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Internet Engineering Task Force                              Mark Allman
INTERNET DRAFT                                              NASA GRC/BBN
File: draft-allman-tcp-lossrec-00.txt                  Hari Balakrishnan
                                                                     MIT
                                                             Sally Floyd
                                                                   ACIRI
                                                              June, 2000
                                                 Expires: December, 2000


                 Enhancing TCP's Loss Recovery Using
               Early Duplicate Acknowledgment Response

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 two new TCP mechanisms that can be used to
    more effectively recover lost segments when a connection's
    congestion window is small, or when a large number of segments are
    lost in a single transmission window.  The first of these
    mechanisms, ``Limited Transmit'', calls for sending a new data
    segment in response to each of the first two duplicate
    acknowledgments that arrive at the sender.  The second mechanism is
    to reduce, in certain special circumstances, the number of duplicate
    acknowledgments required to trigger a fast retransmission.

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, for instance, because there is
    only a limited amount of data to send, or because of the limit
    imposed by the receiver-advertised window, or because of the
    constraints imposed by end-to-end congestion control over a

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    connection with a small bandwidth-delay product
    [Mor97,BPS+98,Bal98,LK98,DMKM00].  When it detects a missing
    segment, TCP 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 [PA00].  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) [RFC2018,FF96].

    TCP's retransmission timeout (RTO) is based on measured round-trip
    times (RTT) between the sender and receiver, as specified in [PA00].
    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
    when 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 or if a large number of segments in a window are lost.  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 of course
    has to continue to disambiguate reordering from genuine loss.  In
    contrast, using the techniques outlined in this document and in
    [Bal98], 25% of the RTO-based retransmissions in that dataset would
    have likely been avoided.  In addition, [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 to recover from any losses that occur.

    The next two sections of this document outline small changes to TCP
    senders that will decrease the reliance on the retransmission timer,
    and thereby improve TCP performance when Fast Retransmit is not
    triggered.  These changes do not adversely affect the performance of
    TCP nor interact adversely with other connections, in other
    circumstances.



2   Modified Response to Duplicate ACKs

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    When a TCP sender has previously unsent data queued for
    transmission, new segments SHOULD use the Limited Transmit
    algorithm, which calls for a TCP sender to transmit new data upon
    the arrival of a duplicate ACK when the following conditions are
    satisfied:

      * The receiver's advertised window allows the transmission of the
        segment.

      * The amount of outstanding data would remain less than the
        congestion window plus the duplicate ACK threshold used to
        trigger Fast Retransmit.  In other words, the sender can only
        send two segments beyond the congestion window (cwnd).

    The congestion window (cwnd) MUST NOT be changed when these new
    segments are transmitted.  Assuming that these new segments and the
    corresponding ACKs are not dropped, this procedure allows the sender
    to infer loss using the standard Fast Retransmit threshold of three
    duplicate ACKs [RFC2581].  This is more robust to reordered packets
    than it would be to retransmit an old packet on the first or second
    duplicate ACK.

    Note: If the connection is using selective acknowledgments
    [RFC2018], the data sender MUST NOT send new segments in response to
    duplicate ACKs that contain no new SACK information, as a
    misbehaving receiver can generate such ACKs to trigger inappropriate
    transmission of data segments.  See [SCWA99] for a discussion of
    attacks by misbehaving receivers.

    Using Limited Transmit follows the ``conservation of packets''
    congestion control principle [Jac88].  Each of the first two
    duplicate ACKs indicate that a segment has left the network.
    Furthermore, the sender has not yet decided that a segment has been
    dropped and therefore has no reason to assume the current congestion
    control state is not accurate.  Therefore, transmitting segments
    does not deviate from the spirit of TCP's congestion control
    principles.

    [BPS99] shows that packet reordering is not a rare network event.
    [RFC2581] does not provide for sending of data on the first two
    duplicate ACKs that arrive at the sender.  This causes a burst of
    segments to be sent when an ACK for new data does arrive.  Using
    Limited Transmit, data packets will be clocked out by incoming ACKs
    and therefore transmission will not be as bursty.

    Note: Limited Transmit is implemented in the ns simulator.
    Researchers wishing to investigate this mechanism further can do so
    by enabling ``singledup_'' for the given TCP connection.




3   Reduction of the Retransmission Threshold

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    In some cases the TCP sender may not have new data queued and ready
    to be transmitted when the first two duplicate ACKs arrive.  In this
    case, the Limited Transmit algorithm outlined in section 2 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 cwnd is small and when no unsent data
    segments are enqueued.  In particular, if cwnd is less than 4
    segments and there are no unsent segments at the sender, the
    duplicate ACK threshold used to trigger Fast Retransmit is reduced
    to cwnd-1 duplicate ACKs (where cwnd is in terms of segments).  In
    other words, when cwnd 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 one segment is lost.
    This mitigation is clearly 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 cwnd was smaller than 4 segments.

    Note that persistent reordering of segments, coupled with an
    application that does not constantly send data, can result in large
    numbers of retransmissions.  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 when using the above algorithm.  However, this would
    only be a problem for long-lived connections from applications that
    transmit in spurts.

    To combat this problem, a TCP connection SHOULD 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.  We note that

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    future research may allow this restriction to be relaxed, and refer
    the reader to Appendix A for a discussion of some alternate
    mechanisms.  While not explicitly recommended by this document, we
    believe that these may prove useful depending on the results of
    further research.

4   Related Work

    Deployment of Explicit Congestion Notification (ECN) [Flo94,RFC2481]
    may benefit connections with small congestion window sizes [SA00].
    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
    [SA00].

