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]
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]
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]
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]
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
ignore subsequent bad blocks
---------
Floyd, et al.              Standards Track                    [Page 5]

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