it cycles
approximately every 4.55 hours, which is much longer than the MSL. approximately every 4.55 hours, which is much longer than the MSL.
This recommended algorithm is further described in RFC 1948 and This recommended algorithm is further described in RFC 6528 [34] and
builds on the basic clock-driven algorithm from RFC 793. builds on the basic clock-driven algorithm from RFC 793.
A TCP MUST use a clock-driven selection of initial sequence numbers A TCP implementation MUST use a clock-driven selection of initial
(MUST-8), and SHOULD generate its Initial Sequence Numbers with the sequence numbers (MUST-8), and SHOULD generate its Initial Sequence
expression: Numbers with the expression:
ISN = M + F(localip, localport, remoteip, remoteport, secretkey) ISN = M + F(localip, localport, remoteip, remoteport, secretkey)
where M is the 4 microsecond timer, and F() is a pseudorandom where M is the 4 microsecond timer, and F() is a pseudorandom
function (PRF) of the connection's identifying parameters ("localip, function (PRF) of the connection's identifying parameters ("localip,
localport, remoteip, remoteport") and a secret key ("secretkey") localport, remoteip, remoteport") and a secret key ("secretkey")
(SHLD-1). F() MUST NOT be computable from the outside (MUST-9), or (SHLD-1). F() MUST NOT be computable from the outside (MUST-9), or
an attacker could still guess at sequence numbers from the ISN used an attacker could still guess at sequence numbers from the ISN used
for some other connection. The PRF could be implemented as a for some other connection. The PRF could be implemented as a
cryptographic has of the concatenation of the TCP connection cryptographic hash of the concatenation of the TCP connection
parameters and some secret data. For discussion of the selection of parameters and some secret data. For discussion of the selection of
a specific hash algorithm and management of the secret key data, a specific hash algorithm and management of the secret key data,
please see Section 3 of [32]. please see Section 3 of [34].
For each connection there is a send sequence number and a receive For each connection there is a send sequence number and a receive
sequence number. The initial send sequence number (ISS) is chosen by sequence number. The initial send sequence number (ISS) is chosen by
the data sending TCP, and the initial receive sequence number (IRS) the data sending TCP peer, and the initial receive sequence number
is learned during the connection establishing procedure. (IRS) is learned during the connection establishing procedure.
For a connection to be established or initialized, the two TCPs must For a connection to be established or initialized, the two TCP peers
synchronize on each other's initial sequence numbers. This is done must synchronize on each other's initial sequence numbers. This is
in an exchange of connection establishing segments carrying a control done in an exchange of connection establishing segments carrying a
bit called "SYN" (for synchronize) and the initial sequence numbers. control bit called "SYN" (for synchronize) and the initial sequence
As a shorthand, segments carrying the SYN bit are also called "SYNs". numbers. As a shorthand, segments carrying the SYN bit are also
Hence, the solution requires a suitable mechanism for picking an called "SYNs". Hence, the solution requires a suitable mechanism for
initial sequence number and a slightly involved handshake to exchange picking an initial sequence number and a slightly involved handshake
the ISN's. to exchange the ISN's.
The synchronization requires each side to send its own initial The synchronization requires each side to send its own initial
sequence number and to receive a confirmation of it in acknowledgment sequence number and to receive a confirmation of it in acknowledgment
from the other side. Each side must also receive the other side's from the remote TCP peer. Each side must also receive the remote
initial sequence number and send a confirming acknowledgment. peer's initial sequence number and send a confirming acknowledgment.
1) A --> B SYN my sequence number is X 1) A --> B SYN my sequence number is X
2) A <-- B ACK your sequence number is X 2) A <-- B ACK your sequence number is X
3) A <-- B SYN my sequence number is Y 3) A <-- B SYN my sequence number is Y
4) A --> B ACK your sequence number is Y 4) A --> B ACK your sequence number is Y
Because steps 2 and 3 can be combined in a single message this is Because steps 2 and 3 can be combined in a single message this is
called the three way (or three message) handshake. called the three way (or three message) handshake.
A three way handshake is necessary because sequence numbers are not A three way handshake is necessary because sequence numbers are not
tied to a global clock in the network, and TCPs may have different tied to a global clock in the network, and TCP implementations may
mechanisms for picking the ISN's. The receiver of the first SYN has have different mechanisms for picking the ISN's. The receiver of the
no way of knowing whether the segment was an old delayed one or not, first SYN has no way of knowing whether the segment was an old
unless it remembers the last sequence number used on the connection delayed one or not, unless it remembers the last sequence number used
(which is not always possible), and so it must ask the sender to on the connection (which is not always possible), and so it must ask
verify this SYN. The three way handshake and the advantages of a the sender to verify this SYN. The three way handshake and the
clock-driven scheme are discussed in [47]. advantages of a clock-driven scheme are discussed in [52].
Knowing When to Keep Quiet Knowing When to Keep Quiet
To be sure that a TCP does not create a segment that carries a To be sure that a TCP implementation does not create a segment
sequence number which may be duplicated by an old segment remaining carrying a sequence number that may be duplicated by an old segment
in the network, the TCP must keep quiet for an MSL before assigning remaining in the network, the TCP endpoint must keep quiet for an MSL
any sequence numbers upon starting up or recovering from a crash in before assigning any sequence numbers upon starting up or recovering
which memory of sequence numbers in use was lost. For this from a situation where memory of sequence numbers in use was lost.
specification the MSL is taken to be 2 minutes. This is an For this specification the MSL is taken to be 2 minutes. This is an
engineering choice, and may be changed if experience indicates it is engineering choice, and may be changed if experience indicates it is
desirable to do so. Note that if a TCP is reinitialized in some desirable to do so. Note that if a TCP endpoint is reinitialized in
sense, yet retains its memory of sequence numbers in use, then it some sense, yet retains its memory of sequence numbers in use, then
need not wait at all; it must only be sure to use sequence numbers it need not wait at all; it must only be sure to use sequence numbers
larger than those recently used. larger than those recently used.
The TCP Quiet Time Concept The TCP Quiet Time Concept
This specification provides that hosts which "crash" without Hosts that for any reason lose knowledge of the last sequence numbers
retaining any knowledge of the last sequence numbers transmitted on transmitted on each active (i.e., not closed) connection shall delay
each active (i.e., not closed) connection shall delay emitting any emitting any TCP segments for at least the agreed MSL in the internet
TCP segments for at least the agreed MSL in the internet system of system that the host is a part of. In the paragraphs below, an
which the host is a part. In the paragraphs below, an explanation explanation for this specification is given. TCP implementors may
for this specification is given. TCP implementors may violate the violate the "quiet time" restriction, but only at the risk of causing
"quiet time" restriction, but only at the risk of causing some old some old data to be accepted as new or new data rejected as old
data to be accepted as new or new data rejected as old duplicated by duplicated by some receivers in the internet system.
some receivers in the internet system.
TCPs consume sequence number space each time a segment is formed and TCP endpoints consume sequence number space each time a segment is
entered into the network output queue at a source host. The formed and entered into the network output queue at a source host.
duplicate detection and sequencing algorithm in the TCP protocol The duplicate detection and sequencing algorithm in the TCP protocol
relies on the unique binding of segment data to sequence space to the relies on the unique binding of segment data to sequence space to the
extent that sequence numbers will not cycle through all 2**32 values extent that sequence numbers will not cycle through all 2**32 values
before the segment data bound to those sequence numbers has been before the segment data bound to those sequence numbers has been
delivered and acknowledged by the receiver and all duplicate copies delivered and acknowledged by the receiver and all duplicate copies
of the segments have "drained" from the internet. Without such an of the segments have "drained" from the internet. Without such an
assumption, two distinct TCP segments could conceivably be assigned assumption, two distinct TCP segments could conceivably be assigned
the same or overlapping sequence numbers, causing confusion at the the same or overlapping sequence numbers, causing confusion at the
receiver as to which data is new and which is old. Remember that receiver as to which data is new and which is old. Remember that
each segment is bound to as many consecutive sequence numbers as each segment is bound to as many consecutive sequence numbers as
there are octets of data and SYN or FIN flags in the segment. there are octets of data and SYN or FIN flags in the segment.
Under normal conditions, TCPs keep track of the next sequence number Under normal conditions, TCP implementations keep track of the next
to emit and the oldest awaiting acknowledgment so as to avoid sequence number to emit and the oldest awaiting acknowledgment so as
mistakenly using a sequence number over before its first use has been to avoid mistakenly using a sequence number over before its first use
acknowledged. This alone does not guarantee that old duplicate data has been acknowledged. This alone does not guarantee that old
is drained from the net, so the sequence space has been made very duplicate data is drained from the net, so the sequence space has
large to reduce the probability that a wandering duplicate will cause been made very large to reduce the probability that a wandering
trouble upon arrival. At 2 megabits/sec. it takes 4.5 hours to use duplicate will cause trouble upon arrival. At 2 megabits/sec. it
up 2**32 octets of sequence space. Since the maximum segment takes 4.5 hours to use up 2**32 octets of sequence space. Since the
lifetime in the net is not likely to exceed a few tens of seconds, maximum segment lifetime in the net is not likely to exceed a few
this is deemed ample protection for foreseeable nets, even if data tens of seconds, this is deemed ample protection for foreseeable
rates escalate to l0's of megabits/sec. At 100 megabits/sec, the nets, even if data rates escalate to l0's of megabits/sec. At 100
cycle time is 5.4 minutes which may be a little short, but still megabits/sec, the cycle time is 5.4 minutes, which may be a little
within reason. short, but still within reason.
The basic duplicate detection and sequencing algorithm in TCP can be The basic duplicate detection and sequencing algorithm in TCP can be
defeated, however, if a source TCP does not have any memory of the defeated, however, if a source TCP endpoint does not have any memory
sequence numbers it last used on a given connection. For example, if of the sequence numbers it last used on a given connection. For
the TCP were to start all connections with sequence number 0, then example, if the TCP implementation were to start all connections with
upon crashing and restarting, a TCP might re-form an earlier sequence number 0, then upon the host rebooting, a TCP peer might re-
connection (possibly after half-open connection resolution) and emit form an earlier connection (possibly after half-open connection
packets with sequence numbers identical to or overlapping with resolution) and emit packets with sequence numbers identical to or
packets still in the network which were emitted on an earlier overlapping with packets still in the network, which were emitted on
incarnation of the same connection. In the absence of knowledge an earlier incarnation of the same connection. In the absence of
about the sequence numbers used on a particular connection, the TCP knowledge about the sequence numbers used on a particular connection,
specification recommends that the source delay for MSL seconds before the TCP specification recommends that the source delay for MSL
emitting segments on the connection, to allow time for segments from seconds before emitting segments on the connection, to allow time for
the earlier connection incarnation to drain from the system. segments from the earlier connection incarnation to drain from the
system.
Even hosts which can remember the time of day and used it to select Even hosts that can remember the time of day and used it to select
initial sequence number values are not immune from this problem initial sequence number values are not immune from this problem
(i.e., even if time of day is used to select an initial sequence (i.e., even if time of day is used to select an initial sequence
number for each new connection incarnation). number for each new connection incarnation).
Suppose, for example, that a connection is opened starting with Suppose, for example, that a connection is opened starting with
sequence number S. Suppose that this connection is not used much and sequence number S. Suppose that this connection is not used much and
that eventually the initial sequence number function (ISN(t)) takes that eventually the initial sequence number function (ISN(t)) takes
on a value equal to the sequence number, say S1, of the last segment on a value equal to the sequence number, say S1, of the last segment
sent by this TCP on a particular connection. Now suppose, at this sent by this TCP endpoint on a particular connection. Now suppose,
instant, the host crashes, recovers, and establishes a new at this instant, the host reboots and establishes a new incarnation
incarnation of the connection. The initial sequence number chosen is of the connection. The initial sequence number chosen is S1 = ISN(t)
S1 = ISN(t) -- last used sequence number on old incarnation of -- last used sequence number on old incarnation of connection! If
connection! If the recovery occurs quickly enough, any old the recovery occurs quickly enough, any old duplicates in the net
duplicates in the net bearing sequence numbers in the neighborhood of bearing sequence numbers in the neighborhood of S1 may arrive and be
S1 may arrive and be treated as new packets by the receiver of the treated as new packets by the receiver of the new incarnation of the
new incarnation of the connection. connection.
The problem is that the recovering host may not know for how long it The problem is that the recovering host may not know for how long it
crashed nor does it know whether there are still old duplicates in was down between rebooting nor does it know whether there are still
the system from earlier connection incarnations. old duplicates in the system from earlier connection incarnations.
One way to deal with this problem is to deliberately delay emitting One way to deal with this problem is to deliberately delay emitting
segments for one MSL after recovery from a crash- this is the "quiet segments for one MSL after recovery from a reboot - this is the
time" specification. Hosts which prefer to avoid waiting are willing "quiet time" specification. Hosts that prefer to avoid waiting are
to risk possible confusion of old and new packets at a given willing to risk possible confusion of old and new packets at a given
destination may choose not to wait for the "quite time". destination may choose not to wait for the "quiet time".
Implementors may provide TCP users with the ability to select on a Implementors may provide TCP users with the ability to select on a
connection by connection basis whether to wait after a crash, or may connection by connection basis whether to wait after a reboot, or may
informally implement the "quite time" for all connections. informally implement the "quiet time" for all connections.
Obviously, even where a user selects to "wait," this is not necessary Obviously, even where a user selects to "wait," this is not necessary
after the host has been "up" for at least MSL seconds. after the host has been "up" for at least MSL seconds.
To summarize: every segment emitted occupies one or more sequence To summarize: every segment emitted occupies one or more sequence
numbers in the sequence space, the numbers occupied by a segment are numbers in the sequence space, the numbers occupied by a segment are
"busy" or "in use" until MSL seconds have passed, upon crashing a "busy" or "in use" until MSL seconds have passed, upon rebooting a
block of space-time is occupied by the octets and SYN or FIN flags of block of space-time is occupied by the octets and SYN or FIN flags of
the last emitted segment, if a new connection is started too soon and the last emitted segment, if a new connection is started too soon and
uses any of the sequence numbers in the space-time footprint of the uses any of the sequence numbers in the space-time footprint of the
last segment of the previous connection incarnation, there is a last segment of the previous connection incarnation, there is a
potential sequence number overlap area which could cause confusion at potential sequence number overlap area that could cause confusion at
the receiver. the receiver.
3.4. Establishing a connection 3.4. Establishing a connection
The "three-way handshake" is the procedure used to establish a The "three-way handshake" is the procedure used to establish a
connection. This procedure normally is initiated by one TCP and connection. This procedure normally is initiated by one TCP peer and
responded to by another TCP. The procedure also works if two TCP responded to by another TCP peer. The procedure also works if two
simultaneously initiate the procedure. When simultaneous attempt TCP peers simultaneously initiate the procedure. When simultaneous
occurs, each TCP receives a "SYN" segment which carries no open occurs, each TCP peer receives a "SYN" segment that carries no
acknowledgment after it has sent a "SYN". Of course, the arrival of acknowledgment after it has sent a "SYN". Of course, the arrival of
an old duplicate "SYN" segment can potentially make it appear, to the an old duplicate "SYN" segment can potentially make it appear, to the
recipient, that a simultaneous connection initiation is in progress. recipient, that a simultaneous connection initiation is in progress.
Proper use of "reset" segments can disambiguate these cases. Proper use of "reset" segments can disambiguate these cases.
Several examples of connection initiation follow. Although these Several examples of connection initiation follow. Although these
examples do not show connection synchronization using data-carrying examples do not show connection synchronization using data-carrying
segments, this is perfectly legitimate, so long as the receiving TCP segments, this is perfectly legitimate, so long as the receiving TCP
doesn't deliver the data to the user until it is clear the data is endpoint doesn't deliver the data to the user until it is clear the
valid (i.e., the data must be buffered at the receiver until the data is valid (e.g., the data is buffered at the receiver until the
connection reaches the ESTABLISHED state). The three-way handshake connection reaches the ESTABLISHED state, given that the three-way
reduces the possibility of false connections. It is the handshake reduces the possibility of false connections). It is the
implementation of a trade-off between memory and messages to provide implementation of a trade-off between memory and messages to provide
information for this checking. information for this checking.
The simplest three-way handshake is shown in Figure 5 below. The The simplest three-way handshake is shown in Figure 5 below. The
figures should be interpreted in the following way. Each line is figures should be interpreted in the following way. Each line is
numbered for reference purposes. Right arrows (-->) indicate numbered for reference purposes. Right arrows (-->) indicate
departure of a TCP segment from TCP A to TCP B, or arrival of a departure of a TCP segment from TCP peer A to TCP peer B, or arrival
segment at B from A. Left arrows (<--), indicate the reverse. of a segment at B from A. Left arrows (<--), indicate the reverse.
Ellipsis (...) indicates a segment which is still in the network Ellipsis (...) indicates a segment that is still in the network
(delayed). An "XXX" indicates a segment which is lost or rejected. (delayed). Comments appear in parentheses. TCP connection states
Comments appear in parentheses. TCP states represent the state AFTER represent the state AFTER the departure or arrival of the segment
the departure or arrival of the segment (whose contents are shown in (whose contents are shown in the center of each line). Segment
the center of each line). Segment contents are shown in abbreviated contents are shown in abbreviated form, with sequence number, control
form, with sequence number, control flags, and ACK field. Other flags, and ACK field. Other fields such as window, addresses,
fields such as window, addresses, lengths, and text have been left lengths, and text have been left out in the interest of clarity.
out in the interest of clarity.
TCP A TCP B TCP Peer A TCP Peer B
1. CLOSED LISTEN 1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED 2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED
3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED 3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED 4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED
5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED 5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED
Figure 5: Basic 3-Way Handshake for Connection Synchronization Figure 5: Basic 3-Way Handshake for Connection Synchronization
In line 2 of Figure 5, TCP A begins by sending a SYN segment In line 2 of Figure 5, TCP Peer A begins by sending a SYN segment
indicating that it will use sequence numbers starting with sequence indicating that it will use sequence numbers starting with sequence
number 100. In line 3, TCP B sends a SYN and acknowledges the SYN it number 100. In line 3, TCP Peer B sends a SYN and acknowledges the
received from TCP A. Note that the acknowledgment field indicates SYN it received from TCP Peer A. Note that the acknowledgment field
TCP B is now expecting to hear sequence 101, acknowledging the SYN indicates TCP Peer B is now expecting to hear sequence 101,
which occupied sequence 100. acknowledging the SYN that occupied sequence 100.
At line 4, TCP A responds with an empty segment containing an ACK for At line 4, TCP Peer A responds with an empty segment containing an
TCP B's SYN; and in line 5, TCP A sends some data. Note that the ACK for TCP Peer B's SYN; and in line 5, TCP Peer A sends some data.
sequence number of the segment in line 5 is the same as in line 4 Note that the sequence number of the segment in line 5 is the same as
because the ACK does not occupy sequence number space (if it did, we in line 4 because the ACK does not occupy sequence number space (if
would wind up ACKing ACK's!). it did, we would wind up ACKing ACK's!).
Simultaneous initiation is only slightly more complex, as is shown in Simultaneous initiation is only slightly more complex, as is shown in
Figure 6. Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to Figure 6. Each TCP peer's connection state cycles from CLOSED to
ESTABLISHED. SYN-SENT to SYN-RECEIVED to ESTABLISHED.
TCP A TCP B TCP Peer A TCP Peer B
1. CLOSED CLOSED 1. CLOSED CLOSED
2. SYN-SENT --> <SEQ=100><CTL=SYN> ... 2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
3. SYN-RECEIVED <-- <SEQ=300><CTL=SYN> <-- SYN-SENT 3. SYN-RECEIVED <-- <SEQ=300><CTL=SYN> <-- SYN-SENT
4. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED 4. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED
5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ... 5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED 6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
7. ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED 7. ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED
Figure 6: Simultaneous Connection Synchronization Figure 6: Simultaneous Connection Synchronization
A TCP MUST support simultaneous open attempts (MUST-10). A TCP implementation MUST support simultaneous open attempts (MUST-
10).
Note that a TCP implementation MUST keep track of whether a Note that a TCP implementation MUST keep track of whether a
connection has reached SYN-RECEIVED state as the result of a passive connection has reached SYN-RECEIVED state as the result of a passive
OPEN or an active OPEN (MUST-11). OPEN or an active OPEN (MUST-11).
The principal reason for the three-way handshake is to prevent old The principal reason for the three-way handshake is to prevent old
duplicate connection initiations from causing confusion. To deal duplicate connection initiations from causing confusion. To deal
with this, a special control message, reset, has been devised. If with this, a special control message, reset, is specified. If the
the receiving TCP is in a non-synchronized state (i.e., SYN-SENT, receiving TCP peer is in a non-synchronized state (i.e., SYN-SENT,
SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset. SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.
If the TCP is in one of the synchronized states (ESTABLISHED, FIN- If the TCP peer is in one of the synchronized states (ESTABLISHED,
WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it
aborts the connection and informs its user. We discuss this latter aborts the connection and informs its user. We discuss this latter
case under "half-open" connections below. case under "half-open" connections below.
TCP A TCP B TCP Peer A TCP Peer B
1. CLOSED LISTEN 1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN> ... 2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
3. (duplicate) ... <SEQ=90><CTL=SYN> --> SYN-RECEIVED 3. (duplicate) ... <SEQ=90><CTL=SYN> --> SYN-RECEIVED
4. SYN-SENT <-- <SEQ=300><ACK=91><CTL=SYN,ACK> <-- SYN-RECEIVED 4. SYN-SENT <-- <SEQ=300><ACK=91><CTL=SYN,ACK> <-- SYN-RECEIVED
5. SYN-SENT --> <SEQ=91><CTL=RST> --> LISTEN 5. SYN-SENT --> <SEQ=91><CTL=RST> --> LISTEN
6. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED 6. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED
7. SYN-SENT <-- <SEQ=400><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED 7. SYN-SENT <-- <SEQ=400><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
8. ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK> --> ESTABLISHED 8. ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK> --> ESTABLISHED
Figure 7: Recovery from Old Duplicate SYN Figure 7: Recovery from Old Duplicate SYN
As a simple example of recovery from old duplicates, consider As a simple example of recovery from old duplicates, consider
Figure 7. At line 3, an old duplicate SYN arrives at TCP B. TCP B Figure 7. At line 3, an old duplicate SYN arrives at TCP Peer B.
cannot tell that this is an old duplicate, so it responds normally TCP Peer B cannot tell that this is an old duplicate, so it responds
(line 4). TCP A detects that the ACK field is incorrect and returns normally (line 4). TCP Peer A detects that the ACK field is
a RST (reset) with its SEQ field selected to make the segment incorrect and returns a RST (reset) with its SEQ field selected to
believable. TCP B, on receiving the RST, returns to the LISTEN make the segment believable. TCP Peer B, on receiving the RST,
state. When the original SYN (pun intended) finally arrives at line returns to the LISTEN state. When the original SYN finally arrives
6, the synchronization proceeds normally. If the SYN at line 6 had at line 6, the synchronization proceeds normally. If the SYN at line
arrived before the RST, a more complex exchange might have occurred 6 had arrived before the RST, a more complex exchange might have
with RST's sent in both directions. occurred with RST's sent in both directions.
Half-Open Connections and Other Anomalies Half-Open Connections and Other Anomalies
An established connection is said to be "half-open" if one of the An established connection is said to be "half-open" if one of the TCP
TCPs has closed or aborted the connection at its end without the peers has closed or aborted the connection at its end without the
knowledge of the other, or if the two ends of the connection have knowledge of the other, or if the two ends of the connection have
become desynchronized owing to a crash that resulted in loss of become desynchronized owing to a failure or reboot that resulted in
memory. Such connections will automatically become reset if an loss of memory. Such connections will automatically become reset if
attempt is made to send data in either direction. However, half-open an attempt is made to send data in either direction. However, half-
connections are expected to be unusual, and the recovery procedure is open connections are expected to be unusual.
mildly involved.
If at site A the connection no longer exists, then an attempt by the If at site A the connection no longer exists, then an attempt by the
user at site B to send any data on it will result in the site B TCP user at site B to send any data on it will result in the site B TCP
receiving a reset control message. Such a message indicates to the endpoint receiving a reset control message. Such a message indicates
site B TCP that something is wrong, and it is expected to abort the to the site B TCP endpoint that something is wrong, and it is
connection. expected to abort the connection.
Assume that two user processes A and B are communicating with one Assume that two user processes A and B are communicating with one
another when a crash occurs causing loss of memory to A's TCP. another when a failure or reboot occurs causing loss of memory to A's
Depending on the operating system supporting A's TCP, it is likely TCP implementation. Depending on the operating system supporting A's
that some error recovery mechanism exists. When the TCP is up again, TCP implementation, it is likely that some error recovery mechanism
A is likely to start again from the beginning or from a recovery exists. When the TCP endpoint is up again, A is likely to start
point. As a result, A will probably try to OPEN the connection again again from the beginning or from a recovery point. As a result, A
or try to SEND on the connection it believes open. In the latter will probably try to OPEN the connection again or try to SEND on the
case, it receives the error message "connection not open" from the connection it believes open. In the latter case, it receives the
local (A's) TCP. In an attempt to establish the connection, A's TCP error message "connection not open" from the local (A's) TCP
will send a segment containing SYN. This scenario leads to the implementation. In an attempt to establish the connection, A's TCP
example shown in Figure 8. After TCP A crashes, the user attempts to implementation will send a segment containing SYN. This scenario
re-open the connection. TCP B, in the meantime, thinks the leads to the example shown in Figure 8. After TCP Peer A reboots,
connection is open. the user attempts to re-open the connection. TCP Peer B, in the
meantime, thinks the connection is open.
