draft-ietf-tcpm-experimental-options-03.txt   draft-ietf-tcpm-experimental-options-04.txt 
TCPM Working Group J. Touch TCPM Working Group J. Touch
Internet Draft USC/ISI Internet Draft USC/ISI
Intended status: Proposed Standard November 28, 2012 Intended status: Proposed Standard February 25, 2013
Expires: May 2013 Expires: August 2013
Shared Use of Experimental TCP Options Shared Use of Experimental TCP Options
draft-ietf-tcpm-experimental-options-03.txt draft-ietf-tcpm-experimental-options-04.txt
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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warranty as described in the Simplified BSD License. warranty as described in the Simplified BSD License.
Abstract Abstract
This document describes how the experimental TCP option codepoints This document describes how the experimental TCP option codepoints
can support concurrent use through the use of a magic number. This can concurrently support multiple TCP extensions, even within the
mechanism avoids the need for a coordinated registry and is same connection. It uses a new IANA TCP experiment identifier, and
backward-compatible with currently known uses. It is recommended for is also robust to experiments that are not registered and those that
all new TCP options that use these codepoints. do not use this sharing mechanism. It is recommended for all new TCP
options that use these codepoints.
Table of Contents Table of Contents
1. Introduction...................................................2 1. Introduction...................................................2
2. Conventions used in this document..............................4 2. Conventions used in this document..............................4
3. TCP Experimental Option Structure..............................4 3. TCP Experimental Option Structure..............................4
3.1. Selecting a Magic Number..................................5 3.1. Selecting an ExID.........................................5
3.2. Impact on TCP Option Processing...........................5 3.2. Impact on TCP Option Processing...........................6
4. Reducing the Impact of False Positives.........................6 4. Reducing the Impact of False Positives.........................6
5. Migration to Assigned Options..................................7 5. Migration to Assigned Options..................................7
6. Security Considerations........................................7 6. Security Considerations........................................7
7. IANA Considerations............................................7 7. IANA Considerations............................................7
8. References.....................................................8 8. References.....................................................8
8.1. Normative References......................................8 8.1. Normative References......................................8
8.2. Informative References....................................8 8.2. Informative References....................................8
9. Acknowledgments................................................9 9. Acknowledgments................................................9
1. Introduction 1. Introduction
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activated only where needed and supported [RFC793]. The space for activated only where needed and supported [RFC793]. The space for
identifying such options is small - 256 values, of which 30 are identifying such options is small - 256 values, of which 30 are
assigned at the time this document was published [IANA]. Two of assigned at the time this document was published [IANA]. Two of
these codepoints are allocated to support experiments (253, 254) these codepoints are allocated to support experiments (253, 254)
[RFC4727]. These values are intended for testing purposes or anytime [RFC4727]. These values are intended for testing purposes or anytime
an assigned codepoint is either not warranted or available, e.g., an assigned codepoint is either not warranted or available, e.g.,
based on the maturity status of the defined capability (i.e., based on the maturity status of the defined capability (i.e.,
Experimental or Informational, rather than Standards Track). Experimental or Informational, rather than Standards Track).
The term "experimental TCP options" refers here to options that use The term "experimental TCP options" refers here to options that use
the experimental TCP option codepoints [RFC4727]. Such experiments the TCP experimental option codepoints [RFC4727]. Such experiments
can be described in any type of RFC - Experimental, Informational, can be described in any type of RFC - Experimental, Informational,
etc., and are intended to be used both in controlled environments etc., and are intended to be used both in controlled environments
and in are allowed in public deployments (when not enabled as and in are allowed in public deployments (when not enabled as
default) [RFC3962]. Nothing prohibits deploying multiple experiments default) [RFC3692]. Nothing prohibits deploying multiple experiments
in the same environment - controlled or public. Further, some in the same environment - controlled or public. Further, some
protocols are specified in Experimental or Informational RFCs, which protocols are specified in Experimental or Informational RFCs, which
either include parameters or design choices not yet understood or either include parameters or design choices not yet understood or
which might not be widely deployed [RFC2026]. TCP options in such which might not be widely deployed [RFC2026]. TCP options in such
RFCs are typically not eligible for assigned TCP option codepoints RFCs are typically not eligible for assigned TCP option codepoints
[RFC2780], and so there is a need to share use of the experimental [RFC2780], and so there is a need to share use of the experimental
option codepoints. option codepoints.
