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Internet Engineering Task Force                              Mark Allman
INTERNET DRAFT                                                      ICIR
File: draft-allman-tcp-early-rexmt-03.txt         Konstantin Avrachenkov
                                                                   INRIA
                                                            Urtzi Ayesta
                                                      France Telecom R&D
                                                            Josh Blanton
                                                         Ohio University
                                                          December, 2003
                                                     Expires: June, 2004


                   Early Retransmit for TCP and SCTP

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of [RFC2026].

    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 at
    http://www.ietf.org/ietf/1id-abstracts.txt

    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html.

Abstract

    This document proposes a new mechanism for TCP and SCTP that can be
    used to more effectively recover lost segments when a connection's
    congestion window is small.  The "Early Retransmit" mechanism allows
    the transport to reduce, in certain special circumstances, the
    number of duplicate acknowledgments required to trigger a fast
    retransmission.  This allows the transport to use fast retransmit to
    recover packet losses that would otherwise require a lengthy
    retransmission timeout.

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 RFC 2119 [RFC2119].

1   Introduction

    A number of researchers have pointed out that the loss recovery

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    strategies employed by TCP [RFC793] and SCTP [RFC2960] do not work
    well when the amount of outstanding data a TCP sender has injected
    into the network is small.  This can happen in a number of
    situations, such as:

    (1) The connection is "application limited" and has only a limited
        amount of data to send.  This can happen any time the
        application does not produce enough data to fill the congestion
        window.  A particular case when all connections become
        application limited is as the connection ends.

    (2) The connection is limited by the receiver-advertised window.

    (3) The connection is constrained by end-to-end congestion control
        when the connection's share of the path is small, the path has a
        small bandwidth-delay product or the transport is ascertaining
        the available bandwidth in the first few round-trip times of
        slow start.

    Many researchers have studied problems with TCP when the congestion
    window is small and have outlined possible mechanisms to mitigate
    these problems [Mor97,BPS+98,Bal98,LK98,RFC3150,AA02].  SCTP's loss
    recovery and congestion control mechanisms are based on TCP and
    therefore the same problems impact the performance of SCTP
    connections.  When the transport detects a missing segment, the
    connection enters a loss recovery phase using one of two methods.
    First, if an acknowledgment (ACK) for a given segment is not
    received in a certain amount of time a retransmission timer fires
    and the segment is resent [RFC2988].  Second, the ``Fast
    Retransmit'' algorithm resends a segment when three duplicate ACKs
    arrive at the sender [Jac88,RFC2581].  However, because duplicate
    ACKs from the receiver are also triggered by packet reordering in
    the Internet, the sender waits for three duplicate ACKs in an
    attempt to disambiguate segment loss from packet reordering.  When
    using small windows it may not be possible to generate the required
    number of duplicate ACKs to trigger Fast Retransmit when a loss does
    happen.

    Once in a loss recovery phase, a number of techniques can be used to
    retransmit lost segments.  TCP can use slow start based recovery or
    Fast Recovery [RFC2581], NewReno [RFC2582], and loss recovery based
    on selective acknowledgments (SACKs) [RFC2018,FF96,RFC3517].  SCTP's
    loss recovery is not as varied due to the built-in selective
    acknowledgments.

    The transport's retransmission timeout (RTO) is based on measured
    round-trip times (RTT) between the sender and receiver, as specified
    in [RFC2988] (for TCP) and [RFC2960] (for SCTP).  To prevent
    spurious retransmissions of segments that are only delayed and not
    lost, the minimum RTO is conservatively chosen to be 1 second.
    Therefore, it behooves TCP senders to detect and recover from as
    many losses as possible without incurring a lengthy timeout during
    which the connection remains idle.  However, if not enough duplicate
    ACKs arrive from the receiver, the Fast Retransmit algorithm is

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    never triggered---this situation occurs when the congestion window
    is small, if a large number of segments in a window are lost or at
    the end of a transfer as data drains from the network.  For
    instance, consider a congestion window (cwnd) of three segments.  If
    one segment is dropped by the network, then at most two duplicate
    ACKs will arrive at the sender, assuming no ACK loss.  Since three
    duplicate ACKs are required to trigger Fast Retransmit, a timeout
    will be required to resend the dropped packet.

    [BPS+98] shows that roughly 56% of retransmissions sent by a busy
    web server are sent after the RTO timer expires, while only 44% are
    handled by Fast Retransmit.  In addition, only 4% of the RTO
    timer-based retransmissions could have been avoided with SACK, which
    has to continue to disambiguate reordering from genuine
    loss.  Furthermore, [All00] shows that for one particular web server
    the median transfer size is less than four segments, indicating that
    more than half of the connections will be forced to rely on the RTO
    timer to recover from any losses that occur.  Thus, loss recovery
    without relying on the conservative RTO is beneficial for short TCP
    transfers.

