Internet Engineering Task Force                            Ethan Blanton
INTERNET DRAFT                                           Ohio university
File: draft-allman-tcp-sack-03.txt                           Mark Allman
                                                            BBN/NASA GRC
                                                          February, 2001
                                                   Expires: August, 2001


       A Conservative SACK-based Loss Recovery Algorithm 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.

    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 presents a conservative loss recovery algorithm for
    TCP that is based on the use of the selective acknowledgment TCP
    option.  The algorithm presented in this document conforms to the
    spirit of the current congestion control specification, but allows
    TCP senders to recover more effectively when multiple segments are
    lost from a single flight of data.

1   Introduction

    This document presents a conservative loss recovery algorithm for
    TCP that is based on the use of the selective acknowledgment TCP
    option.  While the TCP selective acknowledgment (SACK) option
    [RFC2018] is being steadily deployed in the Internet [All00] there
    is evidence that hosts are not using the SACK information when
    making retransmission and congestion control decisions [PF00].  The
    goal of this document is to outline one straightforward method for
    TCP implementations to use SACK information to increase performance.

    [RFC2581] allows advanced loss recovery algorithms to be used by TCP
    [RFC793] provided that they follow the spirit of TCP's congestion
    control algorithms [RFC2581,RFC2914].  [RFC2582] outlines one such
    advanced recovery algorithm called NewReno.  This document outlines

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    a loss recovery algorithm that uses the selective acknowledgment
    (SACK) [RFC2018] TCP option to enhance TCP's loss recovery.  The
    algorithm outlined in this document, heavily based on the algorithm
    detailed in [FF96], is a conservative replacement of the fast
    recovery algorithm [Jac90,RFC2581].  The algorithm specified in this
    document is a straightforward SACK-based loss recovery strategy that
    follows the guidelines set in [RFC2581] and can safely be used in
    TCP implementations.  Alternate SACK-based loss recovery methods can
    be used in TCP as implementers see fit (as long as the alternate
    algorithms follow the guidelines provided in [RFC2581]).  Please
    note, however, that the SACK-based decisions in this document (such
    as what segments are to be sent at what time) are largely decoupled
    from the congestion control algorithms, and as such can be treated
    as separate issues if so desired.

2   Definitions

    The reader is expected to be familiar with the definitions given in
    [RFC2581].

    For the purposes of explaining the SACK-based loss recovery
    algorithm we define two variables that a TCP sender stores:

        ``HighACK'' is the sequence number of the highest cumulative ACK
        received at a given point.

        ``HighData'' is the highest sequence number transmitted at a
        given point.

    For the purposes of this specification we define a ``duplicate
    acknowledgment'' as an acknowledgment (ACK) whose cumulative ACK
    number is equal to the current value of HighACK and also conveys new
    selective acknowledgment information for segment(s) above HighACK.

    We define a variable ``DupThresh'' that holds the number of
    duplicate acknowledgments required to trigger a retransmission.  Per
    [RFC2581] this threshold is defined to be 3 duplicate
    acknowledgments.  However, implementers should consult any updates
    to [RFC2581] to determine the current value for DupThresh (or method
    for determining its value).

3   Keeping Track of SACK Information

    For a TCP sender to implement the algorithm defined in the next
    section it must keep a data structure to store incoming selective
    acknowledgment information on a per connection basis.  Such a data
    structure is commonly called the ``scoreboard''.  For the purposes
    of the algorithm defined in this document the scoreboard MUST
    implement the following functions:

    Update ():

        Each octet that is cumulatively ACKed or SACKed should be marked
        accordingly in the scoreboard data structure, and the total number

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        of octets SACKed should be recorded.  For each octet that has not
        been cumulatively acknowledged, a ``DupSACK'' counter is kept
        indicating how many times an octet of greater sequence number has
        been SACKed.  Record the number of octets whose DupSACK counter is
        incremented to (or past) DupThresh during this process.

        Note: SACK information is advisory and therefore SACKed data
        MUST NOT be removed from TCP's retransmission buffer until the
        data is cumulatively acknowledged [RFC2018].

    MarkRetran ():

        When a retransmission is sent, the scoreboard MUST be updated
        with this information so that data is not repeatedly
        retransmitted by the SACK-based algorithm outlined in this
        document.  Note: If a retransmission is lost it will be repaired
        using TCP's retransmission timer.

