RFC 2018 (rfc2018) - Page 1 of 12

TCP Selective Acknowledgement Options

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Network Working Group                                        M. Mathis
Request for Comments: 2018                                  J. Mahdavi
Category: Standards Track                                          PSC
                                                              S. Floyd
                                                                  LBNL
                                                            A. Romanow
                                                      Sun Microsystems
                                                          October 1996


                  TCP Selective Acknowledgment Options

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.

Abstract

   TCP may experience poor performance when multiple packets are lost
   from one window of data.   With the limited information available
   from cumulative acknowledgments, a TCP sender can only learn about a
   single lost packet per round trip time.  An aggressive sender could
   choose to retransmit packets early, but such retransmitted segments
   may have already been successfully received.

   A Selective Acknowledgment (SACK) mechanism, combined with a
   selective repeat retransmission policy, can help to overcome these
   limitations.  The receiving TCP sends back SACK packets to the sender
   informing the sender of data that has been received. The sender can
   then retransmit only the missing data segments.

   This memo proposes an implementation of SACK and discusses its
   performance and related issues.

Acknowledgements

   Much of the text in this document is taken directly from RFC 1072 "TCP
   Extensions for Long-Delay Paths" by Bob Braden and Van Jacobson.  The
   authors would like to thank Kevin Fall (LBNL), Christian Huitema
   (INRIA), Van Jacobson (LBNL), Greg Miller (MITRE), Greg Minshall
   (Ipsilon), Lixia Zhang (XEROX PARC and UCLA), Dave Borman (BSDI),
   Allison Mankin (ISI) and others for their review and constructive
   comments.




Mathis, et. al.             Standards Track
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