There is currently no mechanism to support shared use of the There is currently no mechanism to support shared use of the TCP
experimental TCP option codepoints. Experimental options 253 and 254 experimental option codepoints, either by different experiments on
are already deployed in operational code to support an early version different connections, or for more than two experimental options in
of TCP authentication. Option 253 is also documented for the the same connection. Experimental options 253 and 254 are already
experimental TCP Cookie Transaction option [RFC6013]. This shared deployed in operational code to support an early version of TCP
use results in collisions in which a single codepoint can appear authentication. Option 253 is also documented for the experimental
multiple times in a single TCP segment and for which each use is TCP Cookie Transaction option [RFC6013]. This shared use results in
ambiguous. collisions in which a single codepoint can appear multiple times in
a single TCP segment and for which each use is ambiguous.
Other codepoints have been used without assignment (known as Other codepoints have been used without assignment (known as
"squatting"), notably 31-32 (TCP cookie transactions, as originally "squatting"), notably 31-32 (TCP cookie transactions, as originally
distributed and in its API doc) and 76-78 (tcpcrypt) [Bi11][Si11]. distributed and in its API doc) and 76-78 (tcpcrypt) [Bi11][Si11].
Commercial products reportedly also use unassigned options 33, 69- Commercial products reportedly also use unassigned options 33, 69-
70, and 76-78 as well. Even though these uses are unauthorized, they 70, and 76-78 as well. Even though these uses are unauthorized, they
currently impact legitimate assignees. currently impact legitimate assignees.
Both such misuses (squatting on both experimental and assigned Both such misuses (squatting on both experimental and assigned
codepoints) are expected to continue, but there are several codepoints) are expected to continue, but there are several
approaches which can alleviate the impact on cooperating protocol approaches which can alleviate the impact on cooperating protocol
designers. One proposal relaxes the requirements for assignment of designers. One proposal relaxes the requirements for assignment of
TCP options, allowing them to be assigned more readily for protocols TCP options, allowing them to be assigned more readily for protocols
that have not been standardized through the IETF process [RFC5226]. that have not been standardized through the IETF process [RFC5226].
Another proposal assigns a larger pool to options and manages their Another proposal assigns a larger pool to the TCP experiment option
sharing through IANA coordination [Ed11]. codepoints and manages their sharing through IANA coordination
[Ed11].
The approach proposed in this document does not require additional The approach proposed in this document does not require additional
codepoints and also avoids IANA involvement. The solution adds a TCP option codepoints, and is robust to those who choose either not
field to the structure of the experimental TCP option. This field is to support it or not to register their experiments. The solution
populated with a fixed "magic number" defined as part of a specific adds a field to the structure of the experimental TCP option. This
option experiment. The magic number helps reduce the probability of field is populated with an "experiment identifier" (ExID) defined as
a collision of independent experimental uses of the same option part of a specific option experiment. The ExID helps reduce the
codepoint, both for those who follow this document (using other probability of a collision of independent experimental uses of the
magic numbers) and those who do not (squatters). same option codepoint, both for those who follow this document
(using registered ExIDs) and those who do not (squatters who either
ignore this extension or do not register their ExIDs).
The solution proposed in this document is recommended for all new The solution proposed in this document is recommended for all new
protocols that use experimental TCP option codepoints. The protocols that use TCP experimental option codepoints. The
techniques used here may also help share other experimental techniques used here may also help share other experimental
codepoints, but that issue is out of scope for this document. codepoints, but that issue is out of scope for this document.
2. Conventions used in this document 2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119]. document are to be interpreted as described in RFC-2119 [RFC2119].
In this document, these words will appear with that interpretation In this document, these words will appear with that interpretation
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01234567 89012345 67890123 45678901 01234567 89012345 67890123 45678901
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Kind | Length | ... | | Kind | Length | ... |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| ... | ...
