Retransmission Timeout Considerations
draft-ietf-tcpm-rto-consider-00

Internet Engineering Task Force                                M. Allman
INTERNET-DRAFT                                                      ICSI
File: draft-ietf-tcpm-rto-consider-00.txt               February 2, 2016
Intended Status: Best Current Practice
Expires: August 2, 2016

    
                 Retransmission Timeout Considerations

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Abstract

    Each implementation of a retransmission timeout mechanism must
    balance correctness and timeliness and therefore no implementation
    suits all situations.  This document provides high-level guidance
    for retransmission timeout schemes appropriate for general use in

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    the Internet.  Within the guidelines, implementations have latitude
    to define particulars that best address each situation.

Terminology

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    document are to be interpreted as described in BCP 14, RFC 2119
    [RFC2119].

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 than a timeout
    for repairing loss (e.g., TCP's fast retransmit [RFC5681], NewReno
    [RFC6582] or selective acknowledgment scheme [RFC2018,RFC6675])
    which require information exchange between components in the system.
    Such communication cannot be guaranteed.  Alternatively, information
    coding---e.g., FEC---can allow the recipient to recover from some
    amount of lost information without use of a retransmission.  This
    latter provides probabilistic reliability.  Finally, negative
    acknowledgment schemes exist that do not depend on continuous
    feedback to trigger retransmissions (e.g., [RFC3940]).  However,
    regardless of these useful alternatives, the only thing we can truly
    depend on is the passage of time and therefore our ultimate backstop
    to ensuring reliability is a timeout.  (Note: There is a case when
    we cannot count on the passage of time, but in this case we believe
    repairing loss will be a moot point and hence we do not further
    consider this case in this document.)

    Various protocols have defined their own timeout mechanisms (e.g.,
    TCP [RFC6298], SCTP [RFC4960]).  Ideally, if we know a segment will
    be lost before reaching the destination, a second copy of it would
    be sent immediately after the first transmission.  However, in
    reality the specifics of retransmission
    timeouts often represent a particular tradeoff between correctness
    and responsiveness [AP99].  In other words we want to
    simultaneously:

      - Wait long enough to ensure the decision to retransmit is
        correct.  

      - Bound the delay we impose on applications before
        retransmitting. 
    
    However, serving both of these goals is difficult as they pull us in
    opposite directions.  I.e., towards either (a) withholding needed
    retransmissions too long or (b) not waiting long enough and sending
    spurious retransmissions.  Given this fundamental tradeoff [AP99],
    we have found that even though the retransmission timeout (RTO)
    procedures are standardized, implementations also often add their
    own subtle imprint on the specifics of the process to tilt the
    tradeoff between correctness and responsiveness in some particular

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    way.

    At this point we recognize that often these specific tweaks are not
    crucial for network safety.  Hence, in this document we outline the
    high-level principles that are crucial for any retransmission
    timeout scheme to follow.  The intent is to then allow
    implementations of protocols and applications to 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 considered consistent with the standards.

    A non-goal of this document is to in any way specify individual
    deviations from standard RTO specifications that any particular
    implementation may exhibit.  Rather, we provide a set of
    over-arching guidelines that all RTO mechanisms should follow.

    Finally, we note the guidelines in this document are applicable to
    any protocol that uses an RTO mechanism.  The examples and
    discussion are framed in terms of TCP, however, that is an artifact
    of where much of our experience with RTOs comes from and should not
    be read as narrowing the scope of the guidelines.

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 latency of a path, the
        RTO MUST be conservatively set to no less than 1 second, per
        TCP's current default RTO [RFC6298].
    
        This guideline ensures two important aspects of the RTO.  First,
        when transmitting into an unknown network, retransmissions will
        not be sent before an ACK would reasonably be expected to arrive
        and hence possibly waste scarce network resources.  Second, as
        noted below, sometimes retransmissions can lead to ambiguities
        in assessing the latency of a network path.  Therefore, it is
        especially important for the first latency sample to be free of
        ambiguities such that there is a baseline for the remainder of
        the communication.

    (2) We specify three guidelines that pertain to the sampling of the
        latency across a path.  

        Often measuring the latency is framed as assessing the
        round-trip time (RTT)---e.g., in TCP's RTO computation
        specification [RFC6298].  This is somewhat mis-leading as the
        latency is better framed as the "feedback time" (FT).  In other
        words, it is not simply a network property, but the length of
        time before we expect an acknowledgment for a given segment.
        For instance, this includes any time an ACK is delayed by the
        recipient [RFC5681].

