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rfc:rfc6994

Internet Engineering Task Force (IETF) J. Touch Request for Comments: 6994 USC/ISI Category: Standards Track August 2013 ISSN: 2070-1721

               Shared Use of Experimental TCP Options

Abstract

 This document describes how the experimental TCP option codepoints
 can concurrently support multiple TCP extensions, even within the
 same connection, using a new IANA TCP experiment identifier.  This
 approach is robust to experiments that are not registered and to
 those that do not use this sharing mechanism.  It is recommended for
 all new TCP options that use these codepoints.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6994.

Copyright Notice

 Copyright (c) 2013 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
 (http://trustee.ietf.org/license-info) 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 Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Touch Standards Track [Page 1] RFC 6994 Shared Use of Experimental TCP Options August 2013

Table of Contents

 1. Introduction ....................................................2
 2. Conventions Used in This Document ...............................3
 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 ..........................7
 5. Migration to Assigned Options ...................................7
 6. Rationale .......................................................8
 7. Security Considerations .........................................9
 8. IANA Considerations .............................................9
 9. References .....................................................10
    9.1. Normative References ......................................10
    9.2. Informative References ....................................10
 10. Acknowledgments ...............................................11

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 of this document's publication [IANA].  Two of
 these codepoints (253, 254) are allocated to support experiments
 [RFC4727].  These values are intended for testing purposes or for use
 when 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).
 Here, the term "experimental TCP options" refers to options that use
 the TCP experimental option codepoints [RFC4727].  Such experiments
 can be described in an RFC of any status (e.g., Experimental,
 Informational, etc.) and are intended to be used in controlled
 environments and are allowed in public deployments (when not enabled
 as default [RFC3692]).  Nothing prohibits the deployment of 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].
 Typically, these TCP options are not eligible to receive assigned
 codepoints [RFC2780], so they need a way to share their use of the
 experimental 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

Touch Standards Track [Page 2] RFC 6994 Shared Use of Experimental TCP Options August 2013

 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.  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 that 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
 [Ed11].
 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).
 The solution proposed in this document is recommended for all new
 protocols that use TCP experimental option codepoints.  The
 techniques described here may also enable shared use of 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",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 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.  Lowercase uses of these words are not to be
 interpreted as carrying RFC 2119 significance.

Touch Standards Track [Page 3] RFC 6994 Shared Use of Experimental TCP Options August 2013

 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
 experiments and is the first field after Kind and Length, as follows:
                  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

Touch Standards Track [Page 4] RFC 6994 Shared Use of Experimental TCP Options August 2013

 This mechanism is encouraged for all TCP options that are not yet
 eligible for assigned codepoints:
 >> 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.
 This mechanism is encouraged for all TCP options using the current
 experimental codepoints in controlled environments:
 >> All protocols using the TCP experimental option codepoints (253,
    254), even those deployed in controlled environments, SHOULD use
    ExIDs as described in this document.
 This mechanism is required for all TCP options using the current
 experimental codepoints that are publicly deployed, whether enabled
 by default or not:
 >> All protocols using the TCP experimental option codepoints (253,
    254) that are deployed outside controlled environments, such as in
    the public Internet, MUST use ExIDs as described in this document.
 Once a TCP option uses the mechanism in this document, registration
 of the ExID with IANA is required:
 >> All protocols using ExIDs as described in this document MUST
    register those ExIDs with IANA.
    Applicants register their desired ExID by contacting IANA [IANA].

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".
 >> All ExIDs MUST be either 16 bits or 32 bits long.
 Use of the ExID, whether 16 bit or 32 bit, helps reduce the
 probability of a false positive collision with those who either do
 not register their experiment or who do not implement this mechanism.