+-------- +--------
Figure 1 TCP Option Structure [RFC793] Figure 1 TCP Option Structure [RFC793]
This document extends the option structure for experimental This document extends the option structure for experimental
codepoints (253, 254) with a magic number, which is typically 4 codepoints (253, 254) with an experiment identifier (ExID), which is
bytes in length. The magic number is used to differentiate different either 2 or 4 bytes in length. The ExID is used to differentiate
experiments, and is the first field after the Kind and Length, as different experiments, and is the first field after the Kind and
follows: Length, as follows:
0 1 2 3 0 1 2 3
01234567 89012345 67890123 45678901 01234567 89012345 67890123 45678901
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Kind | Length | Magic Number | | Kind | Length | ExID |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Magic Number | ... | option contents...
+--------+--------+--------+--- +--------+--------+--------+---
Figure 2 TCP Experimental Option with a Magic Number Figure 2 TCP Experimental Option with a 16-bit ExID
0 1 2 3
01234567 89012345 67890123 45678901
+--------+--------+--------+--------+
| Kind | Length | ExID |
+--------+--------+--------+--------+
| ExID (con't) | option contents...
+--------+--------+--------+---
Figure 3 TCP Experimental Option with a 32-bit ExID
>> Protocols requiring new TCP option codepoints that are not >> Protocols requiring new TCP option codepoints that are not
eligible for assigned values SHOULD use the existing TCP eligible for assigned values SHOULD use the existing TCP
experimental option codepoints (253, 254) with magic numbers as experimental option codepoints (253, 254) with ExIDs as described in
described in this document. this document.
>> All protocols using the TCP experimental option codepoints (253, >> All protocols using the TCP experimental option codepoints (253,
254) SHOULD use magic numbers as described in this document. 254) SHOULD use ExIDs as described in this document.
Magic numbers are used in other protocols, e.g., BOOTP [RFC951] and
DHCP [RFC2131]. In the use proposed in this document they help
ensure that concurrent experiments that share the same TCP option
codepoint do not interfere.
3.1. Selecting a Magic Number
The magic number is selected by the protocol designer when an 3.1. Selecting an ExID
experimental option is defined, i.e., when the specification for
that option is authored. The magic number is selected any of a
variety of ways, e.g., using the Unix time() command or bits
selected by an arbitrary function (such as a hash) of an arbitrary
string (e.g., the document title). This operation occurs only when
the option is specified, and is not implemented as part of the
design of an option.
This document does not proscribe a minimum magic number size. Larger ExIDs are selected at design time, when the protocol designer first
magic numbers reduce the probability of a collision with other implements or specifies the experimental option. ExIDs can be either
options sharing the Kind codepoint, but also increase the option 16-bits or 32-bits. In both cases, the value is stored in the header
size. A suggested size is 32 bits, in network standard byte order: in network-standard (big-endian) byte order. ExIDs combine
properties of IANA registered codepoints with "magic numbers".
>> The magic number size and value SHOULD be selected to reduce the ExIDs are registered with IANA using "first-come, first-served"
probability of collision. priority based on the first two bytes. Those two bytes are thus
sufficient to interpret which experimental option is contained in
the option field.
>> The magic number SHOULD be 32 bits, but MAY be either longer or The second two bytes serve as a "magic number". A magic number is a
shorter. self-selected codepoint whose primary value is its unlikely
collision with values selected by others. Magic numbers are used in
other protocols, e.g., BOOTP [RFC951] and DHCP [RFC2131]. The magic
number helps reduce the probability of a false positive collision
with those who either do not register their experiment or who do not
implement this mechanism. Using the additional magic number bytes
also helps the option contents have the same byte alignment in the
TCP header as they would have if (or when) a conventional (non-
experiment) TCP option codepoint is assigned.
