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Shared Use of Experimental TCP Options

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6994.
Author Dr. Joseph D. Touch
Last updated 2012-12-20 (Latest revision 2012-11-28)
Replaces draft-touch-tcpm-experimental-options
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd (None)
IESG IESG state Became RFC 6994 (Proposed Standard)
Consensus boilerplate Unknown
Telechat date (None)
Needs a YES. Needs 10 more YES or NO OBJECTION positions to pass.
Responsible AD Wesley Eddy
IESG note
Send notices to,
TCPM Working Group                                             J. Touch
Internet Draft                                                 USC/ISI
Intended status: Proposed Standard                    November 28, 2012
Expires: May 2013

                  Shared Use of Experimental TCP Options

Status of this Memo

   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-

   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

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on May 28, 2013.

Copyright Notice

   Copyright (c) 2012 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
   ( 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

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   Section 4.e of the Trust Legal Provisions and are provided without
   warranty as described in the Simplified BSD License.


   This document describes how the experimental TCP option codepoints
   can support concurrent use through the use of a magic number. This
   mechanism avoids the need for a coordinated registry and is
   backward-compatible with currently known uses. It is recommended for
   all new TCP options that use these codepoints.

Table of Contents

   1. Introduction...................................................2
   2. Conventions used in this document..............................4
   3. TCP Experimental Option Structure..............................4
      3.1. Selecting a Magic Number..................................5
      3.2. Impact on TCP Option Processing...........................5
   4. Reducing the Impact of False Positives.........................6
   5. Migration to Assigned Options..................................7
   6. Security Considerations........................................7
   7. IANA Considerations............................................7
   8. References.....................................................8
      8.1. Normative References......................................8
      8.2. Informative References....................................8
   9. Acknowledgments................................................9

1. Introduction

   TCP includes options to enable new protocol capabilities that can be
   activated only where needed and supported [RFC793]. The space for
   identifying such options is small - 256 values, of which 30 are
   assigned at the time this document was published [IANA]. Two of
   these codepoints are allocated to support experiments (253, 254)
   [RFC4727]. These values are intended for testing purposes or anytime
   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).

   The term "experimental TCP options" refers here to options that use
   the experimental TCP option codepoints [RFC4727]. Such experiments
   can be described in any type of RFC - Experimental, Informational,
   etc., and are intended to be used both in controlled environments
   and in are allowed in public deployments (when not enabled as
   default) [RFC3962]. Nothing prohibits deploying multiple experiments

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   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 [RFC2026]. TCP options in such
   RFCs are typically not eligible for assigned TCP option codepoints
   [RFC2780], and so there is a need to share use of the experimental
   option codepoints.

   There is currently no mechanism to support shared use of the
   experimental TCP option codepoints. Experimental options 253 and 254
   are already deployed in operational code to support an early version
   of TCP authentication. Option 253 is also documented for the
   experimental TCP Cookie Transaction option [RFC6013]. 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

   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 as well. 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 which 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 [RFC5226].
   Another proposal assigns a larger pool to options and manages their
   sharing through IANA coordination [Ed11].

   The approach proposed in this document does not require additional
   codepoints and also avoids IANA involvement. The solution adds a
   field to the structure of the experimental TCP option. This field is
   populated with a fixed "magic number" defined as part of a specific
   option experiment. The magic number helps reduce the probability of
   a collision of independent experimental uses of the same option
   codepoint, both for those who follow this document (using other
   magic numbers) and those who do not (squatters).

   The solution proposed in this document is recommended for all new
   protocols that use experimental TCP option codepoints. The
   techniques used here may also help share other experimental
   codepoints, but that issue is out of scope for this document.

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2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [RFC2119].

   In this document, these words will appear with that interpretation
   only when in ALL CAPS. Lower case uses of these words are not to be
   interpreted as carrying RFC-2119 significance.

   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 [RFC793], 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 [RFC793]

   This document extends the option structure for experimental
   codepoints (253, 254) with a magic number, which is typically 4
   bytes in length. The magic number is used to differentiate different
   experiments, and is the first field after the Kind and Length, as

                 0          1          2          3
                 01234567 89012345 67890123 45678901
                |  Kind  | Length |  Magic Number   |
                |  Magic Number   |   ...

           Figure 2 TCP Experimental Option with a Magic Number

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   >> Protocols requiring new TCP option codepoints that are not
   eligible for assigned values SHOULD use the existing TCP
   experimental option codepoints (253, 254) with magic numbers as
   described in this document.

