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        RFC 2883

        Title:      An Extension to the Selective Acknowledgement
                    (SACK) Option for TCP 
        Author(s):  S. Floyd, J. Mahdavi, M. Mathis, M. Podolsky
        Status:     Standards Track
        Date:       July 2000
        Mailbox:    floyd@aciri.org, mahdavi@novell.com,
                    mathis@psc.edu, podolsky@eecs.berkeley.edu
        Pages:      17
        Characters: 35794
        Updates/Obsoletes/SeeAlso:  None

        I-D Tag:    draft-floyd-sack-00.txt

        URL:        ftp://ftp.isi.edu/in-notes/rfc2883.txt


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.

This document is a product of the Transport Area Working Group Working
Group of the IETF.

This is now a Proposed Standard Protocol.

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
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