Network Working Group Network Working Group Internet-Draft D. Borman
Intended Status: InformationalUpdates: 879, 2385 Wind River Systems Intended Status: Informational March 25, 2010 File: draft-ietf-tcpm-tcpmss-02.txt July 13, 2009draft-ietf-tcpm-tcpmss-03.txt TCP Options and MSS Abstract This memo discusses what value to use with the TCP MSS option. Status of This Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts.Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.http://www.ietf.org/shadow.html This Internet-Draft will expire on January 13,September 25, 2010. Copyright Copyright (c) 20092010 IETF Trust and the persons identified as the document authors. All rights reserved. Abstract This memo discusses what value to use with the TCP MSS option.1. INTRODUCTIONIntroduction There has been some confusion as to what value should be filled in the TCP MSS option when using IP and TCP options. RFC-879 [Postel83][RFC879] stated: The MSS counts only data octets in the segment, it does not count the TCP header or the IP header. which is unclear about what to do about IP and TCP options.options, since they are part of the headers. RFC-1122 [Braden89] attempted to clarify this in section 188.8.131.52,[RFC1122] clarified the MSS option (see Appendix A), but there still seems to be some confusion. Clarification was first sent1.1 Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to the TCP Large Windows mailing list [Borman93]be interpreted as described in 1993.RFC 2119 [RFC2119]. 2. TCP Options and MSSThe MSSShort Statement When calculating the value to be sentput in anthe TCP MSS option should be equal tooption, the effectiveMTU minusvalue SHOULD be decreased by only the size of the fixed IP and TCP headers. By ignoring both IPheaders, and SHOULD NOT be decreased to account for any possible IP or TCP options when calculatingoptions; conversely, the value forsender MUST reduce the MSS option, if there areTCP data length to account for any IP or TCP options to be sentthat it is including in a packet, thenthe sender must decreasepackets that it sends. The rest of this document just expounds on that statement, and the goal is to avoid IP level fragmentation of TCP packets. The size of the fixed TCP data accordingly.header is 20 bytes, the size of the fixed IPv4 header is 20 bytes, and the size of the fixed IPv6 header is 40 bytes. The reason for this candetermination of what MTU value should be seenused, especially in the following table: +--------------------+--------------------+ | MSScase of multi-homed hosts, is adjusted |beyond the scope of this document. 3. MSS isn't adjusted | | to include options |in to include options | +----------------+--------------------+--------------------+ | Sender adjusts | Packets are too | Packets areother RFCs 3.1 RFC-879 RFC-897 [RFC879] discusses the | | length for | short | correct length | | options | | | +----------------+--------------------+--------------------+ | Sender doesn't | Packets areMSS option and other topics. In the | Packets are too | | adjust length | correct length | long. | | forintroduction, it states: "THE TCP MAXIMUM SEGMENT SIZE IS THE IP MAXIMUM DATAGRAM SIZE MINUS FORTY." and in section 13, it states: "The definition of the MSS option can be stated: The maximum number of data octets that may be received by the sender of this TCP option in TCP segments with no TCP header options | | | +----------------+--------------------+--------------------+ Sincetransmitted in IP datagrams with no IP header options." These are both correct. However, in the goalnext paragraph in section 14, it then confuses this by stating that the consequence is: "When TCP is toused in a situation when either the IP or TCP headers are not sendminimum and yet the maximum IP datagramsdatagram that have tocan be fragmented,received remains 576 octets then the TCP Maximum Segment Size option must be used to reduce the limit on data octets allowed in a TCP segment." For example, if the IP Security option (11 octets) were in use and packets sent withthe constraintsIP maximum datagram size remained at 576 octets, then the TCP should send the MSS with a value of 525 (536-11)." That is incorrect. The simpler and more correct statement would be: When TCP is used in a situation where either the lower rightIP or TCP headers are not minimum, the sender must reduce the amount of this grid will causeTCP data in any given packet by number of octets used by the IP fragmentation,and TCP options. 