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IETF RFC 6994
Last modified on Thursday, August 8th, 2013
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Internet Engineering Task Force (IETF) J. Touch
Request for Comments: 6994 USC/ISI
Category: Standards Track August 2013
ISSN: 2070-1721
Shared Use of Experimental TCP Options
 Abstract
This document describes how the experimental TCP option codepoints
can concurrently support multiple TCP extensions, even within the
same connection, using a new IANA TCP experiment identifier. This
approach is robust to experiments that are not registered and to
those that do not use this sharing mechanism. It is recommended for
all new TCP options that use these codepoints.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6994.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 6994 Shared Use of Experimental TCP Options August 2013
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................3
3. TCP Experimental Option Structure ...............................4
3.1. Selecting an ExID ..........................................5
3.2. Impact on TCP Option Processing ............................6
4. Reducing the Impact of False Positives ..........................7
5. Migration to Assigned Options ...................................7
6. Rationale .......................................................8
7. Security Considerations .........................................9
8. IANA Considerations .............................................9
9. References .....................................................10
9.1. Normative References ......................................10
9.2. Informative References ....................................10
10. Acknowledgments ...............................................11
1. Introduction
TCP includes options to enable new protocol capabilities that can be
activated only where needed and supported [RFC 793]. The space for
identifying such options is small -- 256 values, of which 30 are
assigned at the time of this document's publication [IANA]. Two of
these codepoints (253, 254) are allocated to support experiments
[RFC 4727]. These values are intended for testing purposes or for use
when an assigned codepoint is either not warranted or available,
e.g., based on the maturity status of the defined capability (i.e.,
Experimental or Informational, rather than Standards Track).
Here, the term "experimental TCP options" refers to options that use
the TCP experimental option codepoints [RFC 4727]. Such experiments
can be described in an RFC of any status (e.g., Experimental,
Informational, etc.) and are intended to be used in controlled
environments and are allowed in public deployments (when not enabled
as default [RFC 3692]). Nothing prohibits the deployment of multiple
experiments in the same environment -- controlled or public.
Further, some protocols are specified in Experimental or
Informational RFCs, which either include parameters or design choices
not yet understood or which might not be widely deployed [RFC 2026].
Typically, these TCP options are not eligible to receive assigned
codepoints [RFC 2780], so they need a way to share their use of the
experimental codepoints.
There is currently no mechanism to support shared use of the TCP
experimental option codepoints, either by different experiments on
different connections or for more than two experimental options in
the same connection. Experimental options 253 and 254 are already
deployed in operational code to support an early version of TCP
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RFC 6994 Shared Use of Experimental TCP Options August 2013
authentication. Option 253 is also documented for the experimental
TCP Cookie Transaction option [RFC 6013]. This shared use results in
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
"squatting"), notably 31-32 (TCP cookie transactions, as originally
distributed and in its API doc) and 76-78 (tcpcrypt) [Bi11] [Si11].
Commercial products reportedly also use unassigned options 33, 69-70,
and 76-78. Even though these uses are unauthorized, they currently
impact legitimate assignees.
Both such misuses (squatting on both experimental and assigned
codepoints) are expected to continue, but there are several
approaches that can alleviate the impact on cooperating protocol
designers. One proposal relaxes the requirements for assignment of
TCP options, allowing them to be assigned more readily for protocols
that have not been standardized through the IETF process [RFC 5226].
Another proposal assigns a larger pool to the TCP experiment option
codepoints and manages their sharing through IANA coordination
[Ed11].
The approach proposed in this document does not require additional
TCP option codepoints and is robust to those who choose either not to
support it or not to register their experiments. The solution adds a
field to the structure of the experimental TCP option. This field is
populated with an "experiment identifier" (ExID) defined as part of a
specific option experiment. The ExID helps reduce the probability of
a collision of independent experimental uses of the 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
protocols that use TCP experimental option codepoints. The
techniques described here may also enable shared use of other
experimental codepoints, but that issue is out of scope for this
document.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
In this document, these words will appear with that interpretation
only when in ALL CAPS. Lowercase uses of these words are not to be
interpreted as carrying RFC 2119 significance.