5   Security Considerations

    The security implications of the changes proposed in this document
    are minimal.  The potential security issues come from the subversion
    of end-to-end congestion control from "false" duplicate ACKs, where
    a "false" duplicate ACK is a duplicate ACK that does not actually
    acknowledge new data received at the TCP receiver.  False duplicate
    ACKs could result from duplicate ACKs that are themselves duplicated
    in the network, or from misbehaving TCP receivers that send false
    duplicate ACKs to subvert end-to-end congestion control
    [SCWA99,RFC2581].

    When the TCP data receiver has agreed to use the SACK option, the
    TCP data sender has fairly strong protection against false duplicate
    ACKs.  In particular, with SACK, a duplicate ACK that acknowledges
    new data arriving at the receiver reports the sequence numbers of
    that new data.  Thus, with SACK, the TCP sender can verify that an
    arriving duplicate ACK acknowledges data that the TCP sender has
    actually sent, and for which no previous acknowledgment has been
    received, before sending new data as a result of that
    acknowledgment.  For further protection, the TCP sender could keep a
    record of packet boundaries for transmitted data packets, and
    recognize at most one valid acknowledgment for each packet (e.g.,
    the first acknowledgment acknowledging the receipt of all of the
    sequence numbers in that packet).

    One could imagine some limited protection against false duplicate
    ACKs for a non-SACK TCP connection, where the TCP sender keeps a
    record of the number of packets transmitted, and recognizes at most
    one acknowledgment per packet to be used for triggering the sending
    of new data.  However, this accounting of packets transmitted and
    acknowledged would require additional state and extra complexity at
    the TCP sender, and does not seem necessary.

    The most important protection against false duplicate ACKs comes
    from the limited potential of duplicate ACKs in subverting
    end-to-end congestion control.  There are two separate cases to
    consider, when the TCP sender receives less than a threshold number

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    of duplicate ACKs, and when the TCP sender receives at least a
    threshold number of duplicate ACKs.

    First we consider the case when the TCP sender receives less than a
    threshold number of duplicate ACKs.  For example, the TCP receiver
    could send two duplicate ACKs after each regular ACK.  One might
    imagine that the TCP sender would send at three times its allowed
    sending rate.  However, using Limited Transmit as outlined in
    section 2 the sender is only allowed to exceed the congestion window
    by less than the duplicate ACK threshold, and thus would not send a
    new packet for each duplicate ACK received.

    We next consider the case when the TCP sender receives at least the
    threshold number of duplicate ACKs.  This is an increased
    possibility with the reduction of the duplicate ACK threshold for
    the special case proposed in Section 3.  However, in addition to
    retransmitting a packet when a threshold number of duplicate ACKs is
    received, the TCP sender also halves its congestion window, thus
    reinforcing the role of end-to-end congestion control.  If the
    retransmitted packet is itself dropped, then it will only be
    retransmitted again after the retransmit timer expires.  Thus, the
    potential drawback of a reduced threshold is not one of congestion
    collapse for the network.  Instead, the potential drawback would be
    that of a single unnecessary retransmission, and an accompanying
    unnecessary reduction of the congestion window, for the TCP
    connection itself.  This is not a security consideration, but a
    performance consideration for the TCP connection itself.  We note
    that the reduced threshold would only apply when the TCP sender does
    not have additional data ready to transmit, so the performance
    penalty would be small.

References

    [All00] Mark Allman.  A Server-Side View of WWW Characteristics.
        May, 2000.  In preparation.

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

    [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.


    [DMKM00] Spencer Dawkins, Gabriel Montenegro, Markku Kojo, Vincent

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        Magret.  End-to-end Performance Implications of Slow Links,
        Internet-Draft draft-ietf-pilc-slow-03.txt, March 2000 (work in
        progress).

    [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.

    [FMM+99] Sally Floyd, Jamshid Mahdavi, Matt Mathis, Matt Podolsky,
        Allyn Romanow, An Extension to the Selective Acknowledgement
        (SACK) Option for TCP, Internet-Draft draft-floyd-sack-00.txt,
        August 1999.

    [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.

    [PA00] Vern Paxson, Mark Allman. Computing TCP's Retransmission
        Timer, April 2000. Internet-Draft draft-paxson-tcp-rto-01.txt
        (work in progress).

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

    [SA00] Jamal Hadi Salim and Uvaiz Ahmed, Performance Evaluation of
        Explicit Congestion Notification (ECN) in IP Networks,
        draft-hadi-jhsua-ecnperf-01.txt, March 2000 (work in progress).

    [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
        1999.

    [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.


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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
    mallman@grc.nasa.gov
    http://roland.grc.nasa.gov/~mallman

    Hari Balakrishnan
    Laboratory for Computer Science
    545 Technology Square
    Massachusetts Institute of Technology
    Cambridge, MA 02139
    hari@lcs.mit.edu
    http://nms.lcs.mit.edu/~hari/

    Sally Floyd
    AT&T Center for Internet Research at ICSI (ACIRI)
    Phone: +1 (510) 666-2989
    floyd@aciri.org
    http://www.aciri.org/floyd/

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 3, has the drawback of making
    Fast Retransmit less robust in the face of minor network reordering.
    As outlined in section 3, this document only allows a TCP to use
    Fast Retransmit one time when the number of duplicate ACKs is less
    than three.  This appendix suggests several methods by which this
    restriction may be removed.  However, these methods need further
    research before they are suggested for use in shared networks.

    Using information provided by the DSACK option [FMM+99], 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 3 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
    segments.

    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

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    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
    sender-side implementation changes.

    A TCP sender can take measures to avoid a 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 3).  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 just limit 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).































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