TCPM Working Group                                             J. Touch
Internet Draft                                                 USC/ISI
Intended status: Proposed Standard                         May 30, 2012
Expires: November 2012

                  Shared Use of Experimental TCP Options

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   Section 4.e of the Trust Legal Provisions and are provided without
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   This document describes how TCP option codepoints can support
   concurrent experiments using a magic number field. This 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
      3.1. Reducing the Impact of False Positives....................5
      3.2. Migration to Assigned Options.............................6
   4. Security Considerations........................................6
   5. IANA Considerations............................................6
   6. References.....................................................6
      6.1. Normative References......................................6
      6.2. Informative References....................................7
   7. Acknowledgments................................................7

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 253 and 254 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 codepoints have been used without assignment, notably 31-32
   (TCP cookie transactions, as originally distributed and in its API

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   doc) and 76-78 (tcpcrypt) [Bi11][Si11]. Commercial products
   reportedly also use unassigned options 33 and 76-78 as well. Even
   though these uses are inappropriate, they can impact legitimate

   There are a variety of proposed approaches to address this issue.
   The first is to relax 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].
   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 involvement. The solution involves
   adding a field to the structure of the experimental TCP option. This
   field is typically 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. This feature increases the number of
   bytes used by 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):

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

                  Figure 1 TCP Option Structure [RFC793]

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

                |  Kind  | Length |  Magic Number   |
                |  Magic Number   |   ...

           Figure 2 TCP Experimental Option with a Magic Number

   >> 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]. Here they help ensure that concurrent experiments
   that share the same TCP option codepoint do not interfere.

   The magic number is selected by the protocol designer when an
   experimental option is defined. 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).

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

   This document does not proscribe a minimum magic number size.
   However, a reasonable suggested size is 32 bits, in network standard
   byte order:

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

   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

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   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.

   >> 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).

   >> Experimental option magic numbers, if used, MUST be present in
   TCP SYN segments.

   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:

   >> Experimental option magic numbers, if used, SHOULD be used in all
   TCP segments where the 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.

3.1. Reducing the Impact of False Positives

   False positives are always possible, where a magic number matches
   the value of a field in the legacy use of these options or a
   protocol that does not implement the mechanism described in this

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

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   cooperating parties of their protocol. Such measures SHOULD
   supplement, rather than substitute for, the use of magic numbers.

   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
   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.

3.2. Migration to Assigned Options

   This document does not address a specific migration plan to avoid
   the use of magic numbers once an experimental TCP option is
   considered for operational deployment, e.g., if it transitions to
   proposed standard. The expectation is that such options would be
   assigned their own TCP codepoints and their specifications updated
   to avoid the need to support the experimental codepoint.

4. Security Considerations

   The mechanism described in this document is not intended to provide
   security for TCP option processing.

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.

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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-02,
             Feb. 20, 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.

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

   [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.

7. 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 Sharf provided detailed feedback.

   This document was prepared using

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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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