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Versions: 00                                                            
Internet Engineering Task Force                             Eddie Kohler
INTERNET-DRAFT                                                      UCLA
draft-kohler-tcpm-extopt-00.txt                        19 September 2004
Expires: March 2005

                     Extended Option Space for TCP

Status of this Memo

    This document is an Internet-Draft.

    By submitting this Internet-Draft, we certify that any applicable
    patent or other IPR claims of which we are aware have been
    disclosed, or will be disclosed, and any of which we become aware
    will be disclosed, in accordance with RFC 3668 (BCP 79).

    By submitting this Internet-Draft, we accept the provisions of
    Section 3 of RFC 3667 (BCP 78).

    Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice

    Copyright (C) The Internet Society (2004). All Rights Reserved.


    This memo describes a reinterpretation of the TCP Data Offset field,
    affecting the previously illegal code points 0-4, that allows

Kohler                                                          [Page 1]

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    endpoints to fit more than 40 bytes of option into TCP segments.

Kohler                                                          [Page 2]

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1.  Introduction

    The TCP datagram format has space for up to 40 bytes of TCP options
    [RFC 793]. Although this is adequate in most cases, a combination of
    options such as TCP MD5 [RFC 2385], SACK (Selective Acknowledgement)
    [RFC 2018], and Timestamp [RFC 1323] will not fit in the currently
    available option space.  In fact, SACK alone could take up more
    space than is available, given a sufficiently complex loss pattern.
    A mechanism supporting larger option space might support currently
    illegal option combinations, simplify the deployment of any future
    TCP options, and discourage kludges that try to fit too much data
    into too little option space.  Further motivation and discussion

    The amount of space used for options is determined by the TCP
    header's 4-bit Data Offset field, or DO.  This number equals the
    offset of application data relative to the start of the TCP header,
    measured in 32-bit words.  The fixed portion of the TCP header is 20
    bytes long, so 5 is the smallest legal value for DO; it indicates
    the absence of options.  The largest possible value, 15, indicates a
    data offset of 60 bytes, and thus 40 bytes of option space.  The
    values 0 through 4 are currently illegal.  The proposed mechanism
    uses these code points to indicate extended option space, taking
    more than 40 bytes.

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    this document are to be interpreted as described in [RFC 2119].

2.  Mechanism

    A TCP implementing this Internet-Draft MUST interpret the TCP
    header's DO field according to the following table.  The
    intepretation of values 5-15 is identical to that of [RFC 793].

     DO    Data Offset   Option Space  Min. TCP Length
     --    -----------   ------------  ---------------
      0         68            48             68
      1         84            64             84
      2        148           128            148
      3        276           256            276
      4     infinity     whole packet        20
    5-15      DO*4        (DO*4)-20        DO*4

    A segment's TCP length MUST equal or exceed the Min. TCP Length
    value indicated by its DO field.  A receiving TCP MUST ignore any
    segment that is too short.

Kohler                                              Section 2.  [Page 3]

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    TCP segments with DO between 0 and 4 are called extended segments.

2.1.  Requesting Extended Segments with SYN

    Extended segments MUST NOT be sent unless their use was approved
    during the TCP three-way handshake.  Approval happens when an
    extended segment (here, the SYN) is acknowledged by another extended
    segment (the SYNACK).

    An endpoint performing active open indicates its desire to use
    extended segments by sending an extended SYN, that is, a SYN with
    DO < 5.  If an extended SYNACK arrives in response, the endpoint
    will send an ACK and continue, using extended and nonextended
    segments as appropriate.  If the connection attempt fails (through a
    timeout, ICMP destination unreachable, or received TCP RST), or the
    received SYNACK is not extended, the active endpoint MUST try again
    with a non-extended SYN.  Unless the connection attempt failed
    through a RST, the active endpoint MUST clean up any remote state
    before retrying, by sending a RST and waiting at least a short
    interval (roughly 1 round-trip time, or 100 ms, if no RTT is
    available) to discourage packet reordering.

    A listening endpoint receiving an extended SYN MUST either respond
    with an extended SYNACK (to allow the use of extended segments), or
    reset the connection with a non-extended RST (to prevent their use).

    Requiring a full handshake to approve the use of extended segments
    has the side effect of ensuring that any middleboxes on both parts
    of the path can handle extended segments (or at least won't drop

    The procedures described in this section can delay connection
    establishment, or definitive connection refusal, by up to a SYN
    timeout (on the order of 3 seconds).

