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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.






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   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
   here is an optimal modification of Fast Recovery for responding to
   partial acknowledgements, for TCPs that are unable to use SACK.
   Based on our experiences with the NewReno modification in the NS



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   simulator [NS], we believe that this modification improves the
   performance of the Fast Retransmit and Fast Recovery algorithms in a
   wide variety of scenarios, and we are simply documenting it for the
   benefit of the IETF community.  We encourage the use of this
   modification to Fast Recovery, and we further encourage feedback
   about operational experiences with this or related modifications.

2. Definitions

   This document assumes that the reader is familiar with the terms
   MAXIMUM SEGMENT SIZE (MSS), CONGESTION WINDOW (cwnd), and FLIGHT SIZE
   (FlightSize) defined in [RFC2581].  FLIGHT SIZE is defined as in
   [RFC2581] as follows:

      FLIGHT SIZE:
         The amount of data that has been sent but not yet acknowledged.

3. The Fast Retransmit and Fast Recovery algorithms in NewReno

   The standard implementation of the Fast Retransmit and Fast Recovery
   algorithms is given in [RFC2581].  The NewReno modification of these
   algorithms is given below.  This NewReno modification differs from
   the implementation in [RFC2581] only in the introduction of the
   variable "recover" in step 1, and in the response to a partial or new
   acknowledgement in step 5.  The modification defines a "Fast Recovery
   procedure" that begins when three duplicate ACKs are received and
   ends when either a retransmission timeout occurs or an ACK arrives
   that acknowledges all of the data up to and including the data that
   was outstanding when the Fast Recovery procedure began.

   1.  When the third duplicate ACK is received and the sender is not
       already in the Fast Recovery procedure, set ssthresh to no more
       than the value given in equation 1 below.  (This is equation 3
       from [RFC2581]).

         ssthresh = max (FlightSize / 2, 2*MSS)           (1)

       Record the highest sequence number transmitted in the variable
       "recover".

   2.  Retransmit the lost segment and set cwnd to ssthresh plus 3*MSS.
       This artificially "inflates" the congestion window by the number
       of segments (three) that have left the network and which the
       receiver has buffered.

   3.  For each additional duplicate ACK received, increment cwnd by
       MSS.  This artificially inflates the congestion window in order
       to reflect the additional segment that has left the network.



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   4.  Transmit a segment, if allowed by the new value of cwnd and the
       receiver's advertised window.

   5.  When an ACK arrives that acknowledges new data, this ACK could be
       the acknowledgment elicited by the retransmission from step 2, or
       elicited by a later retransmission.

       If this ACK acknowledges all of the data up to and including
       "recover", then the ACK acknowledges all the intermediate
       segments sent between the original transmission of the lost
       segment and the receipt of the third duplicate ACK.  Set cwnd to
       either (1) min (ssthresh, FlightSize + MSS); or (2) ssthresh,
       where ssthresh is the value set in step 1; this is termed
       "deflating" the window.  (We note that "FlightSize" in step 1
       referred to the amount of data outstanding in step 1, when Fast
       Recovery was entered, while "FlightSize" in step 5 refers to the
       amount of data outstanding in step 5, when Fast Recovery is
       exited.) If the second option is selected, the implementation
       should take measures to avoid a possible burst of data, in case
       the amount of data outstanding in the network was much less than
       the new congestion window allows [HTH98].  Exit the Fast Recovery
       procedure.

       If this ACK does *not* acknowledge all of the data up to and
       including "recover", then this is a partial ACK.  In this case,
       retransmit the first unacknowledged segment.  Deflate the
       congestion window by the amount of new data acknowledged, then
       add back one MSS and send a new segment if permitted by the new
       value of cwnd.  This "partial window deflation" attempts to
       ensure that, when Fast Recovery eventually ends, approximately
       ssthresh amount of data will be outstanding in the network.  Do
       not exit the Fast Recovery procedure (i.e., if any duplicate ACKs
       subsequently arrive, execute Steps 3 and 4 above).


       For the first partial ACK that arrives during Fast Recovery, also
       reset the retransmit timer.

   Note that in Step 5, the congestion window is deflated when a partial
   acknowledgement is received.  The congestion window was likely to
   have been inflated considerably when the partial acknowledgement was
   received.  In addition, depending on the original pattern of packet
   losses, the partial acknowledgement might acknowledge nearly a window
   of data.  In this case, if the congestion window was not deflated,
   the data sender might be able to send nearly a window of data back-
   to-back.

   There are several possible variants to the simple response to partial



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   acknowledgements described above.  First, there is a question of when
   to reset the retransmit timer after a partial acknowledgement.  This
   is discussed further in Section 4 below.

