Internet Engineering Task Force                              Mark Allman
INTERNET DRAFT                                                      ICSI
File: draft-ietf-tcpm-early-rexmt-00.txt draft-ietf-tcpm-early-rexmt-01.txt          Konstantin Avrachenkov
                                                            Urtzi Ayesta
                                                            Josh Blanton
                                                         Ohio University
                                                              Per Hurtig
                                                     Karlstad University
                                                             August 2008
                                                  Expires: February
                                                            January 2009
                   Early Retransmit for TCP and SCTP

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    This document proposes a new mechanism for TCP and SCTP that can be
    used to 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.


    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    document are to be interpreted as described in RFC 2119 [RFC2119].

1   Introduction

    Many researchers have studied problems with TCP [RFC793,RFC2581]
    when the congestion window is small and have outlined possible
    mechanisms to mitigate these problems
    [Mor97,BPS+98,Bal98,LK98,RFC3150,AA02].  SCTP's [RFC4960] 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.  There are several variants
    of the loss recovery phase depending on the TCP implemention.  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

    All the above variants have two methods for invoking loss recovery.
    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,RFC4960].  Second, the ``Fast
    Retransmit'' algorithm resends a segment when three duplicate ACKs
    arrive at the sender [Jac88,RFC2581].  Duplicate ACKs are triggered
    by out-of-order arrivals at the receiver.  However, because
    duplicate ACKs from the receiver are triggered by both packet loss
    and packet reordering in the network path, the sender waits for
    three duplicate ACKs in an attempt to disambiguate packet loss from
    packet reordering.  When using small congestion windows it may not
    be possible to generate the required number of duplicate ACKs to
    trigger Fast Retransmit when a loss does happen.

    Small windows can occur in a number of situations, such as:

    (1) 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.

    (2) 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.

    (3) The connection is limited by the receiver's advertised window.

    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 [RFC4960] (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
    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. sender.  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
    that does not rely on the conservative RTO is likely to be
    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
    [RFC4960] 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.  In the
    remainder of this document we use "Limited Transmit" to include both
    TCP and SCTP mechanisms for sending in response to the first two
    duplicate ACKs.  By sending these two new segments the 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 or cannot be transmitted due to an
    advertised window limitation.

2   Early Retransmit Algorithm
    The Early Retransmit algorithm calls for lowering the threshold for
    triggering Fast Retransmit when the amount of outstanding data is
    small and when no previously unsent data can be transmitted (such
    that Limited Transmit could be used).  Duplicate ACKs are triggered
    by each arriving out-of-order segment.  Therefore, Fast Retransmit
    will not be invoked when there are less than four outstanding
    segments (assuming only one segment loss in the window).  However,
    TCP and SCTP are not required to track the number of outstanding
    segments, but rather the number of outstanding bytes or messages.
    Therefore, applying the intuitive notion of a transport with less
    than four segments outstanding is more complicated than it first
    appears.  In section 2.1 we describe a "byte-based" variant of Early
    Retransmit that attempts to roughly map the number of outstanding
    bytes to a number of outstanding packets that is then used when
    deciding whether to trigger Early Retransmit.  In section 2.2 we
    describe a "packet-based" variant that represents a more precise
    algorithm for triggering Early Retransmit.  The precision comes at
    the cost of requiring additional state to be kept by the TCP sender.
    In both cases we described describe SACK-based and non-SACK-based versions of
    the scheme (of course, the non-SACK version will not apply to SCTP).

2.1 Byte-based Early Retransmit

    A TCP or SCTP sender MAY use byte-based Early Retransmit.

    A sender employing byte-based Early Retransmit MUST use the
    following two conditions to determine when an Early Retransmit is

    (2.a) The amount of outstanding data (ownd)---data sent but not yet
          acknowledged---is less than 4*SMSS bytes.

          (Note that in the byte-based variant of Early Retransmit
          'ownd' is equivalent to 'FlightSize' defined in [RFC2581].  We
          use different notation because 'ownd' is not consistent with
          FlightSize through this document.)

    (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 a
    retransmission MUST 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, Early Retransmit MUST be used only when "ownd - SMSS" bytes
    have been SACKed.

    When conditions (2.a) and (2.b) do not hold, the transport MUST NOT
    use Early Retransmit, but rather prefer the standard mechanisms,
    including Limited Transmit.

    As noted above, the drawback of this byte-based variant is precision
    [HB08].  We illustrate this with two examples:

      + Consider a non-SACK TCP sender that uses an SMSS of 1460 bytes
        and transmits three segments each with 400 bytes of payload.
        This is a case where Early Retransmit could aid loss recovery if
        one segment is lost.  However, in this case ER_thresh will
        become zero, per equation (1), because the number of outstanding
        bytes is a poor estimate of the number of outstanding packets.
        A similar problem occurs for senders that employ SACK as the
        expression "ownd - SMSS" will become negative.

      + Next, consider a non-SACK TCP sender that uses an SMSS of 1460
        bytes and transmits 10 segments each with 400 bytes of payload.
        In this case ER_thresh will be two, per equation (1).  Thus,
        even though there are enough segments outstanding to trigger
        Fast Retransmit with the standard duplicate ACK threshold Early
        Retransmit will be triggered.  This could cause or exacerbate
        performance problems caused by packet reordering in the network.

2.2 Packet-based Early Retransmit

    A TCP or SCTP sender MAY use packet-based Early Retransmit.

    A sender employing packet-based Early Retransmit MUST use the
    following two conditions to determine when an Early Retransmit is

    (3.a) The number of outstanding segments (oseg)---segments sent but
          not yet acknowledged---is less than four.

