The NewReno Modification to TCP's Fast Recovery Algorithm
RFC 6582

Internet Engineering Task Force (IETF)                      T. Henderson
Request for Comments: 6582                                        Boeing
Obsoletes: 3782                                                 S. Floyd
Category: Standards Track                                           ICSI
ISSN: 2070-1721                                                A. Gurtov
                                                      University of Oulu
                                                              Y. Nishida
                                                            WIDE Project
                                                              April 2012

       The NewReno Modification to TCP's Fast Recovery Algorithm

Abstract

   RFC 5681 documents the following four intertwined TCP congestion
   control algorithms: slow start, congestion avoidance, fast
   retransmit, and fast recovery.  RFC 5681 explicitly allows certain
   modifications of these algorithms, including modifications that use
   the TCP Selective Acknowledgment (SACK) option (RFC 2883), and
   modifications that respond to "partial acknowledgments" (ACKs that
   cover new data, but not all the data outstanding when loss was
   detected) in the absence of SACK.  This document describes a specific
   algorithm for responding to partial acknowledgments, referred to as
   "NewReno".  This response to partial acknowledgments was first
   proposed by Janey Hoe.  This document obsoletes RFC 3782.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc6582.

Henderson, et al.            Standards Track                    [Page 1]
RFC 6582                      TCP NewReno                     April 2012

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1.  Introduction

   For the typical implementation of the TCP fast recovery algorithm
   described in [RFC5681] (first implemented in the 1990 BSD Reno
   release, and referred to as the "Reno algorithm" in [FF96]), the TCP
   data sender only retransmits a packet after a retransmit timeout has
   occurred, or after three duplicate acknowledgments have arrived
   triggering the fast retransmit algorithm.  A single retransmit
   timeout might result in the retransmission of several data packets,
   but each invocation of the fast retransmit algorithm in RFC 5681
   leads to the retransmission of only a single data packet.

   Two problems arise with Reno TCP when multiple packet losses occur in
   a single window.  First, Reno will often take a timeout, as has been
   documented in [Hoe95].  Second, even if a retransmission timeout is
   avoided, multiple fast retransmits and window reductions can occur,
   as documented in [F94].  When multiple packet losses occur, if the
   SACK option [RFC2883] is available, the TCP sender has the
   information to make intelligent decisions about which packets to
   retransmit and which packets not to retransmit during fast recovery.