   >> All protocols using the TCP experimental option codepoints (253,
   254) SHOULD use magic numbers 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
   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
   magic numbers reduce the probability of a collision with other
   options sharing the Kind codepoint, but also increase the option
   size. A suggested size is 32 bits, in network standard byte order:

   >> The magic number size and value SHOULD be selected to reduce the
   probability of collision.

   >> The magic number SHOULD be 32 bits, but MAY be either longer or

3.2. Impact on TCP Option Processing

   The magic number is considered part of the TCP option, not the TCP
   option header. The presence of the magic number increases the
   effective option Length field by the size of the magic number. The
   presence of this magic number is thus transparent to implementations
   that do not support TCP options where it is used.

   During TCP processing, experimental options are matched against both
   the experimental codepoints and the magic number value for each
   implemented protocol.

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   >> Experimental options that have magic numbers that do not match
   implemented protocols MUST be ignored.

   The remainder of the option is specified by the particular
   experimental protocol. This includes the possibility that the magic
   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

   The specification of an experimental option needs to describe
   whether the magic number appears in non-SYN segments. If the magic
   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
   of a connection, SHOULD be used in all TCP segments of that
   connection in which any experimental option is present.

   Use of a magic number 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 magic number of one experiment
   matches the value of an option that does not use magic numbers or if
   two experiments select the same magic number. Such collisions can
   cause an option to be interpreted by the incorrect processing

   >> 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
   magic number while reducing the corresponding increased potential
   for false positives. However this document recommends magic numbers
   are used together with such checksums/signatures, not as a

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   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

   Some experiments may transition from experiment, 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/magic numbers to use of an assigned

   However, once an assigned codepoint is allocated, use of a magic
   number represents unnecessary overhead. As a result:

   >> Once a TCP option codepoint is assigned to a protocol, that
   protocol SHOULD NOT continue to use a magic number as part of that
   assigned codepoint.

   This document does not recommend whether or how an implementation of
   an assigned codepoint should be backward-compatible with use of the
   experimental codepoint/magic number.

   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 limited resources and so to ensure
   conservation of the TCP option space:

   >> A TCP segment MUST NOT contain both an assigned TCP option
   codepoint and an experimental TCP option codepoint/magic number 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.

6. Security Considerations

   The mechanism described in this document is not intended to provide
   (nor does it weaken existing) security for TCP option processing.

7. IANA Considerations

   This document has no IANA considerations. This section should be
   removed prior to publication.

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8. References

8.1. Normative References

   [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
             793, Sep. 1981.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4727] Fenner, B., "Experimental Values in IPv4, IPv6, ICMPv4,
             ICMPv6, UDP, and TCP Headers", RFC 4727, Nov. 2006.

8.2. Informative References

   [Bi11]    Bittau, A., D. Boneh, M. Hamburg, M. Handley, D. Mazieres,
             Q. Slack, "Cryptographic protection of TCP Streams
             (tcpcrypt)", work in progress, draft-bittau-tcp-crypt-03,
             Sep. 3, 2012.

   [Ed11]    Eddy, W., "Additional TCP Experimental-Use Options", work
             in progress, draft-eddy-tcpm-addl-exp-options-00, Aug. 16,

   [IANA]    IANA web pages,

   [RFC951]  Croft, B., J. Gilmore, "BOOTSTRAP PROTOCOL (BOOTP)", RFC
             951, Sept. 1985.

   [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
             3", BCP 9, RFC 2026, Oct. 1996.

   [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
             2131, Mar. 1997.

   [RFC2780] Bradner, S., V. Paxson, "IANA Allocation Guidelines For
             Values In the Internet Protocol and Related Headers", BCP
             37, RFC 2780, Mar. 2000.

   [RFC3962] Narten, T., "Assigning Experimental and Testing Numbers
             Considered Useful", BCP 82, RFC 3962, Jan. 2004.

   [RFC5226] Narten, T., H. Alvestrand, "Guidelines for Writing an IANA
             Considerations Section in RFCs", BCP 26, RFC 5226, May

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   [RFC6013] Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC 6013,
             Jan. 2011.

   [Si11]    Simpson, W., "TCP Cookie Transactions (TCPCT) Sockets
             Application Program Interface (API)", work in progress,
             draft-simpson-tcpct-api-04, Apr. 7, 2011.

9. 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 prepared using

Authors' Addresses

   Joe Touch
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695 U.S.A.

   Phone: +1 (310) 448-9151

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