    The Limited Transmit mechanism introduced in [RFC3042] allows a TCP
    sender to transmit previously unsent data upon the reception of each
    of the two duplicate ACKs that precede a fast retransmit.  SCTP
    [RFC2960] uses SACK information to calculate the number of
    outstanding segments in the network.  Hence, when the first two
    duplicate ACKs arrive at the sender they will indicate that data has
    left the network and allow the sender to transmit new data (if
    available) similar to TCP's Limited Transmit algorithm.

    By sending these two new segments the TCP sender is attempting to
    induce additional duplicate ACKs (if appropriate) so that Fast
    Retransmit will be triggered before the retransmission timeout
    expires.  The "Early Retransmit" mechanism outlined in this document
    covers the case when previously unsent data is not available for
    transmission.

    Section 2 of this document outlines a small change to TCP and SCTP
    senders that will decrease the reliance on the retransmission timer,
    and thereby improve performance when Fast Retransmit cannot
    otherwise be triggered.  Section 3 discusses related work.  Section
    4 sketches security issues.

2   Reduction of the Retransmission Threshold

    The Early Retransmit algorithm calls for lowering the threshold for
    triggering Fast Retransmit when the amount of outstanding data is
    small and when no unsent data segments are enqueued.  We define
    variants of Early Retransmit for connections that do and do not
    support selective acknowledgments (SACK) [RFC2018].  (Note: SCTP
    includes SACK in the base protocol and so there is no need for the
    non-SACK variant of Early Retransmit in SCTP.)

    If the following two conditions hold the sender can use Early

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    Retransmit (regardless of SACK support).

    (2.a) The amount of outstanding data (ownd) is less than 4*SMSS
        bytes.

    (2.b) There is either no unsent data ready for transmission at the
        sender or the advertised window does not permit new segments to
        be transmitted.

    When the above two conditions hold and the connection does not
    support SACK the duplicate ACK threshold used to trigger Fast
    Retransmit MAY be reduced to:

                  ER_thresh = ceiling (ownd/SMSS) - 1                 (1)

    duplicate ACKs, where ownd is in terms of bytes.

    When conditions (2.a) and (2.b) hold and the connection does support
    SACK Fast Retransmit MAY be used when ownd - SMSS bytes have been
    SACKed.

    In other words, when ownd is small enough that losing one segment
    would not trigger Fast Retransmit, the trigger for Fast Retransmit
    is reduced to receiving indications that all but one segment has
    arrived at the receiver.  This mitigation is less robust in the face
    of reordered segments than the standard Fast Retransmit threshold.
    Research shows that a general reduction in the number of duplicate
    ACKs required to trigger fast retransmission of a segment to two
    (rather than three) leads to a reduction in the ratio of good to bad
    retransmits by a factor of three [Pax97].  However, this analysis
    did not include the additional conditioning on the event that the
    ownd was smaller than 4 segments.

    The SACK variant of the Early Retransmit algorithm is preferred to
    the non-SACK variant due to its robustness in the face of ACK loss
    (since SACKs are sent redundantly) and due to interactions with the
    delayed ACK timer.  Consider a flight of three segments, S1...S3,
    with S2 being dropped by the network.  When S1 arrives it is
    in-order and so the receiver may or may not delay the ACK, leading
    to two scenarios:

    (A) The ACK for S1 is delayed.  In this case the arrival of S3 will
        trigger an ACK to be transmitted covering segment S1 (which was
        previously unacknowledged).  In this case Early Retransmit
        without SACK will not prevent an RTO because no duplicate ACKs
        will arrive.  However, with SACK the ACK for S1 will also
        include SACK information indicating that S3 has arrived at the
        receiver.  The sender can then invoke Fast Retransmit on this
        ACK because ownd - SMSS bytes have been SACKed when the ACK
        arrives.

    (B) The ACK for S1 is not delayed.  In this case the arrival of S1
        triggers an ACK and the arrival of S3 triggers a second ACK
        (because it is out-of-order).  Both ACKs will cover the same

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        segment (S1).  Therefore, regardless of whether SACK is used
        Early Retransmit can be performed by the sender (assuming no ACK
        loss).

    We note two "worst case" scenarios for Early Retransmit:

    (1) Persistent reordering of segments, coupled with an application
        that does not constantly send data, can result in large numbers
        of needless retransmissions when using Early Retransmit.  For
        instance, consider an application that sends data two segments
        at a time, followed by an idle period when no data is queued for
        delivery by TCP.  If the network consistently reorders the two
        segments, the sender will needlessly retransmit one out of every
        two unique segments transmitted (and one-third of all segments)
        when using the above algorithm.  However, this would only be a
        problem for long-lived connections from applications that
        transmit in spurts.

