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Internet Engineering Task Force                                M. AllmanINTERNET DRAFT                                                      ICSIFile:draft-allman-tcpm-bump-initcwnd-00.txt           November 15, 2010Initial Congestion Window SpecificationStatus of this Memo    This Internet-Draft is submitted to IETF in full conformance with    the provisions ofBCP 78 andBCP 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-    Drafts.    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 athttp://www.ietf.org/ietf/1id-abstracts.txt.    The list of Internet-Draft Shadow Directories can be accessed athttp://www.ietf.org/shadow.html.    This Internet-Draft will expire on May 15, 2011.Copyright Notice    Copyright (c) 2010 IETF Trust and the persons identified as the    document authors.  All rights reserved.    This document is subject toBCP 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 theTrust Legal Provisions and are provided without    warranty as described in the BSD License.Abstract    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 congestionExpires: May 15, 2011                                           [Page 1]

draft-allman-tcpm-bump-initcwnd-00.txt                     November 2010    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    increases.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 reasonablyExpires: May 15, 2011                                           [Page 2]

draft-allman-tcpm-bump-initcwnd-00.txt                     November 2010    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 inSection 2 are and will be safe for general    Internet use the progression can be canceled by making this document    obsolete.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   NoneNormative References    [RFC793] Postel, J., "Transmission Control Protocol", STD 7,RFC793, 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",http://www.ietf.org/proceedings/75/slides/tcpm-1.pdf, July 2009.Expires: May 15, 2011                                           [Page 3]

draft-allman-tcpm-bump-initcwnd-00.txt                     November 2010    [CDCM10] Chu, J., Dukkipati, N., Cheng, Y. and M. Mathis,        "Increasing TCP's Initial Window", Internet-Draftdraft-ietf-tcpm-initcwnd-00.txt (work in progress), October        2010.Author's Addresses   Mark Allman   ICSI   1947 Center Street   Suite 600   Berkeley, CA 94704-1198   Phone: 440-235-1792   EMail: mallman@icir.orghttp://www.icir.org/mallman/Expires: May 15, 2011                                           [Page 4]

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