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rfc:rfc2883

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].

Floyd, et al. Standards Track [Page 1] RFC 2883 SACK Extension July 2000

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.

Floyd, et al. Standards Track [Page 2] RFC 2883 SACK Extension July 2000

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.

Floyd, et al. Standards Track [Page 3] RFC 2883 SACK Extension July 2000

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.

Floyd, et al. Standards Track [Page 4] RFC 2883 SACK Extension July 2000

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
                                               ---------

Floyd, et al. Standards Track [Page 5] RFC 2883 SACK Extension July 2000

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.

Floyd, et al. Standards Track [Page 6] RFC 2883 SACK Extension July 2000

      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.

Floyd, et al. Standards Track [Page 7] RFC 2883 SACK Extension July 2000

       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.

Floyd, et al. Standards Track [Page 8] RFC 2883 SACK Extension July 2000

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.

Floyd, et al. Standards Track [Page 9] RFC 2883 SACK Extension July 2000

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:

Floyd, et al. Standards Track [Page 10] RFC 2883 SACK Extension July 2000

           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

Floyd, et al. Standards Track [Page 11] RFC 2883 SACK Extension July 2000

 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.

Floyd, et al. Standards Track [Page 12] RFC 2883 SACK Extension July 2000

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

Floyd, et al. Standards Track [Page 13] RFC 2883 SACK Extension July 2000

 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".

Floyd, et al. Standards Track [Page 14] RFC 2883 SACK Extension July 2000

 [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".

Floyd, et al. Standards Track [Page 15] 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
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Floyd, et al. Standards Track [Page 16] RFC 2883 SACK Extension July 2000

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Floyd, et al. Standards Track [Page 17]

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