





Network Working Group                                           S. Floyd
Request for Comments: 2883                                         ACIRI
Category: Standards Track                                     J. Mahdavi
                                                                  Novell
                                                               M. Mathis
                                        Pittsburgh Supercomputing Center
                                                             M. Podolsky
                                                             UC Berkeley
                                                               July 2000


  An Extension to the Selective Acknowledgement (SACK) Option 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 (2000).  All Rights Reserved.

Abstract

   This note defines an extension of the Selective Acknowledgement
   (SACK) Option [RFC2018] for TCP.  RFC 2018 specified the use of the
   SACK option for acknowledging out-of-sequence data not covered by
   TCP's cumulative acknowledgement field.  This note extends RFC 2018
   by specifying the use of the SACK option for acknowledging duplicate
   packets.  This note suggests that when duplicate packets are
   received, the first block of the SACK option field can be used to
   report the sequence numbers of the packet that triggered the
   acknowledgement.  This extension to the SACK option allows the TCP
   sender to infer the order of packets received at the receiver,
   allowing the sender to infer when it has unnecessarily retransmitted
   a packet.  A TCP sender could then use this information for more
   robust operation in an environment of reordered packets [BPS99], ACK
   loss, packet replication, and/or early retransmit timeouts.

1.  Conventions and Acronyms

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [B97].




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2. Introduction

   The Selective Acknowledgement (SACK) option defined in RFC 2018 is
   used by the TCP data receiver to acknowledge non-contiguous blocks of
   data not covered by the Cumulative Acknowledgement field.  However,
   RFC 2018 does not specify the use of the SACK option when duplicate
   segments are received.  This note specifies the use of the SACK
   option when acknowledging the receipt of a duplicate packet [F99].
   We use the term D-SACK (for duplicate-SACK) to refer to a SACK block
   that reports a duplicate segment.

   This document does not make any changes to TCP's use of the
   cumulative acknowledgement field, or to the TCP receiver's decision
   of *when* to send an acknowledgement packet.  This document only
   concerns the contents of the SACK option when an acknowledgement is
   sent.

   This extension is compatible with current implementations of the SACK
   option in TCP.  That is, if one of the TCP end-nodes does not
   implement this D-SACK extension and the other TCP end-node does, we
   believe that this use of the D-SACK extension by one of the end nodes
   will not introduce problems.

   The use of D-SACK does not require separate negotiation between a TCP
   sender and receiver that have already negotiated SACK capability.
   The absence of separate negotiation for D-SACK means that the TCP
   receiver could send D-SACK blocks when the TCP sender does not
   understand this extension to SACK.  In this case, the TCP sender will
   simply discard any D-SACK blocks, and process the other SACK blocks
   in the SACK option field as it normally would.





















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3. The Sack Option Format as defined in RFC 2018

   The SACK option as defined in RFC 2018 is as follows:

                            +--------+--------+
                            | Kind=5 | Length |
          +--------+--------+--------+--------+
          |      Left Edge of 1st Block       |
          +--------+--------+--------+--------+
          |      Right Edge of 1st Block      |
          +--------+--------+--------+--------+
          |                                   |
          /            . . .                  /
          |                                   |
          +--------+--------+--------+--------+
          |      Left Edge of nth Block       |
          +--------+--------+--------+--------+
          |      Right Edge of nth Block      |
          +--------+--------+--------+--------+

   The Selective Acknowledgement (SACK) option in the TCP header
   contains a number of SACK blocks, where each block specifies the left
   and right edge of a block of data received at the TCP receiver.  In
   particular, a block represents a contiguous sequence space of data
   received and queued at the receiver, where the "left edge" of the
   block is the first sequence number of the block, and the "right edge"
   is the sequence number immediately following the last sequence number
   of the block.

