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Versions: 00 01 02 03 04 05 07                                          
Internet Engineering Task Force                              Mark Allman
INTERNET DRAFT                                              NASA GRC/BBN
File: draft-allman-tcp-early-rexmt-00.txt         Konstantin Avrachenkov
                                                            Urtzi Ayesta
                                                      France Telecom R&D
                                                            Josh Blanton
                                                         Ohio University
                                                          February, 2003
                                                   Expires: August, 2003

                        Early Retransmit for TCP

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 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

    The list of Internet-Draft Shadow Directories can be accessed at


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

1   Introduction

    A number of researchers have pointed out that TCP's loss recovery
    strategies do not work well when the congestion window at a TCP
    sender is small.  This can happen in a number of situations, such

    (1) The TCP connection is "application limited" and has only a
        limited amount of data to send.

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

    (3) The TCP 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 TCP is ascertaining
        the available bandwidth in the first few round-trip times of
        slow start.

    (4) The TCP connection is "winding down" at the end of a transfer
        such that data is draining from the network but no new data
        (from the application) is available to transmit.

    Many researchers have studied problems with TCP when the congestion
    window is small and have outlined possible mechanisms to mitigate
    these problems (e.g., [Mor97,BPS+98,Bal98,LK98,RFC3150,AA02]).  When
    TCP 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 timeout occurs 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 TCP sender waits for three
    duplicate ACKs in an attempt to disambiguate segment loss from
    packet reordering.  Once in a loss recovery phase, a number of
    techniques can be used to retransmit lost segments, including slow
    start based recovery or Fast Recovery [RFC2581], NewReno [RFC2582],
    and loss recovery based on selective acknowledgments (SACKs)

    TCP's retransmission timeout (RTO) is based on measured round-trip
    times (RTT) between the sender and receiver, as specified in
    [RFC2988].  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 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 expires, while only 44% are
    handled by Fast Retransmit.  In addition, only 4% of the RTO-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

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    the connections will be forced to rely on the RTO to recover from
    any losses that occur.  Thus, loss recovery without relying on the
    conservative RTO is beneficial for short TCP transfers.  In
    particular, as a consequence of points (3) and (4) above, a single
    segment loss will require TCP to RTO when a loss occurs in small

    The Limited Transmit mechanism introduced in [RFC3042] allows a TCP
    sender to send previously unsent data upon the reception of each of
    the two duplicate ACKs that precede a fast retransmit. 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.

    The next section of this document outlines a small change to TCP
    senders that will decrease the reliance on the retransmission timer,
    and thereby improve TCP performance when Fast Retransmit would not
    otherwise be triggered.

2   Reduction of the Retransmission Threshold

    Limited Transmit [RFC3042] allows the sender to attempt to induce
    enough duplicate ACKs to trigger Fast Retransmit.  However, in some
    cases the TCP sender may not have new data queued and ready to be
    transmitted or may be limited by the advertised window when the
    first two duplicate ACKs arrive.  In these cases, the Limited
    Transmit algorithm cannot be utilized.  If there is a large amount
    of outstanding data in the network, not being able to transmit new
    segments when the first two duplicate ACKs arrive is not a problem,
    as Fast Retransmit will be triggered naturally.  However, when the
    amount of outstanding data is small the sender will have to rely on
    the RTO to repair any lost segments.

    As an example, consider the case when cwnd is three segments and one
    of these segments is dropped by the network.  If the other two
    segments arrive at the receiver and the corresponding ACKs are not
    dropped by the network the sender will receive two duplicate ACKs,
    which is not enough to trigger the Fast Retransmit algorithm.  The
    loss can therefore be repaired only after an RTO.  However, the
    sender has enough information to infer that it cannot expect three
    duplicate ACKs when one segment is dropped.

    The first mitigation of the above problem involves lowering the
    duplicate ACK threshold when the amount of outstanding data is small
    and when no unsent data segments are enqueued.  In particular, if
    the amount of outstanding data (ownd) is less than 4 segments and
    there are no unsent segments ready for transmission at the sender,
    the duplicate ACK threshold used to trigger Fast Retransmit MAY be
    reduced to ownd-1 duplicate ACKs (where ownd is in terms of
    segments).  In other words, when ownd is small enough that losing
    one segment would not trigger Fast Retransmit, we reduce the
    duplicate ACK threshold to the number of duplicate ACKs expected if

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    one segment is lost.  This mitigation is less robust in the face of
    reordered segments than the standard Fast Retransmit threshold of
    three duplicate ACKs.  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.