    (3.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 a
    retransmission MUST be reduced to:

                    ER_thresh = oseg - 1                              (2)

    duplicate ACKs, where oseg represents the number of outstanding
    segments.  (We discuss tracking the number of outstanding segments

    When conditions (3.a) and (3.b) hold and the connection does support
    SACK, Early Retransmit MUST be used only when "oseg - 1" segments
    have been SACKed.

    When conditions (3.a) and (3.b) do not hold, the transport MUST NOT
    use Early Retransmit, but rather prefer the standard mechanisms,
    including Limited Transmit.

    This version of Early Retransmit solves the precision issues
    discussed in the previous section.  As noted previously, the cost is
    that the implementation will have to track packet boundaries to form
    an understanding as to how many actual segments have been
    transmitted, but not acknowledged.  This can be done by tracking the
    boundaries of the three segments on the right side of the current
    window (which involves tracking four sequence numbers in TCP).  This
    could be done by keeping a circular list of the packet boundaries,
    for instance.  Cumulative ACKs that do not fall within this region
    indicate that at least four segments are outstanding and therefore
    Early Retransmit MUST NOT be used.  When the outstanding window
    becomes small enough that Early Retransmit can be invoked, a full
    understanding of the number of outstanding packets will be
    available from the four sequence numbers retained.

3   Discussion

    The SACK variant of the Early Retransmit algorithm is preferred to
    the non-SACK variant in TCP due to its robustness in the face of ACK
    loss (since SACKs are sent redundantly) and due to interactions with
    the delayed ACK timer. timer (SCTP does not have a non-SACK mode and
    therefore naturally supports SACK-based Early Retransmit).  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 Early Retransmit on this
        ACK because only one packet remains outstanding.

    (B) The ACK for S1 is not delayed: In this case the arrival of S1
        triggers an ACK of previously unacknowledged data.  The arrival
        of S3 triggers a duplicate ACK (because it is out-of-order).
        Both ACKs will cover the same segment (S1).  Therefore,
        regardless of whether SACK is used Early Retransmit can be
        performed by the sender (assuming no ACK loss).

    Early Retransmit is less robust in the face of reordered segments
    than when using the standard Fast Retransmit threshold.  Research
    shows that a general reduction in the number of duplicate ACKs
    required to trigger Fast Retransmit 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 and that no new data was available for transmission.

    A number of studies have shown that network reordering is not a rare
    event across some network paths.  Various measurement studies have
    shown that reordering along most paths is negligible, but along
    certain paths can be quite prevalent [Pax97,BPS99,BS02,Pir05].
    Evaluating Early Retransmit in the face of real packet reordering is
    part of the experiment we hope to instigate with this document.

    Next, 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.  If the network consistently reorders the two
        segments, the sender will needlessly retransmit one out of every
        two unique segments transmitted when using the above algorithm
        (meaning that one-third of all segments sent are needless
        retransmissions).  However, this would only be a problem for
        long-lived connections from applications that transmit in

    (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.  However, the worst cases are likely pathological
    and part of the experiments that this document hopes to trigger
    would involve better understanding of whether such theoretical worst
    case scenarios are prevalent in the network and in general to
    explore the tradeoff between spurious fast retransmits and the delay
    imposed by the RTO.  Appendix A does offer a survey of possible
    mitigations that call for curtailing the use of Early Retransmit
    when it is making poor retransmission decisions.

4   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 a transport 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, data, and therefore are potentially wasting network
    resources (at a time when there is a reasonable chance that the
    resources are scarce).

5   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
    Retransmit.  The notion of Early Transmit was originally sketched in
    an Internet-Draft co-authored by Sally Floyd and Hari Balakrishnan.
    Armando Caro and many members of the TSVWG and TCPM working groups
    provided good discussions that helped shape this document.  Our
    thanks to all!

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.

    [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

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

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

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 Web Server's View of the Transport Layer.

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

    [BS02] John Bellardo, Stefan Savage.  Measuring Packet Reordering,
        ACM/USENIX Internet Measurement Workshop, November 2002.

    [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.

    [HB08] Per Hurtig, Anna Brunstrom. Enhancing SCTP Loss Recovery: An
        Experimental Evaluation of Early Retransmit.  Elsevier Computer
        Communication, 2008, to appear.

    [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, San Francisco, CA,
        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.

    [Pir05] N. M. Piratla, "A Theoretical Foundation, Metrics and
        Modeling of Packet Reordering and Methodology of Delay Modeling
        using Inter-packet Gaps," Ph.D. Dissertation, Department of
        Electrical and Computer Engineering, Colorado State University,
        Fort Collins, CO, Fall 2005.

    [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
    International Computer Science Institute
    1947 Center Street, Suite 600
    Berkeley, CA 94704-1198
    Phone: 440-235-1792

    Konstantin Avrachenkov
    2004 route des Lucioles, B.P.93
    06902, Sophia Antipolis
    Phone: 00 33 492 38 7751

    Urtzi Ayesta
    7 Avenue Colonel Roche
    31077 Toulouse

    Josh Blanton
    Ohio University
    301 Stocker Center
    Athens, OH  45701

    Per Hurtig
    Karlstad University
    Department of Computer Science
    Universitetsgatan 2 651 88
    Karlstad Sweden

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 3.  This appendix outlines several schemes that have been
    suggested to mitigate the problems caused by Early Retransmit in the
    face of packet 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 pathological 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 "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.  For example, a connection could
        stop using Early Retransmit after the first spurious retransmit
        is detected.

    MITIGATION A.2: 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.3: Allow a connection to trigger Early Retransmit using
        the criteria given in section 2, in addition to a "small"
        timeout [Pax97].  For instance, a sender may have to wait for 2
        duplicate ACKs and then T msec before Early Retransmit is
        invoked.  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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