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RFC 6994 Shared Use of Experimental TCP Options August 2013
In this document, the characters ">>" preceding an indented line(s)
indicates a compliance requirement statement using the key words
listed above. This convention aids reviewers in quickly identifying
or finding the explicit compliance requirements of this RFC.
3. TCP Experimental Option Structure
TCP options have the current common structure [RFC 793], in which the
first byte is the codepoint (Kind) and the second byte is the length
of the option in bytes (Length):
0 1 2 3
01234567 89012345 67890123 45678901
+--------+--------+--------+--------+
| Kind | Length | ... |
+--------+--------+--------+--------+
| ...
+--------
Figure 1. TCP Option Structure [RFC 793]
This document extends the option structure for experimental
codepoints (253, 254) with an experiment identifier (ExID), which is
either 2 or 4 bytes in length. The ExID is used to differentiate
experiments and is the first field after Kind and Length, as follows:
0 1 2 3
01234567 89012345 67890123 45678901
+--------+--------+--------+--------+
| Kind | Length | ExID |
+--------+--------+--------+--------+
| option contents...
+--------+--------+--------+---
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
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RFC 6994 Shared Use of Experimental TCP Options August 2013
This mechanism is encouraged for all TCP options that are not yet
eligible for assigned codepoints:
>> Protocols requiring new TCP option codepoints that are not
eligible for assigned values SHOULD use the existing TCP
experimental option codepoints (253, 254) with ExIDs as described
in this document.
This mechanism is encouraged for all TCP options using the current
experimental codepoints in controlled environments:
>> All protocols using the TCP experimental option codepoints (253,
254), even those deployed in controlled environments, SHOULD use
ExIDs as described in this document.
This mechanism is required for all TCP options using the current
experimental codepoints that are publicly deployed, whether enabled
by default or not:
>> All protocols using the TCP experimental option codepoints (253,
254) that are deployed outside controlled environments, such as in
the public Internet, MUST use ExIDs as described in this document.
Once a TCP option uses the mechanism in this document, registration
of the ExID with IANA is required:
>> All protocols using ExIDs as described in this document MUST
register those ExIDs with IANA.
Applicants register their desired ExID by contacting IANA [IANA].
3.1. Selecting an ExID
ExIDs are selected at design time, when the protocol designer first
implements or specifies the experimental option. ExIDs can be either
16 bits or 32 bits. In both cases, the value is stored in the header
in network-standard (big-endian) byte order. ExIDs combine
properties of IANA registered codepoints with "magic numbers".
>> All ExIDs MUST be either 16 bits or 32 bits long.
Use of the ExID, whether 16 bit or 32 bit, 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.
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RFC 6994 Shared Use of Experimental TCP Options August 2013
In order to conserve TCP option space, either for use within a
specific option or to be available for other options:
>> Options implementing the mechanism of this document SHOULD use
16-bit ExIDs, except where explicitly motivating the need for
32-bit ExIDs, e.g., to avoid false positives or maintain alignment
with an expected future assigned codepoint.
ExIDs are registered with IANA using "first come, first served"
(FCFS) priority based on the first two bytes. Those two bytes are
thus sufficient to interpret which experimental option is contained
in the option field.
>> All ExIDs MUST be unique based on their first 16 bits.
The second two bytes serve as a "magic number". A magic number is a
self-selected codepoint whose primary value is its unlikely collision
with values selected by others. Magic numbers are used in other
protocols, e.g., bootstrap protocol (BOOTP) [RFC 951] and DHCP
[RFC 2131].
Using the additional magic number bytes 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. Use of the same alignment reduces the potential for
implementation errors, especially in using the same word-alignment
padding, if the same software is later modified to use a conventional
codepoint. Use of the longer, 32-bit ExID further decreases the
probability of such a false positive compared to those using shorter,
16-bit ExIDs.
Use of the ExID does consume TCP option space but enables concurrent
use of the experimental codepoints and provides protection against
false positives, leaving less space for other options (including
other experiments). Use of the longer, 32-bit ExID consumes more
space, but provides more protection against false positives.
