TCP Selective Acknowledgement Options (RFC2018)
Original Publication Date: 1996-Oct-01
Included in the Prior Art Database: 2000-Sep-13
Internet Society Requests For Comment (RFCs)
M. Mathis: AUTHOR [+4]
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.
Network Working Group M. Mathis
Request for Comments: 2018 J. Mahdavi
Category: Standards Track PSC
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.
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.
Much of the text in this document is taken directly from RFC1072 "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
Multiple packet losses from a window of data can have a catastrophic
effect on TCP throughput. TCP [Postel81] uses a cumulative
acknowledgment scheme in which received segments that are not at the
left edge of the receive window are not acknowledged. This forces
the sender to either wait a roundtrip time to find out about each
lost packet, or to unnecessarily retransmit segments which have been
correctly received [Fall95]. With the cumulative acknowledgment
scheme, multiple dropped segments generally cause TCP to lose its
ACK-based clock, reducing overall throughput.
Selective Acknowledgment (SACK) is a strategy which corrects this
behavior in the face of multiple dropped segments. With selective