    (2) Similar to the above, consider the case of 2 segment transfers
        that always experience reordering.  Just as in (1) above, one
        out of every two unique data segments will be retransmitted
        needlessly, therefore one-third of the traffic will be spurious.

    Currently this document offers no suggestion on how to mitigate the
    above problems.  Rather, the authors believe that the community's
    consensus is that Early Retransmit is scoped enough that the worst
    case problems are pathological and do not need mitigation at this
    time.  However, Appendix A offers a survey of possible mitigations.

3   Related Work

    Deployment of Explicit Congestion Notification (ECN) [Flo94,RFC3168]
    may benefit connections with small congestion window sizes
    [RFC2884].  ECN provides a method for indicating congestion to the
    end-host without dropping segments.  While some segment drops may
    still occur, ECN may allow TCP to perform better with small cwnd
    sizes because the sender will be required to detect less segment
    loss [RFC2884].

    [Bal98] outlines another solution to the problem of having no new
    segments to transmit into the network when the first two duplicate
    ACKs arrive.  In response to these duplicate ACKs, a TCP sender
    transmits zero-byte segments to induce additional duplicate ACKs.
    This method preserves the robustness of the standard Fast Retransmit
    algorithm at the cost of injecting segments into the network that do
    not deliver any data (and, therefore are potentially wasting network
    resources).

4   Security Considerations

    The security considerations found in [RFC2581] apply to this
    document.  No additional security problems have been identified with
    Early Retransmit at this time.


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Acknowledgments

    We thank Sally Floyd for her feedback in discussions about Early
    Retransmit.  We also thank Sally Floyd and Hari Balakrishnan who
    helped with a large portion of the text of this document when it was
    part of a separate document.  Armando Caro and many members of the
    tsvwg mailing list provided good discussions that helped shape this
    document.

Normative References

    [RFC793] Jon Postel.  Transmission Control Protocol.  Std 7, RFC
        793.  September 1981.

    [RFC2018] Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn Romanow.
        TCP Selective Acknowledgement Options.  RFC 2018, October 1996.

    [RFC2581] Mark Allman, Vern Paxson, W. Richard Stevens.  TCP
        Congestion Control.  RFC 2581, April 1999.

    [RFC2883] Sally Floyd, Jamshid Mahdavi, Matt Mathis, Matt Podolsky.
        An Extension to the Selective Acknowledgement (SACK) Option for
        TCP.  RFC 2883, July 2000.

    [RFC2960] R. Stewart, Q. Xie, K. Morneault, C. Sharp, H.
        Schwarzbauer, T. Taylor, I. Rytina, M. Kalla, L. Zhang, V.
        Paxson.  Stream Control Transmission Protocol.  October 2000.

    [RFC2988] Vern Paxson, Mark Allman. Computing TCP's Retransmission
        Timer.  RFC 2988, April 2000.

    [RFC3042] Mark Allman, Hari Balakrishnan, Sally Floyd.  Enhancing
        TCP's Loss Recovery Using Limited Transmit.  RFC 3042, January
        2001.

    [RFC3522] Reiner Ludwig, Michael Meyer.  The Eifel Detection
        Algorithm for TCP.  RFC 3522, April 2003.

Informative References

    [AA02] Urtzi Ayesta, Konstantin Avrachenkov, "The Effect of the
        Initial Window Size and Limited Transmit Algorithm on the
        Transient Behavior of TCP Transfers", In Proc. of the 15th ITC
        Internet Specialist Seminar, Wurzburg, July 2002.

    [All00] Mark Allman.  A Server-Side View of WWW Characteristics.
        ACM Computer Communications Review, October 2000.

    [Bal98] Hari Balakrishnan.  Challenges to Reliable Data Transport
        over Heterogeneous Wireless Networks.  Ph.D. Thesis, University
        of California at Berkeley, August 1998.

    [BPS+98] Hari Balakrishnan, Venkata Padmanabhan, Srinivasan Seshan,
        Mark Stemm, and Randy Katz.  TCP Behavior of a Busy Web Server:

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        Analysis and Improvements.  Proc. IEEE INFOCOM Conf., San
        Francisco, CA, March 1998.

    [FF96] Kevin Fall, Sally Floyd.  Simulation-based Comparisons of
        Tahoe, Reno, and SACK TCP.  ACM Computer Communication Review,
        July 1996.

    [Flo94] Sally Floyd.  TCP and Explicit Congestion Notification.  ACM
        Computer Communication Review, October 1994.

