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Network Working Group E. Blanton Request for Comments: 3517 Purdue University Category: Standards Track M. Allman

                                                          BBN/NASA GRC
                                                               K. Fall
                                                        Intel Research
                                                               L. Wang
                                                University of Kentucky
                                                            April 2003
       A Conservative Selective Acknowledgment (SACK)-based
                  Loss Recovery Algorithm for TCP

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.


 This document presents a conservative loss recovery algorithm for TCP
 that is based on the use of the selective acknowledgment (SACK) TCP
 option.  The algorithm presented in this document conforms to the
 spirit of the current congestion control specification (RFC 2581),
 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 BCP 14, RFC 2119

Blanton, et al. Standards Track [Page 1] RFC 3517 SACK-based Loss Recovery for TCP April 2003

1 Introduction

 This document presents a conservative loss recovery algorithm for TCP
 that is based on the use of the selective acknowledgment (SACK) TCP
 option.  While the TCP SACK [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 [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 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

Blanton, et al. Standards Track [Page 2] RFC 3517 SACK-based Loss Recovery for TCP April 2003

    "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
 For the purposes of this specification we define a "duplicate
 acknowledgment" as a segment that arrives with no data and an
 acknowledgment (ACK) number that is equal to the current value of
 HighACK, as described in [RFC2581].
 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
 Note that this document refers to keeping account of (marking)
 individual octets of data transferred across a TCP connection.  A
 real-world implementation of the scoreboard would likely prefer to
 manage this data as sequence number ranges.  The algorithms presented
 here allow this, but require arbitrary sequence number ranges to be
 marked as having been selectively acknowledged.

Blanton, et al. Standards Track [Page 3] RFC 3517 SACK-based Loss Recovery for TCP April 2003

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 ():
    Given the information provided in an ACK, 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].
 IsLost (SeqNum):
    This routine returns whether the given sequence number is
    considered to be lost.  The routine returns true when either
    DupThresh discontiguous SACKed sequences have arrived above
    'SeqNum' or (DupThresh * SMSS) bytes with sequence numbers greater
    than 'SeqNum' have been SACKed.  Otherwise, the routine returns
 SetPipe ():
    This routine traverses the sequence space from HighACK to HighData
    and MUST set the "pipe" variable to an estimate of the number of
    octets that are currently in transit between the TCP sender and
    the TCP receiver.  After initializing pipe to zero the following
    steps are taken for each octet 'S1' in the sequence space between
    HighACK and HighData that has not been SACKed:
    (a) If IsLost (S1) returns false:
       Pipe is incremented by 1 octet.
       The effect of this condition is that pipe is incremented for
       packets that have not been SACKed and have not been determined
       to have been lost (i.e., those segments that are still assumed
       to be in the network).
    (b) If S1 <= HighRxt:
       Pipe is incremented by 1 octet.

Blanton, et al. Standards Track [Page 4] RFC 3517 SACK-based Loss Recovery for TCP April 2003

       The effect of this condition is that pipe is incremented for
       the retransmission of the octet.
    Note that octets retransmitted without being considered lost are
    counted twice by the above mechanism.
 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
    been 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 'S2' that
        meets the following three criteria for determining loss, the
        sequence range of one segment of up to SMSS octets starting
        with S2 MUST be returned.
        (1.a) S2 is greater than HighRxt.
        (1.b) S2 is less than the highest octet covered by any
              received SACK.
        (1.c) IsLost (S2) returns true.
    (2) If no sequence number 'S2' 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 'S3' that meets the criteria for
        detecting loss given in steps (1.a) and (1.b) above
        (specifically excluding step (1.c)) then one segment of up to
        SMSS octets starting with S3 MAY be returned.
        Note that rule (3) is a sort of retransmission "last resort".
        It allows for retransmission of sequence numbers even when the
        sender has less certainty a segment has been lost than as with
        rule (1).  Retransmitting segments via rule (3) will help
        sustain TCP's ACK clock and therefore can potentially help
        avoid retransmission timeouts.  However, in sending these
        segments the sender has two copies of the same data considered
        to be in the network (and also in the Pipe estimate).  When an
        ACK or SACK arrives covering this retransmitted segment, the

