Internet Engineering Task Force                                M. Allman
INTERNET-DRAFT                                                      ICSI
draft-allman-tcpm-rto-consider-00.txt                      March 4, 2011

                 Retransmission Timeout Considerations

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    This document provides for high-level guidance for retransmission
    timeout schemes appropriate for general use in the Internet.


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

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1   Introduction

    Despite our best intentions and most robust mechanisms, reliability
    in networking ultimately requires a timeout and re-try mechanism.
    Often there are more timely and precise mechanisms (e.g., TCP's
    selective acknowledgment scheme [RFC2018,RFC3517]), but these
    require information exchange between components in the system, which
    cannot be guaranteed.  To the contrary, we can always depend on the
    passage of time and therefore our ultimate backstop to ensuring
    reliability is a timeout.

    Various protocols have defined their own timeout mechanisms (e.g.,
    TCP [RFC2988], SCTP [RFC4960], etc.).  Further, while standardized,
    implementations also add their own subtle tweaks to the process.  At
    this point we recognize that often the specifics are not crucial for
    network safety.  In this document we outline the high-level
    principles that are crucial for any retransmission timeout scheme to
    leverage.  The intent is to then allow implementations of protocols
    and applications instantiate mechanisms that best realize their
    specific goals within this framework.  These specific mechanisms
    could be standardized or ad-hoc, but as long as they adhere to the
    guidelines given in this document they would be consistent with the

2   Guidelines

    We now list the four guidelines that apply when utilizing a
    retransmission timeout (RTO).

    (1) In the absence of any knowledge about the round-trip time (RTT)
        of a path the RTO MUST be conservatively set to no less than 1
        second, per TCP's current default RTO [RFC2988bis].

          [Note: The above assumes [RFC2988bis] becomes the TCP standard
           as it seems to the author is likely to happen given that the
           document is in WGLC and has seen no objections thus far.  If
           it ultimately does not pass the above would be revised to 3
           seconds, per RFC 2988.]

    (2) In steady state the RTO MUST be set based on recent observations
        of both the RTT and the variance of the RTT.  Also, RTT
        observations MUST be taken regularly.  Finally, RTT samples MUST
        NOT be ambiguous (i.e., using Karn's algorithm [KP87,RFC2988]
        retransmitted segments produce ambiguous RTT samples unless they
        explicitly carry a timestamp).

        The exact definition of "regularly" is deliberately left vague.

        TCP takes an RTT sample once per RTT, or if using the timestamp
        option [RFC1323] on each acknowledgment arrival.  [AP99] shows
        that taking an RTT sample from each segment transmitted does not
        improve the performance of TCP's RTO estimator.  However, we are
        aware of no empirical evidence that explores sampling less
        frequently than once per RTT.

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        Therefore, for the purpose of this guideline we state that RTT
        samples SHOULD be taken at least every RTT or as frequently as
        data is exchanged and ACKed if that happens less frequently than
        every RTT.  However, we also recognize that it may not always be
        practical to take an RTT sample this often and so state that RTT
        samples MUST be taken no more than 1 second apart (assuming the
        data rate allows).

    (3) Each time the RTO fires and causes a retransmission the value of
        the RTO MUST be exponentially backed off such that the next
        firing requires a longer interval.  The backoff may be removed
        after a successful transmission.

    (4) Retransmission timeouts MUST be taken as indications of
        congestion in the network and the sending rate adapted using a
        standard mechanism (e.g., TCP collapses the congestion window to
        one segment).

3   Discussion

    We note that research has shown the tension between responsiveness
    and correctness of TCP's RTO seems to be a fundamental tradeoff
    [AP99].  That is, making the RTO more aggressive (via the EWMA
    gains, lowering the minimum RTO, etc.) can reduce the time spent
    waiting on needed RTOs.  However, at the same time such
    aggressiveness leads to more needless RTOs, as well.  Therefore,
    being as aggressive as the guidelines sketched in the last section
    allow in any particular situation may not be the best course of
    action (e.g., because an RTO carries a requirement to slow down).

    While the tradeoff between responsiveness and correctness seems
    fundamental, the tradeoff can be made less relevant if the sender
    can detect and recover from spurious RTOs.  Several mechanisms have
    been proposed for this purpose, such as Eifel [RFC3522], F-RTO
    [RFC5682] and DSACK [RFC2883,RFC3708].  Using such mechanisms may
    allow a data originator to tip towards being more responsive without
    incurring the attendant costs of needless retransmits.

    Also, note, that in addition to the experiments discussed in [AP99],
    the Linux TCP implementation has been using various non-standard RTO
    mechanisms for many years seemingly without large scale problems
    (e.g., using different EWMA gains).  Also, a number of
    implementations use minimum RTOs that are less than the 1 second
    specified in [RFC2988].  While the precise implications of this may
    show more spurious retransmits (per [AP99]) we are aware of no large
    scale problems caused by this change to the minimum RTO.

4   Security Considerations



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    This document benefits from years of discussions with Sally Floyd,
    Shawn Ostermann, Vern Paxson and the members of the TCPM and
    TCP-IMPL working groups.

Normative References

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

Informative References

    [AP99] Allman, M., V. Paxson, "On Estimating End-to-End Network Path
        Properties", Proceedings of the ACM SIGCOMM Technical Symposium,
        September 1999.

    [KP87] Karn, P. and C. Partridge, "Improving Round-Trip Time
        Estimates in Reliable Transport Protocols", SIGCOMM 87.

    [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
        Selective Acknowledgment Options", RFC 2018, October 1996.

    [RFC2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
        Extension to the Selective Acknowledgement (SACK) Option for
        TCP", RFC 2883, July 2000.

    [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
        Timer", RFC 2988, November 2000.

    [RFC2988bis] Paxson, V., M. Allman, H.K. Chu, M. Sargent, "Computing
        TCP's Retransmission Timer", Internet-Draft
        draft-paxson-tcpm-rfc2988bis-01.txt (work in progress), December

    [RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
        Conservative Selective Acknowledgment (SACK)-based Loss Recovery
        Algorithm for TCP", RFC 3517, April 2003.

    [RFC3522] Ludwig, R., M. Meyer, "The Eifel Detection Algorithm for
        TCP", RFC 3522, april 2003.

    [RFC3708] Blanton, E., M. Allman, "Using TCP Duplicate Selective
        Acknowledgement (DSACKs) and Stream Control Transmission
        Protocol (SCTP) Duplicate Transmission Sequence Numbers (TSNs)
        to Detect Spurious Retransmissions", RFC 3708, February 2004.

    [RFC4960] Stweart, R., "Stream Control Transmission Protocol", RFC
        4960, September 2007.

    [RFC5682] Sarolahti, P., M. Kojo, K. Yamamoto, M. Hata, "Forward
        RTO-Recovery (F-RTO): An Algorithm for Detecting Spurious
        Retransmission Timeouts with TCP", RFC 5682, September 2009.

Authors' Addresses

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

   Phone: 440-235-1792

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