Internet Engineering Task Force                            Ethan Blanton
INTERNET DRAFT                                           Ohio University
File: draft-allman-tcp-sack-09.txt                           Mark Allman
                                                            BBN/NASA GRC
                                                              Kevin Fall
                                                          Intel Research
                                                          February, 2002
                                                   Expires: August, 2002

       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
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    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
    [RFC2581], but allows TCP senders to recover more effectively
    when multiple segments are lost from a single flight of data.


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

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

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    making retransmission and congestion control decisions [PF01].  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
    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

    The reader is assumed to be familiar with selective acknowledgments
    as specified in [RFC2018].

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

        ``HighACK'' is the sequence number of the highest byte of
        data that has been cumulatively ACKed at a given point.

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

        ``HighRxt'' is the highest sequence number which has been
        retransmitted during the current loss recovery phase.

        ``Pipe'' is a sender's estimate of the number of bytes
        outstanding in the network.  This is used during recovery
        for limiting the sender's sending rate.  The pipe variable
        allows TCP to use a fundamentally different congestion
        control than specified in [RFC2581].  The algorithm is often
        referred to as the ``pipe algorithm''.

    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, as described in

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    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).

    Finally, a range of sequence numbers [A,B] is said to ``cover''
    sequence number S if A <= S <= B.

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''.
    The specifics of the scoreboard data structure are out of scope
    for this document (as long as the implementation can perform all
    functions required by this specification).

    Note that while this document speaks of marking and keeping
    track of octets, a real world implementation would probably want
    to keep track of octet ranges or otherwise collapse the data
    while ensuring that arbitrary ranges are still markable.

4   Processing and Acting Upon SACK Information

    For the purposes of the algorithm defined in this document the
    scoreboard SHOULD 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 of octets SACKed should be recorded.

        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].

    NextSeg ():

        This routine uses the scoreboard data structure maintained
        by the Update() function to determine what to transmit based
        on the SACK information that has arrived from the data
        receiver (and, hence, marked in the scoreboard).  NextSeg ()
        MUST return the sequence number range of the next
        segment that is to be transmitted, per the following rules:

        (1) If there exists a smallest unSACKed sequence number 'S1'
            such that HighRxt < S1 < HighData and there are either
            DupThresh * SMSS octets above S1 which have been SACKed or
            the number of discontiguous SACKed sequence spaces above S1

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            is greater than DupThresh, S1 is presumed to have been lost
            and the sequence range of one segment of up to SMSS octets
            starting with S1 MUST be returned.

        (2) If no sequence number 'S1' per rule (1) exists but there
            exists available unsent data and the receiver's advertised
            window allows, the sequence range of one segment of up to
            SMSS octets of previously unsent data starting with sequence
            number HighData+1 MUST be returned.

        (3) If the conditions for rules (1) and (2) fail, but there
            exists an unSACKed sequence number 'S2' such that HighRxt <
            S2 < HighData, one segment of up to SMSS octets starting
            with S2 MUST be returned.  Note that this segment need not
            meet the additional requirements in (1).

        (4) If the conditions for each of (1), (2), and (3) are not
            met, then NextSeg () MUST indicate failure, and no segment
            is returned.

    AmountSACKed (RangeBegin,RangeEnd):

        This routine MUST return the total number of octets which fall
        between RangeBegin and RangeEnd that have been selectively
        acknowledged by the receiver.

    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

5   Algorithm Details

    Upon the receipt of any ACK containing SACK information, the
    scoreboard MUST be updated via the Update () routine.

    Upon the receipt of the first (DupThresh - 1) duplicate ACKs, the
    scoreboard is to be updated as normal.  Note: The first and second
    duplicate ACKs can also be used to trigger the transmission of
    previously unsent segments using the Limited Transmit algorithm

    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 in doing so the following steps MUST be taken:

    (1) pipe = HighData - HighACK - AmountSACKed (HighACK,HighData)

        Set a ``pipe'' variable to the number of outstanding octets
        currently ``in the pipe''; this is the data which has been

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        sent by the TCP sender but for which no cumulative or
        selective acknowledgment has been received.  This data is
        assumed to be still traversing the network path.