TCP A TCP B TCP Peer A TCP Peer B
1. (CRASH) (send 300,receive 100) 1. (REBOOT) (send 300,receive 100)
2. CLOSED ESTABLISHED 2. CLOSED ESTABLISHED
3. SYN-SENT --> <SEQ=400><CTL=SYN> --> (??) 3. SYN-SENT --> <SEQ=400><CTL=SYN> --> (??)
4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK> <-- ESTABLISHED 4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK> <-- ESTABLISHED
5. SYN-SENT --> <SEQ=100><CTL=RST> --> (Abort!!) 5. SYN-SENT --> <SEQ=100><CTL=RST> --> (Abort!!)
6. SYN-SENT CLOSED 6. SYN-SENT CLOSED
7. SYN-SENT --> <SEQ=400><CTL=SYN> --> 7. SYN-SENT --> <SEQ=400><CTL=SYN> -->
Figure 8: Half-Open Connection Discovery Figure 8: Half-Open Connection Discovery
When the SYN arrives at line 3, TCP B, being in a synchronized state, When the SYN arrives at line 3, TCP Peer B, being in a synchronized
and the incoming segment outside the window, responds with an state, and the incoming segment outside the window, responds with an
acknowledgment indicating what sequence it next expects to hear (ACK acknowledgment indicating what sequence it next expects to hear (ACK
100). TCP A sees that this segment does not acknowledge anything it 100). TCP Peer A sees that this segment does not acknowledge
sent and, being unsynchronized, sends a reset (RST) because it has anything it sent and, being unsynchronized, sends a reset (RST)
detected a half-open connection. TCP B aborts at line 5. TCP A will because it has detected a half-open connection. TCP Peer B aborts at
continue to try to establish the connection; the problem is now line 5. TCP Peer A will continue to try to establish the connection;
reduced to the basic 3-way handshake of Figure 5. the problem is now reduced to the basic 3-way handshake of Figure 5.
An interesting alternative case occurs when TCP A crashes and TCP B An interesting alternative case occurs when TCP Peer A reboots and
tries to send data on what it thinks is a synchronized connection. TCP Peer B tries to send data on what it thinks is a synchronized
This is illustrated in Figure 9. In this case, the data arriving at connection. This is illustrated in Figure 9. In this case, the data
TCP A from TCP B (line 2) is unacceptable because no such connection arriving at TCP Peer A from TCP Peer B (line 2) is unacceptable
exists, so TCP A sends a RST. The RST is acceptable so TCP B because no such connection exists, so TCP Peer A sends a RST. The
processes it and aborts the connection. RST is acceptable so TCP Peer B processes it and aborts the
connection.
TCP A TCP B TCP Peer A TCP Peer B
1. (CRASH) (send 300,receive 100) 1. (REBOOT) (send 300,receive 100)
2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED 2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED
3. --> <SEQ=100><CTL=RST> --> (ABORT!!) 3. --> <SEQ=100><CTL=RST> --> (ABORT!!)
Figure 9: Active Side Causes Half-Open Connection Discovery Figure 9: Active Side Causes Half-Open Connection Discovery
In Figure 10, we find the two TCPs A and B with passive connections In Figure 10, we find the two TCP Peers A and B with passive
waiting for SYN. An old duplicate arriving at TCP B (line 2) stirs B connections waiting for SYN. An old duplicate arriving at TCP Peer B
into action. A SYN-ACK is returned (line 3) and causes TCP A to (line 2) stirs B into action. A SYN-ACK is returned (line 3) and
generate a RST (the ACK in line 3 is not acceptable). TCP B accepts causes TCP A to generate a RST (the ACK in line 3 is not acceptable).
the reset and returns to its passive LISTEN state. TCP Peer B accepts the reset and returns to its passive LISTEN state.
TCP A TCP B TCP Peer A TCP Peer B
1. LISTEN LISTEN 1. LISTEN LISTEN
2. ... <SEQ=Z><CTL=SYN> --> SYN-RECEIVED 2. ... <SEQ=Z><CTL=SYN> --> SYN-RECEIVED
3. (??) <-- <SEQ=X><ACK=Z+1><CTL=SYN,ACK> <-- SYN-RECEIVED 3. (??) <-- <SEQ=X><ACK=Z+1><CTL=SYN,ACK> <-- SYN-RECEIVED
4. --> <SEQ=Z+1><CTL=RST> --> (return to LISTEN!) 4. --> <SEQ=Z+1><CTL=RST> --> (return to LISTEN!)
5. LISTEN LISTEN 5. LISTEN LISTEN
Figure 10: Old Duplicate SYN Initiates a Reset on two Passive Sockets Figure 10: Old Duplicate SYN Initiates a Reset on two Passive Sockets
A variety of other cases are possible, all of which are accounted for A variety of other cases are possible, all of which are accounted for
by the following rules for RST generation and processing. by the following rules for RST generation and processing.
Reset Generation Reset Generation
As a general rule, reset (RST) must be sent whenever a segment As a general rule, reset (RST) must be sent whenever a segment
arrives which apparently is not intended for the current connection. arrives that apparently is not intended for the current connection.
A reset must not be sent if it is not clear that this is the case. A reset must not be sent if it is not clear that this is the case.
There are three groups of states: There are three groups of states:
1. If the connection does not exist (CLOSED) then a reset is sent 1. If the connection does not exist (CLOSED) then a reset is sent
in response to any incoming segment except another reset. In in response to any incoming segment except another reset. In
particular, SYNs addressed to a non-existent connection are particular, SYNs addressed to a non-existent connection are
rejected by this means. rejected by this means.
If the incoming segment has the ACK bit set, the reset takes its If the incoming segment has the ACK bit set, the reset takes its
sequence number from the ACK field of the segment, otherwise the sequence number from the ACK field of the segment, otherwise the
reset has sequence number zero and the ACK field is set to the sum reset has sequence number zero and the ACK field is set to the sum
of the sequence number and segment length of the incoming segment. of the sequence number and segment length of the incoming segment.
The connection remains in the CLOSED state. The connection remains in the CLOSED state.
2. If the connection is in any non-synchronized state (LISTEN, 2. If the connection is in any non-synchronized state (LISTEN,
SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges
something not yet sent (the segment carries an unacceptable ACK), something not yet sent (the segment carries an unacceptable ACK),
or if an incoming segment has a security level or compartment or if an incoming segment has a security level or compartment that
which does not exactly match the level and compartment requested does not exactly match the level and compartment requested for the
for the connection, a reset is sent. connection, a reset is sent.
If the incoming segment has an ACK field, the reset takes its If the incoming segment has an ACK field, the reset takes its
sequence number from the ACK field of the segment, otherwise the sequence number from the ACK field of the segment, otherwise the
reset has sequence number zero and the ACK field is set to the sum reset has sequence number zero and the ACK field is set to the sum
of the sequence number and segment length of the incoming segment. of the sequence number and segment length of the incoming segment.
The connection remains in the same state. The connection remains in the same state.
3. If the connection is in a synchronized state (ESTABLISHED, 3. If the connection is in a synchronized state (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT),
any unacceptable segment (out of window sequence number or any unacceptable segment (out of window sequence number or
unacceptable acknowledgment number) must elicit only an empty unacceptable acknowledgment number) must elicit only an empty
acknowledgment segment containing the current send-sequence number acknowledgment segment containing the current send-sequence number
and an acknowledgment indicating the next sequence number expected and an acknowledgment indicating the next sequence number expected
to be received, and the connection remains in the same state. to be received, and the connection remains in the same state.
If an incoming segment has a security level, or compartment which If an incoming segment has a security level, or compartment that
does not exactly match the level and compartment requested for the does not exactly match the level and compartment requested for the
connection, a reset is sent and the connection goes to the CLOSED connection, a reset is sent and the connection goes to the CLOSED
state. The reset takes its sequence number from the ACK field of state. The reset takes its sequence number from the ACK field of
the incoming segment. the incoming segment.
Reset Processing Reset Processing
In all states except SYN-SENT, all reset (RST) segments are validated In all states except SYN-SENT, all reset (RST) segments are validated
by checking their SEQ-fields. A reset is valid if its sequence by checking their SEQ-fields. A reset is valid if its sequence
number is in the window. In the SYN-SENT state (a RST received in number is in the window. In the SYN-SENT state (a RST received in
skipping to change at page 29, line 5 skipping to change at page 29, line 8
acknowledges the SYN. acknowledges the SYN.
The receiver of a RST first validates it, then changes state. If the The receiver of a RST first validates it, then changes state. If the
receiver was in the LISTEN state, it ignores it. If the receiver was receiver was in the LISTEN state, it ignores it. If the receiver was
in SYN-RECEIVED state and had previously been in the LISTEN state, in SYN-RECEIVED state and had previously been in the LISTEN state,
then the receiver returns to the LISTEN state, otherwise the receiver then the receiver returns to the LISTEN state, otherwise the receiver
aborts the connection and goes to the CLOSED state. If the receiver aborts the connection and goes to the CLOSED state. If the receiver
was in any other state, it aborts the connection and advises the user was in any other state, it aborts the connection and advises the user
and goes to the CLOSED state. and goes to the CLOSED state.
TCP SHOULD allow a received RST segment to include data (SHLD-2). TCP implementations SHOULD allow a received RST segment to include
data (SHLD-2).
3.5. Closing a Connection 3.5. Closing a Connection
CLOSE is an operation meaning "I have no more data to send." The CLOSE is an operation meaning "I have no more data to send." The
notion of closing a full-duplex connection is subject to ambiguous notion of closing a full-duplex connection is subject to ambiguous
interpretation, of course, since it may not be obvious how to treat interpretation, of course, since it may not be obvious how to treat
the receiving side of the connection. We have chosen to treat CLOSE the receiving side of the connection. We have chosen to treat CLOSE
in a simplex fashion. The user who CLOSEs may continue to RECEIVE in a simplex fashion. The user who CLOSEs may continue to RECEIVE
until he is told that the other side has CLOSED also. Thus, a until the TCP receiver is told that the remote peer has CLOSED also.
program could initiate several SENDs followed by a CLOSE, and then Thus, a program could initiate several SENDs followed by a CLOSE, and
continue to RECEIVE until signaled that a RECEIVE failed because the then continue to RECEIVE until signaled that a RECEIVE failed because
other side has CLOSED. We assume that the TCP will signal a user, the remote peer has CLOSED. The TCP implementation will signal a
even if no RECEIVEs are outstanding, that the other side has closed, user, even if no RECEIVEs are outstanding, that the remote peer has
so the user can terminate his side gracefully. A TCP will reliably closed, so the user can terminate his side gracefully. A TCP
deliver all buffers SENT before the connection was CLOSED so a user implementation will reliably deliver all buffers SENT before the
who expects no data in return need only wait to hear the connection connection was CLOSED so a user who expects no data in return need
was CLOSED successfully to know that all his data was received at the only wait to hear the connection was CLOSED successfully to know that
destination TCP. Users must keep reading connections they close for all their data was received at the destination TCP endpoint. Users
sending until the TCP says no more data. must keep reading connections they close for sending until the TCP
implementation indicates there is no more data.
There are essentially three cases: There are essentially three cases:
1) The user initiates by telling the TCP to CLOSE the connection 1) The user initiates by telling the TCP implementation to CLOSE
the connection
2) The remote TCP initiates by sending a FIN control signal 2) The remote TCP endpoint initiates by sending a FIN control
signal
3) Both users CLOSE simultaneously 3) Both users CLOSE simultaneously
Case 1: Local user initiates the close Case 1: Local user initiates the close
In this case, a FIN segment can be constructed and placed on the In this case, a FIN segment can be constructed and placed on the
outgoing segment queue. No further SENDs from the user will be outgoing segment queue. No further SENDs from the user will be
accepted by the TCP, and it enters the FIN-WAIT-1 state. RECEIVEs accepted by the TCP implementation, and it enters the FIN-WAIT-1
are allowed in this state. All segments preceding and including state. RECEIVEs are allowed in this state. All segments
FIN will be retransmitted until acknowledged. When the other TCP preceding and including FIN will be retransmitted until
has both acknowledged the FIN and sent a FIN of its own, the first acknowledged. When the other TCP peer has both acknowledged the
TCP can ACK this FIN. Note that a TCP receiving a FIN will ACK FIN and sent a FIN of its own, the first TCP peer can ACK this
but not send its own FIN until its user has CLOSED the connection FIN. Note that a TCP endpoint receiving a FIN will ACK but not
also. send its own FIN until its user has CLOSED the connection also.
Case 2: TCP receives a FIN from the network Case 2: TCP endpoint receives a FIN from the network
If an unsolicited FIN arrives from the network, the receiving TCP If an unsolicited FIN arrives from the network, the receiving TCP
can ACK it and tell the user that the connection is closing. The endpoint can ACK it and tell the user that the connection is
user will respond with a CLOSE, upon which the TCP can send a FIN closing. The user will respond with a CLOSE, upon which the TCP
to the other TCP after sending any remaining data. The TCP then endpoint can send a FIN to the other TCP peer after sending any
waits until its own FIN is acknowledged whereupon it deletes the remaining data. The TCP endpoint then waits until its own FIN is
connection. If an ACK is not forthcoming, after the user timeout acknowledged whereupon it deletes the connection. If an ACK is
the connection is aborted and the user is told. not forthcoming, after the user timeout the connection is aborted
and the user is told.
Case 3: both users close simultaneously Case 3: Both users close simultaneously
A simultaneous CLOSE by users at both ends of a connection causes A simultaneous CLOSE by users at both ends of a connection causes
FIN segments to be exchanged. When all segments preceding the FIN segments to be exchanged. When all segments preceding the
FINs have been processed and acknowledged, each TCP can ACK the FINs have been processed and acknowledged, each TCP peer can ACK
FIN it has received. Both will, upon receiving these ACKs, delete the FIN it has received. Both will, upon receiving these ACKs,
the connection. delete the connection.
TCP A TCP B TCP Peer A TCP Peer B
1. ESTABLISHED ESTABLISHED 1. ESTABLISHED ESTABLISHED
2. (Close) 2. (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT
3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT 3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT
4. (Close) 4. (Close)
TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK
5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED 5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED
6. (2 MSL) 6. (2 MSL)
CLOSED CLOSED
Figure 11: Normal Close Sequence Figure 11: Normal Close Sequence
TCP A TCP B TCP Peer A TCP Peer B
1. ESTABLISHED ESTABLISHED 1. ESTABLISHED ESTABLISHED
2. (Close) (Close) 2. (Close) (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> ... FIN-WAIT-1 FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> ... FIN-WAIT-1
<-- <SEQ=300><ACK=100><CTL=FIN,ACK> <-- <-- <SEQ=300><ACK=100><CTL=FIN,ACK> <--
... <SEQ=100><ACK=300><CTL=FIN,ACK> --> ... <SEQ=100><ACK=300><CTL=FIN,ACK> -->
3. CLOSING --> <SEQ=101><ACK=301><CTL=ACK> ... CLOSING 3. CLOSING --> <SEQ=101><ACK=301><CTL=ACK> ... CLOSING
<-- <SEQ=301><ACK=101><CTL=ACK> <-- <-- <SEQ=301><ACK=101><CTL=ACK> <--
skipping to change at page 31, line 44 skipping to change at page 31, line 44
The normal TCP close sequence delivers buffered data reliably in both The normal TCP close sequence delivers buffered data reliably in both
directions. Since the two directions of a TCP connection are closed directions. Since the two directions of a TCP connection are closed
independently, it is possible for a connection to be "half closed," independently, it is possible for a connection to be "half closed,"
i.e., closed in only one direction, and a host is permitted to i.e., closed in only one direction, and a host is permitted to
continue sending data in the open direction on a half-closed continue sending data in the open direction on a half-closed
connection. connection.
A host MAY implement a "half-duplex" TCP close sequence, so that an A host MAY implement a "half-duplex" TCP close sequence, so that an
application that has called CLOSE cannot continue to read data from application that has called CLOSE cannot continue to read data from
the connection (MAY-1). If such a host issues a CLOSE call while the connection (MAY-1). If such a host issues a CLOSE call while
received data is still pending in TCP, or if new data is received received data is still pending in the TCP connection, or if new data
after CLOSE is called, its TCP SHOULD send a RST to show that data is received after CLOSE is called, its TCP implementation SHOULD send
was lost (SHLD-3). See [17] section 2.17 for discussion. a RST to show that data was lost (SHLD-3). See [17] section 2.17 for
discussion.
When a connection is closed actively, it MUST linger in TIME-WAIT When a connection is closed actively, it MUST linger in TIME-WAIT
state for a time 2xMSL (Maximum Segment Lifetime) (MUST-13). state for a time 2xMSL (Maximum Segment Lifetime) (MUST-13).
However, it MAY accept a new SYN from the remote TCP to reopen the However, it MAY accept a new SYN from the remote TCP endpoint to
connection directly from TIME-WAIT state (MAY-2), if it: reopen the connection directly from TIME-WAIT state (MAY-2), if it:
(1) assigns its initial sequence number for the new connection to (1) assigns its initial sequence number for the new connection to
be larger than the largest sequence number it used on the previous be larger than the largest sequence number it used on the previous
connection incarnation, and connection incarnation, and
(2) returns to TIME-WAIT state if the SYN turns out to be an old (2) returns to TIME-WAIT state if the SYN turns out to be an old
duplicate. duplicate.
When the TCP Timestamp options are available, an improved algorithm When the TCP Timestamp options are available, an improved algorithm
is described in [30] in order to support higher connection is described in [32] in order to support higher connection
establishment rates. This algorithm for reducing TIME-WAIT is a Best establishment rates. This algorithm for reducing TIME-WAIT is a Best
Current Practice that SHOULD be implemented, since timestamp options Current Practice that SHOULD be implemented, since timestamp options
are commonly used, and using them to reduce TIME-WAIT provides are commonly used, and using them to reduce TIME-WAIT provides
benefits for busy Internet servers (SHLD-4). benefits for busy Internet servers (SHLD-4).
3.6. Precedence and Security 3.6. Segmentation
The IPv4 specification [1] includes a precedence value in the (now
obsoleted) Type of Service field (TOS) field. It was modified in
[15], and then obsoleted by the definition of Differentiated Services
(DiffServ) [5]. Setting and conveying TOS between the network layer,
TCP, and applications is obsolete, and replaced by DiffServ in the
current TCP specification.
In DiffServ the former precedence values are treated as Class
Selector codepoints, and methods for compatible treatment are
described in the DiffServ architecture. The RFC 793/1122 TCP
specification includes logic intending to have connections use the
highest precedence requested by either endpoint application, and to
keep the precedence consistent throughout a connection. This logic
from the obsolete TOS is not applicable for DiffServ, and should not
be included in TCP implementations, though changes to DiffServ values
within a connection are discouraged. For discussion of this, see RFC
7657 (sec 5.1, 5.3, and 6) [38].
The obsoleted TOS processing rules in TCP assumed bidirectional (or
symmetric) precedence values used on a connection, but the DiffServ
architecture is asymmetric. Problems with the old TCP logic in this
regard were described in [18] and the solution described is to ignore
IP precedence in TCP. Since RFC 2873 is a Standards Track document
(although not marked as updating RFC 793), current implementations
are expected to be robust to these conditions. Note that the
DiffServ field value used in each direction is a part of the
interface between TCP and the network layer, and values in use can be
indicated both ways between TCP and the application.
The IP security option (IPSO) and compartment defined in [1] was
refined in RFC 1038 that was later obsoleted by RFC 1108. The
Commercial IP Security Option (CIPSO) is defined in FIPS-188, and is
supported by some vendors and operating systems. RFC 1108 is now
Historic, though RFC 791 itself has not been updated to remove the IP
security option. For IPv6, a similar option (CALIPSO) has been
defined [24]. RFC 793 includes logic that includes the IP security/
compartment information in treatment of TCP segments. References to
the IP "security/compartment" in this document may be relevant for
Multi-Level Secure (MLS) system implementers, but can be ignored for
non-MLS implementations, consistent with running code on the
Internet. See Appendix A.1 for further discussion. Note that RFC
5570 describes some MLS networking scenarios where IPSO, CIPSO, or
CALIPSO may be used. In these special cases, TCP implementers should
see section 7.3.1 of RFC 5570, and follow the guidance in that
document on the relation between IP security.
3.7. Segmentation
The term "segmentation" refers to the activity TCP performs when The term "segmentation" refers to the activity TCP performs when
ingesting a stream of bytes from a sending application and ingesting a stream of bytes from a sending application and
packetizing that stream of bytes into TCP segments. Individual TCP packetizing that stream of bytes into TCP segments. Individual TCP
segments often do not correspond one-for-one to individual send (or segments often do not correspond one-for-one to individual send (or
socket write) calls from the application. Applications may perform socket write) calls from the application. Applications may perform
writes at the granularity of messages in the upper layer protocol, writes at the granularity of messages in the upper layer protocol,
but TCP guarantees no boundary coherence between the TCP segments but TCP guarantees no boundary coherence between the TCP segments
sent and received versus user application data read or write buffer sent and received versus user application data read or write buffer
boundaries. In some specific protocols, such as RDMA using DDP and boundaries. In some specific protocols, such as RDMA using DDP and
MPA [22], there are performance optimizations possible when the MPA [24], there are performance optimizations possible when the
relation between TCP segments and application data units can be relation between TCP segments and application data units can be
controlled, and MPA includes a specific mechanism for detecting and controlled, and MPA includes a specific mechanism for detecting and
verifying this relationship between TCP segments and application verifying this relationship between TCP segments and application
message data strcutures, but this is specific to applications like message data strcutures, but this is specific to applications like
RDMA. In general, multiple goals influence the sizing of TCP RDMA. In general, multiple goals influence the sizing of TCP
segments created by a TCP implementation. segments created by a TCP implementation.
Goals driving the sending of larger segments include: Goals driving the sending of larger segments include:
o Reducing the number of packets in flight within the network. o Reducing the number of packets in flight within the network.
o Increasing processing efficiency and potential performance by o Increasing processing efficiency and potential performance by
enabling a smaller number of interrupts and inter-layer enabling a smaller number of interrupts and inter-layer
interactions. interactions.
o Limiting the overhead of TCP headers. o Limiting the overhead of TCP headers.
Note that the performance benefits of sending larger segments may Note that the performance benefits of sending larger segments may
decrease as the size increases, and there may be boundaries where decrease as the size increases, and there may be boundaries where
advantages are reversed. For instance, on some machines 1025 bytes advantages are reversed. For instance, on some implementation
within a segment could lead to worse performance than 1024 bytes, due architectures, 1025 bytes within a segment could lead to worse
purely to data alignment on copy operations. performance than 1024 bytes, due purely to data alignment on copy
operations.
Goals driving the sending of smaller segments include: Goals driving the sending of smaller segments include:
o Avoiding sending segments larger than the smallest MTU within an o Avoiding sending a TCP segment that would result in an IP datagram
IP network path, because this results in either packet loss or larger than the smallest MTU along an IP network path, because
fragmentation. Making matters worse, some firewalls or this results in either packet loss or packet fragmentation.
middleboxes may drop fragmented packets or ICMP messages related Making matters worse, some firewalls or middleboxes may drop
related to fragmentation. fragmented packets or ICMP messages related related to
fragmentation.
o Preventing delays to the application data stream, especially when o Preventing delays to the application data stream, especially when
TCP is waiting on the application to generate more data, or when TCP is waiting on the application to generate more data, or when
the application is waiting on an event or input from its peer in the application is waiting on an event or input from its peer in
order to generate more data. order to generate more data.
o Enabling "fate sharing" between TCP segments and lower-layer data o Enabling "fate sharing" between TCP segments and lower-layer data
units (e.g. below IP, for links with cell or frame sizes smaller units (e.g. below IP, for links with cell or frame sizes smaller
than the IP MTU). than the IP MTU).
Towards meeting these competing sets of goals, TCP includes several Towards meeting these competing sets of goals, TCP includes several
mechanisms, including the Maximum Segment Size option, Path MTU mechanisms, including the Maximum Segment Size option, Path MTU
Discovery, the Nagle algorithm, and support for IPv6 Jumbograms, as Discovery, the Nagle algorithm, and support for IPv6 Jumbograms, as
discussed in the following subsections. discussed in the following subsections.
3.7.1. Maximum Segment Size Option 3.6.1. Maximum Segment Size Option
TCP MUST implement both sending and receiving the MSS option (MUST- TCP endpoints MUST implement both sending and receiving the MSS
14). option (MUST-14).
TCP SHOULD send an MSS option in every SYN segment when its receive TCP implementations SHOULD send an MSS option in every SYN segment
MSS differs from the default 536 for IPv4 or 1220 for IPv6 (SHLD-5), when its receive MSS differs from the default 536 for IPv4 or 1220
and MAY send it always (MAY-3). for IPv6 (SHLD-5), and MAY send it always (MAY-3).