    NextSeg ():

        This routine MUST return the sequence number range of the oldest
        segment that has not been cumulatively ACKed or SACKed and not
        been retransmitted, per the following rules:

        (1) Look for the lowest sequence number that is not ACKed or
            SACKed, but has a DupSACK counter of at least DupThresh.  If
            such a sequence number ``S'' exists, this routine MUST return
            a sequence number range starting at octet S.

        (2) If we fail to find a segment per rule 1, but the connection
            has unsent data available to be transmitted, NextSeg () MUST
            return a sequence number range corresponding to one segment of
            this new data.

        (3) If rules 1 and 2 fail, this routine MUST return a segment
            that has not been ACKed or SACKed but may not meet the
            DupThresh requirement in 1.

        (4) Finally, if rules 1-3 fail, NextSeg () MUST indicate this
            and no data will be sent.

    AmountSACKed ():

        This routine MUST return the number of octets selectively
        acknowledged by the receiver.


    LeftNetwork ():

        This function MUST return the number of octets in the given
        sequence number range that have left the network.  The algorithm
        checks each octet in the given range and separately keeps track
        of the number of retransmitted octets and the number of octets
        that are cumulatively ACKed but were not SACKed.  Note: it is

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        possible to have octets that fit both categories.  In this case,
        the octets MUST be counted in both categories.  After checking
        the sequence number range given this routine returns the sum of
        the two counters.

    Keeping track of SACK information depends on the TCP sender having
    an accurate measure of the current state of the network, the
    conditions of this connection, and the state of the receiver's
    buffer.  Due to these limitations, [RFC2018] suggests that a TCP
    sender SHOULD expunge the SACK information gathered from a receiver
    upon a retransmission timeout; the assumption here being that the
    receiver may have reneged on its SACK information for some reason.

    Note: The SACK-based loss recovery algorithm outlined in this
    document requires more computational resources than previous TCP
    loss recovery strategies.  However, we believe the scoreboard data
    structure can be implemented in a reasonably efficient manner (both
    in terms of computation complexity and memory usage) in most TCP
    implementations.

4   Algorithm Details

    Upon the receipt of the first DupThresh - 1 duplicate ACKs, the
    scoreboard MUST be updated per the selective acknowledgment
    information contained in the ACK (via the Update () routine).  Note:
    The first and second duplicate ACKs can also be used to trigger the
    transmission of previously unsent segments using the Limited
    Transmit mechanism [ABF00].

    When a TCP sender receives the duplicate ACK corresponding to
    DupThresh ACKs, the scoreboard MUST be updated with the new SACK
    information (via Update ()) and a loss recovery phase SHOULD be
    initiated, per the fast retransmit algorithm outlined in [RFC2581],
    and the following steps MUST be taken:

    (1) Set a ``pipe'' variable to the number of outstanding octets
        (i.e., octets that have been sent but not yet acknowledged), per
        the following equation:

        pipe = HighData - HighACK - AmountSACKed ()

    (2) Set a ``RecoveryPoint'' variable to HighData.  When the TCP
        sender receives a cumulative ACK for this data octet the loss
        recovery phase is terminated.

    (3) The congestion window (cwnd) is reduced to half its current
        value.  The value of the slow start threshold (ssthresh) is set
        to the halved value of cwnd.

    (4) Retransmit the first data segment not covered by HighACK.  Use
        the MarkRetran () function to mark the sequence number range as
        having been retransmitted in the scoreboard.  In order to take
        advantage of potential additional available cwnd, proceed to step
        (D) below.

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    Once a TCP is in the loss recovery phase the following procedure
    MUST be used for each arriving ACK:

    (A) An incoming cumulative ACK for a sequence number greater than or
        equal to RecoveryPoint signals the end of loss recovery and the
        loss recovery phase MUST be terminated.

    (B) Upon receipt of a duplicate ACK the following actions MUST be
        taken:

        (B.1) Use Update () to record the new SACK information conveyed
            by the incoming ACK.

        (B.2) The pipe variable is decremented by the number of newly
            SACKed data octets conveyed in the incoming ACK plus the
            number of octets which whose DupSACK counter exceeded
            DupThresh, as that is the amount of new data presumed to have
            left the network.

