rfc2883.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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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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