If an MSS option is not received at connection setup, TCP MUST assume If an MSS option is not received at connection setup, TCP
a default send MSS of 536 (576-40) for IPv4 or 1220 (1280 - 60) for implementations MUST assume a default send MSS of 536 (576-40) for
IPv6 (MUST-15). IPv4 or 1220 (1280 - 60) for IPv6 (MUST-15).
The maximum size of a segment that TCP really sends, the "effective The maximum size of a segment that TCP endpoint really sends, the
send MSS," MUST be the smaller (MUST-16) of the send MSS (which "effective send MSS," MUST be the smaller (MUST-16) of the send MSS
reflects the available reassembly buffer size at the remote host, the (that reflects the available reassembly buffer size at the remote
EMTU_R [14]) and the largest transmission size permitted by the IP host, the EMTU_R [14]) and the largest transmission size permitted by
layer (EMTU_S [14]): the IP layer (EMTU_S [14]):
Eff.snd.MSS = Eff.snd.MSS =
min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsize min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsize
where: where:
o SendMSS is the MSS value received from the remote host, or the o SendMSS is the MSS value received from the remote host, or the
default 536 for IPv4 or 1220 for IPv6, if no MSS option is default 536 for IPv4 or 1220 for IPv6, if no MSS option is
received. received.
skipping to change at page 35, line 29 skipping to change at page 34, line 31
o IPoptionsize is the size of any IP options associated with a TCP o IPoptionsize is the size of any IP options associated with a TCP
connection. Note that some options may not be included on all connection. Note that some options may not be included on all
packets, but that for each segment sent, the sender should adjust packets, but that for each segment sent, the sender should adjust
the data length accordingly, within the Eff.snd.MSS. the data length accordingly, within the Eff.snd.MSS.
The MSS value to be sent in an MSS option should be equal to the The MSS value to be sent in an MSS option should be equal to the
effective MTU minus the fixed IP and TCP headers. By ignoring both effective MTU minus the fixed IP and TCP headers. By ignoring both
IP and TCP options when calculating the value for the MSS option, if IP and TCP options when calculating the value for the MSS option, if
there are any IP or TCP options to be sent in a packet, then the there are any IP or TCP options to be sent in a packet, then the
sender must decrease the size of the TCP data accordingly. RFC 6691 sender must decrease the size of the TCP data accordingly. RFC 6691
[33] discusses this in greater detail. [35] discusses this in greater detail.
The MSS value to be sent in an MSS option must be less than or equal The MSS value to be sent in an MSS option must be less than or equal
to: to:
MMS_R - 20 MMS_R - 20
where MMS_R is the maximum size for a transport-layer message that where MMS_R is the maximum size for a transport-layer message that
can be received (and reassembled at the IP layer). TCP obtains MMS_R can be received (and reassembled at the IP layer) (MUST-67). TCP
and MMS_S from the IP layer; see the generic call GET_MAXSIZES in obtains MMS_R and MMS_S from the IP layer; see the generic call
Section 3.4 of RFC 1122. These are defined in terms of their IP MTU GET_MAXSIZES in Section 3.4 of RFC 1122. These are defined in terms
equivalents, EMTU_R and EMTU_S [14]. of their IP MTU equivalents, EMTU_R and EMTU_S [14].
When TCP is used in a situation where either the IP or TCP headers When TCP is used in a situation where either the IP or TCP headers
are not fixed, the sender must reduce the amount of TCP data in any are not fixed, the sender must reduce the amount of TCP data in any
given packet by the number of octets used by the IP and TCP options. given packet by the number of octets used by the IP and TCP options.
This has been a point of confusion historically, as explained in RFC This has been a point of confusion historically, as explained in RFC
6691, Section 3.1. 6691, Section 3.1.
3.7.2. Path MTU Discovery 3.6.2. Path MTU Discovery
A TCP implementation may be aware of the MTU on directly connected A TCP implementation may be aware of the MTU on directly connected
links, but will rarely have insight about MTUs across an entire links, but will rarely have insight about MTUs across an entire
network path. For IPv4, RFC 1122 provides an IP-layer recommendation network path. For IPv4, RFC 1122 provides an IP-layer recommendation
on the default effective MTU for sending to be less than or equal to on the default effective MTU for sending to be less than or equal to
576 for destinations not directly connected. For IPv6, this would be 576 for destinations not directly connected. For IPv6, this would be
1280. In all cases, however, implementation of Path MTU Discovery 1280. In all cases, however, implementation of Path MTU Discovery
(PMTUD) and Packetization Layer Path MTU Discovery (PLPMTUD) is (PMTUD) and Packetization Layer Path MTU Discovery (PLPMTUD) is
strongly recommended in order for TCP to improve segmentation strongly recommended in order for TCP to improve segmentation
decisions. Both PMTUD and PLPMTUD help TCP choose segment sizes that decisions. Both PMTUD and PLPMTUD help TCP choose segment sizes that
avoid both on-path (for IPv4) and source fragmentation (IPv4 and avoid both on-path (for IPv4) and source fragmentation (IPv4 and
IPv6). IPv6).
PMTUD for IPv4 [2] or IPv6 [3] is implemented in conjunction between PMTUD for IPv4 [2] or IPv6 [3] is implemented in conjunction between
TCP, IP, and ICMP protocols. It relies both on avoiding source TCP, IP, and ICMP protocols. It relies both on avoiding source
fragmentation and setting the IPv4 DF (don't fragment) flag, the fragmentation and setting the IPv4 DF (don't fragment) flag, the
latter to inhibit on-path fragmentation. It relies on ICMP errors latter to inhibit on-path fragmentation. It relies on ICMP errors
from routers along the path, whenever a segment is too large to from routers along the path, whenever a segment is too large to
traverse a link. Several adjustments to a TCP implementation with traverse a link. Several adjustments to a TCP implementation with
PMTUD are described in RFC 2923 in order to deal with problems PMTUD are described in RFC 2923 in order to deal with problems
experienced in practice [7]. PLPMTUD [19] is a Standards Track experienced in practice [7]. PLPMTUD [21] is a Standards Track
improvement to PMTUD that relaxes the requirement for ICMP support improvement to PMTUD that relaxes the requirement for ICMP support
across a path, and improves performance in cases where ICMP is not across a path, and improves performance in cases where ICMP is not
consistently conveyed, but still tries to avoid source fragmentation. consistently conveyed, but still tries to avoid source fragmentation.
The mechanisms in all four of these RFCs are recommended to be The mechanisms in all four of these RFCs are recommended to be
included in TCP implementations. included in TCP implementations.
The TCP MSS option specifies an upper bound for the size of packets The TCP MSS option specifies an upper bound for the size of packets
that can be received. Hence, setting the value in the MSS option too that can be received. Hence, setting the value in the MSS option too
small can impact the ability for PMTUD or PLPMTUD to find a larger small can impact the ability for PMTUD or PLPMTUD to find a larger
path MTU. RFC 1191 discusses this implication of many older TCP path MTU. RFC 1191 discusses this implication of many older TCP
implementations setting MSS to 536 for non-local destinations, rather implementations setting MSS to 536 for non-local destinations, rather
than deriving it from the MTUs of connected interfaces as than deriving it from the MTUs of connected interfaces as
recommended. recommended.
3.7.3. Interfaces with Variable MTU Values 3.6.3. Interfaces with Variable MTU Values
The effective MTU can sometimes vary, as when used with variable The effective MTU can sometimes vary, as when used with variable
compression, e.g., RObust Header Compression (ROHC) [26]. It is compression, e.g., RObust Header Compression (ROHC) [28]. It is
tempting for TCP to want to advertise the largest possible MSS, to tempting for a TCP implementation to want to advertise the largest
support the most efficient use of compressed payloads. possible MSS, to support the most efficient use of compressed
Unfortunately, some compression schemes occasionally need to transmit payloads. Unfortunately, some compression schemes occasionally need
full headers (and thus smaller payloads) to resynchronize state at to transmit full headers (and thus smaller payloads) to resynchronize
their endpoint compressors/decompressors. If the largest MTU is used state at their endpoint compressors/decompressors. If the largest
to calculate the value to advertise in the MSS option, TCP MTU is used to calculate the value to advertise in the MSS option,
retransmission may interfere with compressor resynchronization. TCP retransmission may interfere with compressor resynchronization.
As a result, when the effective MTU of an interface varies, TCP As a result, when the effective MTU of an interface varies, TCP
SHOULD use the smallest effective MTU of the interface to calculate implementations SHOULD use the smallest effective MTU of the
the value to advertise in the MSS option (SHLD-6). interface to calculate the value to advertise in the MSS option
(SHLD-6).
3.7.4. Nagle Algorithm 3.6.4. Nagle Algorithm
The "Nagle algorithm" was described in RFC 896 [13] and was The "Nagle algorithm" was described in RFC 896 [13] and was
recommended in RFC 1122 [14] for mitigation of an early problem of recommended in RFC 1122 [14] for mitigation of an early problem of
too many small packets being generated. It has been implemented in too many small packets being generated. It has been implemented in
most current TCP code bases, sometimes with minor variations (see most current TCP code bases, sometimes with minor variations (see
Appendix A.3). Appendix A.3).
If there is unacknowledged data (i.e., SND.NXT > SND.UNA), then the If there is unacknowledged data (i.e., SND.NXT > SND.UNA), then the
sending TCP buffers all user data (regardless of the PSH bit), until sending TCP endpoint buffers all user data (regardless of the PSH
the outstanding data has been acknowledged or until the TCP can send bit), until the outstanding data has been acknowledged or until the
a full-sized segment (Eff.snd.MSS bytes). TCP endpoint can send a full-sized segment (Eff.snd.MSS bytes).
A TCP SHOULD implement the Nagle Algorithm to coalesce short segments A TCP implementation SHOULD implement the Nagle Algorithm to coalesce
(SHLD-7). However, there MUST be a way for an application to disable short segments (SHLD-7). However, there MUST be a way for an
the Nagle algorithm on an individual connection (MUST-17). In all application to disable the Nagle algorithm on an individual
cases, sending data is also subject to the limitation imposed by the connection (MUST-17). In all cases, sending data is also subject to
Slow Start algorithm [25]. the limitation imposed by the Slow Start algorithm [27].
3.7.5. IPv6 Jumbograms 3.6.5. IPv6 Jumbograms
In order to support TCP over IPv6 jumbograms, implementations need to In order to support TCP over IPv6 jumbograms, implementations need to
be able to send TCP segments larger than the 64KB limit that the MSS be able to send TCP segments larger than the 64KB limit that the MSS
option can convey. RFC 2675 [6] defines that an MSS value of 65,535 option can convey. RFC 2675 [6] defines that an MSS value of 65,535
bytes is to be treated as infinity, and Path MTU Discovery [3] is bytes is to be treated as infinity, and Path MTU Discovery [3] is
used to determine the actual MSS. used to determine the actual MSS.
3.8. Data Communication The Jumbo Payload option need not be implemented or understood by
IPv6 nodes that do not support attachment to links with a MTU greater
than 65,575 [6], and the present IPv6 Node Requiements does not
include support for Jumbograms [44].
3.7. Data Communication
Once the connection is established data is communicated by the Once the connection is established data is communicated by the
exchange of segments. Because segments may be lost due to errors exchange of segments. Because segments may be lost due to errors
(checksum test failure), or network congestion, TCP uses (checksum test failure), or network congestion, TCP uses
retransmission (after a timeout) to ensure delivery of every segment. retransmission to ensure delivery of every segment. Duplicate
Duplicate segments may arrive due to network or TCP retransmission. segments may arrive due to network or TCP retransmission. As
As discussed in the section on sequence numbers the TCP performs discussed in the section on sequence numbers the TCP implementation
certain tests on the sequence and acknowledgment numbers in the performs certain tests on the sequence and acknowledgment numbers in
segments to verify their acceptability. the segments to verify their acceptability.
The sender of data keeps track of the next sequence number to use in The sender of data keeps track of the next sequence number to use in
the variable SND.NXT. The receiver of data keeps track of the next the variable SND.NXT. The receiver of data keeps track of the next
sequence number to expect in the variable RCV.NXT. The sender of sequence number to expect in the variable RCV.NXT. The sender of
data keeps track of the oldest unacknowledged sequence number in the data keeps track of the oldest unacknowledged sequence number in the
variable SND.UNA. If the data flow is momentarily idle and all data variable SND.UNA. If the data flow is momentarily idle and all data
sent has been acknowledged then the three variables will be equal. sent has been acknowledged then the three variables will be equal.
When the sender creates a segment and transmits it the sender When the sender creates a segment and transmits it the sender
advances SND.NXT. When the receiver accepts a segment it advances advances SND.NXT. When the receiver accepts a segment it advances
skipping to change at page 38, line 14 skipping to change at page 37, line 25
an acknowledgment it advances SND.UNA. The extent to which the an acknowledgment it advances SND.UNA. The extent to which the
values of these variables differ is a measure of the delay in the values of these variables differ is a measure of the delay in the
communication. The amount by which the variables are advanced is the communication. The amount by which the variables are advanced is the
length of the data and SYN or FIN flags in the segment. Note that length of the data and SYN or FIN flags in the segment. Note that
once in the ESTABLISHED state all segments must carry current once in the ESTABLISHED state all segments must carry current
acknowledgment information. acknowledgment information.
The CLOSE user call implies a push function, as does the FIN control The CLOSE user call implies a push function, as does the FIN control
flag in an incoming segment. flag in an incoming segment.
3.8.1. Retransmission Timeout 3.7.1. Retransmission Timeout
Because of the variability of the networks that compose an Because of the variability of the networks that compose an
internetwork system and the wide range of uses of TCP connections the internetwork system and the wide range of uses of TCP connections the
retransmission timeout (RTO) must be dynamically determined. retransmission timeout (RTO) must be dynamically determined.
The RTO MUST be computed according to the algorithm in [9], including The RTO MUST be computed according to the algorithm in [9], including
Karn's algorithm for taking RTT samples (MUST-18). Karn's algorithm for taking RTT samples (MUST-18).
RFC 793 contains an early example procedure for computing the RTO. RFC 793 contains an early example procedure for computing the RTO.
This was then replaced by the algorithm described in RFC 1122, and This was then replaced by the algorithm described in RFC 1122, and
subsequently updated in RFC 2988, and then again in RFC 6298. subsequently updated in RFC 2988, and then again in RFC 6298.
If a retransmitted packet is identical to the original packet (which If a retransmitted packet is identical to the original packet (which
implies not only that the data boundaries have not changed, but also implies not only that the data boundaries have not changed, but also
that the window and acknowledgment fields of the header have not that none of the headers have not changed), then the same IPv4
changed), then the same IP Identification field MAY be used (see Identification field MAY be used (see Section 3.2.1.5 of RFC 1122)
Section 3.2.1.5 of RFC 1122) (MAY-4). (MAY-4).
3.8.2. TCP Congestion Control 3.7.2. TCP Congestion Control
RFC 1122 required implementation of Van Jacobson's congestion control RFC 1122 required implementation of Van Jacobson's congestion control
algorithm combining slow start with congestion avoidance. RFC 2581 algorithm combining slow start with congestion avoidance. RFC 2581
provided IETF Standards Track description of this, along with fast provided IETF Standards Track description of this, along with fast
retransmit and fast recovery. RFC 5681 is the current description of retransmit and fast recovery. RFC 5681 is the current description of
these algorithms and is the current standard for TCP congestion these algorithms and is the current standard for TCP congestion
control. control.
A TCP MUST implement RFC 5681 (MUST-19). A TCP endpoint MUST implement RFC 5681 (MUST-19).
Explicit Congestion Notification (ECN) was defined in RFC 3168 and is Explicit Congestion Notification (ECN) was defined in RFC 3168 and is
an IETF Standards Track enhancement that has many benefits [39]. an IETF Standards Track enhancement that has many benefits [41].
A TCP SHOULD implement ECN as described in RFC 3168 (SHLD-8). A TCP endpoint SHOULD implement ECN as described in RFC 3168 (SHLD-
8).
3.8.3. TCP Connection Failures 3.7.3. TCP Connection Failures
Excessive retransmission of the same segment by TCP indicates some Excessive retransmission of the same segment by a TCP endpoint
failure of the remote host or the Internet path. This failure may be indicates some failure of the remote host or the Internet path. This
of short or long duration. The following procedure MUST be used to failure may be of short or long duration. The following procedure
handle excessive retransmissions of data segments (MUST-20): MUST be used to handle excessive retransmissions of data segments
(MUST-20):
(a) There are two thresholds R1 and R2 measuring the amount of (a) There are two thresholds R1 and R2 measuring the amount of
retransmission that has occurred for the same segment. R1 and R2 retransmission that has occurred for the same segment. R1 and R2
might be measured in time units or as a count of retransmissions. might be measured in time units or as a count of retransmissions.
(b) When the number of transmissions of the same segment reaches (b) When the number of transmissions of the same segment reaches
or exceeds threshold R1, pass negative advice (see [14] or exceeds threshold R1, pass negative advice (see [14]
Section 3.3.1.4) to the IP layer, to trigger dead-gateway Section 3.3.1.4) to the IP layer, to trigger dead-gateway
diagnosis. diagnosis.
(c) When the number of transmissions of the same segment reaches a (c) When the number of transmissions of the same segment reaches a
threshold R2 greater than R1, close the connection. threshold R2 greater than R1, close the connection.
(d) An application MUST (MUST-21) be able to set the value for R2 (d) An application MUST (MUST-21) be able to set the value for R2
for a particular connection. For example, an interactive for a particular connection. For example, an interactive
application might set R2 to "infinity," giving the user control application might set R2 to "infinity," giving the user control
over when to disconnect. over when to disconnect.
(e) TCP SHOULD inform the application of the delivery problem (e) TCP implementations SHOULD inform the application of the
(unless such information has been disabled by the application; see delivery problem (unless such information has been disabled by the
Asynchronous Reports section), when R1 is reached and before R2 application; see Asynchronous Reports section), when R1 is reached
(SHLD-9). This will allow a remote login (User Telnet) and before R2 (SHLD-9). This will allow a remote login (User
application program to inform the user, for example. Telnet) application program to inform the user, for example.
The value of R1 SHOULD correspond to at least 3 retransmissions, at The value of R1 SHOULD correspond to at least 3 retransmissions, at
the current RTO (SHLD-10). The value of R2 SHOULD correspond to at the current RTO (SHLD-10). The value of R2 SHOULD correspond to at
least 100 seconds (SHLD-11). least 100 seconds (SHLD-11).
An attempt to open a TCP connection could fail with excessive An attempt to open a TCP connection could fail with excessive
retransmissions of the SYN segment or by receipt of a RST segment or retransmissions of the SYN segment or by receipt of a RST segment or
an ICMP Port Unreachable. SYN retransmissions MUST be handled in the an ICMP Port Unreachable. SYN retransmissions MUST be handled in the
general way just described for data retransmissions, including general way just described for data retransmissions, including
notification of the application layer. notification of the application layer.
However, the values of R1 and R2 may be different for SYN and data However, the values of R1 and R2 may be different for SYN and data
segments. In particular, R2 for a SYN segment MUST be set large segments. In particular, R2 for a SYN segment MUST be set large
enough to provide retransmission of the segment for at least 3 enough to provide retransmission of the segment for at least 3
minutes. The application can close the connection (i.e., give up on minutes (MUST-23). The application can close the connection (i.e.,
the open attempt) sooner, of course. give up on the open attempt) sooner, of course.
3.8.4. TCP Keep-Alives 3.7.4. TCP Keep-Alives
Implementors MAY include "keep-alives" in their TCP implementations Implementors MAY include "keep-alives" in their TCP implementations
(MAY-5), although this practice is not universally accepted. If (MAY-5), although this practice is not universally accepted. Some
TCP implementations, however, have included a keep-alive mechanism.
To confirm that an idle connection is still active, these
implementations send a probe segment designed to elicit a response
from the TCP peer. Such a segment generally contains SEG.SEQ =
SND.NXT-1 and may or may not contain one garbage octet of data. If
keep-alives are included, the application MUST be able to turn them keep-alives are included, the application MUST be able to turn them
on or off for each TCP connection (MUST-24), and they MUST default to on or off for each TCP connection (MUST-24), and they MUST default to
off (MUST-25). off (MUST-25).
Keep-alive packets MUST only be sent when no data or acknowledgement Keep-alive packets MUST only be sent when no data or acknowledgement
packets have been received for the connection within an interval packets have been received for the connection within an interval
(MUST-26). This interval MUST be configurable (MUST-27) and MUST (MUST-26). This interval MUST be configurable (MUST-27) and MUST
default to no less than two hours (MUST-28). default to no less than two hours (MUST-28).
It is extremely important to remember that ACK segments that contain It is extremely important to remember that ACK segments that contain
no data are not reliably transmitted by TCP. Consequently, if a no data are not reliably transmitted by TCP. Consequently, if a
keep-alive mechanism is implemented it MUST NOT interpret failure to keep-alive mechanism is implemented it MUST NOT interpret failure to
respond to any specific probe as a dead connection (MUST-29). respond to any specific probe as a dead connection (MUST-29).
An implementation SHOULD send a keep-alive segment with no data An implementation SHOULD send a keep-alive segment with no data
(SHLD-12); however, it MAY be configurable to send a keep-alive (SHLD-12); however, it MAY be configurable to send a keep-alive
segment containing one garbage octet (MAY-6), for compatibility with segment containing one garbage octet (MAY-6), for compatibility with
erroneous TCP implementations. erroneous TCP implementations.
3.8.5. The Communication of Urgent Information 3.7.5. The Communication of Urgent Information
As a result of implementation differences and middlebox interactions, As a result of implementation differences and middlebox interactions,
new applications SHOULD NOT employ the TCP urgent mechanism (SHLD- new applications SHOULD NOT employ the TCP urgent mechanism (SHLD-
13). However, TCP implementations MUST still include support for the 13). However, TCP implementations MUST still include support for the
urgent mechanism (MUST-30). Details can be found in RFC 6093 [29]. urgent mechanism (MUST-30). Details can be found in RFC 6093 [31].
The objective of the TCP urgent mechanism is to allow the sending The objective of the TCP urgent mechanism is to allow the sending
user to stimulate the receiving user to accept some urgent data and user to stimulate the receiving user to accept some urgent data and
to permit the receiving TCP to indicate to the receiving user when to permit the receiving TCP endpoint to indicate to the receiving
all the currently known urgent data has been received by the user. user when all the currently known urgent data has been received by
the user.
This mechanism permits a point in the data stream to be designated as This mechanism permits a point in the data stream to be designated as
the end of urgent information. Whenever this point is in advance of the end of urgent information. Whenever this point is in advance of
the receive sequence number (RCV.NXT) at the receiving TCP, that TCP the receive sequence number (RCV.NXT) at the receiving TCP endpoint,
must tell the user to go into "urgent mode"; when the receive that TCP must tell the user to go into "urgent mode"; when the
sequence number catches up to the urgent pointer, the TCP must tell receive sequence number catches up to the urgent pointer, the TCP
user to go into "normal mode". If the urgent pointer is updated implementation must tell user to go into "normal mode". If the
while the user is in "urgent mode", the update will be invisible to urgent pointer is updated while the user is in "urgent mode", the
the user. update will be invisible to the user.
The method employs a urgent field which is carried in all segments The method employs a urgent field that is carried in all segments
transmitted. The URG control flag indicates that the urgent field is transmitted. The URG control flag indicates that the urgent field is
meaningful and must be added to the segment sequence number to yield meaningful and must be added to the segment sequence number to yield
the urgent pointer. The absence of this flag indicates that there is the urgent pointer. The absence of this flag indicates that there is
no urgent data outstanding. no urgent data outstanding.
To send an urgent indication the user must also send at least one To send an urgent indication the user must also send at least one
data octet. If the sending user also indicates a push, timely data octet. If the sending user also indicates a push, timely
delivery of the urgent information to the destination process is delivery of the urgent information to the destination process is
enhanced. enhanced.
A TCP MUST support a sequence of urgent data of any length (MUST-31). A TCP implementation MUST support a sequence of urgent data of any
[14] length (MUST-31). [14]
The urgent pointer MUST point to the sequence number of the octet The urgent pointer MUST point to the sequence number of the octet
following the urgent data (MUST-62). following the urgent data (MUST-62).
A TCP MUST (MUST-32) inform the application layer asynchronously A TCP implementation MUST (MUST-32) inform the application layer
whenever it receives an Urgent pointer and there was previously no asynchronously whenever it receives an Urgent pointer and there was
pending urgent data, or whenvever the Urgent pointer advances in the previously no pending urgent data, or whenvever the Urgent pointer
data stream. There MUST (MUST-33) be a way for the application to advances in the data stream. There MUST (MUST-33) be a way for the
learn how much urgent data remains to be read from the connection, or application to learn how much urgent data remains to be read from the
at least to determine whether or not more urgent data remains to be connection, or at least to determine whether or not more urgent data
read. [14] remains to be read. [14]
3.8.6. Managing the Window 3.7.6. Managing the Window
The window sent in each segment indicates the range of sequence The window sent in each segment indicates the range of sequence
numbers the sender of the window (the data receiver) is currently numbers the sender of the window (the data receiver) is currently
prepared to accept. There is an assumption that this is related to prepared to accept. There is an assumption that this is related to
the currently available data buffer space available for this the currently available data buffer space available for this
connection. connection.