   RFC 2018 implies that the first SACK block specify the segment that
   triggered the acknowledgement.  From RFC 2018, when the data receiver
   chooses to send a SACK option, "the first SACK block ... MUST specify
   the contiguous block of data containing the segment which triggered
   this ACK, unless that segment advanced the Acknowledgment Number
   field in the header."

   However, RFC 2018 does not address the use of the SACK option when
   acknowledging a duplicate segment.  For example, RFC 2018 specifies
   that "each block represents received bytes of data that are
   contiguous and isolated".  RFC 2018 further specifies that "if sent
   at all, SACK options SHOULD be included in all ACKs which do not ACK
   the highest sequence number in the data receiver's queue."  RFC 2018
   does not specify the use of the SACK option when a duplicate segment
   is received, and the cumulative acknowledgement field in the ACK
   acknowledges all of the data in the data receiver's queue.






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4. Use of the SACK option for reporting a duplicate segment

   This section specifies the use of SACK blocks when the SACK option is
   used in reporting a duplicate segment.  When D-SACK is used, the
   first block of the SACK option should be a D-SACK block specifying
   the sequence numbers for the duplicate segment that triggers the
   acknowledgement.  If the duplicate segment is part of a larger block
   of non-contiguous data in the receiver's data queue, then the
   following SACK block should be used to specify this larger block.
   Additional SACK blocks can be used to specify additional non-
   contiguous blocks of data, as specified in RFC 2018.

   The guidelines for reporting duplicate segments are summarized below:

   (1) A D-SACK block is only used to report a duplicate contiguous
   sequence of data received by the receiver in the most recent packet.

   (2) Each duplicate contiguous sequence of data received is reported
   in at most one D-SACK block.  (I.e., the receiver sends two identical
   D-SACK blocks in subsequent packets only if the receiver receives two
   duplicate segments.)

   (3) The left edge of the D-SACK block specifies the first sequence
   number of the duplicate contiguous sequence, and the right edge of
   the D-SACK block specifies the sequence number immediately following
   the last sequence in the duplicate contiguous sequence.

   (4) If the D-SACK block reports a duplicate contiguous sequence from
   a (possibly larger) block of data in the receiver's data queue above
   the cumulative acknowledgement, then the second SACK block in that
   SACK option should specify that (possibly larger) block of data.

   (5) Following the SACK blocks described above for reporting duplicate
   segments, additional SACK blocks can be used for reporting additional
   blocks of data, as specified in RFC 2018.

   Note that because each duplicate segment is reported in only one ACK
   packet, information about that duplicate segment will be lost if that
   ACK packet is dropped in the network.

4.1  Reporting Full Duplicate Segments

   We illustrate these guidelines with three examples.  In each example,
   we assume that the data receiver has first received eight segments of
   500 bytes each, and has sent an acknowledgement with the cumulative
   acknowledgement field set to 4000 (assuming the first sequence number
   is zero).  The D-SACK block is underlined in each example.




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4.1.1.  Example 1: Reporting a duplicate segment.

   Because several ACK packets are lost, the data sender retransmits
   packet 3000-3499, and the data receiver subsequently receives a
   duplicate segment with sequence numbers 3000-3499.  The receiver
   sends an acknowledgement with the cumulative acknowledgement field
   set to 4000, and the first, D-SACK block specifying sequence numbers
   3000-3500.

        Transmitted    Received    ACK Sent
        Segment        Segment     (Including SACK Blocks)

        3000-3499      3000-3499   3500 (ACK dropped)
        3500-3999      3500-3999   4000 (ACK dropped)
        3000-3499      3000-3499   4000, SACK=3000-3500
                                              ---------
4.1.2.  Example 2:  Reporting an out-of-order segment and a duplicate
        segment.