3.2. Impact on TCP Option Processing 3.2. Impact on TCP Option Processing
The magic number is considered part of the TCP option, not the TCP The ExID number is considered part of the TCP option, not the TCP
option header. The presence of the magic number increases the option header. The presence of the ExID increases the effective
effective option Length field by the size of the magic number. The option Length field by the size of the ExID. The presence of this
presence of this magic number is thus transparent to implementations ExID is thus transparent to implementations that do not support TCP
that do not support TCP options where it is used. options where it is used.
During TCP processing, experimental options are matched against both During TCP processing, ExIDs in experimental options are matched
the experimental codepoints and the magic number value for each against the ExIDs for each implemented protocol. The remainder of
implemented protocol. the option is specified by the particular experimental protocol.
>> Experimental options that have magic numbers that do not match >> Experimental options that have ExIDs that do not match
implemented protocols MUST be ignored. implemented protocols MUST be ignored.
The remainder of the option is specified by the particular The ExID mechanism must be coordinated during connection
experimental protocol. This includes the possibility that the magic establishment, just as with any TCP option.
number could appear in only a subset of instances of the option.
Because TCP option capabilities are negotiated during connection
establishment, the magic number might be omitted afterwards (e.g.,
in non-SYN segments).
>> TCP experimental option magic numbers, if used in any TCP segment
of a connection, MUST be present in TCP SYN segments of that
connection.
The specification of an experimental option needs to describe >> TCP ExID, if used in any TCP segment of a connection, MUST be
whether the magic number appears in non-SYN segments. If the magic present in TCP SYN segments of that connection.
number does not appear in all segments, the experimental option may
need to be rejected during connection negotiation because options
for different experiments in non-SYN segments may not be
distinguishable. As a result, this document recommends that:
>> TCP experimental option magic numbers, if used in any TCP segment >> TCP experimental option ExIDS, if used in any TCP segment of a
of a connection, SHOULD be used in all TCP segments of that connection, SHOULD be used in all TCP segments of that connection in
connection in which any experimental option is present. which any experimental option is present.
Use of a magic number uses additional space in the TCP header and Use of an ExID uses additional space in the TCP header and requires
requires additional protocol processing by experimental protocols. additional protocol processing by experimental protocols. Because
Because these are experiments, neither consideration is a these are experiments, neither consideration is a substantial
substantial impediment; a finalized protocol can avoid both issues impediment; a finalized protocol can avoid both issues with the
with the assignment of a dedicated option codepoint later. assignment of a dedicated option codepoint later.
4. Reducing the Impact of False Positives 4. Reducing the Impact of False Positives
False positives occur where the magic number of one experiment False positives occur where the ExID of one experiment matches the
matches the value of an option that does not use magic numbers or if value of an option that does not use ExIDs or if two experiments
two experiments select the same magic number. Such collisions can select the same ExID. Such collisions can cause an option to be
cause an option to be interpreted by the incorrect processing interpreted by the incorrect processing routine. Use of checksums or
routine. signatures may help an experiment use a shorter ExID while reducing
the corresponding increased potential for false positives.
>> Experiments that are not robust to magic number false positives
SHOULD implement other detection measures, such as checksums or
digital signatures.
Use of checksums or signatures may help an experiment use a shorter >> Experiments that are not robust to ExID false positives SHOULD
magic number while reducing the corresponding increased potential implement other detection measures, such as checksums or minimal
for false positives. However this document recommends magic numbers digital signatures over the experimental options they support.
are used together with such checksums/signatures, not as a
substitute thereof. Magic numbers are static and thus more easily
identify the experiment using the experimental option; they can also
be more efficiently interpreted at the TCP receiver.
5. Migration to Assigned Options 5. Migration to Assigned Options
Some experiments may transition from experiment, and become eligible Some experiments may transition from experiment, and become eligible
for an assigned TCP option codepoint. This document does not for an assigned TCP option codepoint. This document does not
recommend a specific migration plan to transition from use of the recommend a specific migration plan to transition from use of the
experimental TCP options/magic numbers to use of an assigned experimental TCP options/ExIDs to use of an assigned codepoint.
codepoint.
