Network Working Group S. Floyd
Request for Comments: 2883 ACIRI
Category: Standards Track J. Mahdavi
Pittsburgh Supercomputing Center
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 (C) The Internet Society (2000). All Rights Reserved.
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 20002. 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
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 20003. 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 |