   Following a lost data packet, the receiver receives an out-of-order
   data segment, which triggers the SACK option as specified in  RFC
   2018.  Because of several lost ACK packets, the sender then
   retransmits a data packet.  The receiver receives the duplicate
   packet, and reports it in the first, D-SACK block:

        Transmitted    Received    ACK Sent
        Segment        Segment     (Including SACK Blocks)

        3000-3499      3000-3499   3500 (ACK dropped)
        3500-3999      3500-3999   4000 (ACK dropped)
        4000-4499      (data packet dropped)
        4500-4999      4500-4999   4000, SACK=4500-5000 (ACK dropped)
        3000-3499      3000-3499   4000, SACK=3000-3500, 4500-5000
                                                 ---------

















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4.1.3.  Example 3:  Reporting a duplicate of an out-of-order segment.

   Because of a lost data packet, the receiver receives two out-of-order
   segments.  The receiver next receives a duplicate segment for one of
   these out-of-order segments:

        Transmitted    Received    ACK Sent
        Segment        Segment     (Including SACK Blocks)

        3500-3999      3500-3999   4000
        4000-4499      (data packet dropped)
        4500-4999      4500-4999   4000, SACK=4500-5000
        5000-5499      5000-5499   4000, SACK=4500-5500
                       (duplicated packet)
                       5000-5499   4000, SACK=5000-5500, 4500-5500
                                              ---------
4.2.  Reporting Partial Duplicate Segments

   It may be possible that a sender transmits a packet that includes one
   or more duplicate sub-segments--that is, only part but not all of the
   transmitted packet has already arrived at the receiver.  This can
   occur when the size of the sender's transmitted segments increases,
   which can occur when the PMTU increases in the middle of a TCP
   session, for example.  The guidelines in Section 4 above apply to
   reporting partial as well as full duplicate segments.  This section
   gives examples of these guidelines when reporting partial duplicate
   segments.

   When the SACK option is used for reporting partial duplicate
   segments, the first D-SACK block reports the first duplicate sub-
   segment.  If the data packet being acknowledged contains multiple
   partial duplicate sub-segments, then only the first such duplicate
   sub-segment is reported in the SACK option.  We illustrate this with
   the examples below.

4.2.1.  Example 4:  Reporting a single duplicate subsegment.

   The sender increases the packet size from 500 bytes to 1000 bytes.
   The receiver subsequently receives a 1000-byte packet containing one
   500-byte subsegment that has already been received and one which has
   not.  The receiver reports only the already received subsegment using
   a single D-SACK block.









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        Transmitted    Received    ACK Sent
        Segment        Segment     (Including SACK Blocks)

        500-999        500-999     1000
        1000-1499      (delayed)
        1500-1999      (data packet dropped)
        2000-2499      2000-2499   1000, SACK=2000-2500
        1000-2000      1000-1499   1500, SACK=2000-2500
                       1000-2000   2500, SACK=1000-1500
                                              ---------

4.2.2.  Example 5:  Two non-contiguous duplicate subsegments covered by
        the cumulative acknowledgement.

   After the sender increases its packet size from 500 bytes to 1500
   bytes, the receiver receives a packet containing two non-contiguous
   duplicate 500-byte subsegments which are less than the cumulative
   acknowledgement field.  The receiver reports the first such duplicate
   segment in a single D-SACK block.

         Transmitted    Received    ACK Sent
         Segment        Segment     (Including SACK Blocks)

         500-999        500-999     1000
         1000-1499      (delayed)
         1500-1999      (data packet dropped)
         2000-2499      (delayed)
         2500-2999      (data packet dropped)
         3000-3499      3000-3499   1000, SACK=3000-3500
         1000-2499      1000-1499   1500, SACK=3000-3500
                        2000-2499   1500, SACK=2000-2500, 3000-3500
                        1000-2499   2500, SACK=1000-1500, 3000-3500
                                               ---------

4.2.3.  Example 6:  Two non-contiguous duplicate subsegments not covered
        by the cumulative acknowledgement.

   This example is similar to Example 5, except that after the sender
   increases the packet size, the receiver receives a packet containing
   two non-contiguous duplicate subsegments which are above the
   cumulative acknowledgement field, rather than below.  The first, D-
   SACK block reports the first duplicate subsegment, and the second,
   SACK block reports the larger block of non-contiguous data that it
   belongs to.