Blanton, et al. Standards Track [Page 5] RFC 3517 SACK-based Loss Recovery for TCP April 2003

        sender cannot be sure exactly how much data left the network
        (one of the two transmissions of the packet or both
        transmissions of the packet).  Therefore the sender may
        underestimate Pipe by considering both segments to have left
        the network when it is possible that only one of the two has.
        We believe that the triggering of rule (3) will be rare and
        that the implications are likely limited to corner cases
        relative to the entire recovery algorithm.  Therefore we leave
        the decision of whether or not to use rule (3) to
    (4) If the conditions for each of (1), (2), and (3) are not met,
        then NextSeg () MUST indicate failure, and no segment is
 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 ()).  If no previous loss event has occurred
 on the connection or the cumulative acknowledgment point is beyond
 the last value of RecoveryPoint, a loss recovery phase SHOULD be
 initiated, per the fast retransmit algorithm outlined in [RFC2581].
 The following steps MUST be taken:
 (1) RecoveryPoint = HighData
     When the TCP sender receives a cumulative ACK for this data octet
     the loss recovery phase is terminated.

Blanton, et al. Standards Track [Page 6] RFC 3517 SACK-based Loss Recovery for TCP April 2003

 (2) ssthresh = cwnd = (FlightSize / 2)
     The congestion window (cwnd) and slow start threshold (ssthresh)
     are reduced to half of FlightSize per [RFC2581].
 (3) 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 segment.
 (4) Run SetPipe ()
     Set a "pipe" variable  to the number of outstanding octets
     currently "in the pipe"; this is the data which has been sent by
     the TCP sender but for which no cumulative or selective
     acknowledgment has been received and the data has not been
     determined to have been dropped in the network.  It is assumed
     that the data is still traversing the network path.
 (5) In order to take advantage of potential additional available
     cwnd, proceed to step (C) 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 an ACK that does not cover RecoveryPoint the
     following actions MUST be taken:
     (B.1) Use Update () to record the new SACK information conveyed
     by the incoming ACK.
     (B.2) Use SetPipe () to re-calculate the number of octets still
     in the network.
 (C) If cwnd - pipe >= 1 SMSS the sender SHOULD transmit one or more
     segments as follows:
     (C.1) The scoreboard MUST be queried via NextSeg () for the
     sequence number range of the next segment to transmit (if any),

Blanton, et al. Standards Track [Page 7] RFC 3517 SACK-based Loss Recovery for TCP April 2003

     and the given segment sent.  If NextSeg () returns failure (no
     data to send) return without sending anything (i.e., terminate
     steps C.1 -- C.5).
     (C.2) If any of the data octets sent in (C.1) are below HighData,
     HighRxt MUST be set to the highest sequence number of the
     retransmitted segment.
     (C.3) If any of the data octets sent in (C.1) are above HighData,
     HighData must be updated to reflect the transmission of
     previously unsent data.
     (C.4) The estimate of the amount of data outstanding in the
     network must be updated by incrementing pipe by the number of
     octets transmitted in (C.1).
     (C.5) If cwnd - pipe >= 1 SMSS, return to (C.1)

5.1 Retransmission Timeouts

 In order to avoid memory deadlocks, the TCP receiver is allowed to
 discard data that has already been selectively acknowledged.  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.
 If an RTO occurs during loss recovery as specified in this document,
 RecoveryPoint MUST be set to HighData.  Further, the new value of
 RecoveryPoint MUST be preserved and the loss recovery algorithm
 outlined in this document MUST be terminated.  In addition, a new
 recovery phase (as described in section 5) MUST NOT be initiated
 until HighACK is greater than or equal to the new value of
 As described in Sections 4 and 5, Update () SHOULD 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
 processing of the retransmitted segment, the corresponding ACK will
 contain SACK information.  In this case, a TCP sender SHOULD use this
 SACK information when determining what data should be sent in each

Blanton, et al. Standards Track [Page 8] RFC 3517 SACK-based Loss Recovery for TCP April 2003

 segment of the slow start.  The exact algorithm for this selection is
 not specified in this document (specifically NextSeg () is
 inappropriate during slow start after an RTO).  A relatively
 straightforward approach to "filling in" the sequence space reported
 as missing should be a reasonable approach.