The sending TCP packages the data to be transmitted into segments The sending TCP endpoint packages the data to be transmitted into
which fit the current window, and may repackage segments on the segments that fit the current window, and may repackage segments on
retransmission queue. Such repackaging is not required, but may be the retransmission queue. Such repackaging is not required, but may
helpful. be helpful.
In a connection with a one-way data flow, the window information will In a connection with a one-way data flow, the window information will
be carried in acknowledgment segments that all have the same sequence be carried in acknowledgment segments that all have the same sequence
number so there will be no way to reorder them if they arrive out of number so there will be no way to reorder them if they arrive out of
order. This is not a serious problem, but it will allow the window order. This is not a serious problem, but it will allow the window
information to be on occasion temporarily based on old reports from information to be on occasion temporarily based on old reports from
the data receiver. A refinement to avoid this problem is to act on the data receiver. A refinement to avoid this problem is to act on
the window information from segments that carry the highest the window information from segments that carry the highest
acknowledgment number (that is segments with acknowledgment number acknowledgment number (that is segments with acknowledgment number
equal or greater than the highest previously received). equal or greater than the highest previously received).
Indicating a large window encourages transmissions. If more data Indicating a large window encourages transmissions. If more data
arrives than can be accepted, it will be discarded. This will result arrives than can be accepted, it will be discarded. This will result
in excessive retransmissions, adding unnecessarily to the load on the in excessive retransmissions, adding unnecessarily to the load on the
network and the TCPs. Indicating a small window may restrict the network and the TCP endpoints. Indicating a small window may
transmission of data to the point of introducing a round trip delay restrict the transmission of data to the point of introducing a round
between each new segment transmitted. trip delay between each new segment transmitted.
The mechanisms provided allow a TCP to advertise a large window and The mechanisms provided allow a TCP endpoint to advertise a large
to subsequently advertise a much smaller window without having window and to subsequently advertise a much smaller window without
accepted that much data. This, so called "shrinking the window," is having accepted that much data. This, so called "shrinking the
strongly discouraged. The robustness principle [14] dictates that window," is strongly discouraged. The robustness principle [14]
TCPs will not shrink the window themselves, but will be prepared for dictates that TCP peers will not shrink the window themselves, but
such behavior on the part of other TCPs. will be prepared for such behavior on the part of other TCP peers.
A TCP receiver SHOULD NOT shrink the window, i.e., move the right A TCP receiver SHOULD NOT shrink the window, i.e., move the right
window edge to the left (SHLD-14). However, a sending TCP MUST be window edge to the left (SHLD-14). However, a sending TCP peer MUST
robust against window shrinking, which may cause the "useable window" be robust against window shrinking, which may cause the "useable
(see Section 3.8.6.2.1) to become negative (MUST-34). window" (see Section 3.7.6.2.1) to become negative (MUST-34).
If this happens, the sender SHOULD NOT send new data (SHLD-15), but If this happens, the sender SHOULD NOT send new data (SHLD-15), but
SHOULD retransmit normally the old unacknowledged data between SHOULD retransmit normally the old unacknowledged data between
SND.UNA and SND.UNA+SND.WND (SHLD-16). The sender MAY also SND.UNA and SND.UNA+SND.WND (SHLD-16). The sender MAY also
retransmit old data beyond SND.UNA+SND.WND (MAY-7), but SHOULD NOT retransmit old data beyond SND.UNA+SND.WND (MAY-7), but SHOULD NOT
time out the connection if data beyond the right window edge is not time out the connection if data beyond the right window edge is not
acknowledged (SHLD-17). If the window shrinks to zero, the TCP MUST acknowledged (SHLD-17). If the window shrinks to zero, the TCP
probe it in the standard way (described below) (MUST-35). implementation MUST probe it in the standard way (described below)
(MUST-35).
3.8.6.1. Zero Window Probing 3.7.6.1. Zero Window Probing
The sending TCP must be prepared to accept from the user and send at The sending TCP peer must be prepared to accept from the user and
least one octet of new data even if the send window is zero. The send at least one octet of new data even if the send window is zero.
sending TCP must regularly retransmit to the receiving TCP even when The sending TCP peer must regularly retransmit to the receiving TCP
the window is zero, in order to "probe" the window. Two minutes is peer even when the window is zero, in order to "probe" the window.
recommended for the retransmission interval when the window is zero. Two minutes is recommended for the retransmission interval when the
This retransmission is essential to guarantee that when either TCP window is zero. This retransmission is essential to guarantee that
has a zero window the re-opening of the window will be reliably when either TCP peer has a zero window the re-opening of the window
reported to the other. This is referred to as Zero-Window Probing will be reliably reported to the other. This is referred to as Zero-
(ZWP) in other documents. Window Probing (ZWP) in other documents.
Probing of zero (offered) windows MUST be supported (MUST-36). Probing of zero (offered) windows MUST be supported (MUST-36).
A TCP MAY keep its offered receive window closed indefinitely (MAY- A TCP implementation MAY keep its offered receive window closed
8). As long as the receiving TCP continues to send acknowledgments indefinitely (MAY-8). As long as the receiving TCP peer continues to
in response to the probe segments, the sending TCP MUST allow the send acknowledgments in response to the probe segments, the sending
connection to stay open (MUST-37). This enables TCP to function in TCP peer MUST allow the connection to stay open (MUST-37). This
scenarios such as the "printer ran out of paper" situation described enables TCP to function in scenarios such as the "printer ran out of
in Section 4.2.2.17 of RFC1122. The behavior is subject to the paper" situation described in Section 4.2.2.17 of RFC1122. The
implementation's resource management concerns, as noted in [31]. behavior is subject to the implementation's resource management
concerns, as noted in [33].
When the receiving TCP has a zero window and a segment arrives it When the receiving TCP peer has a zero window and a segment arrives
must still send an acknowledgment showing its next expected sequence it must still send an acknowledgment showing its next expected
number and current window (zero). sequence number and current window (zero).
The transmitting host SHOULD send the first zero-window probe when a The transmitting host SHOULD send the first zero-window probe when a
zero window has existed for the retransmission timeout period (SHLD- zero window has existed for the retransmission timeout period (SHLD-
29) (see Section 3.8.1), and SHOULD increase exponentially the 29) (see Section 3.7.1), and SHOULD increase exponentially the
interval between successive probes (SHLD-30). interval between successive probes (SHLD-30).
3.8.6.2. Silly Window Syndrome Avoidance 3.7.6.2. Silly Window Syndrome Avoidance
The "Silly Window Syndrome" (SWS) is a stable pattern of small The "Silly Window Syndrome" (SWS) is a stable pattern of small
incremental window movements resulting in extremely poor TCP incremental window movements resulting in extremely poor TCP
performance. Algorithms to avoid SWS are described below for both performance. Algorithms to avoid SWS are described below for both
the sending side and the receiving side. RFC 1122 contains more the sending side and the receiving side. RFC 1122 contains more
detailed discussion of the SWS problem. Note that the Nagle detailed discussion of the SWS problem. Note that the Nagle
algorithm and the sender SWS avoidance algorithm play complementary algorithm and the sender SWS avoidance algorithm play complementary
roles in improving performance. The Nagle algorithm discourages roles in improving performance. The Nagle algorithm discourages
sending tiny segments when the data to be sent increases in small sending tiny segments when the data to be sent increases in small
increments, while the SWS avoidance algorithm discourages small increments, while the SWS avoidance algorithm discourages small
segments resulting from the right window edge advancing in small segments resulting from the right window edge advancing in small
increments. increments.
3.8.6.2.1. Sender's Algorithm - When to Send Data 3.7.6.2.1. Sender's Algorithm - When to Send Data
A TCP MUST include a SWS avoidance algorithm in the sender (MUST-38). A TCP implementation MUST include a SWS avoidance algorithm in the
sender (MUST-38).
The Nagle algorithm from Section 3.7.4 additionally describes how to The Nagle algorithm from Section 3.6.4 additionally describes how to
coalesce short segments. coalesce short segments.
The sender's SWS avoidance algorithm is more difficult than the The sender's SWS avoidance algorithm is more difficult than the
receivers's, because the sender does not know (directly) the receivers's, because the sender does not know (directly) the
receiver's total buffer space RCV.BUFF. An approach which has been receiver's total buffer space RCV.BUFF. An approach that has been
found to work well is for the sender to calculate Max(SND.WND), the found to work well is for the sender to calculate Max(SND.WND), the
maximum send window it has seen so far on the connection, and to use maximum send window it has seen so far on the connection, and to use
this value as an estimate of RCV.BUFF. Unfortunately, this can only this value as an estimate of RCV.BUFF. Unfortunately, this can only
be an estimate; the receiver may at any time reduce the size of be an estimate; the receiver may at any time reduce the size of
RCV.BUFF. To avoid a resulting deadlock, it is necessary to have a RCV.BUFF. To avoid a resulting deadlock, it is necessary to have a
timeout to force transmission of data, overriding the SWS avoidance timeout to force transmission of data, overriding the SWS avoidance
algorithm. In practice, this timeout should seldom occur. algorithm. In practice, this timeout should seldom occur.
The "useable window" is: The "useable window" is:
U = SND.UNA + SND.WND - SND.NXT U = SND.UNA + SND.WND - SND.NXT
i.e., the offered window less the amount of data sent but not i.e., the offered window less the amount of data sent but not
acknowledged. If D is the amount of data queued in the sending TCP acknowledged. If D is the amount of data queued in the sending TCP
but not yet sent, then the following set of rules is recommended. endpoint but not yet sent, then the following set of rules is
recommended.
Send data: Send data:
(1) if a maximum-sized segment can be sent, i.e, if: (1) if a maximum-sized segment can be sent, i.e, if:
min(D,U) >= Eff.snd.MSS; min(D,U) >= Eff.snd.MSS;
(2) or if the data is pushed and all queued data can be sent now, (2) or if the data is pushed and all queued data can be sent now,
i.e., if: i.e., if:
skipping to change at page 44, line 30 skipping to change at page 43, line 46
[SND.NXT = SND.UNA and] [SND.NXT = SND.UNA and]
min(D.U) >= Fs * Max(SND.WND); min(D.U) >= Fs * Max(SND.WND);
(4) or if data is PUSHed and the override timeout occurs. (4) or if data is PUSHed and the override timeout occurs.
Here Fs is a fraction whose recommended value is 1/2. The override Here Fs is a fraction whose recommended value is 1/2. The override
timeout should be in the range 0.1 - 1.0 seconds. It may be timeout should be in the range 0.1 - 1.0 seconds. It may be
convenient to combine this timer with the timer used to probe zero convenient to combine this timer with the timer used to probe zero
windows (Section Section 3.8.6.1). windows (Section Section 3.7.6.1).
3.8.6.2.2. Receiver's Algorithm - When to Send a Window Update 3.7.6.2.2. Receiver's Algorithm - When to Send a Window Update
A TCP MUST include a SWS avoidance algorithm in the receiver (MUST- A TCP implementation MUST include a SWS avoidance algorithm in the
39). receiver (MUST-39).
The receiver's SWS avoidance algorithm determines when the right The receiver's SWS avoidance algorithm determines when the right
window edge may be advanced; this is customarily known as "updating window edge may be advanced; this is customarily known as "updating
the window". This algorithm combines with the delayed ACK algorithm the window". This algorithm combines with the delayed ACK algorithm
(see Section 3.8.6.3) to determine when an ACK segment containing the (see Section 3.7.6.3) to determine when an ACK segment containing the
current window will really be sent to the receiver. current window will really be sent to the receiver.
The solution to receiver SWS is to avoid advancing the right window The solution to receiver SWS is to avoid advancing the right window
edge RCV.NXT+RCV.WND in small increments, even if data is received edge RCV.NXT+RCV.WND in small increments, even if data is received
from the network in small segments. from the network in small segments.
Suppose the total receive buffer space is RCV.BUFF. At any given Suppose the total receive buffer space is RCV.BUFF. At any given
moment, RCV.USER octets of this total may be tied up with data that moment, RCV.USER octets of this total may be tied up with data that
has been received and acknowledged but which the user process has not has been received and acknowledged but that the user process has not
yet consumed. When the connection is quiescent, RCV.WND = RCV.BUFF yet consumed. When the connection is quiescent, RCV.WND = RCV.BUFF
and RCV.USER = 0. and RCV.USER = 0.
Keeping the right window edge fixed as data arrives and is Keeping the right window edge fixed as data arrives and is
acknowledged requires that the receiver offer less than its full acknowledged requires that the receiver offer less than its full
buffer space, i.e., the receiver must specify a RCV.WND that keeps buffer space, i.e., the receiver must specify a RCV.WND that keeps
RCV.NXT+RCV.WND constant as RCV.NXT increases. Thus, the total RCV.NXT+RCV.WND constant as RCV.NXT increases. Thus, the total
buffer space RCV.BUFF is generally divided into three parts: buffer space RCV.BUFF is generally divided into three parts:
|<------- RCV.BUFF ---------------->| |<------- RCV.BUFF ---------------->|
skipping to change at page 45, line 31 skipping to change at page 45, line 7
The suggested SWS avoidance algorithm for the receiver is to keep The suggested SWS avoidance algorithm for the receiver is to keep
RCV.NXT+RCV.WND fixed until the reduction satisfies: RCV.NXT+RCV.WND fixed until the reduction satisfies:
RCV.BUFF - RCV.USER - RCV.WND >= RCV.BUFF - RCV.USER - RCV.WND >=
min( Fr * RCV.BUFF, Eff.snd.MSS ) min( Fr * RCV.BUFF, Eff.snd.MSS )
where Fr is a fraction whose recommended value is 1/2, and where Fr is a fraction whose recommended value is 1/2, and
Eff.snd.MSS is the effective send MSS for the connection (see Eff.snd.MSS is the effective send MSS for the connection (see
Section 3.7.1). When the inequality is satisfied, RCV.WND is set to Section 3.6.1). When the inequality is satisfied, RCV.WND is set to
RCV.BUFF-RCV.USER. RCV.BUFF-RCV.USER.
Note that the general effect of this algorithm is to advance RCV.WND Note that the general effect of this algorithm is to advance RCV.WND
in increments of Eff.snd.MSS (for realistic receive buffers: in increments of Eff.snd.MSS (for realistic receive buffers:
Eff.snd.MSS < RCV.BUFF/2). Note also that the receiver must use its Eff.snd.MSS < RCV.BUFF/2). Note also that the receiver must use its
own Eff.snd.MSS, assuming it is the same as the sender's. own Eff.snd.MSS, assuming it is the same as the sender's.
3.8.6.3. Delayed Acknowledgements - When to Send an ACK Segment 3.7.6.3. Delayed Acknowledgements - When to Send an ACK Segment
A host that is receiving a stream of TCP data segments can increase A host that is receiving a stream of TCP data segments can increase
efficiency in both the Internet and the hosts by sending fewer than efficiency in both the Internet and the hosts by sending fewer than
one ACK (acknowledgment) segment per data segment received; this is one ACK (acknowledgment) segment per data segment received; this is
known as a "delayed ACK". known as a "delayed ACK".
A TCP SHOULD implement a delayed ACK (SHLD-18), but an ACK should not A TCP endpoint SHOULD implement a delayed ACK (SHLD-18), but an ACK
be excessively delayed; in particular, the delay MUST be less than should not be excessively delayed; in particular, the delay MUST be
0.5 seconds (MUST-40), and in a stream of full-sized segments there less than 0.5 seconds (MUST-40), and in a stream of full-sized
SHOULD be an ACK for at least every second segment (SHLD-19). segments there SHOULD be an ACK for at least every second segment
(SHLD-19). Excessive delays on ACK's can disturb the round-trip
Excessive delays on ACK's can disturb the round-trip timing and timing and packet "clocking" algorithms.
packet "clocking" algorithms.
3.9. Interfaces 3.8. Interfaces
There are of course two interfaces of concern: the user/TCP interface There are of course two interfaces of concern: the user/TCP interface
and the TCP/lower-level interface. We have a fairly elaborate model and the TCP/lower-level interface. We have a fairly elaborate model
of the user/TCP interface, but the interface to the lower level of the user/TCP interface, but the interface to the lower level
protocol module is left unspecified here, since it will be specified protocol module is left unspecified here, since it will be specified
in detail by the specification of the lower level protocol. For the in detail by the specification of the lower level protocol. For the
case that the lower level is IP we note some of the parameter values case that the lower level is IP we note some of the parameter values
that TCPs might use. that TCP implementations might use.
3.9.1. User/TCP Interface 3.8.1. User/TCP Interface
The following functional description of user commands to the TCP is, The following functional description of user commands to the TCP
at best, fictional, since every operating system will have different implementation is, at best, fictional, since every operating system
facilities. Consequently, we must warn readers that different TCP will have different facilities. Consequently, we must warn readers
implementations may have different user interfaces. However, all that different TCP implementations may have different user
TCPs must provide a certain minimum set of services to guarantee that interfaces. However, all TCP implementations must provide a certain
all TCP implementations can support the same protocol hierarchy. minimum set of services to guarantee that all TCP implementations can
This section specifies the functional interfaces required of all TCP support the same protocol hierarchy. This section specifies the
implementations. functional interfaces required of all TCP implementations.
Section 3.1 of [43] also identifies primitives provided by TCP, and
could be used as an additional reference for implementers.
TCP User Commands TCP User Commands
The following sections functionally characterize a USER/TCP The following sections functionally characterize a USER/TCP
interface. The notation used is similar to most procedure or interface. The notation used is similar to most procedure or
function calls in high level languages, but this usage is not function calls in high level languages, but this usage is not
meant to rule out trap type service calls. meant to rule out trap type service calls.
The user commands described below specify the basic functions the The user commands described below specify the basic functions the
TCP must perform to support interprocess communication. TCP implementation must perform to support interprocess
Individual implementations must define their own exact format, and communication. Individual implementations must define their own
may provide combinations or subsets of the basic functions in exact format, and may provide combinations or subsets of the basic
single calls. In particular, some implementations may wish to functions in single calls. In particular, some implementations
automatically OPEN a connection on the first SEND or RECEIVE may wish to automatically OPEN a connection on the first SEND or
issued by the user for a given connection. RECEIVE issued by the user for a given connection.
In providing interprocess communication facilities, the TCP must In providing interprocess communication facilities, the TCP
not only accept commands, but must also return information to the implementation must not only accept commands, but must also return
processes it serves. The latter consists of: information to the processes it serves. The latter consists of:
(a) general information about a connection (e.g., interrupts, (a) general information about a connection (e.g., interrupts,
remote close, binding of unspecified foreign socket). remote close, binding of unspecified remote socket).
(b) replies to specific user commands indicating success or (b) replies to specific user commands indicating success or
various types of failure. various types of failure.
Open Open
Format: OPEN (local port, foreign socket, active/passive [, Format: OPEN (local port, remote socket, active/passive [,
timeout] [, DiffServ field] [, security/compartment] [local IP timeout] [, DiffServ field] [, security/compartment] [local IP
address,] [, options]) -> local connection name address,] [, options]) -> local connection name
We assume that the local TCP is aware of the identity of the We assume that the local TCP endpoint is aware of the identity
processes it serves and will check the authority of the process of the processes it serves and will check the authority of the
to use the connection specified. Depending upon the process to use the connection specified. Depending upon the
implementation of the TCP, the local network and TCP implementation of the TCP endpoint, the local network and TCP
identifiers for the source address will either be supplied by identifiers for the source address will either be supplied by
the TCP or the lower level protocol (e.g., IP). These the TCP endpoint or the lower level protocol (e.g., IP). These
considerations are the result of concern about security, to the considerations are the result of concern about security, to the
extent that no TCP be able to masquerade as another one, and so extent that no TCP peer be able to masquerade as another one,
on. Similarly, no process can masquerade as another without and so on. Similarly, no process can masquerade as another
the collusion of the TCP. without the collusion of the TCP implementation.
If the active/passive flag is set to passive, then this is a If the active/passive flag is set to passive, then this is a
call to LISTEN for an incoming connection. A passive open may call to LISTEN for an incoming connection. A passive open may
have either a fully specified foreign socket to wait for a have either a fully specified remote socket to wait for a
particular connection or an unspecified foreign socket to wait particular connection or an unspecified remote socket to wait
for any call. A fully specified passive call can be made for any call. A fully specified passive call can be made
active by the subsequent execution of a SEND. active by the subsequent execution of a SEND.
A transmission control block (TCB) is created and partially A transmission control block (TCB) is created and partially
filled in with data from the OPEN command parameters. filled in with data from the OPEN command parameters.
Every passive OPEN call either creates a new connection record Every passive OPEN call either creates a new connection record
in LISTEN state, or it returns an error; it MUST NOT affect any in LISTEN state, or it returns an error; it MUST NOT affect any
previously created connection record (MUST-41). previously created connection record (MUST-41).
A TCP that supports multiple concurrent users MUST provide an A TCP implementation that supports multiple concurrent users
OPEN call that will functionally allow an application to LISTEN MUST provide an OPEN call that will functionally allow an
on a port while a connection block with the same local port is application to LISTEN on a port while a connection block with
in SYN-SENT or SYN-RECEIVED state (MUST-42). the same local port is in SYN-SENT or SYN-RECEIVED state (MUST-
42).
On an active OPEN command, the TCP will begin the procedure to On an active OPEN command, the TCP endpoint will begin the
synchronize (i.e., establish) the connection at once. procedure to synchronize (i.e., establish) the connection at
once.
The timeout, if present, permits the caller to set up a timeout The timeout, if present, permits the caller to set up a timeout
for all data submitted to TCP. If data is not successfully for all data submitted to TCP. If data is not successfully
delivered to the destination within the timeout period, the TCP delivered to the destination within the timeout period, the TCP
will abort the connection. The present global default is five endpoint will abort the connection. The present global default
minutes. is five minutes.
The TCP or some component of the operating system will verify The TCP implementation or some component of the operating
the users authority to open a connection with the specified system will verify the users authority to open a connection
DiffServ field value or security/compartment. The absence of a with the specified DiffServ field value or security/
DiffServ field value or security/compartment specification in compartment. The absence of a DiffServ field value or
the OPEN call indicates the default values must be used. security/compartment specification in the OPEN call indicates
the default values must be used.
TCP will accept incoming requests as matching only if the TCP will accept incoming requests as matching only if the
security/compartment information is exactly the same as that security/compartment information is exactly the same as that
requested in the OPEN call. requested in the OPEN call.
The DiffServ field value indicated by the user only impacts The DiffServ field value indicated by the user only impacts
outgoing packets, may be altered en route through the network, outgoing packets, may be altered en route through the network,
and has no direct bearing or relation to received packets. and has no direct bearing or relation to received packets.
A local connection name will be returned to the user by the A local connection name will be returned to the user by the TCP
TCP. The local connection name can then be used as a short implementation. The local connection name can then be used as
hand term for the connection defined by the <local socket, a short hand term for the connection defined by the <local
foreign socket> pair. socket, remote socket> pair.
The optional "local IP address" parameter MUST be supported to The optional "local IP address" parameter MUST be supported to
allow the specification of the local IP address (MUST-43). allow the specification of the local IP address (MUST-43).
This enables applications that need to select the local IP This enables applications that need to select the local IP
address used when multihoming is present. address used when multihoming is present.
A passive OPEN call with a specified "local IP address" A passive OPEN call with a specified "local IP address"
parameter will await an incoming connection request to that parameter will await an incoming connection request to that
address. If the parameter is unspecified, a passive OPEN will address. If the parameter is unspecified, a passive OPEN will
await an incoming connection request to any local IP address, await an incoming connection request to any local IP address,
and then bind the local IP address of the connection to the and then bind the local IP address of the connection to the
particular address that is used. particular address that is used.
For an active OPEN call, a specified "local IP address" For an active OPEN call, a specified "local IP address"
parameter will be used for opening the connection. If the parameter will be used for opening the connection. If the
parameter is unspecified, the host will choose an appropriate parameter is unspecified, the host will choose an appropriate
local IP address (see RFC 1122 section 3.3.4.2). local IP address (see RFC 1122 section 3.3.4.2).
If an application on a multihomed host does not specify the If an application on a multihomed host does not specify the
local IP address when actively opening a TCP connection, then local IP address when actively opening a TCP connection, then
the TCP MUST ask the IP layer to select a local IP address the TCP implementation MUST ask the IP layer to select a local
before sending the (first) SYN (MUST-44). See the function IP address before sending the (first) SYN (MUST-44). See the
GET_SRCADDR() in Section 3.4 of RFC 1122. function GET_SRCADDR() in Section 3.4 of RFC 1122.
At all other times, a previous segment has either been sent or At all other times, a previous segment has either been sent or
received on this connection, and TCP MUST use the same local received on this connection, and TCP implementations MUST use
address is used that was used in those previous segments (MUST- the same local address is used that was used in those previous
45). segments (MUST-45).
A TCP implementation MUST reject as an error a local OPEN call A TCP implementation MUST reject as an error a local OPEN call
for an invalid remote IP address (e.g., a broadcast or for an invalid remote IP address (e.g., a broadcast or
multicast address) (MUST-46). multicast address) (MUST-46).