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         Transmitted    Received    ACK Sent
         Segment        Segment     (Including SACK Blocks)

         500-999        500-999     1000
         1000-1499      (data packet dropped)
         1500-1999      (delayed)
         2000-2499      (data packet dropped)
         2500-2999      (delayed)
         3000-3499      (data packet dropped)
         3500-3999      3500-3999   1000, SACK=3500-4000
         1000-1499      (data packet dropped)
         1500-2999      1500-1999   1000, SACK=1500-2000, 3500-4000
                        2000-2499   1000, SACK=2000-2500, 1500-2000,
                                            3500-4000
                        1500-2999   1000, SACK=1500-2000, 1500-3000,
                                               ---------
                                            3500-4000

4.3.  Interaction Between D-SACK and PAWS

   RFC 1323 [RFC1323] specifies an algorithm for Protection Against
   Wrapped Sequence Numbers (PAWS).  PAWS gives a method for
   distinguishing between sequence numbers for new data, and sequence
   numbers from a previous cycle through the sequence number space.
   Duplicate segments might be detected by PAWS as belonging to a
   previous cycle through the sequence number space.

   RFC 1323 specifies that for such packets, the receiver should do the
   following:

      Send an acknowledgement in reply as specified in RFC 793 page 69,
      and drop the segment.

   Since PAWS still requires sending an ACK, there is no harmful
   interaction between PAWS and the use of D-SACK.  The D-SACK block can
   be included in the SACK option of the ACK, as outlined in Section 4,
   independently of the use of PAWS by the TCP receiver, and
   independently of the determination by PAWS of the validity or
   invalidity of the data segment.

   TCP senders receiving D-SACK blocks should be aware that a segment
   reported as a duplicate segment could possibly have been from a prior
   cycle through the sequence number space.  This is independent of the
   use of PAWS by the TCP data receiver.  We do not anticipate that this
   will present significant problems for senders using D-SACK
   information.





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5. Detection of Duplicate Packets

   This extension to the SACK option enables the receiver to accurately
   report the reception of duplicate data.  Because each receipt of a
   duplicate packet is reported in only one ACK packet, the loss of a
   single ACK can prevent this information from reaching the sender.  In
   addition, we note that the sender can not necessarily trust the
   receiver to send it accurate information [SCWA99].

   In order for the sender to check that the first (D)SACK block of an
   acknowledgement in fact acknowledges duplicate data, the sender
   should compare the sequence space in the first SACK block to the
   cumulative ACK which is carried IN THE SAME PACKET.  If the SACK
   sequence space is less than this cumulative ACK, it is an indication
   that the segment identified by the SACK block has been received more
   than once by the receiver.  An implementation MUST NOT compare the
   sequence space in the SACK block to the TCP state variable snd.una
   (which carries the total cumulative ACK), as this may result in the
   wrong conclusion if ACK packets are reordered.

   If the sequence space in the first SACK block is greater than the
   cumulative ACK, then the sender next compares the sequence space in
   the first SACK block with the sequence space in the second SACK
   block, if there is one.  This comparison can determine if the first
   SACK block is reporting duplicate data that lies above the cumulative
   ACK.

   TCP implementations which follow RFC 2581 [RFC2581] could see
   duplicate packets in each of the following four situations.  This
   document does not specify what action a TCP implementation should
   take in these cases.  The extension to the SACK option simply enables
   the sender to detect each of these cases.  Note that these four
   conditions are not an exhaustive list of possible cases for duplicate
   packets, but are representative of the most common/likely cases.
   Subsequent documents will describe experimental proposals for sender
   responses to the detection of unnecessary retransmits due to
   reordering, lost ACKS, or early retransmit timeouts.