6 Managing the RTO Timer

 The standard TCP RTO estimator is defined in [RFC2988].  Due to the
 fact that the SACK algorithm in this document can have an impact on
 the behavior of the estimator, implementers may wish to consider how
 the timer is managed.  [RFC2988] calls for the RTO timer to be
 re-armed each time an ACK arrives that advances the cumulative ACK
 point.  Because the algorithm presented in this document can keep the
 ACK clock going through a fairly significant loss event,
 (comparatively longer than the algorithm described in [RFC2581]), on
 some networks the loss event could last longer than the RTO.  In this
 case the RTO timer would expire prematurely and a segment that need
 not be retransmitted would be resent.
 Therefore we give implementers the latitude to use the standard
 [RFC2988] style RTO management or, optionally, a more careful variant
 that re-arms the RTO timer on each retransmission that is sent during
 recovery MAY be used.  This provides a more conservative timer than
 specified in [RFC2988], and so may not always be an attractive
 alternative.  However, in some cases it may prevent needless
 retransmissions, go-back-N transmission and further reduction of the
 congestion window.

7 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

8 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.

Blanton, et al. Standards Track [Page 9] RFC 3517 SACK-based Loss Recovery for TCP April 2003


 The authors wish to thank Sally Floyd for encouraging this document
 and commenting on early drafts.  The algorithm described in this
 document is loosely 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,
 Ken Calvert, Tom Henderson, Reiner Ludwig, Jamshid Mahdavi, Matt
 Mathis, Shawn Ostermann, Vern Paxson and Venkat Venkatsubra provided
 valuable feedback on earlier versions of this document.  We thank
 Matt Mathis and Jamshid Mahdavi for implementing the scoreboard in ns
 and hence guiding our thinking in keeping track of SACK state.
 The first author would like to thank Ohio University and the Ohio
 University Internetworking Research Group for supporting the bulk of
 his work on this project.

Normative References

 [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
           793, September 1981.
 [RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
           Selective Acknowledgment Options", RFC 2018, October 1996.
 [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
           3", BCP 9, RFC 2026, October 1996.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2581] Allman, M., Paxson, V. and R. Stevens, "TCP Congestion
           Control", RFC 2581, April 1999.

Informative 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.

Blanton, et al. Standards Track [Page 10] RFC 3517 SACK-based Loss Recovery for TCP April 2003

 [Jac90]   Van Jacobson.  Modified TCP Congestion Avoidance Algorithm.
           Technical Report, LBL, April 1990.
 [PF01]    Jitendra Padhye, Sally Floyd.  Identifying the TCP Behavior
           of Web Servers, ACM SIGCOMM, August 2001.
 [RFC2582] Floyd, S. and T. Henderson, "The NewReno Modification to
           TCP's Fast Recovery Algorithm", RFC 2582, April 1999.
 [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
           2914, September 2000.
 [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
           Timer", RFC 2988, November 2000.
 [RFC3042] Allman, M., Balakrishnan, H, and S. Floyd, "Enhancing TCP's
           Loss Recovery Using Limited Transmit", RFC 3042, January

Intellectual Property Rights Notice

 The IETF takes no position regarding the validity or scope of any
 intellectual property or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; neither does it represent that it
 has made any effort to identify any such rights.  Information on the
 IETF's procedures with respect to rights in standards-track and
 standards-related documentation can be found in BCP-11.  Copies of
 claims of rights made available for publication and any assurances of
 licenses to be made available, or the result of an attempt made to
 obtain a general license or permission for the use of such
 proprietary rights by implementors or users of this specification can
 be obtained from the IETF Secretariat.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights which may cover technology that may be required to practice
 this standard.  Please address the information to the IETF Executive

Blanton, et al. Standards Track [Page 11] RFC 3517 SACK-based Loss Recovery for TCP April 2003

Authors' Addresses

 Ethan Blanton
 Purdue University Computer Sciences
 1398 Computer Science Building
 West Lafayette, IN  47907
 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
 Lili Wang
 Laboratory for Advanced Networking
 210 Hardymon Building
 University of Kentucky
 Lexington, KY 40506-0495

Blanton, et al. Standards Track [Page 12] RFC 3517 SACK-based Loss Recovery for TCP April 2003

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 Internet Society.

Blanton, et al. Standards Track [Page 13]

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