Internet DRAFT - draft-allman-tcpm-no-initwin


Internet Engineering Task Force                                M. Allman
INTERNET-DRAFT                                                      ICSI
File: draft-allman-tcpm-no-initwin-00.txt              November 24, 2015
Intended Status: Best Current Practice
Expires: May 24, 2016

                Removing TCP's Initial Congestion Window

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    This specification removes the specification of TCP's initial
    congestion window.


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    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    document are to be interpreted as described in BCP 14, RFC 2119

1   Introduction

    TCP connections may choose the initial value of the congestion
    window (cwnd) and are not beholden to [RFC6928] or previous
    specifications of the initial cwnd, provided that:

    (1) The 3WHS MUST complete without a retransmission.

    (2) For initial windows of more than 10 segments, the initial
        window of segments MUST be paced evenly across the first
        round-trip time (as measured during the 3WHS).

    (3) Since the initial cwnd has no relationship to the available
        capacity of the network path, in the case of loss within the
        initial window of segments sent, the cwnd MUST be set to
        SMSS * ((IW - R) / 2) instead of simply halving the cwnd.  Here,
        the IW is the size of the initial cwnd (in segments) and R is
        the number of retransmitted segments within the initial
        transmission window.

    (4) The initial cwnd MUST be bounded by the receiver's advertised

2   Reasoning    

    The reasoning behind this proposal is mostly taken from [LAJW07].

    (a) The author thinks that talking about the initial window for the
        better part of two decades is probably enough.  And, definitely 

    (b) Traffic is heavy tailed and most TCP connections cannot use an
        overly large IW as they are short. 

    (c) An overly aggressive IW is likely to congestion local networks
        before burdening remote portions of the path.

    (d) Routers should be using Active Queue Management [RFC2309] to
        protect from overly aggressive flows.

    (e) Receivers cannot be overrun as they can exercise control via the
        advertised window.

    (f) TCP's congestion control algorithms remain in force and
        therefore even if a sender transmits too aggressively, this
        aggression will not be a prolonged event.

    (g) Ultimately, being egregiously overly aggressive will not be in 

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        the sender's best interest---e.g., there will be a fight for
        local resources among the sender's own connections---and
        therefore there is an incentive to be reasonable.

3   Security Considerations

    A large IW allows TCP to send a large burst of traffic, but an
    attacker that can tune a TCP to do this can also simply send a large
    amount of traffic.

Normative References
    [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
        Requirement Levels", BCP 14, RFC 2119, March 1997.

    [RFC6928] J. Chu, N. Dukkipati, Y. Cheng, M. Mathis.  Increasing
        TCP's Initial Window, RFC 6928, April 2013.
Informative References

    [LAJW07] Dan Liu, Mark Allman, Shudong Jin, Limin Wang.  Congestion
        Control Without a Startup Phase.  Workshop on Protocols for Fast
        Long-Distance Networks (PFLDnet), February 2007.
    [RFC2309] B. Braden, D. Clark, J. Crowcroft, B. Davie, S. Deering,
        D. Estrin, S. Floyd, V. Jacobson, G. Minshall, C. Partridge,
        L. Peterson, K. Ramakrishnan, S. Shenker, J. Wroclawski,
        L. Zhang.  Recommendations on Queue Management and Congestion
        Avoidance in the Internet, RFC 2309, April 1998.

Authors' Addresses

   Mark Allman
   International Computer Science Institute
   1947 Center St.  Suite 600
   Berkeley, CA  94704


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