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5.1.  Replication by the network

   If a packet is replicated in the network, this extension to the SACK
   option can identify this.  For example:

             Transmitted    Received    ACK Sent
             Segment        Segment     (Including SACK Blocks)

             500-999        500-999     1000
             1000-1499      1000-1499   1500
                            (replicated)
                            1000-1499   1500, SACK=1000-1500
                                                   ---------

   In this case, the second packet was replicated in the network.  An
   ACK containing a D-SACK block which is lower than its ACK field and
   is not identical to a previously retransmitted segment is indicative
   of a replication by the network.

   WITHOUT D-SACK:

   If D-SACK was not used and the last ACK was piggybacked on a data
   packet, the sender would not know that a packet had been replicated
   in the network.  If D-SACK was not used and neither of the last two
   ACKs was piggybacked on a data packet, then the sender could
   reasonably infer that either some data packet *or* the final ACK
   packet had been replicated in the network.  The receipt of the D-SACK
   packet gives the sender positive knowledge that this data packet was
   replicated in the network (assuming that the receiver is not lying).

   RESEARCH ISSUES:

   The current SACK option already allows the sender to identify
   duplicate ACKs that do not acknowledge new data, but the D-SACK
   option gives the sender a stronger basis for inferring that a
   duplicate ACK does not acknowledge new data.  The knowledge that a
   duplicate ACK does not acknowledge new data allows the sender to
   refrain from using that duplicate ACKs to infer packet loss (e.g.,
   Fast Retransmit) or to send more data (e.g., Fast Recovery).

5.2.  False retransmit due to reordering

   If packets are reordered in the network such that a segment arrives
   more than 3 packets out of order, TCP's Fast Retransmit algorithm
   will retransmit the out-of-order packet.  An example of this is shown
   below:





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             Transmitted    Received    ACK Sent
             Segment        Segment     (Including SACK Blocks)

             500-999        500-999     1000
             1000-1499      (delayed)
             1500-1999      1500-1999   1000, SACK=1500-2000
             2000-2499      2000-2499   1000, SACK=1500-2500
             2500-2999      2500-2999   1000, SACK=1500-3000
             1000-1499      1000-1499   3000
                            1000-1499   3000, SACK=1000-1500
                                                   ---------

   In this case, an ACK containing a SACK block which is lower than its
   ACK field and identical to a previously retransmitted segment is
   indicative of a significant reordering followed by a false
   (unnecessary) retransmission.

   WITHOUT D-SACK:

   With the use of D-SACK illustrated above, the sender knows that
   either the first transmission of segment 1000-1499 was delayed in the
   network, or the first transmission of segment 1000-1499 was dropped
   and the second transmission of segment 1000-1499 was duplicated.
   Given that no other segments have been duplicated in the network,
   this second option can be considered unlikely.

   Without the use of D-SACK, the sender would only know that either the
   first transmission of segment 1000-1499 was delayed in the network,
   or that either one of the data segments or the final ACK was
   duplicated in the network.  Thus, the use of D-SACK allows the sender
   to more reliably infer that the first transmission of segment
   1000-1499 was not dropped.

   [AP99], [L99], and [LK00] note that the sender could unambiguously
   detect an unnecessary retransmit with the use of the timestamp
   option.  [LK00] proposes a timestamp-based algorithm that minimizes
   the penalty for an unnecessary retransmit.  [AP99] proposes a
   heuristic for detecting an unnecessary retransmit in an environment
   with neither timestamps nor SACK.  [L99] also proposes a two-bit
   field as an alternate to the timestamp option for unambiguously
   marking the first three retransmissions of a packet.  A similar idea
   was proposed in [ISO8073].