   There is a related question of how many packets to retransmit after
   each partial acknowledgement.  The algorithm described above
   retransmits a single packet after each partial acknowledgement.  This
   is the most conservative alternative, in that it is the least likely
   to result in an unnecessarily-retransmitted packet.  A variant that
   would recover faster from a window with many packet drops would be to
   effectively Slow-Start, requiring less than N roundtrip times to
   recover from N losses [Hoe96].  With this slightly-more-aggressive
   response to partial acknowledgements, it would be advantageous to
   reset the retransmit timer after each retransmission.  Because we
   have not experimented with this variant in our simulator, we do not
   discuss this variant further in this document.

   A third question involves avoiding multiple Fast Retransmits caused
   by the retransmission of packets already received by the receiver.
   This is discussed in Section 5 below.  Avoiding multiple Fast
   Retransmits is particularly important if more aggressive responses to
   partial acknowledgements are implemented, because in this case the
   sender is more likely to retransmit packets already received by the
   receiver.

   As a final note, we would observe that in the absence of the SACK
   option, the data sender is working from limited information.  One
   could spend a great deal of time considering exactly which variant of
   Fast Recovery is optimal for which scenario in this case.  When the
   issue of recovery from multiple dropped packets from a single window
   of data is of particular importance, the best alternative would be to
   use the SACK option.

4. Resetting the retransmit timer.

   The algorithm in Section 3 resets the retransmit timer only after the
   first partial ACK.  In this case, if a large number of packets were
   dropped from a window of data, the TCP data sender's retransmit timer
   will ultimately expire, and the TCP data sender will invoke Slow-
   Start.  (This is illustrated on page 12 of [F98].)  We call this the
   Impatient variant of NewReno.

   In contrast, the NewReno simulations in [FF96] illustrate the
   algorithm described above, with the modification that the retransmit
   timer is reset after each partial acknowledgement.  We call this the
   Slow-but-Steady variant of NewReno.  In this case, for a window with
   a large number of packet drops, the TCP data sender retransmits at
   most one packet per roundtrip time.  (This behavior is illustrated in



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   the New-Reno TCP simulation of Figure 5 in [FF96], and on page 11 of
   [F98].)

   For TCP implementations where the Retransmission Timeout Value (RTO)
   is generally not much larger than the round-trip time (RTT), the
   Impatient variant can result in a retransmit timeout even in a
   scenario with a small number of packet drops.  For TCP
   implementations where the Retransmission Timeout Value (RTO) is
   usually considerably larger than the round-trip time (RTT), the Slow-
   but-Steady variant can remain in Fast Recovery for a long time when
   multiple packets have been dropped from a window of data.  Neither of
   these variants are optimal; one possibility for a more optimal
   algorithm might be one that recovered more quickly from multiple
   packet drops, and combined this with the Slow-but-Steady variant in
   terms of resetting the retransmit timers.  We note, however, that
   there is a limitation to the potential performance in this case in
   the absence of the SACK option.

5. Avoiding Multiple Fast Retransmits

   In the absence of the SACK option, a duplicate acknowledgement
   carries no information to identify the data packet or packets at the
   TCP data receiver that triggered that duplicate acknowledgement.  The
   TCP data sender is unable to distinguish between a duplicate
   acknowledgement that results from a lost or delayed data packet, and
   a duplicate acknowledgement that results from the sender's
   retransmission of a data packet that had already been received at the
   TCP data receiver.  Because of this, multiple segment losses from a
   single window of data can sometimes result in unnecessary multiple
   Fast Retransmits (and multiple reductions of the congestion window)
   [Flo94].

   With the Fast Retransmit and Fast Recovery algorithms in Reno or
   NewReno TCP, the performance problems caused by multiple Fast
   Retransmits are relatively minor (compared to the potential problems
   with Tahoe TCP, which does not implement Fast Recovery).
   Nevertheless, unnecessary Fast Retransmits can occur with Reno or
   NewReno TCP, particularly if a Retransmit Timeout occurs during Fast
   Recovery.  (This is illustrated for Reno on page 6 of [F98], and for
   NewReno on page 8 of [F98].)  With NewReno, the data sender remains
   in Fast Recovery until either a Retransmit Timeout, or until all of
   the data outstanding when Fast Retransmit was entered has been
   acknowledged.  Thus with NewReno, the problem of multiple Fast
   Retransmits from a single window of data can only occur after a
   Retransmit Timeout.

   The following modification to the algorithms in Section 3 eliminates
   the problem of multiple Fast Retransmits.  (This modification is



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