Send Send
Format: SEND (local connection name, buffer address, byte Format: SEND (local connection name, buffer address, byte
count, PUSH flag (optional), URGENT flag [,timeout]) count, PUSH flag (optional), URGENT flag [,timeout])
This call causes the data contained in the indicated user This call causes the data contained in the indicated user
buffer to be sent on the indicated connection. If the buffer to be sent on the indicated connection. If the
connection has not been opened, the SEND is considered an connection has not been opened, the SEND is considered an
error. Some implementations may allow users to SEND first; in error. Some implementations may allow users to SEND first; in
which case, an automatic OPEN would be done. For example, this which case, an automatic OPEN would be done. For example, this
might be one way for application data to be included in SYN might be one way for application data to be included in SYN
segments. If the calling process is not authorized to use this segments. If the calling process is not authorized to use this
connection, an error is returned. connection, an error is returned.
A TCP MAY implement PUSH flags on SEND calls (MAY-15). If PUSH A TCP endpoint MAY implement PUSH flags on SEND calls (MAY-15).
flags are not implemented, then the sending TCP: (1) MUST NOT If PUSH flags are not implemented, then the sending TCP peer:
buffer data indefinitely (MUST-60), and (2) MUST set the PSH (1) MUST NOT buffer data indefinitely (MUST-60), and (2) MUST
bit in the last buffered segment (i.e., when there is no more set the PSH bit in the last buffered segment (i.e., when there
queued data to be sent) (MUST-61). The remaining description is no more queued data to be sent) (MUST-61). The remaining
below assumes the PUSH flag is supported on SEND calls. description below assumes the PUSH flag is supported on SEND
calls.
If the PUSH flag is set, the application intends the data to be If the PUSH flag is set, the application intends the data to be
transmitted promptly to the receiver, and the PUSH bit will be transmitted promptly to the receiver, and the PUSH bit will be
set in the last TCP segment created from the buffer. When an set in the last TCP segment created from the buffer. When an
application issues a series of SEND calls without setting the application issues a series of SEND calls without setting the
PUSH flag, the TCP MAY aggregate the data internally without PUSH flag, the TCP implementation MAY aggregate the data
sending it (MAY-16). internally without sending it (MAY-16).
The PSH bit is not a record marker and is independent of The PSH bit is not a record marker and is independent of
segment boundaries. The transmitter SHOULD collapse successive segment boundaries. The transmitter SHOULD collapse successive
bits when it packetizes data, to send the largest possible bits when it packetizes data, to send the largest possible
segment (SHLD-27). segment (SHLD-27).
If the PUSH flag is not set, the data may be combined with data If the PUSH flag is not set, the data may be combined with data
from subsequent SENDs for transmission efficiency. Note that from subsequent SENDs for transmission efficiency. Note that
when the Nagle algorithm is in use, TCP may buffer the data when the Nagle algorithm is in use, TCP implementations may
before sending, without regard to the PUSH flag (see buffer the data before sending, without regard to the PUSH flag
Section 3.7.4). (see Section 3.6.4).
An application program is logically required to set the PUSH An application program is logically required to set the PUSH
flag in a SEND call whenever it needs to force delivery of the flag in a SEND call whenever it needs to force delivery of the
data to avoid a communication deadlock. However, a TCP SHOULD data to avoid a communication deadlock. However, a TCP
send a maximum-sized segment whenever possible (SHLD-28), to implementation SHOULD send a maximum-sized segment whenever
improve performance (see Section 3.8.6.2.1). possible (SHLD-28), to improve performance (see
Section 3.7.6.2.1).
New applications SHOULD NOT set the URGENT flag [29] due to New applications SHOULD NOT set the URGENT flag [31] due to
implementation differences and middlebox issues (SHLD-13). implementation differences and middlebox issues (SHLD-13).
If the URGENT flag is set, segments sent to the destination TCP If the URGENT flag is set, segments sent to the destination TCP
will have the urgent pointer set. The receiving TCP will peer will have the urgent pointer set. The receiving TCP peer
signal the urgent condition to the receiving process if the will signal the urgent condition to the receiving process if
urgent pointer indicates that data preceding the urgent pointer the urgent pointer indicates that data preceding the urgent
has not been consumed by the receiving process. The purpose of pointer has not been consumed by the receiving process. The
urgent is to stimulate the receiver to process the urgent data purpose of urgent is to stimulate the receiver to process the
and to indicate to the receiver when all the currently known urgent data and to indicate to the receiver when all the
urgent data has been received. The number of times the sending currently known urgent data has been received. The number of
user's TCP signals urgent will not necessarily be equal to the times the sending user's TCP implementation signals urgent will
number of times the receiving user will be notified of the not necessarily be equal to the number of times the receiving
presence of urgent data. user will be notified of the presence of urgent data.
If no foreign socket was specified in the OPEN, but the If no remote socket was specified in the OPEN, but the
connection is established (e.g., because a LISTENing connection connection is established (e.g., because a LISTENing connection
has become specific due to a foreign segment arriving for the has become specific due to a remote segment arriving for the
local socket), then the designated buffer is sent to the local socket), then the designated buffer is sent to the
implied foreign socket. Users who make use of OPEN with an implied remote socket. Users who make use of OPEN with an
unspecified foreign socket can make use of SEND without ever unspecified remote socket can make use of SEND without ever
explicitly knowing the foreign socket address. explicitly knowing the remote socket address.
However, if a SEND is attempted before the foreign socket However, if a SEND is attempted before the remote socket
becomes specified, an error will be returned. Users can use becomes specified, an error will be returned. Users can use
the STATUS call to determine the status of the connection. In the STATUS call to determine the status of the connection.
some implementations the TCP may notify the user when an Some TCP implementations may notify the user when an
unspecified socket is bound. unspecified socket is bound.
If a timeout is specified, the current user timeout for this If a timeout is specified, the current user timeout for this
connection is changed to the new one. connection is changed to the new one.
In the simplest implementation, SEND would not return control In the simplest implementation, SEND would not return control
to the sending process until either the transmission was to the sending process until either the transmission was
complete or the timeout had been exceeded. However, this complete or the timeout had been exceeded. However, this
simple method is both subject to deadlocks (for example, both simple method is both subject to deadlocks (for example, both
sides of the connection might try to do SENDs before doing any sides of the connection might try to do SENDs before doing any
RECEIVEs) and offers poor performance, so it is not RECEIVEs) and offers poor performance, so it is not
recommended. A more sophisticated implementation would return recommended. A more sophisticated implementation would return
immediately to allow the process to run concurrently with immediately to allow the process to run concurrently with
network I/O, and, furthermore, to allow multiple SENDs to be in network I/O, and, furthermore, to allow multiple SENDs to be in
progress. Multiple SENDs are served in first come, first progress. Multiple SENDs are served in first come, first
served order, so the TCP will queue those it cannot service served order, so the TCP endpoint will queue those it cannot
immediately. service immediately.
We have implicitly assumed an asynchronous user interface in We have implicitly assumed an asynchronous user interface in
which a SEND later elicits some kind of SIGNAL or pseudo- which a SEND later elicits some kind of SIGNAL or pseudo-
interrupt from the serving TCP. An alternative is to return a interrupt from the serving TCP endpoint. An alternative is to
response immediately. For instance, SENDs might return return a response immediately. For instance, SENDs might
immediate local acknowledgment, even if the segment sent had return immediate local acknowledgment, even if the segment sent
not been acknowledged by the distant TCP. We could had not been acknowledged by the distant TCP endpoint. We
optimistically assume eventual success. If we are wrong, the could optimistically assume eventual success. If we are wrong,
connection will close anyway due to the timeout. In the connection will close anyway due to the timeout. In
implementations of this kind (synchronous), there will still be implementations of this kind (synchronous), there will still be
some asynchronous signals, but these will deal with the some asynchronous signals, but these will deal with the
connection itself, and not with specific segments or buffers. connection itself, and not with specific segments or buffers.
In order for the process to distinguish among error or success In order for the process to distinguish among error or success
indications for different SENDs, it might be appropriate for indications for different SENDs, it might be appropriate for
the buffer address to be returned along with the coded response the buffer address to be returned along with the coded response
to the SEND request. TCP-to-user signals are discussed below, to the SEND request. TCP-to-user signals are discussed below,
indicating the information which should be returned to the indicating the information that should be returned to the
calling process. calling process.
Receive Receive
Format: RECEIVE (local connection name, buffer address, byte Format: RECEIVE (local connection name, buffer address, byte
count) -> byte count, urgent flag, push flag (optional) count) -> byte count, urgent flag, push flag (optional)
This command allocates a receiving buffer associated with the This command allocates a receiving buffer associated with the
specified connection. If no OPEN precedes this command or the specified connection. If no OPEN precedes this command or the
calling process is not authorized to use this connection, an calling process is not authorized to use this connection, an
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urgent pointer (non-urgent data) cannot be delivered to the urgent pointer (non-urgent data) cannot be delivered to the
user in the same buffer with preceding urgent data unless the user in the same buffer with preceding urgent data unless the
boundary is clearly marked for the user. boundary is clearly marked for the user.
To distinguish among several outstanding RECEIVEs and to take To distinguish among several outstanding RECEIVEs and to take
care of the case that a buffer is not completely filled, the care of the case that a buffer is not completely filled, the
return code is accompanied by both a buffer pointer and a byte return code is accompanied by both a buffer pointer and a byte
count indicating the actual length of the data received. count indicating the actual length of the data received.
Alternative implementations of RECEIVE might have the TCP Alternative implementations of RECEIVE might have the TCP
allocate buffer storage, or the TCP might share a ring buffer endpoint allocate buffer storage, or the TCP endpoint might
with the user. share a ring buffer with the user.
Close Close
Format: CLOSE (local connection name) Format: CLOSE (local connection name)
This command causes the connection specified to be closed. If This command causes the connection specified to be closed. If
the connection is not open or the calling process is not the connection is not open or the calling process is not
authorized to use this connection, an error is returned. authorized to use this connection, an error is returned.
Closing connections is intended to be a graceful operation in Closing connections is intended to be a graceful operation in
the sense that outstanding SENDs will be transmitted (and the sense that outstanding SENDs will be transmitted (and
retransmitted), as flow control permits, until all have been retransmitted), as flow control permits, until all have been
serviced. Thus, it should be acceptable to make several SEND serviced. Thus, it should be acceptable to make several SEND
calls, followed by a CLOSE, and expect all the data to be sent calls, followed by a CLOSE, and expect all the data to be sent
to the destination. It should also be clear that users should to the destination. It should also be clear that users should
continue to RECEIVE on CLOSING connections, since the other continue to RECEIVE on CLOSING connections, since the remote
side may be trying to transmit the last of its data. Thus, peer may be trying to transmit the last of its data. Thus,
CLOSE means "I have no more to send" but does not mean "I will CLOSE means "I have no more to send" but does not mean "I will
not receive any more." It may happen (if the user level not receive any more." It may happen (if the user level
protocol is not well thought out) that the closing side is protocol is not well thought out) that the closing side is
unable to get rid of all its data before timing out. In this unable to get rid of all its data before timing out. In this
event, CLOSE turns into ABORT, and the closing TCP gives up. event, CLOSE turns into ABORT, and the closing TCP peer gives
up.
The user may CLOSE the connection at any time on his own The user may CLOSE the connection at any time on his own
initiative, or in response to various prompts from the TCP initiative, or in response to various prompts from the TCP
(e.g., remote close executed, transmission timeout exceeded, implementation (e.g., remote close executed, transmission
destination inaccessible). timeout exceeded, destination inaccessible).
Because closing a connection requires communication with the Because closing a connection requires communication with the
foreign TCP, connections may remain in the closing state for a remote TCP peer, connections may remain in the closing state
short time. Attempts to reopen the connection before the TCP for a short time. Attempts to reopen the connection before the
replies to the CLOSE command will result in error responses. TCP peer replies to the CLOSE command will result in error
responses.
Close also implies push function. Close also implies push function.
Status Status
Format: STATUS (local connection name) -> status data Format: STATUS (local connection name) -> status data
This is an implementation dependent user command and could be This is an implementation dependent user command and could be
excluded without adverse effect. Information returned would excluded without adverse effect. Information returned would
typically come from the TCB associated with the connection. typically come from the TCB associated with the connection.
This command returns a data block containing the following This command returns a data block containing the following
information: information:
local socket, local socket,
foreign socket, remote socket,
local connection name, local connection name,
receive window, receive window,
send window, send window,
connection state, connection state,
number of buffers awaiting acknowledgment, number of buffers awaiting acknowledgment,
number of buffers pending receipt, number of buffers pending receipt,
urgent state, urgent state,
DiffServ field value, DiffServ field value,
security/compartment, security/compartment,
and transmission timeout. and transmission timeout.
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authorized to use this connection, an error is returned. This authorized to use this connection, an error is returned. This
prevents unauthorized processes from gaining information about prevents unauthorized processes from gaining information about
a connection. a connection.
Abort Abort
Format: ABORT (local connection name) Format: ABORT (local connection name)
This command causes all pending SENDs and RECEIVES to be This command causes all pending SENDs and RECEIVES to be
aborted, the TCB to be removed, and a special RESET message to aborted, the TCB to be removed, and a special RESET message to
be sent to the TCP on the other side of the connection. be sent to the remote TCP peer of the connection. Depending on
Depending on the implementation, users may receive abort the implementation, users may receive abort indications for
indications for each outstanding SEND or RECEIVE, or may simply each outstanding SEND or RECEIVE, or may simply receive an
receive an ABORT-acknowledgment. ABORT-acknowledgment.
Flush Flush
Some TCP implementations have included a FLUSH call, which will Some TCP implementations have included a FLUSH call, which will
empty the TCP send queue of any data for which the user has empty the TCP send queue of any data that the user has issued
issued SEND calls but which is still to the right of the SEND calls but is still to the right of the current send
current send window. That is, it flushes as much queued send window. That is, it flushes as much queued send data as
data as possible without losing sequence number possible without losing sequence number synchronization. The
synchronization. The FLUSH call MAY be implemented (MAY-14). FLUSH call MAY be implemented (MAY-14).
Asynchronous Reports Asynchronous Reports
There MUST be a mechanism for reporting soft TCP error There MUST be a mechanism for reporting soft TCP error
conditions to the application (MUST-47). Generically, we conditions to the application (MUST-47). Generically, we
assume this takes the form of an application-supplied assume this takes the form of an application-supplied
ERROR_REPORT routine that may be upcalled asynchronously from ERROR_REPORT routine that may be upcalled asynchronously from
the transport layer: the transport layer:
ERROR_REPORT(local connection name, reason, subreason) ERROR_REPORT(local connection name, reason, subreason)
The precise encoding of the reason and subreason parameters is The precise encoding of the reason and subreason parameters is
not specified here. However, the conditions that are reported not specified here. However, the conditions that are reported
asynchronously to the application MUST include: asynchronously to the application MUST include:
* ICMP error message arrived (see Section 3.9.2.2 for * ICMP error message arrived (see Section 3.8.2.2 for
description of handling each ICMP message type, since some description of handling each ICMP message type, since some
message types need to be suppressed from generating reports message types need to be suppressed from generating reports
to the application) to the application)
* Excessive retransmissions (see Section 3.8.3) (TODO - the * Excessive retransmissions (see Section 3.7.3)
MUST here is inconsistent with SHOULD in the section
describing excessive retransmissions. Both conflicting bits
of text are direct from 1122)
* Urgent pointer advance (see Section 3.8.5) (MUST-32). * Urgent pointer advance (see Section 3.7.5)
However, an application program that does not want to receive However, an application program that does not want to receive
such ERROR_REPORT calls SHOULD be able to effectively disable such ERROR_REPORT calls SHOULD be able to effectively disable
these calls (SHLD-20). these calls (SHLD-20).
Set Differentiated Services Field (IPv4 TOS or IPv6 Traffic Class) Set Differentiated Services Field (IPv4 TOS or IPv6 Traffic Class)
The application layer MUST be able to specify the The application layer MUST be able to specify the
Differentiated Services field for segments that are sent on a Differentiated Services field for segments that are sent on a
connection (MUST-48). The Differentiated Services field connection (MUST-48). The Differentiated Services field
includes the 6-bit Differentiated Services Code Point (DSCP) includes the 6-bit Differentiated Services Code Point (DSCP)
value. It is not required, but the application SHOULD be able value. It is not required, but the application SHOULD be able
to change the Differentiated Services field during the to change the Differentiated Services field during the
connection lifetime (SHLD-21). TCP SHOULD pass the current connection lifetime (SHLD-21). TCP implementations SHOULD pass
Differentiated Services field value without change to the IP the current Differentiated Services field value without change
layer, when it sends segments on the connection (SHLD-22). to the IP layer, when it sends segments on the connection
(SHLD-22).
The Differentiated Services field will be specified The Differentiated Services field will be specified
independently in each direction on the connection, so that the independently in each direction on the connection, so that the
receiver application will specify the Differentiated Services receiver application will specify the Differentiated Services
field used for ACK segments. field used for ACK segments.
TCP MAY pass the most recently received Differentiated Services TCP implementations MAY pass the most recently received
field up to the application (MAY-9). Differentiated Services field up to the application (MAY-9).
3.9.2. TCP/Lower-Level Interface 3.8.2. TCP/Lower-Level Interface
The TCP calls on a lower level protocol module to actually send and The TCP endpoint calls on a lower level protocol module to actually
receive information over a network. The two current standard send and receive information over a network. The two current
Internet Protocol (IP) versions layered below TCP are IPv4 [1] and standard Internet Protocol (IP) versions layered below TCP are IPv4
IPv6 [11]. [1] and IPv6 [11].
If the lower level protocol is IPv4 it provides arguments for a type If the lower level protocol is IPv4 it provides arguments for a type
of service (used within the Differentiated Services field) and for a of service (used within the Differentiated Services field) and for a
time to live. TCP uses the following settings for these parameters: time to live. TCP uses the following settings for these parameters:
DiffServ field: The IP header value for the DiffServ field is DiffServ field: The IP header value for the DiffServ field is
given by the user. This includes the bits of the DiffServ Code given by the user. This includes the bits of the DiffServ Code
Point (DSCP). Point (DSCP).
Time to Live (TTL): The TTL value used to send TCP segments MUST Time to Live (TTL): The TTL value used to send TCP segments MUST
skipping to change at page 55, line 41 skipping to change at page 55, line 21
that a segment be destroyed if it cannot be delivered by the that a segment be destroyed if it cannot be delivered by the
internet system within one minute. RFC 1122 changed this internet system within one minute. RFC 1122 changed this
specification to require that the TTL be configurable. specification to require that the TTL be configurable.
Note that the DiffServ field is permitted to change during a Note that the DiffServ field is permitted to change during a
connection (section 4.2.4.2 of RFC 1122). However, the connection (section 4.2.4.2 of RFC 1122). However, the
application interface might not support this ability, and the application interface might not support this ability, and the
application does not have knowledge about individual TCP application does not have knowledge about individual TCP
segments, so this can only be done on a coarse granularity, at segments, so this can only be done on a coarse granularity, at
best. This limitation is further discussed in RFC 7657 (sec best. This limitation is further discussed in RFC 7657 (sec
5.1, 5.3, and 6) [38]. Generally, an application SHOULD NOT 5.1, 5.3, and 6) [40]. Generally, an application SHOULD NOT
change the DiffServ field value during the course of a change the DiffServ field value during the course of a
connection (SHLD-23). connection (SHLD-23).
Any lower level protocol will have to provide the source address, Any lower level protocol will have to provide the source address,
destination address, and protocol fields, and some way to determine destination address, and protocol fields, and some way to determine
the "TCP length", both to provide the functional equivalent service the "TCP length", both to provide the functional equivalent service
of IP and to be used in the TCP checksum. of IP and to be used in the TCP checksum.
When received options are passed up to TCP from the IP layer, TCP When received options are passed up to TCP from the IP layer, TCP
MUST ignore options that it does not understand (MUST-50). implementations MUST ignore options that it does not understand
(MUST-50).
A TCP MAY support the Time Stamp (MAY-10) and Record Route (MAY-11) A TCP implementation MAY support the Time Stamp (MAY-10) and Record
options. Route (MAY-11) options.
3.9.2.1. Source Routing 3.8.2.1. Source Routing
If the lower level is IP (or other protocol that provides this If the lower level is IP (or other protocol that provides this
feature) and source routing is used, the interface must allow the feature) and source routing is used, the interface must allow the
route information to be communicated. This is especially important route information to be communicated. This is especially important
so that the source and destination addresses used in the TCP checksum so that the source and destination addresses used in the TCP checksum
be the originating source and ultimate destination. It is also be the originating source and ultimate destination. It is also
important to preserve the return route to answer connection requests. important to preserve the return route to answer connection requests.
An application MUST be able to specify a source route when it An application MUST be able to specify a source route when it
actively opens a TCP connection (MUST-51), and this MUST take actively opens a TCP connection (MUST-51), and this MUST take
precedence over a source route received in a datagram (MUST-52). precedence over a source route received in a datagram (MUST-52).
When a TCP connection is OPENed passively and a packet arrives with a When a TCP connection is OPENed passively and a packet arrives with a
completed IP Source Route option (containing a return route), TCP completed IP Source Route option (containing a return route), TCP
MUST save the return route and use it for all segments sent on this implementations MUST save the return route and use it for all
connection (MUST-53). If a different source route arrives in a later segments sent on this connection (MUST-53). If a different source
segment, the later definition SHOULD override the earlier one (SHLD- route arrives in a later segment, the later definition SHOULD
24). override the earlier one (SHLD-24).
3.9.2.2. ICMP Messages 3.8.2.2. ICMP Messages
TCP MUST act on an ICMP error message passed up from the IP layer, TCP implementations MUST act on an ICMP error message passed up from
directing it to the connection that created the error (MUST-54). The the IP layer, directing it to the connection that created the error
necessary demultiplexing information can be found in the IP header (MUST-54). The necessary demultiplexing information can be found in
contained within the ICMP message. the IP header contained within the ICMP message.
This applies to ICMPv6 in addition to IPv4 ICMP. This applies to ICMPv6 in addition to IPv4 ICMP.
[23] contains discussion of specific ICMP and ICMPv6 messages [25] contains discussion of specific ICMP and ICMPv6 messages
classified as either "soft" or "hard" errors that may bear different classified as either "soft" or "hard" errors that may bear different
responses. Treatment for classes of ICMP messages is described responses. Treatment for classes of ICMP messages is described
below: below:
Source Quench Source Quench
TCP MUST silently discard any received ICMP Source Quench messages TCP implementations MUST silently discard any received ICMP Source
(MUST-55). See [10] for discussion. Quench messages (MUST-55). See [10] for discussion.
Soft Errors Soft Errors
For ICMP these include: Destination Unreachable -- codes 0, 1, 5, For ICMP these include: Destination Unreachable -- codes 0, 1, 5,
Time Exceeded -- codes 0, 1, and Parameter Problem. Time Exceeded -- codes 0, 1, and Parameter Problem.
For ICMPv6 these include: Destination Unreachable -- codes 0 and 3, For ICMPv6 these include: Destination Unreachable -- codes 0 and 3,
Time Exceeded -- codes 0, 1, and Parameter Problem -- codes 0, 1, 2 Time Exceeded -- codes 0, 1, and Parameter Problem -- codes 0, 1, 2
Since these Unreachable messages indicate soft error conditions, Since these Unreachable messages indicate soft error conditions,
TCP MUST NOT abort the connection (MUST-56), and it SHOULD make the TCP implementations MUST NOT abort the connection (MUST-56), and it
information available to the application (SHLD-25). SHOULD make the information available to the application (SHLD-25).
Hard Errors Hard Errors
For ICMP these include Destination Unreachable -- codes 2-4"> For ICMP these include Destination Unreachable -- codes 2-4">
These are hard error conditions, so TCP SHOULD abort the connection These are hard error conditions, so TCP implementations SHOULD
(SHLD-26). [23] notes that some implementations do not abort abort the connection (SHLD-26). [25] notes that some
connections when an ICMP hard error is received for a connection implementations do not abort connections when an ICMP hard error is
that is in any of the synchronized states. received for a connection that is in any of the synchronized
states.
Note that [23] section 4 describes widespread implementation behavior Note that [25] section 4 describes widespread implementation behavior
that treats soft errors as hard errors during connection that treats soft errors as hard errors during connection
establishment. establishment.
3.9.2.3. Remote Address Validation 3.8.2.3. Remote Address Validation
RFC 1122 requires addresses to be validated in incoming SYN packets: RFC 1122 requires addresses to be validated in incoming SYN packets:
An incoming SYN with an invalid source address MUST be ignored An incoming SYN with an invalid source address MUST be ignored
either by TCP or by the IP layer (MUST-63) (see Section 3.2.1.3 of either by TCP or by the IP layer (MUST-63) (see Section 3.2.1.3 of
[14]). [14]).
A TCP implementation MUST silently discard an incoming SYN segment A TCP implementation MUST silently discard an incoming SYN segment
that is addressed to a broadcast or multicast address (MUST-57). that is addressed to a broadcast or multicast address (MUST-57).
This prevents connection state and replies from being erroneously This prevents connection state and replies from being erroneously
generated, and implementers should note that this guidance is generated, and implementers should note that this guidance is
applicable to all incoming segments, not just SYNs, as specifically applicable to all incoming segments, not just SYNs, as specifically
indicated in RFC 1122. indicated in RFC 1122.
3.10. Event Processing 3.9. Event Processing
The processing depicted in this section is an example of one possible The processing depicted in this section is an example of one possible
implementation. Other implementations may have slightly different implementation. Other implementations may have slightly different
processing sequences, but they should differ from those in this processing sequences, but they should differ from those in this
section only in detail, not in substance. section only in detail, not in substance.
