TCPM Working Group                                             J. Touch
Internet Draft                                                  USC/ISI
Intended status: Informational                         October 24, 2011
Expires: April 2012

                  Shared Use of Experimental TCP Options

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   This document describes how TCP option codepoints can support
   concurrent experiments. The suggested mechanism avoids the need for
   a coordinated registry, and is backward-compatible with currently
   known uses.

Table of Contents

   1. Introduction...................................................2
   2. Conventions used in this document..............................3
   3. TCP Experimental Option Structure..............................3
   4. Security Considerations........................................4
   5. IANA Considerations............................................5
   6. References.....................................................5
      6.1. Normative References......................................5
      6.2. Informative References....................................5
   7. Acknowledgments................................................6

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 31 are
   assigned at the time this document was published [IANA]. Two of
   these codepoints are allocated to support experiments (253, 254)
   [RFC4727]. These numbers are intended for testing purposes, and
   implementations need to assume they can be used for other purposes,
   but this is often not the case.

   There is no mechanism to support shared use of the experimental
   option codepoints. Experimental options 245 and 255 are 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 each use is ambiguous.

   Other options have been used without assignment, 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 and 76-78 as well.

   There are a variety of proposed approaches to address this issue.
   The first is to relax the requirements for assignment of TCP

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   options, allowing them to be assigned more readily for protocols
   that have not been standardized through the IETF process [RFC5226].
   A second would be to assign a larger pool to options, and to manage
   their sharing through IANA coordination [Ed11].

   This document proposes a solution that does not require additional
   codepoints and also avoids IANA participation. A short nonce is
   added to the structure of the experimental TCP option structure. The
   nonce helps reduce the probability of collision of independent
   experimental uses of the same option codepoint. This feature
   increases the size of experimental options, but the size can be
   reduced when the experiment is converted to a standard protocol with
   a conventional codepoint assignment.

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, where the first byte
   is the codepoint (Kind) and the second is the length of the option
   in bytes (Length):

                |  Kind  | Length |       ...       |
                |    ...

                  Figure 1 TCP Option Structure [RFC793]

   This document extends the option structure for experimental
   codepoints (253, 254) as follows:

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                |  Kind  | Length |     Nonce       |
                |      Nonce      |   ...

               Figure 2 TCP Experimental Option with a Nonce

   >> Protocols using the TCP experimental option codepoints (253, 254)
   SHOULD use nonces as described in this document.

   The nonce is selected by the protocol designer when the experimental
   option is defined. The Nonce 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).

   >> The nonce SHOULD be selected to reduce the probability of

   The length of the nonce is intended to be 32 bit in network standard
   byte order. It can be shorter if desired (e.g., 16 bits), with a
   corresponding increased probability of collision and thus false

   During TCP processing, experimental options are matched against both
   the experimental codepoints and the Nonce value for each implemented

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

   The remainder of the option is specified by the particular
   experimental protocol.

   Use of a nonce 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. Security Considerations

   The mechanism described in this document is not intended to provide
   security for TCP option processing. False positives are always
   possible, where a Nonce matches a legacy use of these options or a
   protocol that does not implement the mechanism described in this

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   >> Protocols that are not robust to such false positives SHOULD
   implement other measures to ensure they process options for their
   protocol only, such as checksums or digital signatures among
   cooperating parties of their protocol.

5. IANA Considerations

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

6. References

6.1. 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.

6.2. 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-01,
             Aug. 29, 2011.

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

   [IANA]    IANA web pages,

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

   [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.

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7. Acknowledgments

   This document was motivated by discussions on the IETF TCPM mailing
   list and by Wes Eddy's proposal [Ed11].

   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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