The NewReno Modification to TCP's Fast Recovery Algorithm
RFC 2582

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Network Working Group                                           S. Floyd
Request for Comments: 2582                                         ACIRI
Category: Experimental                                      T. Henderson
                                                           U.C. Berkeley
                                                              April 1999

       The NewReno Modification to TCP's Fast Recovery Algorithm

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   RFC 2001 [RFC2001] documents the following four intertwined TCP
   congestion control algorithms: Slow Start, Congestion Avoidance, Fast
   Retransmit, and Fast Recovery.  RFC 2581 [RFC2581] explicitly allows
   certain modifications of these algorithms, including modifications
   that use the TCP Selective Acknowledgement (SACK) option [MMFR96],
   and modifications that respond to "partial acknowledgments" (ACKs
   which 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 in [Hoe95].

1. Introduction

   For the typical implementation of the TCP Fast Recovery algorithm
   described in [RFC2581] (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 acknowledgements 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 Reno Fast Retransmit algorithm leads to
   the retransmission of only a single data packet.

Floyd & Henderson             Experimental                      [Page 1]
RFC 2582      NewReno Modification to TCP's Fast Recovery     April 1999

   Problems can arise, therefore, when multiple packets have been
   dropped from a single window of data and the Fast Retransmit and Fast
   Recovery algorithms are invoked.  In this case, if the SACK option 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.  This document applies only for TCP
   connections that are unable to use the TCP Selective Acknowledgement
   (SACK) option.

   In the absence of SACK, there is little information available to the
   TCP sender in making retransmission decisions during Fast Recovery.
   From the three duplicate acknowledgements, the sender infers a packet
   loss, and retransmits the indicated packet.  After this, the data
   sender could receive additional duplicate acknowledgements, as the
   data receiver acknowledges additional data packets that were already
   in flight when the sender entered Fast Retransmit.

   In the case of multiple packets dropped from a single window of data,
   the first new information available to the sender comes when the
   sender receives an acknowledgement for the retransmitted packet (that
   is the packet retransmitted when Fast Retransmit was first entered).
   If there had been a single packet drop, then the acknowledgement for
   this packet will acknowledge all of the packets transmitted before
   Fast Retransmit was entered (in the absence of reordering).  However,
   when there were multiple packet drops, then the acknowledgement for
   the retransmitted packet will acknowledge some but not all of the
   packets transmitted before the Fast Retransmit.  We call this packet
   a partial acknowledgment.

   Along with several other suggestions, [Hoe95] suggested that during
   Fast Recovery the TCP data sender respond to a partial acknowledgment
   by inferring that the indicated packet has been lost, and
   retransmitting that packet.  This document describes a modification
   to the Fast Recovery algorithm in Reno TCP that incorporates a
   response to partial acknowledgements received during Fast Recovery.
   We call this modified Fast Recovery algorithm NewReno, because it is
   a slight but significant variation of the basic Reno algorithm.  This
   document does not discuss the other suggestions in [Hoe95] and
   [Hoe96], such as a change to the ssthresh parameter during Slow-
   Start, or the proposal to send a new packet for every two duplicate
   acknowledgements during Fast Recovery.  The version of NewReno in
   this document also draws on other discussions of NewReno in the
   literature [LM97].

   We do not claim that the NewReno version of Fast Recovery described