The activity of the TCP can be characterized as responding to events. The activity of the TCP endpoint can be characterized as responding
The events that occur can be cast into three categories: user calls, to events. The events that occur can be cast into three categories:
arriving segments, and timeouts. This section describes the user calls, arriving segments, and timeouts. This section describes
processing the TCP does in response to each of the events. In many the processing the TCP endpoint does in response to each of the
cases the processing required depends on the state of the connection. events. In many cases the processing required depends on the state
of the connection.
Events that occur: Events that occur:
User Calls User Calls
OPEN OPEN
SEND SEND
RECEIVE RECEIVE
CLOSE CLOSE
ABORT ABORT
STATUS STATUS
Arriving Segments Arriving Segments
SEGMENT ARRIVES SEGMENT ARRIVES
skipping to change at page 58, line 26 skipping to change at page 58, line 10
USER TIMEOUT USER TIMEOUT
RETRANSMISSION TIMEOUT RETRANSMISSION TIMEOUT
TIME-WAIT TIMEOUT TIME-WAIT TIMEOUT
The model of the TCP/user interface is that user commands receive an The model of the TCP/user interface is that user commands receive an
immediate return and possibly a delayed response via an event or immediate return and possibly a delayed response via an event or
pseudo interrupt. In the following descriptions, the term "signal" pseudo interrupt. In the following descriptions, the term "signal"
means cause a delayed response. means cause a delayed response.
Error responses are given as character strings. For example, user Error responses in this document are identified by character strings.
commands referencing connections that do not exist receive "error: For example, user commands referencing connections that do not exist
connection not open". receive "error: connection not open".
Please note in the following that all arithmetic on sequence numbers, Please note in the following that all arithmetic on sequence numbers,
acknowledgment numbers, windows, et cetera, is modulo 2**32 the size acknowledgment numbers, windows, et cetera, is modulo 2**32 the size
of the sequence number space. Also note that "=<" means less than or of the sequence number space. Also note that "=<" means less than or
equal to (modulo 2**32). equal to (modulo 2**32).
A natural way to think about processing incoming segments is to A natural way to think about processing incoming segments is to
imagine that they are first tested for proper sequence number (i.e., imagine that they are first tested for proper sequence number (i.e.,
that their contents lie in the range of the expected "receive window" that their contents lie in the range of the expected "receive window"
in the sequence number space) and then that they are generally queued in the sequence number space) and then that they are generally queued
and processed in sequence number order. and processed in sequence number order.
When a segment overlaps other already received segments we When a segment overlaps other already received segments we
reconstruct the segment to contain just the new data, and adjust the reconstruct the segment to contain just the new data, and adjust the
header fields to be consistent. header fields to be consistent.
Note that if no state change is mentioned the TCP stays in the same Note that if no state change is mentioned the TCP connection stays in
state. the same state.
OPEN Call OPEN Call
CLOSED STATE (i.e., TCB does not exist) CLOSED STATE (i.e., TCB does not exist)
Create a new transmission control block (TCB) to hold Create a new transmission control block (TCB) to hold
connection state information. Fill in local socket identifier, connection state information. Fill in local socket identifier,
foreign socket, DiffServ field, security/compartment, and user remote socket, DiffServ field, security/compartment, and user
timeout information. Note that some parts of the foreign timeout information. Note that some parts of the remote socket
socket may be unspecified in a passive OPEN and are to be may be unspecified in a passive OPEN and are to be filled in by
filled in by the parameters of the incoming SYN segment. the parameters of the incoming SYN segment. Verify the
Verify the security and DiffServ value requested are allowed security and DiffServ value requested are allowed for this
for this user, if not return "error: precedence not allowed" or user, if not return "error: precedence not allowed" or "error:
"error: security/compartment not allowed." If passive enter security/compartment not allowed." If passive enter the LISTEN
the LISTEN state and return. If active and the foreign socket state and return. If active and the remote socket is
is unspecified, return "error: foreign socket unspecified"; if unspecified, return "error: remote socket unspecified"; if
active and the foreign socket is specified, issue a SYN active and the remote socket is specified, issue a SYN segment.
segment. An initial send sequence number (ISS) is selected. A An initial send sequence number (ISS) is selected. A SYN
SYN segment of the form <SEQ=ISS><CTL=SYN> is sent. Set segment of the form <SEQ=ISS><CTL=SYN> is sent. Set SND.UNA to
SND.UNA to ISS, SND.NXT to ISS+1, enter SYN-SENT state, and ISS, SND.NXT to ISS+1, enter SYN-SENT state, and return.
return.
If the caller does not have access to the local socket If the caller does not have access to the local socket
specified, return "error: connection illegal for this process". specified, return "error: connection illegal for this process".
If there is no room to create a new connection, return "error: If there is no room to create a new connection, return "error:
insufficient resources". insufficient resources".
LISTEN STATE LISTEN STATE
If active and the foreign socket is specified, then change the If active and the remote socket is specified, then change the
connection from passive to active, select an ISS. Send a SYN connection from passive to active, select an ISS. Send a SYN
segment, set SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT segment, set SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT
state. Data associated with SEND may be sent with SYN segment state. Data associated with SEND may be sent with SYN segment
or queued for transmission after entering ESTABLISHED state. or queued for transmission after entering ESTABLISHED state.
The urgent bit if requested in the command must be sent with The urgent bit if requested in the command must be sent with
the data segments sent as a result of this command. If there the data segments sent as a result of this command. If there
is no room to queue the request, respond with "error: is no room to queue the request, respond with "error:
insufficient resources". If Foreign socket was not specified, insufficient resources". If Foreign socket was not specified,
then return "error: foreign socket unspecified". then return "error: remote socket unspecified".
SYN-SENT STATE SYN-SENT STATE
SYN-RECEIVED STATE SYN-RECEIVED STATE
ESTABLISHED STATE ESTABLISHED STATE
FIN-WAIT-1 STATE FIN-WAIT-1 STATE
FIN-WAIT-2 STATE FIN-WAIT-2 STATE
CLOSE-WAIT STATE CLOSE-WAIT STATE
CLOSING STATE CLOSING STATE
LAST-ACK STATE LAST-ACK STATE
TIME-WAIT STATE TIME-WAIT STATE
skipping to change at page 61, line 16 skipping to change at page 61, line 16
CLOSED STATE (i.e., TCB does not exist) CLOSED STATE (i.e., TCB does not exist)
If the user does not have access to such a connection, then If the user does not have access to such a connection, then
return "error: connection illegal for this process". return "error: connection illegal for this process".
Otherwise, return "error: connection does not exist". Otherwise, return "error: connection does not exist".
LISTEN STATE LISTEN STATE
If the foreign socket is specified, then change the connection If the remote socket is specified, then change the connection
from passive to active, select an ISS. Send a SYN segment, set from passive to active, select an ISS. Send a SYN segment, set
SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data
associated with SEND may be sent with SYN segment or queued for associated with SEND may be sent with SYN segment or queued for
transmission after entering ESTABLISHED state. The urgent bit transmission after entering ESTABLISHED state. The urgent bit
if requested in the command must be sent with the data segments if requested in the command must be sent with the data segments
sent as a result of this command. If there is no room to queue sent as a result of this command. If there is no room to queue
the request, respond with "error: insufficient resources". If the request, respond with "error: insufficient resources". If
Foreign socket was not specified, then return "error: foreign Foreign socket was not specified, then return "error: remote
socket unspecified". socket unspecified".
SYN-SENT STATE SYN-SENT STATE
SYN-RECEIVED STATE SYN-RECEIVED STATE
Queue the data for transmission after entering ESTABLISHED Queue the data for transmission after entering ESTABLISHED
state. If no space to queue, respond with "error: insufficient state. If no space to queue, respond with "error: insufficient
resources". resources".
ESTABLISHED STATE ESTABLISHED STATE
skipping to change at page 62, line 37 skipping to change at page 62, line 37
request, queue the request. If there is no queue space to request, queue the request. If there is no queue space to
remember the RECEIVE, respond with "error: insufficient remember the RECEIVE, respond with "error: insufficient
resources". resources".
Reassemble queued incoming segments into receive buffer and Reassemble queued incoming segments into receive buffer and
return to user. Mark "push seen" (PUSH) if this is the case. return to user. Mark "push seen" (PUSH) if this is the case.
If RCV.UP is in advance of the data currently being passed to If RCV.UP is in advance of the data currently being passed to
the user notify the user of the presence of urgent data. the user notify the user of the presence of urgent data.
When the TCP takes responsibility for delivering data to the When the TCP endpoint takes responsibility for delivering data
user that fact must be communicated to the sender via an to the user that fact must be communicated to the sender via an
acknowledgment. The formation of such an acknowledgment is acknowledgment. The formation of such an acknowledgment is
described below in the discussion of processing an incoming described below in the discussion of processing an incoming
segment. segment.
CLOSE-WAIT STATE CLOSE-WAIT STATE
Since the remote side has already sent FIN, RECEIVEs must be Since the remote side has already sent FIN, RECEIVEs must be
satisfied by text already on hand, but not yet delivered to the satisfied by text already on hand, but not yet delivered to the
user. If no text is awaiting delivery, the RECEIVE will get a user. If no text is awaiting delivery, the RECEIVE will get a
"error: connection closing" response. Otherwise, any remaining "error: connection closing" response. Otherwise, any remaining
skipping to change at page 68, line 13 skipping to change at page 68, line 13
Return "state = TIME-WAIT", and the TCB pointer. Return "state = TIME-WAIT", and the TCB pointer.
SEGMENT ARRIVES SEGMENT ARRIVES
If the state is CLOSED (i.e., TCB does not exist) then If the state is CLOSED (i.e., TCB does not exist) then
all data in the incoming segment is discarded. An incoming all data in the incoming segment is discarded. An incoming
segment containing a RST is discarded. An incoming segment not segment containing a RST is discarded. An incoming segment not
containing a RST causes a RST to be sent in response. The containing a RST causes a RST to be sent in response. The
acknowledgment and sequence field values are selected to make acknowledgment and sequence field values are selected to make
the reset sequence acceptable to the TCP that sent the the reset sequence acceptable to the TCP endpoint that sent the
offending segment. offending segment.
If the ACK bit is off, sequence number zero is used, If the ACK bit is off, sequence number zero is used,
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK> <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If the ACK bit is on, If the ACK bit is on,
<SEQ=SEG.ACK><CTL=RST> <SEQ=SEG.ACK><CTL=RST>
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other control or text should be queued for processing later. other control or text should be queued for processing later.
ISS should be selected and a SYN segment sent of the form: ISS should be selected and a SYN segment sent of the form:
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection
state should be changed to SYN-RECEIVED. Note that any state should be changed to SYN-RECEIVED. Note that any
other incoming control or data (combined with SYN) will be other incoming control or data (combined with SYN) will be
processed in the SYN-RECEIVED state, but processing of SYN processed in the SYN-RECEIVED state, but processing of SYN
and ACK should not be repeated. If the listen was not fully and ACK should not be repeated. If the listen was not fully
specified (i.e., the foreign socket was not fully specified (i.e., the remote socket was not fully specified),
specified), then the unspecified fields should be filled in then the unspecified fields should be filled in now.
now.
fourth other text or control fourth other text or control
Any other control or text-bearing segment (not containing Any other control or text-bearing segment (not containing
SYN) must have an ACK and thus would be discarded by the ACK SYN) must have an ACK and thus would be discarded by the ACK
processing. An incoming RST segment could not be valid, processing. An incoming RST segment could not be valid,
since it could not have been sent in response to anything since it could not have been sent in response to anything
sent by this incarnation of the connection. So you are sent by this incarnation of the connection. So, if this
unlikely to get here, but if you do, drop the segment, and unlikely condition is reached, the correct behavior is to
return. drop the segment and return.
If the state is SYN-SENT then If the state is SYN-SENT then
first check the ACK bit first check the ACK bit
If the ACK bit is set If the ACK bit is set
If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset
(unless the RST bit is set, if so drop the segment and (unless the RST bit is set, if so drop the segment and
return) return)
<SEQ=SEG.ACK><CTL=RST> <SEQ=SEG.ACK><CTL=RST>
and discard the segment. Return. and discard the segment. Return.
If SND.UNA < SEG.ACK =< SND.NXT then the ACK is If SND.UNA < SEG.ACK =< SND.NXT then the ACK is
acceptable. Some deployed TCP code has used the check acceptable. Some deployed TCP code has used the check
SEG.ACK == SND.NXT (using "==" rather than "=<", but this SEG.ACK == SND.NXT (using "==" rather than "=<", but this
is not appropriate when the stack is capable of sending is not appropriate when the stack is capable of sending
data on the SYN, because the peer TCP may not accept and data on the SYN, because the TCP peer may not accept and
acknowledge all of the data on the SYN. acknowledge all of the data on the SYN.
second check the RST bit second check the RST bit
If the RST bit is set
If the RST bit is set
A potential blind reset attack is described in RFC 5961 A potential blind reset attack is described in RFC 5961
[28], with the mitigation that a TCP implementation [30], with the mitigation that a TCP implementation
SHOULD first check that the sequence number exactly SHOULD first check that the sequence number exactly
matches RCV.NXT prior to executing the action in the next matches RCV.NXT prior to executing the action in the next
paragraph. paragraph.
If the ACK was acceptable then signal the user "error: If the ACK was acceptable then signal the user "error:
connection reset", drop the segment, enter CLOSED state, connection reset", drop the segment, enter CLOSED state,
delete TCB, and return. Otherwise (no ACK) drop the delete TCB, and return. Otherwise (no ACK) drop the
segment and return. segment and return.
third check the security third check the security
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fourth check the SYN bit fourth check the SYN bit
This step should be reached only if the ACK is ok, or there This step should be reached only if the ACK is ok, or there
is no ACK, and it the segment did not contain a RST. is no ACK, and it the segment did not contain a RST.
If the SYN bit is on and the security/compartment is If the SYN bit is on and the security/compartment is
acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to
SEG.SEQ. SND.UNA should be advanced to equal SEG.ACK (if SEG.SEQ. SND.UNA should be advanced to equal SEG.ACK (if
there is an ACK), and any segments on the retransmission there is an ACK), and any segments on the retransmission
queue which are thereby acknowledged should be removed. queue that are thereby acknowledged should be removed.
If SND.UNA > ISS (our SYN has been ACKed), change the If SND.UNA > ISS (our SYN has been ACKed), change the
connection state to ESTABLISHED, form an ACK segment connection state to ESTABLISHED, form an ACK segment
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
and send it. Data or controls which were queued for and send it. Data or controls that were queued for
transmission may be included. If there are other controls transmission may be included. If there are other controls
or text in the segment then continue processing at the sixth or text in the segment then continue processing at the sixth
step below where the URG bit is checked, otherwise return. step below where the URG bit is checked, otherwise return.
Otherwise enter SYN-RECEIVED, form a SYN,ACK segment Otherwise enter SYN-RECEIVED, form a SYN,ACK segment
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
and send it. Set the variables: and send it. Set the variables:
SND.WND <- SEG.WND SND.WND <- SEG.WND
SND.WL1 <- SEG.SEQ SND.WL1 <- SEG.SEQ
SND.WL2 <- SEG.ACK SND.WL2 <- SEG.ACK
If there are other controls or text in the segment, queue If there are other controls or text in the segment, queue
them for processing after the ESTABLISHED state has been them for processing after the ESTABLISHED state has been
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If there are other controls or text in the segment, queue If there are other controls or text in the segment, queue
them for processing after the ESTABLISHED state has been them for processing after the ESTABLISHED state has been
reached, return. reached, return.
Note that it is legal to send and receive application data Note that it is legal to send and receive application data
on SYN segments (this is the "text in the segment" mentioned on SYN segments (this is the "text in the segment" mentioned
above. There has been significant misinformation and above. There has been significant misinformation and
misunderstanding of this topic historically. Some firewalls misunderstanding of this topic historically. Some firewalls
and security devices consider this suspicious. However, the and security devices consider this suspicious. However, the
capability was used in T/TCP [16] and is used in TCP Fast capability was used in T/TCP [16] and is used in TCP Fast
Open (TFO) [36], so is important for implementations and Open (TFO) [38], so is important for implementations and
network devices to permit. network devices to permit.
fifth, if neither of the SYN or RST bits is set then drop the fifth, if neither of the SYN or RST bits is set then drop the
segment and return. segment and return.
Otherwise, Otherwise,
first check sequence number first check sequence number
SYN-RECEIVED STATE SYN-RECEIVED STATE
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TIME-WAIT STATE TIME-WAIT STATE
Segments are processed in sequence. Initial tests on Segments are processed in sequence. Initial tests on
arrival are used to discard old duplicates, but further arrival are used to discard old duplicates, but further
processing is done in SEG.SEQ order. If a segment's processing is done in SEG.SEQ order. If a segment's
contents straddle the boundary between old and new, only the contents straddle the boundary between old and new, only the
new parts should be processed. new parts should be processed.
In general, the processing of received segments MUST be In general, the processing of received segments MUST be
implemented to aggregate ACK segments whenever possible implemented to aggregate ACK segments whenever possible
(MUST-58). For example, if the TCP is processing a series (MUST-58). For example, if the TCP endpoint is processing a
of queued segments, it MUST process them all before sending series of queued segments, it MUST process them all before
any ACK segments (MUST-59). sending any ACK segments (MUST-59).
There are four cases for the acceptability test for an There are four cases for the acceptability test for an
incoming segment: incoming segment:
Segment Receive Test Segment Receive Test
Length Window Length Window
------- ------- ------------------------------------------- ------- ------- -------------------------------------------
0 0 SEG.SEQ = RCV.NXT 0 0 SEG.SEQ = RCV.NXT
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If an incoming segment is not acceptable, an acknowledgment If an incoming segment is not acceptable, an acknowledgment
should be sent in reply (unless the RST bit is set, if so should be sent in reply (unless the RST bit is set, if so
drop the segment and return): drop the segment and return):
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
After sending the acknowledgment, drop the unacceptable After sending the acknowledgment, drop the unacceptable
segment and return. segment and return.
Note that for the TIME-WAIT state, there is an improved Note that for the TIME-WAIT state, there is an improved
algorithm described in [30] for handling incoming SYN algorithm described in [32] for handling incoming SYN
segments, that utilizes timestamps rather than relying on segments, that utilizes timestamps rather than relying on
the sequence number check described here. When the improved the sequence number check described here. When the improved
algorithm is implemented, the logic above is not applicable algorithm is implemented, the logic above is not applicable
for incoming SYN segments with timestamp options, received for incoming SYN segments with timestamp options, received
on a connection in the TIME-WAIT state. on a connection in the TIME-WAIT state.
In the following it is assumed that the segment is the In the following it is assumed that the segment is the
idealized segment that begins at RCV.NXT and does not exceed idealized segment that begins at RCV.NXT and does not exceed
the window. One could tailor actual segments to fit this the window. One could tailor actual segments to fit this
assumption by trimming off any portions that lie outside the assumption by trimming off any portions that lie outside the
skipping to change at page 73, line 24 skipping to change at page 73, line 21
For stacks implementing RFC 5961, the three checks below For stacks implementing RFC 5961, the three checks below
apply, otherwise processesing for these states is indicated apply, otherwise processesing for these states is indicated
further below. further below.
1) If the RST bit is set and the sequence number is 1) If the RST bit is set and the sequence number is
outside the current receive window, silently drop the outside the current receive window, silently drop the
segment. segment.
2) If the RST bit is set and the sequence number exactly 2) If the RST bit is set and the sequence number exactly
matches the next expected sequence number (RCV.NXT), then matches the next expected sequence number (RCV.NXT), then
TCP MUST reset the connection in the manner prescribed TCP endpoints MUST reset the connection in the manner
below according to the connection state. prescribed below according to the connection state.
3) If the RST bit is set and the sequence number does not 3) If the RST bit is set and the sequence number does not
exactly match the next expected sequence value, yet is exactly match the next expected sequence value, yet is
within the current receive window, TCP MUST send an within the current receive window, TCP endpoints MUST
acknowledgement (challenge ACK): send an acknowledgement (challenge ACK):
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
After sending the challenge ACK, TCP MUST drop the After sending the challenge ACK, TCP endpoints MUST drop
unacceptable segment and stop processing the incoming the unacceptable segment and stop processing the incoming
packet further. Note that RFC 5961 and Errata ID 4772 packet further. Note that RFC 5961 and Errata ID 4772
contain additional considerations for ACK throttling in contain additional considerations for ACK throttling in
an implementation. an implementation.
SYN-RECEIVED STATE SYN-RECEIVED STATE
If the RST bit is set If the RST bit is set
If this connection was initiated with a passive OPEN If this connection was initiated with a passive OPEN
(i.e., came from the LISTEN state), then return this (i.e., came from the LISTEN state), then return this
skipping to change at page 75, line 28 skipping to change at page 75, line 26
FIN-WAIT STATE-2 FIN-WAIT STATE-2
CLOSE-WAIT STATE CLOSE-WAIT STATE
CLOSING STATE CLOSING STATE
LAST-ACK STATE LAST-ACK STATE
TIME-WAIT STATE TIME-WAIT STATE
If the SYN bit is set in these synchronized states, it If the SYN bit is set in these synchronized states, it
may be either a legitimate new connection attempt (e.g. may be either a legitimate new connection attempt (e.g.
in the case of TIME-WAIT), an error where the connection in the case of TIME-WAIT), an error where the connection
should be reset, or the result of an attack attempt, as should be reset, or the result of an attack attempt, as
described in RFC 5961 [28]. For the TIME-WAIT state, new described in RFC 5961 [30]. For the TIME-WAIT state, new
connections can be accepted if the timestamp option is connections can be accepted if the timestamp option is
used and meets expectations (per [30]). For all other used and meets expectations (per [32]). For all other
caess, RFC 5961 provides a mitigation that SHOULD be caess, RFC 5961 provides a mitigation that SHOULD be
implemented, though there are alternatives (see implemented, though there are alternatives (see
Section 6). RFC 5961 recommends that in these Section 6). RFC 5961 recommends that in these
synchronized states, if the SYN bit is set, irrespective synchronized states, if the SYN bit is set, irrespective
of the sequence number, TCP MUST send a "challenge ACK" of the sequence number, TCP endpoints MUST send a
to the remote peer: "challenge ACK" to the remote peer:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
After sending the acknowledgement, TCP MUST drop the After sending the acknowledgement, TCP implementations
unacceptable segment and stop processing further. Note MUST drop the unacceptable segment and stop processing
that RFC 5961 and Errata ID 4772 contain additional ACK further. Note that RFC 5961 and Errata ID 4772 contain
throttling notes for an implementation. additional ACK throttling notes for an implementation.
For implementations that do not follow RFC 5961, the For implementations that do not follow RFC 5961, the
original RFC 793 behavior follows in this paragraph. If original RFC 793 behavior follows in this paragraph. If
the SYN is in the window it is an error, send a reset, the SYN is in the window it is an error, send a reset,
any outstanding RECEIVEs and SEND should receive "reset" any outstanding RECEIVEs and SEND should receive "reset"
responses, all segment queues should be flushed, the user responses, all segment queues should be flushed, the user
should also receive an unsolicited general "connection should also receive an unsolicited general "connection
reset" signal, enter the CLOSED state, delete the TCB, reset" signal, enter the CLOSED state, delete the TCB,
and return. and return.
skipping to change at page 77, line 4 skipping to change at page 76, line 50
form a reset segment, form a reset segment,
<SEQ=SEG.ACK><CTL=RST> <SEQ=SEG.ACK><CTL=RST>
and send it. and send it.
ESTABLISHED STATE ESTABLISHED STATE
If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <-
SEG.ACK. Any segments on the retransmission queue SEG.ACK. Any segments on the retransmission queue
which are thereby entirely acknowledged are removed. that are thereby entirely acknowledged are removed.
Users should receive positive acknowledgments for Users should receive positive acknowledgments for
buffers which have been SENT and fully acknowledged buffers that have been SENT and fully acknowledged
(i.e., SEND buffer should be returned with "ok" (i.e., SEND buffer should be returned with "ok"
response). If the ACK is a duplicate (SEG.ACK =< response). If the ACK is a duplicate (SEG.ACK =<
SND.UNA), it can be ignored. If the ACK acks SND.UNA), it can be ignored. If the ACK acks
something not yet sent (SEG.ACK > SND.NXT) then send something not yet sent (SEG.ACK > SND.NXT) then send
an ACK, drop the segment, and return. an ACK, drop the segment, and return.
If SND.UNA =< SEG.ACK =< SND.NXT, the send window If SND.UNA =< SEG.ACK =< SND.NXT, the send window
should be updated. If (SND.WL1 < SEG.SEQ or (SND.WL1 should be updated. If (SND.WL1 < SEG.SEQ or (SND.WL1
= SEG.SEQ and SND.WL2 =< SEG.ACK)), set SND.WND <- = SEG.SEQ and SND.WL2 =< SEG.ACK)), set SND.WND <-
SEG.WND, set SND.WL1 <- SEG.SEQ, and set SND.WL2 <- SEG.WND, set SND.WL1 <- SEG.SEQ, and set SND.WL2 <-
skipping to change at page 77, line 29 skipping to change at page 77, line 26
Note that SND.WND is an offset from SND.UNA, that Note that SND.WND is an offset from SND.UNA, that
SND.WL1 records the sequence number of the last SND.WL1 records the sequence number of the last
segment used to update SND.WND, and that SND.WL2 segment used to update SND.WND, and that SND.WL2
records the acknowledgment number of the last segment records the acknowledgment number of the last segment
used to update SND.WND. The check here prevents using used to update SND.WND. The check here prevents using
old segments to update the window. old segments to update the window.