   RESEARCH ISSUES:

   The use of D-SACK allows the sender to detect some cases (e.g., when
   no ACK packets have been lost) when a a Fast Retransmit was due to
   packet reordering instead of packet loss.  This allows the TCP sender



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   to adjust the duplicate acknowledgment threshold, to prevent such
   unnecessary Fast Retransmits in the future.  Coupled with this, when
   the sender determines, after the fact, that it has made an
   unnecessary window reduction, the sender has the option of "undoing"
   that reduction in the congestion window by resetting ssthresh to the
   value of the old congestion window, and slow-starting until the
   congestion window has reached that point.

   Any proposal for "undoing" a reduction in the congestion window would
   have to address the possibility that the TCP receiver could be lying
   in its reports of received packets [SCWA99].

5.3.  Retransmit Timeout Due to ACK Loss

   If an entire window of ACKs is lost, a timeout will result.  An
   example of this is given below:

             Transmitted    Received    ACK Sent
             Segment        Segment     (Including SACK Blocks)

             500-999        500-999     1000 (ACK dropped)
             1000-1499      1000-1499   1500 (ACK dropped)
             1500-1999      1500-1999   2000 (ACK dropped)
             2000-2499      2000-2499   2500 (ACK dropped)
             (timeout)
             500-999        500-999     2500, SACK=500-1000
                                                   --------

   In this case, all of the ACKs are dropped, resulting in a timeout.
   This condition can be identified because the first ACK received
   following the timeout carries a D-SACK block indicating duplicate
   data was received.

   WITHOUT D-SACK:

   Without the use of D-SACK, the sender in this case would be unable to
   decide that no data packets has been dropped.

   RESEARCH ISSUES:

   For a TCP that implements some form of ACK congestion control
   [BPK97], this ability to distinguish between dropped data packets and
   dropped ACK packets would be particularly useful.  In this case, the
   connection could implement congestion control for the return (ACK)
   path independently from the congestion control on the forward (data)
   path.





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5.4.  Early Retransmit Timeout

   If the sender's RTO is too short, an early retransmission timeout can
   occur when no packets have in fact been dropped in the network.  An
   example of this is given below:

             Transmitted    Received    ACK Sent
             Segment        Segment     (Including SACK Blocks)

             500-999        (delayed)
             1000-1499      (delayed)
             1500-1999      (delayed)
             2000-2499      (delayed)
             (timeout)
             500-999        (delayed)
                            500-999     1000
             1000-1499      (delayed)
                            1000-1499   1500
             ...
                            1500-1999   2000
                            2000-2499   2500
                            500-999     2500, SACK=500-1000
                                                   --------
                            1000-1499   2500, SACK=1000-1500
                                                   ---------
                            ...

   In this case, the first packet is retransmitted following the
   timeout.  Subsequently, the original window of packets arrives at the
   receiver, resulting in ACKs for these segments.  Following this, the
   retransmissions of these segments arrive, resulting in ACKs carrying
   SACK blocks which identify the duplicate segments.

   This can be identified as an early retransmission timeout because the
   ACK for byte 1000 is received after the timeout with no SACK
   information, followed by an ACK which carries SACK information (500-
   999) indicating that the retransmitted segment had already been
   received.

   WITHOUT D-SACK:

   If D-SACK was not used and one of the duplicate ACKs was piggybacked
   on a data packet, the sender would not know how many duplicate
   packets had been received.  If D-SACK was not used and none of the
   duplicate ACKs were piggybacked on a data packet, then the sender
   would have sent N duplicate packets, for some N, and received N
   duplicate ACKs.  In this case, the sender could reasonably infer that




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   some data or ACK packet had been replicated in the network, or that
   an early retransmission timeout had occurred (or that the receiver is
   lying).

   RESEARCH ISSUES:

   After the sender determines that an unnecessary (i.e., early)
   retransmit timeout has occurred, the sender could adjust parameters
   for setting the RTO, to prevent more unnecessary retransmit timeouts.
   Coupled with this, when the sender determines, after the fact, that
   it has made an unnecessary window reduction, the sender has the
   option of "undoing" that reduction in the congestion window.