However, once an assigned codepoint is allocated, use of a magic However, once an assigned codepoint is allocated, use of an ExID
number represents unnecessary overhead. As a result: represents unnecessary overhead. As a result:
>> Once a TCP option codepoint is assigned to a protocol, that >> Once a TCP option codepoint is assigned to a protocol, that
protocol SHOULD NOT continue to use a magic number as part of that protocol SHOULD NOT continue to use an ExID as part of that assigned
assigned codepoint. codepoint.
This document does not recommend whether or how an implementation of This document does not recommend whether or how an implementation of
an assigned codepoint should be backward-compatible with use of the an assigned codepoint can be backward-compatible with use of the
experimental codepoint/magic number. experimental codepoint/ ExID.
However, some implementers may be tempted to include both the However, some implementers may be tempted to include both the
experimental and assigned codepoint in the same segment, e.g., in a experimental and assigned codepoint in the same segment, e.g., in a
SYN to support backward-compatibility during connection SYN to support backward-compatibility during connection
establishment. This is a poor use limited resources and so to ensure establishment. This is a poor use limited resources and so to ensure
conservation of the TCP option space: conservation of the TCP option space:
>> A TCP segment MUST NOT contain both an assigned TCP option >> A TCP segment MUST NOT contain both an assigned TCP option
codepoint and an experimental TCP option codepoint/magic number for codepoint and a TCP experimental option codepoint for the same
the same protocol. protocol.
Instead, a TCP that intends backward compatibility might send Instead, a TCP that intends backward compatibility might send
multiple SYNs with alternates of the same option and discard all but multiple SYNs with alternates of the same option and discard all but
the most desired successful connection. the most desired successful connection. Although this approach may
resolve more slowly or require additional effort at the endpoints,
it is preferable to excessively consuming TCP option space.
6. Security Considerations 6. Security Considerations
The mechanism described in this document is not intended to provide The mechanism described in this document is not intended to provide
(nor does it weaken existing) security for TCP option processing. (nor does it weaken existing) security for TCP option processing.
7. IANA Considerations 7. IANA Considerations
This document has no IANA considerations. This section should be This document calls for IANA to create a new TCP experimental option
removed prior to publication. Experiment Identifier (ExID) registry.
That registry should allow 16-bit and 32-bit entries, where entries
are "first-come, first-served" on the first two bytes of the value
in network-standard byte order (big endian), in which the entry
should indicate the entire ExID value. Known overlapping uses -
whether of the first-come portion or the entire value - should also
be listed and highlighted as collisions.
IANA should impose no requirements on making a registration other
than indicating the desired codepoint and providing a point of
contact. A short description or acronym for the use is desired, but
should not be required.
8. References 8. References
8.1. Normative References 8.1. Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC [RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, Sep. 1981. 793, Sep. 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
skipping to change at page 8, line 44 skipping to change at page 9, line 9
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, Oct. 1996. 3", BCP 9, RFC 2026, Oct. 1996.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, Mar. 1997. 2131, Mar. 1997.
[RFC2780] Bradner, S., V. Paxson, "IANA Allocation Guidelines For [RFC2780] Bradner, S., V. Paxson, "IANA Allocation Guidelines For
Values In the Internet Protocol and Related Headers", BCP Values In the Internet Protocol and Related Headers", BCP
37, RFC 2780, Mar. 2000. 37, RFC 2780, Mar. 2000.
[RFC3962] Narten, T., "Assigning Experimental and Testing Numbers [RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3962, Jan. 2004. Considered Useful", BCP 82, RFC 3692, Jan. 2004.
[RFC5226] Narten, T., H. Alvestrand, "Guidelines for Writing an IANA [RFC5226] Narten, T., H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 5226, May Considerations Section in RFCs", BCP 26, RFC 5226, May
2008. 2008.
[RFC6013] Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC 6013, [RFC6013] Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC 6013,
Jan. 2011. Jan. 2011.
[Si11] Simpson, W., "TCP Cookie Transactions (TCPCT) Sockets [Si11] Simpson, W., "TCP Cookie Transactions (TCPCT) Sockets
Application Program Interface (API)", work in progress, Application Program Interface (API)", work in progress,
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