FIN-WAIT-1 STATE FIN-WAIT-1 STATE
In addition to the processing for the ESTABLISHED In addition to the processing for the ESTABLISHED
state, if our FIN is now acknowledged then enter FIN- state, if the FIN segment is now acknowledged then
WAIT-2 and continue processing in that state. enter FIN-WAIT-2 and continue processing in that
state.
FIN-WAIT-2 STATE FIN-WAIT-2 STATE
In addition to the processing for the ESTABLISHED In addition to the processing for the ESTABLISHED
state, if the retransmission queue is empty, the state, if the retransmission queue is empty, the
user's CLOSE can be acknowledged ("ok") but do not user's CLOSE can be acknowledged ("ok") but do not
delete the TCB. delete the TCB.
CLOSE-WAIT STATE CLOSE-WAIT STATE
skipping to change at page 78, line 46 skipping to change at page 78, line 44
seventh, process the segment text, seventh, process the segment text,
ESTABLISHED STATE ESTABLISHED STATE
FIN-WAIT-1 STATE FIN-WAIT-1 STATE
FIN-WAIT-2 STATE FIN-WAIT-2 STATE
Once in the ESTABLISHED state, it is possible to deliver Once in the ESTABLISHED state, it is possible to deliver
segment text to user RECEIVE buffers. Text from segments segment text to user RECEIVE buffers. Text from segments
can be moved into buffers until either the buffer is full can be moved into buffers until either the buffer is full
or the segment is empty. If the segment empties and or the segment is empty. If the segment empties and
carries an PUSH flag, then the user is informed, when the carries a PUSH flag, then the user is informed, when the
buffer is returned, that a PUSH has been received. buffer is returned, that a PUSH has been received.
When the TCP takes responsibility for delivering the data When the TCP endpoint takes responsibility for delivering
to the user it must also acknowledge the receipt of the the data to the user it must also acknowledge the receipt
data. of the data.
Once the TCP takes responsibility for the data it Once the TCP endpoint takes responsibility for the data
advances RCV.NXT over the data accepted, and adjusts it advances RCV.NXT over the data accepted, and adjusts
RCV.WND as appropriate to the current buffer RCV.WND as appropriate to the current buffer
availability. The total of RCV.NXT and RCV.WND should availability. The total of RCV.NXT and RCV.WND should
not be reduced. not be reduced.
A TCP MAY send an ACK segment acknowledging RCV.NXT when A TCP implementation MAY send an ACK segment
a valid segment arrives that is in the window but not at acknowledging RCV.NXT when a valid segment arrives that
the left window edge (MAY-13). is in the window but not at the left window edge (MAY-
13).
Please note the window management suggestions in Please note the window management suggestions in
Section 3.8. Section 3.7.
Send an acknowledgment of the form: Send an acknowledgment of the form:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
This acknowledgment should be piggybacked on a segment This acknowledgment should be piggybacked on a segment
being transmitted if possible without incurring undue being transmitted if possible without incurring undue
delay. delay.
CLOSE-WAIT STATE CLOSE-WAIT STATE
skipping to change at page 82, line 5 skipping to change at page 82, line 5
For any state if the retransmission timeout expires on a For any state if the retransmission timeout expires on a
segment in the retransmission queue, send the segment at the segment in the retransmission queue, send the segment at the
front of the retransmission queue again, reinitialize the front of the retransmission queue again, reinitialize the
retransmission timer, and return. retransmission timer, and return.
TIME-WAIT TIMEOUT TIME-WAIT TIMEOUT
If the time-wait timeout expires on a connection delete the If the time-wait timeout expires on a connection delete the
TCB, enter the CLOSED state and return. TCB, enter the CLOSED state and return.
3.11. Glossary 3.10. Glossary
ACK ACK
A control bit (acknowledge) occupying no sequence space, A control bit (acknowledge) occupying no sequence space,
which indicates that the acknowledgment field of this segment which indicates that the acknowledgment field of this segment
specifies the next sequence number the sender of this segment specifies the next sequence number the sender of this segment
is expecting to receive, hence acknowledging receipt of all is expecting to receive, hence acknowledging receipt of all
previous sequence numbers. previous sequence numbers.
connection connection
A logical communication path identified by a pair of sockets. A logical communication path identified by a pair of sockets.
datagram datagram
A message sent in a packet switched computer communications A message sent in a packet switched computer communications
network. network.
Destination Address Destination Address
The destination address, usually the network and host The network layer address of the remote endpoint.
identifiers.
FIN FIN
A control bit (finis) occupying one sequence number, which A control bit (finis) occupying one sequence number, which
indicates that the sender will send no more data or control indicates that the sender will send no more data or control
occupying sequence space. occupying sequence space.
fragment fragment
A portion of a logical unit of data, in particular an A portion of a logical unit of data, in particular an
internet fragment is a portion of an internet datagram. internet fragment is a portion of an internet datagram.
skipping to change at page 82, line 47 skipping to change at page 82, line 46
host host
A computer. In particular a source or destination of A computer. In particular a source or destination of
messages from the point of view of the communication network. messages from the point of view of the communication network.
Identification Identification
An Internet Protocol field. This identifying value assigned An Internet Protocol field. This identifying value assigned
by the sender aids in assembling the fragments of a datagram. by the sender aids in assembling the fragments of a datagram.
internet address internet address
A source or destination address specific to the host level. A network layer address.
internet datagram internet datagram
The unit of data exchanged between an internet module and the The unit of data exchanged between an internet module and the
higher level protocol together with the internet header. higher level protocol together with the internet header.
internet fragment internet fragment
A portion of the data of an internet datagram with an A portion of the data of an internet datagram with an
internet header. internet header.
IP IP
skipping to change at page 83, line 28 skipping to change at page 83, line 26
on a connection, (either ISS or IRS). Selected in a way that on a connection, (either ISS or IRS). Selected in a way that
is unique within a given period of time and is unpredictable is unique within a given period of time and is unpredictable
to attackers. to attackers.
ISS ISS
The Initial Send Sequence number. The first sequence number The Initial Send Sequence number. The first sequence number
used by the sender on a connection. used by the sender on a connection.
left sequence left sequence
This is the next sequence number to be acknowledged by the This is the next sequence number to be acknowledged by the
data receiving TCP (or the lowest currently unacknowledged data receiving TCP endpoint (or the lowest currently
sequence number) and is sometimes referred to as the left unacknowledged sequence number) and is sometimes referred to
edge of the send window. as the left edge of the send window.
module module
An implementation, usually in software, of a protocol or An implementation, usually in software, of a protocol or
other procedure. other procedure.
MSL MSL
Maximum Segment Lifetime, the time a TCP segment can exist in Maximum Segment Lifetime, the time a TCP segment can exist in
the internetwork system. Arbitrarily defined to be 2 the internetwork system. Arbitrarily defined to be 2
minutes. minutes.
octet octet
An eight bit byte. An eight bit byte.
Options Options
An Option field may contain several options, and each option An Option field may contain several options, and each option
may be several octets in length. may be several octets in length.
packet packet
A package of data with a header which may or may not be A package of data with a header that may or may not be
logically complete. More often a physical packaging than a logically complete. More often a physical packaging than a
logical packaging of data. logical packaging of data.
port port
The portion of a socket that specifies which logical input or The portion of a connection identifier used for
output channel of a process is associated with the data. demultiplexing connections at an endpoint.
process process
A program in execution. A source or destination of data from A program in execution. A source or destination of data from
the point of view of the TCP or other host-to-host protocol. the point of view of the TCP endpoint or other host-to-host
protocol.
PUSH PUSH
A control bit occupying no sequence space, indicating that A control bit occupying no sequence space, indicating that
this segment contains data that must be pushed through to the this segment contains data that must be pushed through to the
receiving user. receiving user.
RCV.NXT RCV.NXT
receive next sequence number receive next sequence number
RCV.UP RCV.UP
receive urgent pointer receive urgent pointer
RCV.WND RCV.WND
receive window receive window
receive next sequence number receive next sequence number
This is the next sequence number the local TCP is expecting This is the next sequence number the local TCP endpoint is
to receive. expecting to receive.
receive window receive window
This represents the sequence numbers the local (receiving) This represents the sequence numbers the local (receiving)
TCP is willing to receive. Thus, the local TCP considers TCP endpoint is willing to receive. Thus, the local TCP
that segments overlapping the range RCV.NXT to RCV.NXT + endpoint considers that segments overlapping the range
RCV.WND - 1 carry acceptable data or control. Segments RCV.NXT to RCV.NXT + RCV.WND - 1 carry acceptable data or
containing sequence numbers entirely outside of this range control. Segments containing sequence numbers entirely
are considered duplicates and discarded. outside of this range are considered duplicates and
discarded.
RST RST
A control bit (reset), occupying no sequence space, A control bit (reset), occupying no sequence space,
indicating that the receiver should delete the connection indicating that the receiver should delete the connection
without further interaction. The receiver can determine, without further interaction. The receiver can determine,
based on the sequence number and acknowledgment fields of the based on the sequence number and acknowledgment fields of the
incoming segment, whether it should honor the reset command incoming segment, whether it should honor the reset command
or ignore it. In no case does receipt of a segment or ignore it. In no case does receipt of a segment
containing RST give rise to a RST in response. containing RST give rise to a RST in response.
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segment segment
A logical unit of data, in particular a TCP segment is the A logical unit of data, in particular a TCP segment is the
unit of data transfered between a pair of TCP modules. unit of data transfered between a pair of TCP modules.
segment acknowledgment segment acknowledgment
The sequence number in the acknowledgment field of the The sequence number in the acknowledgment field of the
arriving segment. arriving segment.
segment length segment length
The amount of sequence number space occupied by a segment, The amount of sequence number space occupied by a segment,
including any controls which occupy sequence space. including any controls that occupy sequence space.
segment sequence segment sequence
The number in the sequence field of the arriving segment. The number in the sequence field of the arriving segment.
send sequence send sequence
This is the next sequence number the local (sending) TCP will This is the next sequence number the local (sending) TCP
use on the connection. It is initially selected from an endpoint will use on the connection. It is initially
initial sequence number curve (ISN) and is incremented for selected from an initial sequence number curve (ISN) and is
each octet of data or sequenced control transmitted. incremented for each octet of data or sequenced control
transmitted.
send window send window
This represents the sequence numbers which the remote This represents the sequence numbers that the remote
(receiving) TCP is willing to receive. It is the value of (receiving) TCP endpoint is willing to receive. It is the
the window field specified in segments from the remote (data value of the window field specified in segments from the
receiving) TCP. The range of new sequence numbers which may remote (data receiving) TCP endpoint. The range of new
be emitted by a TCP lies between SND.NXT and SND.UNA + sequence numbers that may be emitted by a TCP implementation
SND.WND - 1. (Retransmissions of sequence numbers between lies between SND.NXT and SND.UNA + SND.WND - 1.
SND.UNA and SND.NXT are expected, of course.) (Retransmissions of sequence numbers between SND.UNA and
SND.NXT are expected, of course.)
SND.NXT SND.NXT
send sequence send sequence
SND.UNA SND.UNA
left sequence left sequence
SND.UP SND.UP
send urgent pointer send urgent pointer
SND.WL1 SND.WL1
segment sequence number at last window update segment sequence number at last window update
SND.WL2 SND.WL2
segment acknowledgment number at last window update segment acknowledgment number at last window update
SND.WND SND.WND
send window send window
socket (or socket number) socket (or socket number, or socket address, or socket identifier)
An address which specifically includes a port identifier, An address that specifically includes a port identifier, that
that is, the concatenation of an Internet Address with a TCP is, the concatenation of an Internet Address with a TCP port.
port.
Source Address Source Address
The source address, usually the network and host identifiers. The network layer address of the sending endpoint.
SYN SYN
A control bit in the incoming segment, occupying one sequence A control bit in the incoming segment, occupying one sequence
number, used at the initiation of a connection, to indicate number, used at the initiation of a connection, to indicate
where the sequence numbering will start. where the sequence numbering will start.
TCB TCB
Transmission control block, the data structure that records Transmission control block, the data structure that records
the state of a connection. the state of a connection.
TCP TCP
Transmission Control Protocol: A host-to-host protocol for Transmission Control Protocol: A host-to-host protocol for
reliable communication in internetwork environments. reliable communication in internetwork environments.
TOS TOS
Type of Service, an obsoleted IPv4 field. The same header Type of Service, an obsoleted IPv4 field. The same header
bits currently are used for the Differentiated Services field bits currently are used for the Differentiated Services field
[5] containing the Differentiated Services Code Point (DSCP) [5] containing the Differentiated Services Code Point (DSCP)
value and two unused bits. value and the 2-bit ECN codepoint [8].
Type of Service Type of Service
An Internet Protocol field which indicates the type of An Internet Protocol field that indicates the type of service
service for this internet fragment. for this internet fragment.
URG URG
A control bit (urgent), occupying no sequence space, used to A control bit (urgent), occupying no sequence space, used to
indicate that the receiving user should be notified to do indicate that the receiving user should be notified to do
urgent processing as long as there is data to be consumed urgent processing as long as there is data to be consumed
with sequence numbers less than the value indicated in the with sequence numbers less than the value indicated in the
urgent pointer. urgent pointer.
urgent pointer urgent pointer
A control field meaningful only when the URG bit is on. This A control field meaningful only when the URG bit is on. This
field communicates the value of the urgent pointer which field communicates the value of the urgent pointer that
indicates the data octet associated with the sending user's indicates the data octet associated with the sending user's
urgent call. urgent call.
4. Changes from RFC 793 4. Changes from RFC 793
This document obsoletes RFC 793 as well as RFC 6093 and 6528, which This document obsoletes RFC 793 as well as RFC 6093 and 6528, which
updated 793. In all cases, only the normative protocol specification updated 793. In all cases, only the normative protocol specification
and requirements have been incorporated into this document, and some and requirements have been incorporated into this document, and some
informational text with background and rationale may not have been informational text with background and rationale may not have been
carried in. The informational content of those documents is still carried in. The informational content of those documents is still
skipping to change at page 91, line 31 skipping to change at page 91, line 33
was missing. was missing.
The -13 revision contains only changes in the inline editor notes. The -13 revision contains only changes in the inline editor notes.
The -14 revision includes updates with regard to several comments The -14 revision includes updates with regard to several comments
from the mailing list, including editorial fixes, adding IANA from the mailing list, including editorial fixes, adding IANA
considerations for the header flags, improving figure title considerations for the header flags, improving figure title
placement, and breaking up the "Terminology" section into more placement, and breaking up the "Terminology" section into more
appropriately titled subsections. appropriately titled subsections.
The -15 revision has many technical and editorial corrections from
Gorry Fairhurst's review, and subsequent discussion on the TCPM list,
as well as some other collected clarifications and improvements from
mailing list discussion.
Some other suggested changes that will not be incorporated in this Some other suggested changes that will not be incorporated in this
793 update unless TCPM consensus changes with regard to scope are: 793 update unless TCPM consensus changes with regard to scope are:
1. Tony Sabatini's suggestion for describing DO field 1. Tony Sabatini's suggestion for describing DO field
2. Per discussion with Joe Touch (TAPS list, 6/20/2015), the 2. Per discussion with Joe Touch (TAPS list, 6/20/2015), the
description of the API could be revisited description of the API could be revisited
Early in the process of updating RFC 793, Scott Brim mentioned that Early in the process of updating RFC 793, Scott Brim mentioned that
this should include a PERPASS/privacy review. This may be something this should include a PERPASS/privacy review. This may be something
for the chairs or AD to request during WGLC or IETF LC. for the chairs or AD to request during WGLC or IETF LC.
skipping to change at page 92, line 16 skipping to change at page 92, line 24
Bit Name Reference Bit Name Reference
--- ---- --------- --- ---- ---------
10 Urgent Pointer field significant (URG) (this document) 10 Urgent Pointer field significant (URG) (this document)
11 Acknowledgment field significant (ACK) (this document) 11 Acknowledgment field significant (ACK) (this document)
12 Push Function (PSH) (this document) 12 Push Function (PSH) (this document)
13 Reset the connection (RST) (this document) 13 Reset the connection (RST) (this document)
14 Synchronize sequence numbers (SYN) (this document) 14 Synchronize sequence numbers (SYN) (this document)
15 No more data from sender (FIN) (this document) 15 No more data from sender (FIN) (this document)
This TCP Header Flags registry should also be moved to a sub-registry
under the global "Transmission Control Protocol (TCP) Parameters
registry (https://www.iana.org/assignments/tcp-parameters/tcp-
parameters.xhtml).
6. Security and Privacy Considerations 6. Security and Privacy Considerations
The TCP design includes only rudimentary security features that The TCP design includes only rudimentary security features that
improve the robustness and reliability of connections and application improve the robustness and reliability of connections and application
data transfer, but there are no built-in cryptographic capabilities data transfer, but there are no built-in cryptographic capabilities
to support any form of privacy, authentication, or other typical to support any form of privacy, authentication, or other typical
security functions. Non-cryptographic enhancements (e.g. [28]) have security functions. Non-cryptographic enhancements (e.g. [30]) have
been developed to improve robustness of TCP connections to particular been developed to improve robustness of TCP connections to particular
types of attacks, but the applicability and protections of non- types of attacks, but the applicability and protections of non-
cryptographic enhancements are limited (e.g. see section 1.1 of cryptographic enhancements are limited (e.g. see section 1.1 of
[28]). Applications typically utilize lower-layer (e.g. IPsec) and [30]). Applications typically utilize lower-layer (e.g. IPsec) and
upper-layer (e.g. TLS) protocols to provide security and privacy for upper-layer (e.g. TLS) protocols to provide security and privacy for
TCP connections and application data carried in TCP. Methods based TCP connections and application data carried in TCP. Methods based
on TCP options have been developed as well, to support some security on TCP options have been developed as well, to support some security
capabilities. capabilities.
In order to fully protect TCP connections (including their control In order to fully protect TCP connections (including their control
flags) IPsec or the TCP Authentication Option (TCP-AO) [27] are the flags) IPsec or the TCP Authentication Option (TCP-AO) [29] are the
only current effective methods. Other methods discussed in this only current effective methods. Other methods discussed in this
section may protect the payload, but either only a subset of the section may protect the payload, but either only a subset of the
fields (e.g. tcpcrypt) or none at all (e.g. TLS). Other security fields (e.g. tcpcrypt) or none at all (e.g. TLS). Other security
features that have been added to TCP (e.g. ISN generation, sequence features that have been added to TCP (e.g. ISN generation, sequence
number checks, etc.) are only capable of partially hindering attacks. number checks, etc.) are only capable of partially hindering attacks.
Applications using long-lived TCP flows have been vulnerable to Applications using long-lived TCP flows have been vulnerable to
attacks that exploit the processing of control flags described in attacks that exploit the processing of control flags described in
earlier TCP specifications [21]. TCP-MD5 was a commonly implemented earlier TCP specifications [23]. TCP-MD5 was a commonly implemented
TCP option to support authentication for some of these connections, TCP option to support authentication for some of these connections,
but had flaws and is now deprecated. TCP-AO provides a capability to but had flaws and is now deprecated. TCP-AO provides a capability to
protect long-lived TCP connections from attacks, and has superior protect long-lived TCP connections from attacks, and has superior
properties to TCP-MD5. It does not provide any privacy for properties to TCP-MD5. It does not provide any privacy for
application data, nor for the TCP headers. application data, nor for the TCP headers.
The "tcpcrypt" [45]Experimental extension to TCP provides the ability The "tcpcrypt" [49] Experimental extension to TCP provides the
to cryptographically protect connection data. Metadata aspects of ability to cryptographically protect connection data. Metadata
the TCP flow are still visible, but the application stream is well- aspects of the TCP flow are still visible, but the application stream
protected. Within the TCP header, only the urgent pointer and FIN is well-protected. Within the TCP header, only the urgent pointer
flag are protected through tcpcrypt. and FIN flag are protected through tcpcrypt.
The TCP Roadmap [37] includes notes about several RFCs related to TCP The TCP Roadmap [39] includes notes about several RFCs related to TCP
security. Many of the enhancements provided by these RFCs have been security. Many of the enhancements provided by these RFCs have been
integrated into the present document, including ISN generation, integrated into the present document, including ISN generation,
mitigating blind in-window attacks, and improving handling of soft mitigating blind in-window attacks, and improving handling of soft
errors and ICMP packets. These are all discussed in greater detail errors and ICMP packets. These are all discussed in greater detail
in the referenced RFCs that originally described the changes needed in the referenced RFCs that originally described the changes needed
to earlier TCP specifications. Additionally, see RFC 6093 [29] for to earlier TCP specifications. Additionally, see RFC 6093 [31] for
discussion of security considerations related to the urgent pointer discussion of security considerations related to the urgent pointer
field, that has been deprecated. field, that has been deprecated.
Since TCP is often used for bulk transfer flows, some attacks are Since TCP is often used for bulk transfer flows, some attacks are
possible that abuse the TCP congestion control logic. An example is possible that abuse the TCP congestion control logic. An example is
"ACK-division" attacks. Updates that have been made to the TCP "ACK-division" attacks. Updates that have been made to the TCP
congestion control specifications include mechanisms like Appropriate congestion control specifications include mechanisms like Appropriate
Byte Counting (ABC) that act as mitigations to these attacks. Byte Counting (ABC) [19] that act as mitigations to these attacks.
Other attacks are focused on exhausting the resources of a TCP Other attacks are focused on exhausting the resources of a TCP
server. Examples include SYN flooding [20] or wasting resources on server. Examples include SYN flooding [22] or wasting resources on
non-progressing connections [31]. Operating systems commonly non-progressing connections [33]. Operating systems commonly
implement mitigations for these attacks. Some common defenses also implement mitigations for these attacks. Some common defenses also
utilize proxies, stateful firewalls, and other technologies outside utilize proxies, stateful firewalls, and other technologies outside
of the end-host TCP implementation. of the end-host TCP implementation.
7. Acknowledgements 7. Acknowledgements
This document is largely a revision of RFC 793, which Jon Postel was This document is largely a revision of RFC 793, which Jon Postel was
the editor of. Due to his excellent work, it was able to last for the editor of. Due to his excellent work, it was able to last for
three decades before we felt the need to revise it. three decades before we felt the need to revise it.
skipping to change at page 93, line 50 skipping to change at page 94, line 16
chairs, over the course of work on this document: chairs, over the course of work on this document:
Michael Scharf Michael Scharf
Yoshifumi Nishida Yoshifumi Nishida
Pasi Sarolahti Pasi Sarolahti
Michael Tuexen Michael Tuexen
During early discussion of this work on the TCPM mailing list, and at During early discussion of this work on the TCPM mailing list, and at
the IETF 88 meeting in Vancouver, and following adoption by the TCPM the IETF 88 meeting in Vancouver, and following adoption by the TCPM
working group, helpful comments, critiques, and reviews were received working group, helpful comments, critiques, and reviews were received
from (listed alphabetically): David Borman, Mohamed Boucadair, from (listed alphabetically): David Borman, Mohamed Boucadair, Bob
Yuchung Cheng, Martin Duke, Ted Faber, Rodney Grimes, Kevin Lahey, Briscoe, Neal Cardwell, Yuchung Cheng, Martin Duke, Ted Faber, Gorry
Kevin Mason, Matt Mathis, Tommy Pauly, Hagen Paul Pfeifer, Anthony Fairhurst, Rodney Grimes, Mike Kosek, Kevin Lahey, Kevin Mason, Matt
Mathis, Jonathan Morton, Tommy Pauly, Hagen Paul Pfeifer, Anthony
Sabatini, Michael Scharf, Greg Skinner, Joe Touch, Reji Varghese, Tim Sabatini, Michael Scharf, Greg Skinner, Joe Touch, Reji Varghese, Tim
Wicinski, Lloyd Wood, and Alex Zimmermann. Joe Touch provided Wicinski, Lloyd Wood, and Alex Zimmermann. Joe Touch provided
additional help in clarifying the description of segment size additional help in clarifying the description of segment size
parameters and PMTUD/PLPMTUD recommendations. parameters and PMTUD/PLPMTUD recommendations.
This document includes content from errata that were reported by This document includes content from errata that were reported by
(listed chronologically): Yin Shuming, Bob Braden, Morris M. Keesan, (listed chronologically): Yin Shuming, Bob Braden, Morris M. Keesan,
Pei-chun Cheng, Constantin Hagemeier, Vishwas Manral, Mykyta Pei-chun Cheng, Constantin Hagemeier, Vishwas Manral, Mykyta
Yevstifeyev, EungJun Yi, Botong Huang. Yevstifeyev, EungJun Yi, Botong Huang.
skipping to change at page 96, line 10 skipping to change at page 96, line 24
J., Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known J., Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known
TCP Implementation Problems", RFC 2525, TCP Implementation Problems", RFC 2525,
DOI 10.17487/RFC2525, March 1999, DOI 10.17487/RFC2525, March 1999,
<https://www.rfc-editor.org/info/rfc2525>. <https://www.rfc-editor.org/info/rfc2525>.