6. Security Considerations

   This document neither strengthens nor weakens TCP's current security
   properties.

7. Acknowledgements

   We would like to thank Mark Handley, Reiner Ludwig, and Venkat
   Padmanabhan for conversations on these issues, and to thank Mark
   Allman for helpful feedback on this document.

8. References

   [AP99]    Mark Allman and Vern Paxson, On Estimating End-to-End
             Network Path Properties, SIGCOMM 99, August 1999.  URL
             "http://www.acm.org/sigcomm/sigcomm99/papers/session7-
             3.html".

   [BPS99]   J.C.R. Bennett, C. Partridge, and N. Shectman, Packet
             Reordering is Not Pathological Network Behavior, IEEE/ACM
             Transactions on Networking, Vol. 7, No. 6, December 1999,
             pp. 789-798.

   [BPK97]   Hari Balakrishnan, Venkata Padmanabhan, and Randy H. Katz,
             The Effects of Asymmetry on TCP Performance, Third ACM/IEEE
             Mobicom Conference, Budapest, Hungary, Sep 1997.  URL
             "http://www.cs.berkeley.edu/~padmanab/
             index.html#Publications".

   [F99]     Floyd, S., Re: TCP and out-of-order delivery, Message ID
             <199902030027.QAA06775@owl.ee.lbl.gov> to the end-to-end-
             interest mailing list, February 1999.  URL
             "http://www.aciri.org/floyd/notes/TCP_Feb99.email".





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   [ISO8073] ISO/IEC, Information-processing systems - Open Systems
             Interconnection - Connection Oriented Transport Protocol
             Specification, Internation Standard ISO/IEC 8073, December
             1988.

   [L99]     Reiner Ludwig, A Case for Flow Adaptive Wireless links,
             Technical Report UCB//CSD-99-1053, May 1999.  URL
             "http://iceberg.cs.berkeley.edu/papers/Ludwig-
             FlowAdaptive/".

   [LK00]    Reiner Ludwig and Randy H. Katz, The Eifel Algorithm:
             Making TCP Robust Against Spurious Retransmissions, SIGCOMM
             Computer Communication Review, V. 30, N. 1, January 2000.
             URL "http://www.acm.org/sigcomm/ccr/archive/ccr-toc/ccr-
             toc-2000.html".

   [RFC1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for
             High Performance", RFC 1323, May 1992.

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

   [RFC2581] Allman, M., Paxson,V. and W. Stevens, "TCP Congestion
             Control", RFC 2581, April 1999.

   [SCWA99]  Stefan Savage, Neal Cardwell, David Wetherall, Tom
             Anderson, TCP Congestion Control with a Misbehaving
             Receiver, ACM Computer Communications Review, pp. 71-78, V.
             29, N. 5, October, 1999.  URL
             "http://www.acm.org/sigcomm/ccr/archive/ccr-toc/ccr-toc-
             99.html".




















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RFC 2883                     SACK Extension                    July 2000


Authors' Addresses

   Sally Floyd
   AT&T Center for Internet Research at ICSI (ACIRI)

   Phone: +1 510-666-6989
   EMail: floyd@aciri.org
   URL:  http://www.aciri.org/floyd/


   Jamshid Mahdavi
   Novell

   Phone: 1-408-967-3806
   EMail: mahdavi@novell.com


   Matt Mathis
   Pittsburgh Supercomputing Center

   Phone: 412 268-3319
   EMail: mathis@psc.edu
   URL: http://www.psc.edu/~mathis/


   Matthew Podolsky
   UC Berkeley Electrical Engineering & Computer Science Dept.

   Phone: 510-649-8914
   EMail: podolsky@eecs.berkeley.edu
   URL: http://www.eecs.berkeley.edu/~podolsky




















Floyd, et al.               Standards Track                    [Page 16]

RFC 2883                     SACK Extension                    July 2000


Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
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   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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