2.2.  Requesting Extended Segments with SYNACK

    A passive, listening endpoint MAY also request the use of extended
    segments, by sending an extended SYNACK in response to a non-
    extended SYN.  Approval is granted if the response ACK is extended.
    This procedure is riskier than requesting extended segments on the
    SYN, however.  An active endpoint with a "legacy" implementation
    might reset the connection in response to the extended SYNACK, and
    not retry.  Furthermore, a listening endpoint implementing this
    procedure must distinguish SYN transmissions from retransmissions,
    preventing the use of SYN cookies [SYNCOOKIES].

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    A listening endpoint receiving a non-extended SYN MAY respond with
    an extended SYNACK to request the use of extended segments.  If an
    extended ACK arrives in response, the endpoint will continue using
    extended and nonextended segments as appropriate.  If the extended
    SYNACK transmission fails (a timeout occurs, a retransmitted non-
    extended SYN is received, or a non-extended RST is received), it
    MUST try again with a non-extended SYNACK.  If a non-extended ACK is
    received, it MUST send a non-extended SYNACK retransmission; the
    hope is that the active endpoint will use any options specified on
    the retransmission.

    An active-open endpoint that sent a non-extended SYN, but received
    an extended SYNACK, MUST either respond with an extended ACK (to
    allow the use of extended segments), or reset the connection with a
    non-extended RST (to prevent their use).

3.  Stability Considerations

    Existing "legacy" TCP implementations -- both those in end hosts,
    and those in middleboxes such as firewalls -- clearly will not
    process extended segments according to this memo.  On encountering
    an extended segment, legacy implementations might drop the segment
    as erroneous, act as if the segment had no options, reset the
    connection, or even conceivably crash.  Even if endpoints were able
    to complete an extended-segment handshake, a path change (perhaps
    induced by mobility) might introduce a legacy middlebox into the
    connection, leading to possible connection reset.  For these
    reasons, TCP connections SHOULD NOT use extended segments, or the
    extended segment handshake, unless it is considered required.  APIs
    SHOULD let applications allow the use of extended segments; this API
    SHOULD be off by default.

    Legacy endpoints that treat extended segments as if they have DO 5
    are particularly problematic.  The risk is that any options on the
    packet, including the mandatory MSS option, will be ignored; and
    that any options on retransmitted SYN or SYNACK packets will
    likewise be ignored.  This risk should be investigated further.
    Modern open-source operating systems, at least, appear to drop
    extended segments.

4.  Security Considerations

    TCP implementations that follow this document will respond more
    slowly to some received RSTs, specifically those sent in response to
    extended SYNs and SYNACKs.  Endpoints that implement the algorithm
    in Section 2.2 cannot use SYN cookies to protect against SYN-flood
    denial-of-service attacks.  (Others?)

Kohler                                              Section 4.  [Page 5]

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5.  Acknowledgements

    This mechanism was developed in conversation with Mark Allman,
    following conversation with Wes Eddy.

Normative References

    [RFC 793] J. Postel, editor.  Transmission Control Protocol.
        RFC 793.

    [RFC 2119] S. Bradner.  Key Words For Use in RFCs to Indicate
        Requirement Levels.  RFC 2119.

Informative References

    [RFC 1323] V. Jacobson, R. Braden, and D.  Borman.  TCP Extensions
        for High Performance.  RFC 1323, May 1992.

    [RFC 2018] M. Mathis, J. Mahdavi, S. Floyd, and A. Romanow.  TCP
        Selective Acknowledgement Options.  RFC 2018, October 1996.

    [RFC 2385] A. Heffernan.  Protection of BGP Sessions via the TCP MD5
        Signature Option.  RFC 2385, August 1998.

    [RFC 3168] K.K. Ramakrishnan, S. Floyd, and D. Black.  The Addition
        of Explicit Congestion Notification (ECN) to IP.  RFC 3168.

    [RFC 3360] S. Floyd.  Inappropriate TCP Resets Considered Harmful.
        RFC 3360.

    [RFC 3517] E. Blanton, M. Allman, K. Fall, and L. Wang.  A
        Conservative Selective Acknowledgment (SACK)-based Loss Recovery
        Algorithm for TCP.  RFC 3517.

    [SB00] Alex C. Snoeren and Hari Balakrishnan.  An End-to-End
        Approach to Host Mobility.  Proc. 6th Annual ACM/IEEE
        International Conference on Mobile Computing and Networking
        (MOBICOM '00), August 2000.

    [SYNCOOKIES] Daniel J. Bernstein.  SYN Cookies.
        http://cr.yp.to/syncookies.html, as of July 2003.

Authors' Addresses

Kohler                                                          [Page 6]

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    Eddie Kohler <kohler@cs.ucla.edu>
    4531C Boelter Hall
    UCLA Computer Science Department
    Los Angeles, CA 90095

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Kohler                                                          [Page 8]