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        (a) In steady state the RTO MUST be set based on recent
            observations of both the FT and the variance of the FT.

            In other words, the RTO should be based on a reasonable
            amount of time that the sender should wait for an
            acknowledgment of the data before retransmitting the given
            data.
        
        (b) FT observations MUST be taken regularly.  

            The exact definition of "regularly" is deliberately left
            vague.  TCP takes a FT sample roughly once per RTT, or if
            using the timestamp option [RFC7323] on each acknowledgment
            arrival.  [AP99] shows that both these approaches result in
            roughly equivalent performance for the RTO estimator.
            Additionally, [AP99] shows that taking only a single FT
            sample per TCP connection is suboptimal.  Therefore, for the
            purpose of this guideline we state that FT samples SHOULD be
            taken at least once per 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 a FT sample this often in all
            cases and hence this requirement is explicitly a "SHOULD"
            and not a "MUST".
            
        (c) FT samples used in the computation of the RTO MUST NOT be
            ambiguous.  

            Assume two copies of some segment X are transmitted at times
            t0 and t1 and then segment X is acknowledged at time t2.  In
            some cases, it is not clear which copy of X triggered the
            ACK and hence the actual FT is either t2-t1 or t2-t0, but
            which is a mystery.  Therefore, in this situation an
            implementation MUST use Karn's algorithm [KP87,RFC6298] and
            use neither version of the FT sample and hence not update
            the RTO.

            There are cases where two copies of some data are
            transmitted in a way whereby the sender can tell which is
            being acknowledged by an incoming ACK.  E.g., TCP's
            timestamp option [RFC7323] allows for segments to be
            uniquely identified and hence avoid the ambiguity.  In such
            cases there is no ambiguity and the resulting samples can
            update the RTO.

    (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 the successful transmission of non-retransmitted data.

        A maximum value MAY be placed on the RTO provided it is at least
        60 seconds (a la [RFC6298]).

        This ensures network safety.

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    (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 [RFC5681]).

        This ensures network safety.

        An exception is made to this rule if a standard mechanism is
        used to determine that a particular loss is due to a
        non-congestion event (e.g., bit errors or packet reordering).
        In such a case a congestion control action is not required.

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 TCP's RTO more aggressive (via the EWMA
    gains, lowering the minimum RTO, etc.) can reduce the time spent
    waiting on needed retransmissions.  However, at the same time such
    aggressiveness leads to more needless retransmissions, 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 expiration 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 (as much of) 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 [RFC6298].  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.

    Finally, we note that while allowing implementations to be more
    aggressive may in fact increase the number of needless
    retransmissions the above guidelines fail safe in that they insist
    on exponential backoff of the RTO and a transmission rate reduction.
    Therefore, allowing implementers latitude in their instantiations of
    an RTO mechanism does not somehow open the flood gates to aggressive
    behavior.  Since there is a downside to being aggressive the
    incentives for proper behavior are retained in the mechanism.

4   Security Considerations

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    This document does not alter the security properties of 
    retransmission timeout mechanisms.  See [RFC6298] for a discussion
    of these within the context of TCP.

Acknowledgments

    This document benefits from years of discussions with Ethan Blanton,
    Sally Floyd, Shawn Ostermann, Vern Paxson and the members of the
    TCPM and TCP-IMPL working groups.  Ran Atkinson, Yuchung Cheng,
    Jonathan Looney and Michael Scharf provided useful comments on a
    previous version of this draft.

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.
    
    [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.

    [RFC3940] Adamson, B., C. Bormann, M. Handley, J. Macker,
        "Negative-acknowledgment (NACK)-Oriented Reliable Multicast
        (NORM) Protocol", November 2004, RFC 3940.

    [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.

    [RFC6298] Paxson, V., M. Allman, H.K. Chu, M. Sargent, "Computing

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        TCP's Retransmission Timer", June 2011, RFC 6298.

    [RFC6582] Henderson, T., S. Floyd, A. Gurtov, Y. Nishida, "The
        NewReno Modification to TCP's Fast Recovery Algorithm", April
        2012, RFC 6582. 
    
    [RFC6675] Blanton, E., M. Allman, L. Wang, I. Jarvinen, M.  Kojo,
        Y. Nishida, "A Conservative Loss Recovery Algorithm Based on
        Selective Acknowledgment (SACK) for TCP", August 2012, RFC 6675.

    [RFC7323] Borman D., B. Braden, V. Jacobson, R. Scheffenegger, "TCP
        Extensions for High Performance", September 2014, RFC 7323.

Authors' Addresses

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

   EMail: mallman@icir.org
   http://www.icir.org/mallman

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