TCPM Working Group                                             J. Touch
Internet Draft                                                  USC/ISI
Intended status: Proposed Standard                     January 17, 2012
Expires: July 2012

                  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 July 17, 2012.

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

Touch, (TBD)            Expires July 17, 2012                  [Page 1]

Internet-Draft  Shared Use of Experimental TCP Options     January 2012

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


   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........................................5
   5. IANA Considerations............................................5
   6. References.....................................................5
      6.1. Normative References......................................5
      6.2. Informative References....................................6
   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.

Touch, (TBD)            Expires July 17, 2012                  [Page 2]

Internet-Draft  Shared Use of Experimental TCP Options     January 2012

   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 participation. A short magic number
   is added to the structure of the experimental TCP option structure.
   The magic number 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) with a magic number. The magic number is used

Touch, (TBD)            Expires July 17, 2012                  [Page 3]

Internet-Draft  Shared Use of Experimental TCP Options     January 2012

   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 value SHOULD be selected to reduce the
   probability of collision.

   The length of the magic number is a 32 bit value 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

   >> The magic number SHOULD be 32 bits long; it MAY be as few as 16
   bits if desired.

   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.

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

Touch, (TBD)            Expires July 17, 2012                  [Page 4]

Internet-Draft  Shared Use of Experimental TCP Options     January 2012

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

   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.

   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. 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 document.

   >> 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.

Touch, (TBD)            Expires July 17, 2012                  [Page 5]

Internet-Draft  Shared Use of Experimental TCP Options     January 2012

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,

   [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].

   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

Touch, (TBD)            Expires July 17, 2012                  [Page 6]