Touch Standards Track [Page 5] RFC 6994 Shared Use of Experimental TCP Options August 2013

 In order to conserve TCP option space, either for use within a
 specific option or to be available for other options:
 >> Options implementing the mechanism of this document SHOULD use
    16-bit ExIDs, except where explicitly motivating the need for
    32-bit ExIDs, e.g., to avoid false positives or maintain alignment
    with an expected future assigned codepoint.
 ExIDs are registered with IANA using "first come, first served"
 (FCFS) priority based on the first two bytes.  Those two bytes are
 thus sufficient to interpret which experimental option is contained
 in the option field.
 >> All ExIDs MUST be unique based on their first 16 bits.
 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., bootstrap protocol (BOOTP) [RFC951] and DHCP
 [RFC2131].
 Using the additional magic number bytes 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.  Use of the same alignment reduces the potential for
 implementation errors, especially in using the same word-alignment
 padding, if the same software is later modified to use a conventional
 codepoint.  Use of the longer, 32-bit ExID further decreases the
 probability of such a false positive compared to those using shorter,
 16-bit ExIDs.
 Use of the ExID does consume TCP option space but enables concurrent
 use of the experimental codepoints and provides protection against
 false positives, leaving less space for other options (including
 other experiments).  Use of the longer, 32-bit ExID consumes more
 space, but provides more protection against false positives.

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.
 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.

Touch Standards Track [Page 6] RFC 6994 Shared Use of Experimental TCP Options August 2013

 >> Experimental options with 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 registered ExID of an experiment
 matches the value of an option that does not use ExIDs.  Such
 collisions can cause an option to be interpreted by the incorrect
 processing routine.  Use of checksums or signatures may help an
 experiment use the shorter ExID while reducing the corresponding
 increased potential for false positives.
 >> 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 away from being experimental 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 codepoint.

Touch Standards Track [Page 7] RFC 6994 Shared Use of Experimental TCP Options August 2013

 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 of limited resources; 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
    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.  Although this approach may
 resolve more slowly or require additional effort at the endpoints, it
 is preferable to excessively consuming TCP option space.

6. Rationale

 The ExIDs described in this document combine properties of IANA
 FCFS-registered values with magic numbers.  Although IANA FCFS
 registries are common, so too are those who either fail to register
 or who 'squat' by deliberately using codepoints that are assigned to
 others.  The approach in this document is intended to recognize this
 reality and be more robust to its consequences than would be a
 conventional IANA FCFS registry.
 Existing ID spaces were considered as ExIDs in the development of
 this mechanism, including IEEE Organizationally Unique Identifier
 (OUI) and IANA Private Enterprise Numbers (PENs) [IEEE802] [OUI]
 [RFC1155].
 OUIs are 24-bit identifiers that are combined with 24 to 40 bits of
 privately assigned space to create identifiers that are commonly
 assigned to a unique piece of hardware.  OUIs are already longer than
 the smaller ExID value, and obtaining an OUI is costly (currently
 $1,885.00 USD).  An OUI could be obtained for each experiment, but
 this could be considered expensive.  An OUI already assigned to an
 organization could be shared if extended (to support multiple
 experiments within an organization), but this would either require
 coordination within an organization or an IANA registry; the former
 is prohibitive, and the latter is more complicated than having IANA
 manage the entire space.

Touch Standards Track [Page 8] RFC 6994 Shared Use of Experimental TCP Options August 2013

 PENs were originally used in the Simple Network Management Protocol
 (SNMP) [RFC1157].  PENs are identifiers that can be obtained without
 cost from IANA [PEN].  Despite the current registry, the size of the
 PEN assignment space is currently undefined and has only recently
 been proposed (as 32 bits) [IANA-PEN].  PENs are currently assigned
 to organizations, and there is no current process for assigning them
 to individuals.  Finally, if the PENs are 32 bits as expected, they
 would be larger than needed in many cases.

7. Security Considerations

 The mechanism described in this document is not intended to enhance,
 nor does it weaken the existing state of security for TCP option
 processing.