    (2) RecoveryPoint = HighData

        When the TCP sender receives a cumulative ACK for this data
        octet the loss recovery phase is terminated.

    (3) ssthresh = cwnd = (FlightSize / 2)

        The congestion window (cwnd) and slow start threshold
        (sstrhesh) are reduced to half of FlightSize per [RFC2581].

    (4) Retransmit the first data segment presumed dropped -- the
        segment starting with sequence number HighACK + 1.  To
        prevent repeated retransmission of the same data, set
        HighRxt to the highest sequence number in the retransmitted

    (5) In order to take advantage of potential additional available
        cwnd, proceed to step (D) below.

    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
        RecoveryPoint signals the end of loss recovery and the loss
        recovery phase MUST be terminated.  Any information contained in
        the scoreboard for sequence numbers greater than the new value
        of HighACK SHOULD NOT be cleared when leaving the loss recovery

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

        (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 (i.e., those
            octets that are being SACKed for the first time), 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.

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        (C.3) The value of pipe MUST be decremented by the number of
            octets that have left the network path using the following

            pipe = pipe - ((HighACK - OldHighACK) -
                           AmountSACKed (OldHighACK + 1, HighACK))

        (C.4) The value of pipe MUST be decremented by the number of
            newly SACKed data octets conveyed in the incoming ACK (i.e.,
            those octets that are being SACKed for the first time), as
            these octets represent data that has left the network.

    (D) While pipe is less than cwnd 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 sent.

        (D.2) If any of the data octets sent in (D.1) are above
            HighData, the pipe variable MUST be incremented by the
            number of data octets previously unsent in (D.1).

        (D.3) If any of the data octets sent in (D.1) are below
            HighData, HighRxt MUST be set to the highest sequence number
            of the segment retransmitted.

        (D.4) If any of the data octets sent in (D.1) are above
            HighData, HighData must be updated to reflect the
            transmission of previously unsent data.

        (D.5) If cwnd - pipe is greater than 1 SMSS, return to (D.1)

5.1 Retransmission Timeouts

    In order to avoid memory deadlocks, the TCP receiver is allowed to
    discard data that has already been acknowledged with a selective
    acknowledgment.  As a result [RFC2018] suggests that a TCP sender
    SHOULD expunge the SACK information gathered from a receiver upon a
    retransmission timeout ``since the timeout might indicate that the
    data receiver has reneged.''  Additionally, a TCP sender MUST
    ``ignore prior SACK information in determining which data to
    retransmit.''  However, a SACK TCP sender SHOULD still use all SACK
    information made available during the slow start phase of loss
    recovery following an RTO.

    As described in Sections 4 and 5, Update () MAY continue to be
    used appropriately upon receipt of ACKs.  This will allow the
    slow start recovery period to benefit from all available
    information provided by the receiver, despite the fact that SACK
    information was expunged due to the RTO.

    If there are segments missing from the receiver's buffer following

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    processing of the retransmitted segment, the corresponding ACK will
    contain SACK information.  In this case, a TCP sender SHOULD use
    this SACK information by using the NextSeg () routine to determine
    what data should be sent in each segment of the slow start.

6   Research

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

7   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 or not a particular ACK is legitimate.
    While not fool-proof, SACK does provide some amount of protection in
    this area.


    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 text.  Murali Bashyam,
    Reiner Ludwig, Jamshid Mahdavi, Matt Mathis, Shawn Ostermann, 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.


    [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

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

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    [PF01] Jitendra Padhye, Sally Floyd.  Identifying the TCP Behavior
        of Web Servers, ACM SIGCOMM, August 2001.

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

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

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

    [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
        Requirement Levels", BCP 14, RFC 2119, March 1997.

    [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

    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

    Kevin Fall
    Intel Research
    2150 Shattuck Ave., PH Suite
    Berkeley, CA 94704

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