3.2. Impact on TCP Option Processing
The ExID number is considered part of the TCP option, not the TCP
option header. The presence of the ExID increases the effective
option Length field by the size of the ExID. The presence of this
ExID is thus transparent to implementations that do not support TCP
options.
During TCP processing, ExIDs in experimental options are matched
against the ExIDs for each implemented protocol. The remainder of
the option is specified by the particular experimental protocol.
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RFC 6994 Shared Use of Experimental TCP Options August 2013
>> Experimental options with ExIDs that do not match implemented
protocols MUST be ignored.
The ExID mechanism must be coordinated during connection
establishment, just as with any TCP option.
>> TCP ExID, if used in any TCP segment of a connection, MUST be
present in TCP SYN segments of that connection.
>> TCP experimental option ExIDs, if used in any TCP segment of a
connection, SHOULD be used in all TCP segments of that connection
in which any experimental option is present.
Use of an ExID uses additional space in the TCP header and requires
additional protocol processing by experimental protocols. Because
these are experiments, neither consideration is a substantial
impediment; a finalized protocol can avoid both issues with the
assignment of a dedicated option codepoint later.
4. Reducing the Impact of False Positives
False positives occur where the registered ExID of an experiment
matches the value of an option that does not use ExIDs. Such
collisions can cause an option to be interpreted by the incorrect
processing routine. Use of checksums or signatures may help an
experiment use the shorter ExID while reducing the corresponding
increased potential for false positives.
>> Experiments that are not robust to ExID false positives SHOULD
implement other detection measures, such as checksums or minimal
digital signatures over the experimental options they support.
5. Migration to Assigned Options
Some experiments may transition away from being experimental and
become eligible for an assigned TCP option codepoint. This document
does not recommend a specific migration plan to transition from use
of the experimental TCP options/ExIDs to use of an assigned
codepoint.
However, once an assigned codepoint is allocated, use of an ExID
represents unnecessary overhead. As a result:
>> Once a TCP option codepoint is assigned to a protocol, that
protocol SHOULD NOT continue to use an ExID as part of that
assigned codepoint.
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RFC 6994 Shared Use of Experimental TCP Options August 2013
This document does not recommend whether or how an implementation of
an assigned codepoint can be backward compatible with use of the
experimental codepoint/ExID.
However, some implementers may be tempted to include both the
experimental and assigned codepoint in the same segment, e.g., in a
SYN to support backward compatibility during connection
establishment. This is a poor use of limited resources; so, to
ensure conservation of the TCP option space:
>> A TCP segment MUST NOT contain both an assigned TCP option
codepoint and a TCP experimental option codepoint for the same
protocol.
Instead, a TCP that intends backward compatibility might send
multiple SYNs with alternates of the same option and discard all but
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. Rationale
The ExIDs described in this document combine properties of IANA
FCFS-registered values with magic numbers. Although IANA FCFS
registries are common, so too are those who either fail to register
or who 'squat' by deliberately using codepoints that are assigned to
others. The approach in this document is intended to recognize this
reality and be more robust to its consequences than would be a
conventional IANA FCFS registry.
Existing ID spaces were considered as ExIDs in the development of
this mechanism, including IEEE Organizationally Unique Identifier
(OUI) and IANA Private Enterprise Numbers (PENs) [IEEE802] [OUI]
[RFC 1155].
OUIs are 24-bit identifiers that are combined with 24 to 40 bits of
privately assigned space to create identifiers that are commonly
assigned to a unique piece of hardware. OUIs are already longer than
the smaller ExID value, and obtaining an OUI is costly (currently
$1,885.00 USD). An OUI could be obtained for each experiment, but
this could be considered expensive. An OUI already assigned to an
organization could be shared if extended (to support multiple
experiments within an organization), but this would either require
coordination within an organization or an IANA registry; the former
is prohibitive, and the latter is more complicated than having IANA
manage the entire space.