    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 TCP 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.  Appendix A offers a survey of possible mitigations.
    However, the authors would like further input before choosing one of
    these options (or, deciding that the worst case scenarios listed
    above are sufficiently rare that Early Retransmit can be used
    without modification).

3   Related Work

    Deployment of Explicit Congestion Notification (ECN) [Flo94,RFC2481]
    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].

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.


    We thank Sally Floyd for her feedback in discussions about Early

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    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 seperate effort.


    [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.

    [AP99] Mark Allman, Vern Paxson. On Estimating End-to-End Network
        Path Properties. ACM SIGCOMM, September 1999.

    [BAFW02] Ethan Blanton, Mark Allman, Kevin Fall, Lili Wang.  A
        Conservative SACK-based Loss Recovery Algorithm for TCP, October
        2002.  Internet-Draft draft-allman-tcp-sack-13.txt (work in

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

    [BPS99] Jon Bennett, Craig Partridge, Nicholas Shectman.  Packet
        Reordering is Not Pathological Network Behavior.  IEEE/ACM
        Transactions on Networking, December 1999.

    [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.

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    [SCWA99] Stefan Savage, Neal Cardwell, David Wetherall, Tom
        Anderson.  TCP Congestion Control with a Misbehaving Receiver.
        ACM Computer Communications Review, October 1999.

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

    [RFC2481] K. K. Ramakrishnan, Sally Floyd.  A Proposal to Add
        Explicit Congestion Notification (ECN) to IP.  RFC 2481, January

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

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

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

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

Author's Addresses:

    Mark Allman
    NASA Glenn Research Center/BBN Technologies
    Lewis Field
    21000 Brookpark Rd.  MS 54-2
    Cleveland, OH  44135
    Phone: 216-433-6586
    Fax: 216-433-8705

    Konstantin Avrachenkov
    2004 route des Lucioles, B.P.93
    06902, Sophia Antipolis

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    Phone: 00 33 492 38 7751
    Email: k.avrachenkov@inria.fr

    Urtzi Ayesta
    France Telecom R&D
    905 rue Albert Einstein
    06921 Sophia Antipolis
    Email: Urtzi.Ayesta@francetelecom.com

    Josh Blanton
    Ohio University
    301 Stocker Center
    Athens, OH  45701

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 use in shared networks.

    One possible mitigation for the damge of spurious retransmits is to
    allow a TCP connection to only send one retransmission using a
    duplicate ACK threshold of less than three.  This allows for
    enhanced recovery for short connections and protects the network
    from longer connections that could possibly use this algorithm to
    send many needless retransmissions.

    Using information provided by the DSACK option [RFC2883], a TCP
    sender can determine when its Fast Retransmit threshold is too low,
    causing needless retransmissions due to reordering in the network.
    Coupling the information provided by DSACKs with the algorithm
    outlined in section 2 may provide a further enhancement.
    Specifically, the proposed reduction in the duplicate ACK threshold
    would not be taken if the network path is known to be reordering

    The next method is to detect needless retransmits based on the time
    between the retransmission and the next ACK received.  As outlined
    in [AP99] if this time is less than half of the minimum RTT observed
    thus far the retransmission was likely unnecessary.  When using less
    than three duplicate ACKs as the threshold to trigger Fast
    Retransmit, a TCP sender could attempt to determine whether the
    retransmission was needed or not.  In the case when it was
    unnecessary, the sender could refrain from further use of Fast
    Retransmit with a threshold of less than three duplicate ACKs.  This
    method of detecting bad retransmits is not as robust as using
    DSACKs.  However, the advantage is that this mechanism only requires

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    sender-side implementation changes.

    A TCP sender can take measures to avoid the case where a large
    percentage of the unique segments transmitted are being needlessly
    retransmitted due to the use of a low duplicate ACK threshold (such
    as the one outlined in section 2).  Specifically, the sender can
    limit the percentage of retransmits based on a duplicate ACK
    threshold of less than three.  This allows the mechanism to be used
    throughout a long lived connection, but at the same time protecting
    the network from potentially wasteful needless retransmissions.
    However, this solution does not attempt to address the underlying
    problem, but rather limits the damage the algorithm can cause.

    Finally, [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 [Bal98].  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).

    Even with the introduction of the Early Retransmit mechanism, the
    loss of the last segment of a transfer will lead to a timeout.  To
    overcome this TCP can send an extra segment at the end of the
    session containing no data.  One may expect this would introduce
    less aditional load than the proposal of [Bal98], but requires more
    research before such a mechanism can be recommended.

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