    [Jac88] Van Jacobson.  Congestion Avoidance and Control.  ACM
        SIGCOMM 1988.

    [LK98] Dong Lin, H.T. Kung.  TCP Fast Recovery Strategies: Analysis
        and Improvements.  Proceedings of InfoCom, March 1998.

    [Mor97] Robert Morris.  TCP Behavior with Many Flows.  Proceedings
        of the Fifth IEEE International Conference on Network Protocols.
        October 1997.

    [Pax97] Vern Paxson.  End-to-End Internet Packet Dynamics.  ACM
        SIGCOMM, September 1997.

    [RFC2582] Sally Floyd, Tom Henderson.  The NewReno Modification to
        TCP's Fast Recovery Algorithm.  RFC 2582, April 1999.

    [RFC2884] Jamal Hadi Salim and Uvaiz Ahmed. Performance Evaluation
        of Explicit Congestion Notification (ECN) in IP Networks.  RFC
        2884, July 2000.

    [RFC3150] Spencer Dawkins, Gabriel Montenegro, Markku Kojo, Vincent
        Magret.  End-to-end Performance Implications of Slow Links.  RFC
        3150, July 2001.

    [RFC3168] K. K. Ramakrishnan, Sally Floyd, David Black.  The
        Addition of Explicit Congestion Notification (ECN) to IP.  RFC
        3168, September 2001.

    [RFC3517] Ethan Blanton, Mark Allman, Kevin Fall, Lili Wang.  A
        Conservative Selective Acknowledgment (SACK)-based Loss Recovery
        Algorithm for TCP.  RFC 3517, April 2003.

Author's Addresses:

    Mark Allman
    ICSI Center for Internet Research (ICIR)
    1947 Center Street, Suite 600
    Berkeley, CA 94704-1198
    Phone: 216-243-7361
    mallman@icir.org
    http://www.icir.org/mallman/

    Konstantin Avrachenkov
    INRIA

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    2004 route des Lucioles, B.P.93
    06902, Sophia Antipolis
    France
    Phone: 00 33 492 38 7751
    Email: k.avrachenkov@sophia.inria.fr
    http://www.inria.fr/mistral/personnel/K.Avrachenkov/moi.html

    Urtzi Ayesta
    France Telecom R&D
    905 rue Albert Einstein
    06921 Sophia Antipolis
    France
    Email: Urtzi.Ayesta@francetelecom.com
    http://www.inria.fr/mistral/personnel/Urtzi.Ayesta/me.html

    Josh Blanton
    Ohio University
    301 Stocker Center
    Athens, OH  45701
    jblanton@irg.cs.ohiou.edu

Appendix A: Research Issues in Adjusting the Duplicate ACK Threshold

    Decreasing the number of duplicate ACKs required to trigger Fast
    Retransmit, as suggested in section 2, has the drawback of making
    Fast Retransmit less robust in the face of minor network reordering.
    Two egregious examples of problems caused by reordering are given in
    section 2.  This appendix outlines several schemes that have been
    suggested to mitigate the problems caused to Early Retransmit by
    reordering.  These methods need further research before they are
    suggested for general use (and, current consensus is that the cases
    that make Early Retransmit unnecessarily retransmit a large amount
    of data are patalogical and therefore these mitigations are not
    generally required).

    MITIGATION A.1: Allow a connection to use Early Retransmit as long
    as the algorithm is not injecting a "too much" spurious data into
    the network.  For instance, using the information provided by TCP's
    DSACK option [RFC2883] or SCTP's Duplicate-TSN notification, a
    sender can determine when segments sent via Early Retransmit are
    needless.  Likewise, using Eifel [RFC3522] the sender can detect
    spurious Early Retransmits.  Once spurious Early Retransmits are
    detected the sender can either eliminate the use of Early Retransmit
    or limit the use of the algorithm to ensure that an acceptably small
    fraction of the connection's transmissions are not spurious.

    Alternatively, if a sender cannot reliably determine if an Early
    Retransmitted segment is spurious or not the sender could simply
    limit Early Retransmits either to some fixed number per connection
    (e.g., Early Retransmit is allowed only once per connection) or to
    some small percentage of the total traffic being transmitted.

    MITIGATION A.2: Allow a connection to trigger Early Retransmit using
    the criteria given in section 2, in addition to a "small" timeout

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    [Pax97].  For instance, a sender may have to wait for 2 duplicate
    ACKs and then T msec before Early Retransmitting a segment.  The
    added time gives reordered acknowledgments time to arrive at the
    sender and avoid a needless retransmit.  Designing a method for
    choosing an appropriate timeout is part of the research that would
    need to be involved in this scheme.

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