    (C) When a ``partial ACK'' (an ACK that increases the HighACK point,
        but does not terminate loss recovery) arrives, the following
        actions MUST be performed:

        (C.1) Before updating HighACK based on the received cumulative
            ACK, save HighACK as OldHighACK.

        (C.2) The scoreboard MUST be updated based on the cumulative ACK
            and any new SACK information that is included in the ACK via
            the Update () routine.

        (C.3) The value of pipe MUST be decremented by the number of
            octets returned by the LeftNetwork () routine when given the
            sequence number range OldHighACK-HighACK.

    (D) While pipe is less than cwnd and the receiver's advertised window
        permits, the TCP sender SHOULD transmit one or more segments
        as follows:

        (D.1) The scoreboard MUST be queried via NextSeg () for the
            sequence number range of the next segment to transmit, and
            the given segment is sent.

        (D.2) The pipe variable MUST be incremented by the number of
            data octets sent in (D.1).

        (D.3) If any of the data octets sent in (D.1) are below HighData,
            they MUST be marked as retransmitted via Update ().

        (D.4) If cwnd - pipe is greater than 1 SMSS, return to (D.1)

5   Research

    The algorithm specified in this document is analyzed in [FF96],

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    which shows that the above algorithm is effective in reducing
    transfer time over standard TCP Reno [RFC2581] when multiple
    segments are dropped from a window of data (especially as the number
    of drops increases).  [AHKO97] shows that the algorithm defined in
    this document can greatly improve throughput in connections
    traversing satellite channels.

6   Security Considerations

    The algorithm presented in this paper shares security considerations
    with [RFC2581].  A key difference is that an algorithm based on
    SACKs is more robust against attackers forging duplicate ACKs to
    force the TCP sender to reduce cwnd.  With SACKs TCP senders have an
    additional check on whether the ACK is legitimate or not.  While not
    fool-proof, SACK provides some amount of protection in this area.

Acknowledgments

    The authors wish to thank Sally Floyd for encouraging this document
    and commenting on an early draft.  The algorithm described in this
    document is largely based on an algorithm outlined by Kevin Fall and
    Sally Floyd in [FF96] (although the authors of this document assume
    responsibility for any mistakes in the above).  Murali Bashyam,
    Jamshid Mahdavi, Matt Mathis, Vern Paxson and Venkat Venkatsubra
    provided valuable feedback on earlier versions of this document.
    Finally, we thank Matt Mathis and Jamshid Mahdavi for implementing
    the scoreboard in ns and hence guiding our thinking in keeping track
    of SACK state.

References

    [AHKO97] Mark Allman, Chris Hayes, Hans Kruse, Shawn Ostermann. TCP
        Performance Over Satellite Links.  Proceedings of the Fifth
        International Conference on Telecommunications Systems,
        Nashville, TN, March, 1997.

    [All00] Mark Allman. A Web Server's View of the Transport Layer. ACM
        Computer Communication Review, 30(5), October 2000.

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

    [Jac90] Van Jacobson.  Modified TCP Congestion Avoidance Algorithm.
        Technical Report, LBL, April 1990.

    [PF00] Jitendra Padhye, Sally Floyd.  TBIT, the TCP Behavior
        Inference Tool, October 2000.  http://www.aciri.org/tbit/.

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

    [RFC2018] M. Mathis, J. Mahdavi, S. Floyd, A. Romanow. TCP Selective
        Acknowledgement Options. RFC 2018, October 1996

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    [RFC2026] Scott Bradner. The Internet Standards Process -- Revision
        3, RFC 2026, October 1996

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

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

    [RFC2914] Sally Floyd.  Congestion Control Principles, RFC 2914,
        September 2000.

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



Author's Addresses:

    Ethan Blanton
    Ohio University Internetworking Research Lab
    Stocker Center
    Athens, OH  45701
    eblanton@cs.ohiou.edu

    Mark Allman
    BBN Technologies/NASA Glenn Research Center
    Lewis Field
    21000 Brookpark Rd.  MS 54-5
    Cleveland, OH  44135
    Phone: 216-433-6586
    Fax: 216-433-8705
    mallman@bbn.com
    http://roland.grc.nasa.gov/~mallman



















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