[18] Xiao, X., Hannan, A., Paxson, V., and E. Crabbe, "TCP [18] Xiao, X., Hannan, A., Paxson, V., and E. Crabbe, "TCP
Processing of the IPv4 Precedence Field", RFC 2873, Processing of the IPv4 Precedence Field", RFC 2873,
DOI 10.17487/RFC2873, June 2000, DOI 10.17487/RFC2873, June 2000,
<https://www.rfc-editor.org/info/rfc2873>. <https://www.rfc-editor.org/info/rfc2873>.
[19] Mathis, M. and J. Heffner, "Packetization Layer Path MTU [19] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
2003, <https://www.rfc-editor.org/info/rfc3465>.
[20] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727,
DOI 10.17487/RFC4727, November 2006,
<https://www.rfc-editor.org/info/rfc4727>.
[21] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>. <https://www.rfc-editor.org/info/rfc4821>.
[20] Eddy, W., "TCP SYN Flooding Attacks and Common [22] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007, Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>. <https://www.rfc-editor.org/info/rfc4987>.
[21] Touch, J., "Defending TCP Against Spoofing Attacks", [23] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, DOI 10.17487/RFC4953, July 2007, RFC 4953, DOI 10.17487/RFC4953, July 2007,
<https://www.rfc-editor.org/info/rfc4953>. <https://www.rfc-editor.org/info/rfc4953>.
[22] Culley, P., Elzur, U., Recio, R., Bailey, S., and J. [24] Culley, P., Elzur, U., Recio, R., Bailey, S., and J.
Carrier, "Marker PDU Aligned Framing for TCP Carrier, "Marker PDU Aligned Framing for TCP
Specification", RFC 5044, DOI 10.17487/RFC5044, October Specification", RFC 5044, DOI 10.17487/RFC5044, October
2007, <https://www.rfc-editor.org/info/rfc5044>. 2007, <https://www.rfc-editor.org/info/rfc5044>.
[23] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461, [25] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
DOI 10.17487/RFC5461, February 2009, DOI 10.17487/RFC5461, February 2009,
<https://www.rfc-editor.org/info/rfc5461>. <https://www.rfc-editor.org/info/rfc5461>.
[24] StJohns, M., Atkinson, R., and G. Thomas, "Common [26] StJohns, M., Atkinson, R., and G. Thomas, "Common
Architecture Label IPv6 Security Option (CALIPSO)", Architecture Label IPv6 Security Option (CALIPSO)",
RFC 5570, DOI 10.17487/RFC5570, July 2009, RFC 5570, DOI 10.17487/RFC5570, July 2009,
<https://www.rfc-editor.org/info/rfc5570>. <https://www.rfc-editor.org/info/rfc5570>.
[25] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion [27] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>. <https://www.rfc-editor.org/info/rfc5681>.
[26] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust [28] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795, Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010, DOI 10.17487/RFC5795, March 2010,
<https://www.rfc-editor.org/info/rfc5795>. <https://www.rfc-editor.org/info/rfc5795>.
[27] Touch, J., Mankin, A., and R. Bonica, "The TCP [29] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925, Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>. June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[28] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's [30] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
Robustness to Blind In-Window Attacks", RFC 5961, Robustness to Blind In-Window Attacks", RFC 5961,
DOI 10.17487/RFC5961, August 2010, DOI 10.17487/RFC5961, August 2010,
<https://www.rfc-editor.org/info/rfc5961>. <https://www.rfc-editor.org/info/rfc5961>.
[29] Gont, F. and A. Yourtchenko, "On the Implementation of the [31] Gont, F. and A. Yourtchenko, "On the Implementation of the
TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093, TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093,
January 2011, <https://www.rfc-editor.org/info/rfc6093>. January 2011, <https://www.rfc-editor.org/info/rfc6093>.
[30] Gont, F., "Reducing the TIME-WAIT State Using TCP [32] Gont, F., "Reducing the TIME-WAIT State Using TCP
Timestamps", BCP 159, RFC 6191, DOI 10.17487/RFC6191, Timestamps", BCP 159, RFC 6191, DOI 10.17487/RFC6191,
April 2011, <https://www.rfc-editor.org/info/rfc6191>. April 2011, <https://www.rfc-editor.org/info/rfc6191>.
[31] Bashyam, M., Jethanandani, M., and A. Ramaiah, "TCP Sender [33] Bashyam, M., Jethanandani, M., and A. Ramaiah, "TCP Sender
Clarification for Persist Condition", RFC 6429, Clarification for Persist Condition", RFC 6429,
DOI 10.17487/RFC6429, December 2011, DOI 10.17487/RFC6429, December 2011,
<https://www.rfc-editor.org/info/rfc6429>. <https://www.rfc-editor.org/info/rfc6429>.
[32] Gont, F. and S. Bellovin, "Defending against Sequence [34] Gont, F. and S. Bellovin, "Defending against Sequence
Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
2012, <https://www.rfc-editor.org/info/rfc6528>. 2012, <https://www.rfc-editor.org/info/rfc6528>.
[33] Borman, D., "TCP Options and Maximum Segment Size (MSS)", [35] Borman, D., "TCP Options and Maximum Segment Size (MSS)",
RFC 6691, DOI 10.17487/RFC6691, July 2012, RFC 6691, DOI 10.17487/RFC6691, July 2012,
<https://www.rfc-editor.org/info/rfc6691>. <https://www.rfc-editor.org/info/rfc6691>.
[34] Touch, J., "Shared Use of Experimental TCP Options", [36] Touch, J., "Shared Use of Experimental TCP Options",
RFC 6994, DOI 10.17487/RFC6994, August 2013, RFC 6994, DOI 10.17487/RFC6994, August 2013,
<https://www.rfc-editor.org/info/rfc6994>. <https://www.rfc-editor.org/info/rfc6994>.
[35] Borman, D., Braden, B., Jacobson, V., and R. [37] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance", Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014, RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>. <https://www.rfc-editor.org/info/rfc7323>.
[36] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP [38] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>. <https://www.rfc-editor.org/info/rfc7413>.
[37] Duke, M., Braden, R., Eddy, W., Blanton, E., and A. [39] Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
Zimmermann, "A Roadmap for Transmission Control Protocol Zimmermann, "A Roadmap for Transmission Control Protocol
(TCP) Specification Documents", RFC 7414, (TCP) Specification Documents", RFC 7414,
DOI 10.17487/RFC7414, February 2015, DOI 10.17487/RFC7414, February 2015,
<https://www.rfc-editor.org/info/rfc7414>. <https://www.rfc-editor.org/info/rfc7414>.
[38] Black, D., Ed. and P. Jones, "Differentiated Services [40] Black, D., Ed. and P. Jones, "Differentiated Services
(Diffserv) and Real-Time Communication", RFC 7657, (Diffserv) and Real-Time Communication", RFC 7657,
DOI 10.17487/RFC7657, November 2015, DOI 10.17487/RFC7657, November 2015,
<https://www.rfc-editor.org/info/rfc7657>. <https://www.rfc-editor.org/info/rfc7657>.
[39] Fairhurst, G. and M. Welzl, "The Benefits of Using [41] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087, Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC8087, March 2017, DOI 10.17487/RFC8087, March 2017,
<https://www.rfc-editor.org/info/rfc8087>. <https://www.rfc-editor.org/info/rfc8087>.
[40] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind, [42] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
Ed., "Services Provided by IETF Transport Protocols and Ed., "Services Provided by IETF Transport Protocols and
Congestion Control Mechanisms", RFC 8095, Congestion Control Mechanisms", RFC 8095,
DOI 10.17487/RFC8095, March 2017, DOI 10.17487/RFC8095, March 2017,
<https://www.rfc-editor.org/info/rfc8095>. <https://www.rfc-editor.org/info/rfc8095>.
[41] IANA, "Transmission Control Protocol (TCP) Parameters, [43] Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
https://www.iana.org/assignments/tcp-parameters/ Transport Features Provided by IETF Transport Protocols",
tcp-parameters.xhtml", 2019. RFC 8303, DOI 10.17487/RFC8303, February 2018,
<https://www.rfc-editor.org/info/rfc8303>.
[42] IANA, "Transmission Control Protocol (TCP) Header Flags, [44] Chown, T., Loughney, J., and T. Winters, "IPv6 Node
https://www.iana.org/assignments/tcp-header-flags/ Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
tcp-header-flags.xhtml", 2019. January 2019, <https://www.rfc-editor.org/info/rfc8504>.
[43] Gont, F., "Processing of IP Security/Compartment and [45] IANA, "Transmission Control Protocol (TCP) Parameters,
https://www.iana.org/assignments/tcp-parameters/tcp-
parameters.xhtml", 2019.
[46] IANA, "Transmission Control Protocol (TCP) Header Flags,
https://www.iana.org/assignments/tcp-header-flags/tcp-
header-flags.xhtml", 2019.
[47] Gont, F., "Processing of IP Security/Compartment and
Precedence Information by TCP", draft-gont-tcpm-tcp- Precedence Information by TCP", draft-gont-tcpm-tcp-
seccomp-prec-00 (work in progress), March 2012. seccomp-prec-00 (work in progress), March 2012.
[44] Gont, F. and D. Borman, "On the Validation of TCP Sequence [48] Gont, F. and D. Borman, "On the Validation of TCP Sequence
Numbers", draft-gont-tcpm-tcp-seq-validation-02 (work in Numbers", draft-gont-tcpm-tcp-seq-validation-02 (work in
progress), March 2015. progress), March 2015.
[45] Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack, [49] Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
Q., and E. Smith, "Cryptographic protection of TCP Streams Q., and E. Smith, "Cryptographic protection of TCP Streams
(tcpcrypt)", draft-ietf-tcpinc-tcpcrypt-09 (work in (tcpcrypt)", draft-ietf-tcpinc-tcpcrypt-09 (work in
progress), November 2017. progress), November 2017.
[46] Minshall, G., "A Proposed Modification to Nagle's [50] Touch, J. and W. Eddy, "TCP Extended Data Offset Option",
draft-ietf-tcpm-tcp-edo-10 (work in progress), July 2018.
[51] Minshall, G., "A Proposed Modification to Nagle's
Algorithm", draft-minshall-nagle-01 (work in progress), Algorithm", draft-minshall-nagle-01 (work in progress),
June 1999. June 1999.
[47] Dalal, Y. and C. Sunshine, "Connection Management in [52] Dalal, Y. and C. Sunshine, "Connection Management in
Transport Protocols", Computer Networks Vol. 2, No. 6, pp. Transport Protocols", Computer Networks Vol. 2, No. 6, pp.
454-473, December 1978. 454-473, December 1978.
Appendix A. Other Implementation Notes Appendix A. Other Implementation Notes
This section includes additional notes and references on TCP This section includes additional notes and references on TCP
implementation decisions that are currently not a part of the RFC implementation decisions that are currently not a part of the RFC
series or included within the TCP standard. These items can be series or included within the TCP standard. These items can be
considered by implementers, but there was not yet a consensus to considered by implementers, but there was not yet a consensus to
include them in the standard. include them in the standard.
A.1. IP Security Compartment and Precedence A.1. IP Security Compartment and Precedence
The IPv4 specification [1] includes a precedence value in the (now
obsoleted) Type of Service field (TOS) field. It was modified in
[15], and then obsoleted by the definition of Differentiated Services
(DiffServ) [5]. Setting and conveying TOS between the network layer,
TCP implementation, and applications is obsolete, and replaced by
DiffServ in the current TCP specification.
RFC 793 requires checking the IP security compartment and precedence RFC 793 requires checking the IP security compartment and precedence
on incoming TCP segments for consistency within a connection, and on incoming TCP segments for consistency within a connection, and
with application requests. Each of these aspects of IP have become with application requests. Each of these aspects of IP have become
outdated, without specific updates to RFC 793. The issues with outdated, without specific updates to RFC 793. The issues with
precedence were fixed by [18] which is Standards Track, and so this precedence were fixed by [18], which is Standards Track, and so this
present TCP specification includes those changes. However, the state present TCP specification includes those changes. However, the state
of IP security options that may be used by MLS systems is not as of IP security options that may be used by MLS systems is not as
clean. clean.
Implementers of MLS systems that use IP security options (e.g. IPSO,
CIPSO, or CALIPSO) should implement any additional logic appropriate
for their requirements.
Reseting connections when incoming packets do not meet expected Reseting connections when incoming packets do not meet expected
security compartment or precedence expectations has been recognized security compartment or precedence expectations has been recognized
as a possible attack vector [43], and there has been discussion about as a possible attack vector [47], and there has been discussion about
ammending the TCP specification to prevent connections from being ammending the TCP specification to prevent connections from being
aborted due to non-matching IP security compartment and DiffServ aborted due to non-matching IP security compartment and DiffServ
codepoint values. codepoint values.
A.1.1. Precedence
In DiffServ the former precedence values are treated as Class
Selector codepoints, and methods for compatible treatment are
described in the DiffServ architecture. The RFC 793/1122 TCP
specification includes logic intending to have connections use the
highest precedence requested by either endpoint application, and to
keep the precedence consistent throughout a connection. This logic
from the obsolete TOS is not applicable for DiffServ, and should not
be included in TCP implementations, though changes to DiffServ values
within a connection are discouraged. For discussion of this, see RFC
7657 (sec 5.1, 5.3, and 6) [40].
The obsoleted TOS processing rules in TCP assumed bidirectional (or
symmetric) precedence values used on a connection, but the DiffServ
architecture is asymmetric. Problems with the old TCP logic in this
regard were described in [18] and the solution described is to ignore
IP precedence in TCP. Since RFC 2873 is a Standards Track document
(although not marked as updating RFC 793), current implementations
are expected to be robust to these conditions. Note that the
DiffServ field value used in each direction is a part of the
interface between TCP and the network layer, and values in use can be
indicated both ways between TCP and the application.
A.1.2. MLS Systems
The IP security option (IPSO) and compartment defined in [1] was
refined in RFC 1038 that was later obsoleted by RFC 1108. The
Commercial IP Security Option (CIPSO) is defined in FIPS-188, and is
supported by some vendors and operating systems. RFC 1108 is now
Historic, though RFC 791 itself has not been updated to remove the IP
security option. For IPv6, a similar option (CALIPSO) has been
defined [26]. RFC 793 includes logic that includes the IP security/
compartment information in treatment of TCP segments. References to
the IP "security/compartment" in this document may be relevant for
Multi-Level Secure (MLS) system implementers, but can be ignored for
non-MLS implementations, consistent with running code on the
Internet. See Appendix A.1 for further discussion. Note that RFC
5570 describes some MLS networking scenarios where IPSO, CIPSO, or
CALIPSO may be used. In these special cases, TCP implementers should
see section 7.3.1 of RFC 5570, and follow the guidance in that
document.
A.2. Sequence Number Validation A.2. Sequence Number Validation
There are cases where the TCP sequence number validation rules can There are cases where the TCP sequence number validation rules can
prevent ACK fields from being processed. This can result in prevent ACK fields from being processed. This can result in
connection issues, as described in [44], which includes descriptions connection issues, as described in [48], which includes descriptions
of potential problems in conditions of simultaneous open, self- of potential problems in conditions of simultaneous open, self-
connects, simultaneous close, and simultaneous window probes. The connects, simultaneous close, and simultaneous window probes. The
document also describes potential changes to the TCP specification to document also describes potential changes to the TCP specification to
mitigate the issue by expanding the acceptable sequence numbers. mitigate the issue by expanding the acceptable sequence numbers.
In Internet usage of TCP, these conditions are rarely occuring. In Internet usage of TCP, these conditions are rarely occuring.
Common operating systems include different alternative mitigations, Common operating systems include different alternative mitigations,
and the standard has not been updated yet to codify one of them, but and the standard has not been updated yet to codify one of them, but
implementers should consider the problems described in [44]. implementers should consider the problems described in [48].
A.3. Nagle Modification A.3. Nagle Modification
In common operating systems, both the Nagle algorithm and delayed In common operating systems, both the Nagle algorithm and delayed
acknowledgements are implemented and enabled by default. TCP is used acknowledgements are implemented and enabled by default. TCP is used
by many applications that have a request-response style of by many applications that have a request-response style of
communication, where the combination of the Nagle algorithm and communication, where the combination of the Nagle algorithm and
delayed acknowledgements can result in poor application performance. delayed acknowledgements can result in poor application performance.
A modification to the Nagle algorithm is described in [46] that A modification to the Nagle algorithm is described in [51] that
improves the situation for these applications. improves the situation for these applications.
This modification is implemented in some common operating systems, This modification is implemented in some common operating systems,
and does not impact TCP interoperability. Additionally, many and does not impact TCP interoperability. Additionally, many
applications simply disable Nagle, since this is generally supported applications simply disable Nagle, since this is generally supported
by a socket option. The TCP standard has not been updated to include by a socket option. The TCP standard has not been updated to include
this Nagle modification, but implementers may find it beneficial to this Nagle modification, but implementers may find it beneficial to
consider. consider.
A.4. Low Water Mark A.4. Low Water Mark Settings
TODO - mention the low watermark function that is in Linux -
suggested by Michael Welzl
SO_SNDLOWAT and SO_RCVLOWAT would be potential enhancements to the Some operating system kernel TCP implementations include socket
abstract TCP API options that allow specifying the number of bytes in the buffer until
the socket layer will pass sent data to TCP (SO_SNDLOWAT) or to the
application on receiving (SO_RCVLOWAT).
TCP_NOTSENT_LOWAT is what Michael is talking about, that helps a In addition, another socket option (TCP_NOTSENT_LOWAT) can be used to
sending TCP application to help avoid creating large amounts of control the amount of unsent bytes in the write queue. This can help
buffered data (and corresponding latency). This is useful for a sending TCP application to avoid creating large amounts of buffered
applications that are multiplexing data from multiple upper level data (and corresponding latency). As an example, this may be useful
for applications that are multiplexing data from multiple upper level
streams onto a connection, especially when streams may be a mix of streams onto a connection, especially when streams may be a mix of
interactive/realtime and bulk data transfer. interactive/realtime and bulk data transfer.
Appendix B. TCP Requirement Summary Appendix B. TCP Requirement Summary
This section is adapted from RFC 1122. This section is adapted from RFC 1122.
Note that there is no requirement related to PLPMTUD in this list, Note that there is no requirement related to PLPMTUD in this list,
but that PLPMTUD is recommended. but that PLPMTUD is recommended.
skipping to change at page 101, line 23 skipping to change at page 103, line 4
- Retransmit old unacked data within window | SHLD-16| |x| | | | - Retransmit old unacked data within window | SHLD-16| |x| | | |
- Time out conn for data past right edge | SHLD-17| | | |x| | - Time out conn for data past right edge | SHLD-17| | | |x| |
Robust against shrinking window | MUST-34|x| | | | | Robust against shrinking window | MUST-34|x| | | | |
Receiver's window closed indefinitely | MAY-8 | | |x| | | Receiver's window closed indefinitely | MAY-8 | | |x| | |
Use standard probing logic | MUST-35|x| | | | | Use standard probing logic | MUST-35|x| | | | |
Sender probe zero window | MUST-36|x| | | | | Sender probe zero window | MUST-36|x| | | | |
First probe after RTO | SHLD-29| |x| | | | First probe after RTO | SHLD-29| |x| | | |
Exponential backoff | SHLD-30| |x| | | | Exponential backoff | SHLD-30| |x| | | |
Allow window stay zero indefinitely | MUST-37|x| | | | | Allow window stay zero indefinitely | MUST-37|x| | | | |
Retransmit old data beyond SND.UNA+SND.WND | MAY-7 | | |x| | | Retransmit old data beyond SND.UNA+SND.WND | MAY-7 | | |x| | |
Process RST and URG even with zero window | MUST-66|x| | | | |
| | | | | | | | | | | | | |
Urgent Data | | | | | | | Urgent Data | | | | | | |
Include support for urgent pointer | MUST-30|x| | | | | Include support for urgent pointer | MUST-30|x| | | | |
Pointer indicates first non-urgent octet | MUST-62|x| | | | | Pointer indicates first non-urgent octet | MUST-62|x| | | | |
Arbitrary length urgent data sequence | MUST-31|x| | | | | Arbitrary length urgent data sequence | MUST-31|x| | | | |
Inform ALP asynchronously of urgent data | MUST-32|x| | | | |1 Inform ALP asynchronously of urgent data | MUST-32|x| | | | |1
ALP can learn if/how much urgent data Q'd | MUST-33|x| | | | |1 ALP can learn if/how much urgent data Q'd | MUST-33|x| | | | |1
ALP employ the urgent mechanism | SHLD-13| | | |x| | ALP employ the urgent mechanism | SHLD-13| | | |x| |
| | | | | | | | | | | | | |
TCP Options | | | | | | | TCP Options | | | | | | |
Support the mandatory option set | MUST-4 |x| | | | | Support the mandatory option set | MUST-4 |x| | | | |
Receive TCP option in any segment | MUST-5 |x| | | | | Receive TCP option in any segment | MUST-5 |x| | | | |
Ignore unsupported options | MUST-6 |x| | | | | Ignore unsupported options | MUST-6 |x| | | | |
Cope with illegal option length | MUST-7 |x| | | | | Cope with illegal option length | MUST-7 |x| | | | |
Process options regardless of word alignment | MUST-64|x| | | | |
Implement sending & receiving MSS option | MUST-14|x| | | | | Implement sending & receiving MSS option | MUST-14|x| | | | |
IPv4 Send MSS option unless 536 | SHLD-5 | |x| | | | IPv4 Send MSS option unless 536 | SHLD-5 | |x| | | |
IPv6 Send MSS option unless 1220 | SHLD-5 | |x| | | | IPv6 Send MSS option unless 1220 | SHLD-5 | |x| | | |
Send MSS option always | MAY-3 | | |x| | | Send MSS option always | MAY-3 | | |x| | |
IPv4 Send-MSS default is 536 | MUST-15|x| | | | | IPv4 Send-MSS default is 536 | MUST-15|x| | | | |
IPv6 Send-MSS default is 1220 | MUST-15|x| | | | | IPv6 Send-MSS default is 1220 | MUST-15|x| | | | |
Calculate effective send seg size | MUST-16|x| | | | | Calculate effective send seg size | MUST-16|x| | | | |
MSS accounts for varying MTU | SHLD-6 | |x| | | | MSS accounts for varying MTU | SHLD-6 | |x| | | |
MSS not sent on non-SYN segments | MUST-65| | | | |x|
MSS value based on MMS_R | MUST-67|x| | | | |
| | | | | | | | | | | | | |
TCP Checksums | | | | | | | TCP Checksums | | | | | | |
Sender compute checksum | MUST-2 |x| | | | | Sender compute checksum | MUST-2 |x| | | | |
Receiver check checksum | MUST-3 |x| | | | | Receiver check checksum | MUST-3 |x| | | | |
| | | | | | | | | | | | | |
ISN Selection | | | | | | | ISN Selection | | | | | | |
Include a clock-driven ISN generator component | MUST-8 |x| | | | | Include a clock-driven ISN generator component | MUST-8 |x| | | | |
Secure ISN generator with a PRF component | SHLD-1 | |x| | | | Secure ISN generator with a PRF component | SHLD-1 | |x| | | |
PRF computable from outside the host | MUST-9 | | | | |x| PRF computable from outside the host | MUST-9 | | | | |x|
| | | | | | | | | | | | | |
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Error Report mechanism | MUST-47|x| | | | | Error Report mechanism | MUST-47|x| | | | |
ALP can disable Error Report Routine | SHLD-20| |x| | | | ALP can disable Error Report Routine | SHLD-20| |x| | | |
ALP can specify DiffServ field for sending | MUST-48|x| | | | | ALP can specify DiffServ field for sending | MUST-48|x| | | | |
Passed unchanged to IP | SHLD-22| |x| | | | Passed unchanged to IP | SHLD-22| |x| | | |
ALP can change DiffServ field during connection| SHLD-21| |x| | | | ALP can change DiffServ field during connection| SHLD-21| |x| | | |
ALP generally changing DiffServ during conn. | SHLD-23| | | |x| | ALP generally changing DiffServ during conn. | SHLD-23| | | |x| |
Pass received DiffServ field up to ALP | MAY-9 | | |x| | | Pass received DiffServ field up to ALP | MAY-9 | | |x| | |
FLUSH call | MAY-14 | | |x| | | FLUSH call | MAY-14 | | |x| | |
Optional local IP addr parm. in OPEN | MUST-43|x| | | | | Optional local IP addr parm. in OPEN | MUST-43|x| | | | |
| | | | | | | | | | | | | |
RFC 5961 Support: | | | | | | | RFC 5961 Support: | | | | | | |
Implement data injection protection | MAY-12 | | |x| | | Implement data injection protection | MAY-12 | | |x| | |
| | | | | | | | | | | | | |
Explicit Congestion Notification: | | | | | | | Explicit Congestion Notification: | | | | | | |
Support ECN | SHLD-8 | |x| | | | Support ECN | SHLD-8 | |x| | | |
-------------------------------------------------|--------|-|-|-|-|-|-- -------------------------------------------------|--------|-|-|-|-|-|-
FOOTNOTES: (1) "ALP" means Application-Layer program. FOOTNOTES: (1) "ALP" means Application-Layer program.
Author's Address Author's Address
Wesley M. Eddy (editor) Wesley M. Eddy (editor)
MTI Systems MTI Systems
US US
Email: wes@mti-systems.com Email: wes@mti-systems.com
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