TCPM Working Group                                             J. Touch
Internet Draft                                                 USC/ISI
Intended status: Proposed Standard                    February 25, 2013
Expires: August 2013

                  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 the experimental TCP option codepoints
   can concurrently support multiple TCP extensions, even within the
   same connection. It uses a new IANA TCP experiment identifier, and
   is also robust to experiments that are not registered and those that
   do not use this sharing mechanism. 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 an ExID.........................................5
      3.2. Impact on TCP Option Processing...........................6
   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 TCP experimental 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

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   default) [RFC3692]. Nothing prohibits deploying 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 [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 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
   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 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 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 the TCP experiment option
   codepoints and manages their sharing through IANA coordination

   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).

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   The solution proposed in this document is recommended for all new
   protocols that use TCP experimental option codepoints. The
   techniques used here may also help share 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",
   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 an experiment identifier (ExID), which is
   either 2 or 4 bytes in length. The ExID is used to differentiate
   different experiments, and is the first field after the Kind and
   Length, as follows:

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                 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

   >> 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.

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

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".

   ExIDs are registered with IANA using "first-come, first-served"
   priority based on the first two bytes. Those two bytes are thus
   sufficient to interpret which experimental option is contained in
   the option field.

   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., BOOTP [RFC951] and DHCP [RFC2131]. The magic
   number helps reduce the probability of a false positive collision
   with those who either do not register their experiment or who do not

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   implement this mechanism. Using the additional magic number bytes
   also 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.

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 where it is used.

   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.

   >> Experimental options that have 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 ExID of one experiment matches the
   value of an option that does not use ExIDs or if two experiments
   select the same ExID. Such collisions can cause an option to be
   interpreted by the incorrect processing routine. Use of checksums or
   signatures may help an experiment use a shorter ExID while reducing
   the corresponding increased potential for false positives.

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   >> 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 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/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

   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 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 a TCP experimental option codepoint for the same

   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. 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 calls for IANA to create a new TCP experimental option
   Experiment Identifier (ExID) registry.

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   That registry should allow 16-bit and 32-bit entries, where entries
   are "first-come, first-served" on the first two bytes of the value
   in network-standard byte order (big endian), in which the entry
   should indicate the entire ExID value. Known overlapping uses -
   whether of the first-come portion or the entire value - should also
   be listed and highlighted as collisions.

   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.

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.

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

   [RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
             Considered Useful", BCP 82, RFC 3692, Jan. 2004.

   [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.

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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