8. IANA Considerations

 IANA has created a "TCP Experimental Option Experiment Identifiers
 (TCP ExIDs)" registry.  The registry records both 16-bit and 32-bit
 ExIDs, as well as a reference (description, document pointer, or
 assignee name and e-mail contact) for each entry.  ExIDs are
 registered for use with both of the TCP experimental option
 codepoints, i.e., with TCP options with values of 253 and 254.
 Entries are assigned on a First Come, First Served (FCFS) basis
 [RFC5226].  The registry operates FCFS on the first two bytes of the
 ExID (in network-standard order) but records the entire ExID (in
 network-standard order).  Some examples are:
 o  0x12340000 collides with a previous registration of 0x1234abcd
 o  0x5678 collides with a previous registration of 0x56780123
 o  0xabcd1234 collides with a previous registration of 0xabcd
 IANA will advise applicants of duplicate entries to select an
 alternate value, as per typical FCFS processing.
 IANA will record known duplicate uses to assist the community in both
 debugging assigned uses as well as correcting unauthorized duplicate
 uses.
 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.

Touch Standards Track [Page 9] RFC 6994 Shared Use of Experimental TCP Options August 2013

9. References

9.1. Normative References

 [RFC793]   Postel, J., "Transmission Control Protocol", STD 7, RFC
            793, September 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, November 2006.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.

9.2. Informative References

 [Bi11]     Bittau, A., Boneh, D., Hamburg, M., Handley, M., Mazieres,
            D., and Q. Slack, "Cryptographic protection of TCP
            Streams", Work in Progress, September 2012.
 [Ed11]
            Eddy, W., "Additional TCP Experimental-Use Options", Work
            in Progress, August 2011.
 [IANA]     IANA, <http://www.iana.org/>.
 [IANA-PEN] Liang, P. and A. Melnikov, "Private Enterprise Number
            (PEN) Practices and Internet Assigned Numbers: Authority
            (IANA) Considerations for Registration Procedures", Work
            in Progress, June 2012.
 [IEEE802]  IEEE, "IEEE Standard for Local and Metropolitan Area
            Networks: Overview and Architecture", IEEE 802-2001, 8
            March 2002.
 [OUI]      IEEE, "Organizationally Unique Identifier (OUI) or
            'Company_ID'",
            <http://standards.ieee.org/develop/regauth/oui/>.
 [PEN]      IANA, "Private Enterprise Numbers",
            <http://www.iana.org/assignments/enterprise-numbers>.
 [RFC951]   Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
            September 1985.

Touch Standards Track [Page 10] RFC 6994 Shared Use of Experimental TCP Options August 2013

 [RFC1155]  Rose, M. and K. McCloghrie, "Structure and Identification
            of Management Information for TCP/IP-Based Internets", STD
            16, RFC 1155, May 1990.
 [RFC1157]  Case, J., Fedor, M., Schoffstall, M., and J. Davin,
            "Simple Network Management Protocol (SNMP)", RFC 1157, May
            1990.
 [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
            3", BCP 9, RFC 2026, October 1996.
 [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
            2131, March 1997.
 [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
            Values In the Internet Protocol and Related Headers", BCP
            37, RFC 2780, March 2000.
 [RFC3692]  Narten, T., "Assigning Experimental and Testing Numbers
            Considered Useful", BCP 82, RFC 3692, January 2004.
 [RFC6013]  Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC 6013,
            January 2011.
 [Si11]     Simpson, W., "TCP Cookie Transactions (TCPCT) Sockets
            Application Program Interface (API)", Work in Progress,
            April 2011.

10. 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 originally prepared using 2-Word-v2.0.template.dot.

Author's Address

 Joe Touch
 USC/ISI
 4676 Admiralty Way
 Marina del Rey, CA 90292-6695 U.S.A.
 Phone: +1 (310) 448-9151
 EMail: touch@isi.edu

Touch Standards Track [Page 11]

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