Internet Engineering Task Force                                M. Allman
INTERNET DRAFT                                                      ICSI
File: draft-allman-tcpm-bump-initcwnd-00.txt           November 15, 2010

                Initial Congestion Window Specification

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    This document specifies the progression of initial TCP congestion
    window sizes over the next nine years.

1   Introduction

    This document proposes a series of increases to TCP's [RFC790]
    initial congestion window.  For the first time in roughly nine years
    the IETF is currently considering increasing the initial congestion

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    window (IW).  The current proposal is to increase IW from the
    standard two to four segments [RFC5681] to ten segments [CDCM10].
    While there is much evidence ([Chu09,CDCM10] and references therein)
    that the increase has benefit, there is also some unease within the
    community that stems from a lack of solid understanding of the
    dynamics that the increase would cause system-wide.  Such an
    understanding is nearly impossible to apprehend.  This document
    presents an alternate approach that slowly increases the size of the
    allowed IW over the next nine years---attaining a value of ten
    segments over the course of the first three years and increasing to
    15 segments by the end of the inflationary period.  By making slow
    and steady changes the community can continually assess the dynamics
    and short-circuit the increases as necessary.  We believe this
    offers the best tradeoff between (1) assuring network safety and (2)
    allowing for reasonable performance increases as network capacity

2   Initial Congestion Window Values

    This document specifies experimental and standard values for the
    initial window as a function of time in the following chart.

      Year        Exp. IW                Std. IW
      pre-1998                           (1,1*SMSS)
      1998        (2--4,4380) [RFC2414]  (1,1*SMSS)
      1999                               (2,2*SMSS) [RFC2581]
      2002                               (2--4,4380) [RFC3390]
      2009                               (2--4,4380) [RFC5681]
      2011        (6,8760)
      2012        (8,11680)              (6,8760)
      2013        (10,14600)             (8,11680)
      2014        (11,16060)             (10,14600)
      2015        (12,17520)             (11,16060)
      2016        (13,18980)             (12,17520)
      2017        (14,20440)             (13,18980)
      2018        (15,21900)             (14,20440)
      2019                               (15,21900)

    The (X,Y) tuples indicate the IW in terms of the maximum number of
    segments, X, and the maximum number of octets, Y.  The smaller of
    these two values is used as the IW.  In other words, (6,8760)
    indicates that six segments of 1460 bytes each can be sent.  If the
    packet size is larger than 1460 bytes then the IW is bounded at 8760
    bytes.  If the segment size is smaller than 1460 bytes then the IW
    is bounded by six segments.

    Each IW value spends one year as experimental and assuming the
    community does not find any broad problems because standard the
    following year.

    We take larger steps at the beginning of the time period than at the
    end due to our confidence in each step.  That is, [CDCM10] (and
    references therein) shows that an IW of ten segments is reasonably

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    safe today.  Therefore, our first step to six segments seems
    conservative, as does the overall progression to ten segments over
    the next three years.  After that the empirical basis for further
    increases, which exists in some form today [CDCM10], is less
    compelling and therefore further steps are increases of only a
    single segment at a time.

    We stress that the above is an upper bound on the allowable IW and
    not a requirement to use the given value.  Hosts may indeed have
    reasons to be less aggressive in certain situations.

3   Short Circuiting

    The table in the last section can be short-circuited if the IETF
    finds particular issues with a given IW.  While our expectation is
    the IW values given in Section 2 are and will be safe for general
    Internet use the progression can be canceled by making this document

4   Security Considerations

    This document discusses the initial congestion window permitted for
    TCP connections.  Changing this value does not raise any known new
    security issues with TCP.

5  IANA Considerations


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.

    [RFC2414] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
        Initial Window Size", RFC 2414, September 1998.

    [RFC2581] Allman, M., Paxson V. and W. Stevens, "TCP Congestion
        Control", RFC 2581, April 1999.

    [RFC3390] Allman, M., Floyd, S., C. Partridge, "Increasing TCP's
        Initial Window", RFC 3390, October 2002.

    [RFC5681] Allman, M., Paxson V. and E. Blanton, "TCP Congestion
        Control", RFC 5681, September 2009.

Non-Normative References

    [Chu09] Chu, J., "Tuning TCP Parameters for the 21st Century",, July 2009.

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    [CDCM10] Chu, J., Dukkipati, N., Cheng, Y. and M. Mathis,
        "Increasing TCP's Initial Window", Internet-Draft
        draft-ietf-tcpm-initcwnd-00.txt (work in progress), October

Author's Addresses

   Mark Allman
   1947 Center Street
   Suite 600
   Berkeley, CA 94704-1198

   Phone: 440-235-1792

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