Early Retransmit for TCP and Stream Control Transmission Protocol (SCTP)
RFC 5827

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• 04

Internet Engineering Task Force (IETF)                         M. Allman
Request for Comments: 5827                                          ICSI
Category: Experimental                                    K. Avrachenkov
ISSN: 2070-1721                                                    INRIA
                                                               U. Ayesta
                                           BCAM-IKERBASQUE and LAAS-CNRS
                                                              J. Blanton
                                                         Ohio University
                                                               P. Hurtig
                                                     Karlstad University
                                                              April 2010

                        Early Retransmit for TCP
            and Stream Control Transmission Protocol (SCTP)

Abstract

   This document proposes a new mechanism for TCP and Stream Control
   Transmission Protocol (SCTP) that can be used to recover lost
   segments when a connection's congestion window is small.  The "Early
   Retransmit" mechanism allows the transport to reduce, in certain
   special circumstances, the number of duplicate acknowledgments
   required to trigger a fast retransmission.  This allows the transport
   to use fast retransmit to recover segment losses that would otherwise
   require a lengthy retransmission timeout.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5827.

Allman, et al.                Experimental                      [Page 1]
RFC 5827            Early Retransmit for TCP and SCTP         April 2010

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   document authors.  All rights reserved.

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   Contributions published or made publicly available before November
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   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

1.  Introduction

   Many researchers have studied the problems with TCP's loss recovery
   [RFC793, RFC5681] when the congestion window is small, and they have
   outlined possible mechanisms to mitigate these problems
   [Mor97, BPS+98, Bal98, LK98, RFC3150, AA02].  SCTP's [RFC4960] loss
   recovery and congestion control mechanisms are based on TCP, and
   therefore the same problems impact the performance of SCTP
   connections.  When the transport detects a missing segment, the
   connection enters a loss recovery phase.  There are several variants
   of the loss recovery phase depending on the TCP implementation.  TCP
   can use slow-start-based recovery or fast recovery [RFC5681], NewReno
   [RFC3782], and loss recovery, based on selective acknowledgments
   (SACKs) [RFC2018, FF96, RFC3517].  SCTP's loss recovery is not as
   varied due to the built-in selective acknowledgments.

   All of the above variants have two methods for invoking loss
   recovery.  First, if an acknowledgment (ACK) for a given segment is
   not received in a certain amount of time, a retransmission timer
   fires, and the segment is resent [RFC2988, RFC4960].  Second, the