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RFC 6994 Shared Use of Experimental TCP Options August 2013
PENs were originally used in the Simple Network Management Protocol
(SNMP) [RFC 1157]. PENs are identifiers that can be obtained without
cost from IANA [PEN]. Despite the current registry, the size of the
PEN assignment space is currently undefined and has only recently
been proposed (as 32 bits) [IANA-PEN]. PENs are currently assigned
to organizations, and there is no current process for assigning them
to individuals. Finally, if the PENs are 32 bits as expected, they
would be larger than needed in many cases.
7. Security Considerations
The mechanism described in this document is not intended to enhance,
nor does it weaken the existing state of security for TCP option
processing.
8. IANA Considerations
IANA has created a "TCP Experimental Option Experiment Identifiers
(TCP ExIDs)" registry. The registry records both 16-bit and 32-bit
ExIDs, as well as a reference (description, document pointer, or
assignee name and e-mail contact) for each entry. ExIDs are
registered for use with both of the TCP experimental option
codepoints, i.e., with TCP options with values of 253 and 254.
Entries are assigned on a First Come, First Served (FCFS) basis
[RFC 5226]. The registry operates FCFS on the first two bytes of the
ExID (in network-standard order) but records the entire ExID (in
network-standard order). Some examples are:
o 0x12340000 collides with a previous registration of 0x1234abcd
o 0x5678 collides with a previous registration of 0x56780123
o 0xabcd1234 collides with a previous registration of 0xabcd
IANA will advise applicants of duplicate entries to select an
alternate value, as per typical FCFS processing.
IANA will record known duplicate uses to assist the community in both
debugging assigned uses as well as correcting unauthorized duplicate
uses.
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.
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RFC 6994 Shared Use of Experimental TCP Options August 2013
9. References
9.1. Normative References
[RFC 793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
9.2. Informative References
[Bi11] Bittau, A., Boneh, D., Hamburg, M., Handley, M., Mazieres,
D., and Q. Slack, "Cryptographic protection of TCP
Streams", Work in Progress, September 2012.
[Ed11]
Eddy, W., "Additional TCP Experimental-Use Options", Work
in Progress, August 2011.
[IANA] IANA, <http://www.iana.org/>.
[IANA-PEN] Liang, P. and A. Melnikov, "Private Enterprise Number
(PEN) Practices and Internet Assigned Numbers: Authority
(IANA) Considerations for Registration Procedures", Work
in Progress, June 2012.
[IEEE802] IEEE, "IEEE Standard for Local and Metropolitan Area
Networks: Overview and Architecture", IEEE 802-2001, 8
March 2002.
[OUI] IEEE, "Organizationally Unique Identifier (OUI) or
'Company_ID'",
<http://standards.ieee.org/develop/regauth/oui/>.
[PEN] IANA, "Private Enterprise Numbers",
<http://www.iana.org/assignments/enterprise-numbers>.
[RFC 951] Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
September 1985.
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RFC 6994 Shared Use of Experimental TCP Options August 2013
[RFC 1155] Rose, M. and K. McCloghrie, "Structure and Identification
of Management Information for TCP/IP-Based Internets", STD
16, RFC 1155, May 1990.
[RFC 1157] Case, J., Fedor, M., Schoffstall, M., and J. Davin,
"Simple Network Management Protocol (SNMP)", RFC 1157, May
1990.
[RFC 2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC 2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
Values In the Internet Protocol and Related Headers", BCP
37, RFC 2780, March 2000.
[RFC 3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
[RFC 6013] Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC 6013,
January 2011.
[Si11] Simpson, W., "TCP Cookie Transactions (TCPCT) Sockets
Application Program Interface (API)", Work in Progress,
April 2011.
10. Acknowledgments
This document was motivated by discussions on the IETF TCPM mailing
list and by Wes Eddy's proposal [Ed11]. Yoshifumi Nishida, Pasi
Sarolathi, and Michael Scharf provided detailed feedback.
This document was originally prepared using 2-Word-v2.0.template.dot.
Author's Address
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695 U.S.A.
Phone: +1 (310) 448-9151
EMail: touch@isi.edu
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RFC TOTAL SIZE: 25577 bytes
PUBLICATION DATE: Thursday, August 8th, 2013
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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