3.2 RFC-2385 RFC-2385 [RFC2385] incorrectly states in section 4.3: "As with other options that are added to every segment, the size of the MD5 option must be factored into the MSS offered to the other side during connection negotiation. Specifically, the size of the header to subtract from the MTU (whether it is the MTU of the outgoing interface or IP's minimal MTU of 576 bytes) is now at least 18 bytes larger." This is incorrect, the value for the MSS option is only wayadjusted by the fixed IP and TCP headers. 4. Clarification from the TCP Large Windows mailing list Additional clarification was sent to the TCP Large Windows mailing list in 1993 [Borman93]. 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 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 sender must decrease the size of the TCP data accordingly. The reason for this can be seen in the following table: +--------------------+--------------------+ | MSS is adjusted | MSS isn't adjusted | | to include options | to include options | +--------------------+--------------------+--------------------+ | Sender adjusts | Packets are too | Packets are the | | packet length | short | correct length | | for options | | | +--------------------+--------------------+--------------------+ | Sender doesn't | Packets are the | Packets are too | | adjust packet | correct length | long. | | length for options | | | +--------------------+--------------------+--------------------+ Since the goal is to not send IP datagrams that have to be fragmented, and packets sent with the constraints in the lower right of this grid will cause IP fragmentation, the only way to guarantee that this doesn't happen is for the data sender to decrease the TCP data length by the size of the IP and TCP options. It follows then, that since the sender will be adjusting the TCP data length when sending IP and TCP options, there is no need to include the IP and TCP option lengths in the MSS value. Another argument against including IP or TCP options in the determination of the MSS value is that the MSS is a fixed value, and by their very nature the length of IP and TCP options are variable, so the MSS value can never accurately reflect all possible IP and TCP option combinations, the only one that knows for sure how many IP and TCP options are in any given packet is the sender, hence the sender should be doing the adjustment to the TCP data length to account for any IP and TCP options. 5. Additional considerations 5.1 Path MTU Discovery 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 small can impact the ability for Path MTU discovery to find a larger Path MTU. For more information on Path MTU Discovery, see: RFC-1191 "Path MTU Discovery" [RFC1191] RFC-2923 "TCP Problems with Path MTU Discovery" [RFC2923] RFC-4821 "Packetization Layer Path MTU Discovery" [RFC4821] 5.2 Interfaces with Variable MSS values The effective MTU can sometimes vary, as when used with variable compression, e.g., ROHC [RFC5795]. It is tempting for TCP to want to advertise the largest possible MSS, to support the most efficient use of compressed payloads. Unfortunately, some compression schemes occasionally need to transmit full headers (and thus smaller payloads) to resynchronize state at their endpoint compressor/decompressors. If the largest MTU is used to calculate the value to advertise in the MSS option, TCP retransmission may interfere with compressor resynchronization. As a result, when the effective MTU of an interface varies, TCP SHOULD use the smallest effective MTU of the interface to calculate the value to advertise in the MSS option. 5.3 IPv6 Jumbograms In order to support TCP over IPv6 Jumbograms, implementations need to be able to send TCP segments larger than 64K. RFC-2675 [RFC2675] defines that a value of 65,535 is to be treated as infinity, and Path MTU Discovery [RFC1981] is used to determine the actual MSS. 6. Security Considerations Packets that are too long will either be fragmented or dropped. If packets are fragmented, intermediary firewalls or middle boxes may drop the fragmented packets. In either case, when packets are dropped the connection can fail; hence it is best to avoid generating fragments. 7. IANA Considerations This document has no actions for IANA. 8. References Informative References [Borman93] Borman, D., "TCP MSS & Timestamps", Message to the tcplw mailing list, Jan 7, 1993. [RFC793] Postel, J., "Transmission Control Protocol," RFC-793, September 1981. [RFC879] Postel, J., "The TCP Maximum Segment Size and Related Topics", RFC-879, ISI, November 1983. [RFC1122] Braden, R., editor, "Requirements for Internet Hosts -- Communication Layers", RFC-1122, October, 1989. [RFC1191] Mogul, J.C., Deering, S.E., "Path MTU discovery" RFC-1191, November 1990. [RFC1981] McCann, J., Deering, S. and Mogul, J.,, "Path MTU Discovery for IP Version 6", RFC-1981, August 1986. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC-2119, March 1997. [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC-2385, August 1988. [RFC2675] Borman, D., Deering, S., Hinden, R., "IPv6 Jumbograms", RFC-2675, August, 1999. [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC-2923, September 2000. [RFC4821] Mathis, M., Heffner, J., "Packetization Layer Path MTU Discovery", RFC-4821, March 2007. [RFC5795] Sandlund, K., Gelletier, G. and Jonsson, L-E., "The RObust Header Compression (ROHC) Framework", RFC-5795, March 2010. Appendix A: Details from RFC-793 and RFC-1122 RFC-793 [RFC793] defines the MSS option: Maximum Segment Size Option Data: 16 bits If this option is present, then it communicates the maximum receive segment size at the TCP which sends this segment. This field must only be sent in the initial connection request (i.e., in segments with the SYN control bit set). If this option is not used, any segment size is allowed. RFC-1122 [RFC1122] provides additional clarification in section 184.108.40.206, pages 85-86. First, it changes the default behavior when the MSS option is not present: If an MSS option is not received at connection setup, TCP MUST assume a default send MSS of 536 (576-40) [TCP:4]. Then it clarifies how to determine the value to use in the MSS option: The MSS value to be sent in an MSS option must be less than or equal to: MMS_R - 20 where MMS_R is the maximum size for a transport-layer message that can be received (and reassembled). TCP obtains MMS_R and MMS_S from the IP layer; see the generic call GET_MAXSIZES in Section 3.4. What is implied, but not explicitly stated, is that the 20 is the size of the fixed TCP header. The definition of MMS_R is found in section 3.3.2 on page 57: MMS_R is given by: MMS_R = EMTU_R - 20 since 20 is the minimum size of an IP header. and on page 56, also section 3.3.2: We designate the largest datagram size that can be reassembled by EMTU_R ("Effective MTU to receive"); this is sometimes called the "reassembly buffer size". EMTU_R MUST be greater than or equal to 576, SHOULD be either configurable or indefinite, and SHOULD be greater than or equal to guarantee that this doesn't happen is forthe data sender to decreaseMTU of the TCP data length byconnected network(s). What should be noted here is that EMTU_R is the largest datagram size ofthat can be reassembled, not the IP and TCP options. It follows then,largest datagram size that can be received without fragmentation. Taking this literally, since the sender willmost modern TCP/IP implementations can reassemble a full 64K IP packet, implementations should be adjustingusing 65535 - 20 - 20, or 65495 for the TCP data length when sending IP and TCP options,MSS option. But there is no needmore to include the IP and TCP option lengthsit than that, in RFC-1122 on page 86 it also states: The choice of TCP segment size has a strong effect on performance. Larger segments increase throughput by amortizing header size and per-datagram processing overhead over more data bytes; however, if the MSS value. 3. Security Considerations Packetspacket is so large that it causes IP fragmentation, efficiency drops sharply if any fragments are too longlost [IP:9]. Since it is guaranteed that any IP datagram that is larger than the MTU of the connected network will eitherhave to be fragmented to be received, implementations ignore the "greater than or" part of "SHOULD be greater than or dropped. If packets are fragmented, intermediary firewalls or middle boxes may dropequal to the fragmented packets. In either case, when packets are droppedMTU of the connection can fail; hence it is bestconnected network(s)". Thus, the MSS value to avoid generating fragments. 4. IANA Considerations This document has no actions for IANA. 5. References Informative References [Braden89] Braden, R., editor, "Requirementsbe sent in an MSS option must be less than or equal to: EMTU_R - FixedIPhdrsize - FixedTCPhdrsize where FixedTCPhdrsize is 20, and FixedIPhdrsize is 20 for Internet Hosts -- Communication Layers", RFC-1122, October, 1989 [Postel83] Postel, J., "The TCP Maximum Segment SizeIPv4 and Related Topics", RFC-879, ISI, November 1983. [Borman93] Borman, D., "TCP MSS & Timestamps", Message to tcplw mailing list, Jan 7, 1993.40 for IPv6. Authors' Addresses David Borman Wind River Systems Mendota Heights, MN 55120 Phone: (651) 454-3052 Email: firstname.lastname@example.org Full Copyright Statement Copyright (c) 20092010 IETF Trust and the persons identified as the document authors. All rights reserved. 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