path: root/doc/rfc/rfc3782.txt

Network Working Group                                           S. Floyd
Request for Comments: 3782                                          ICSI
Obsoletes: 2582                                             T. Henderson
Category: Standards Track                                         Boeing
                                                               A. Gurtov
                                                             TeliaSonera
                                                              April 2004


       The NewReno Modification to TCP's Fast Recovery Algorithm

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.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   The purpose of this document is to advance NewReno TCP's  Fast
   Retransmit and Fast Recovery algorithms in RFC 2582 from Experimental
   to Standards Track status.

   The main change in this document relative to RFC 2582 is to specify
   the Careful variant of NewReno's Fast Retransmit and Fast Recovery
   algorithms.  The base algorithm described in RFC 2582 did not attempt
   to avoid unnecessary multiple Fast Retransmits that can occur after a
   timeout.  However, RFC 2582 also defined "Careful" and "Less Careful"
   variants that avoid these unnecessary Fast Retransmits, and
   recommended the Careful variant.  This document specifies the
   previously-named "Careful" variant as the basic version of NewReno
   TCP.













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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 Fast Retransmit algorithm in RFC 2581
   leads to the retransmission of only a single data packet.

   Problems can arise, therefore, when multiple packets are 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, either because the option is not locally supported or
   because the TCP peer did not indicate a willingness to use SACK.

   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 is a single packet drop and no reordering, then the
   acknowledgement for this packet will acknowledge all of the packets
   transmitted before Fast Retransmit was entered.  However, if there
   are 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 acknowledgement
   a partial acknowledgment.

   Along with several other suggestions, [Hoe95] suggested that during
   Fast Recovery the TCP data sender responds to a partial
   acknowledgment by inferring that the next in-sequence packet has been
   lost, and retransmitting that packet.  This document describes a
   modification to the Fast Recovery algorithm in RFC 2581 that
   incorporates a response to partial acknowledgements received during



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   Fast Recovery.  We call this modified Fast Recovery algorithm
   NewReno, because it is a slight but significant variation of the
   basic Reno algorithm in RFC 2581.  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, Hen98].

   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 TCP connections that are unable to use
   SACK.  Based on our experiences with the NewReno modification in the
   NS simulator [NS] and with numerous implementations of NewReno, we
   believe that this modification improves the performance of the Fast
   Retransmit and Fast Recovery algorithms in a wide variety of
   scenarios.

2.  Terminology and Definitions

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119].  This RFC indicates requirement levels for compliant TCP
   implementations implementing the NewReno Fast Retransmit and Fast
   Recovery algorithms described in this document.

   This document assumes that the reader is familiar with the terms
   SENDER MAXIMUM SEGMENT SIZE (SMSS), 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].  This section specifies the basic
   NewReno algorithm.  Sections 4 through 6 describe some optional
   variants, and the motivations behind them, that an implementor may
   want to consider when tuning performance for certain network
   scenarios.  Sections 7 and 8 provide some guidance to implementors
   based on experience with NewReno implementations.

   The NewReno modification concerns the 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



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   of the data up to and including the data that was outstanding when
   the Fast Recovery procedure began.

   The NewReno algorithm specified in this document differs from the
   implementation in [RFC2581] in the introduction of the variable
   "recover" in step 1, in the response to a partial or new
   acknowledgement in step 5, and in modifications to step 1 and the
   addition of step 6 for avoiding multiple Fast Retransmits caused by
   the retransmission of packets already received by the receiver.

   The algorithm specified in this document uses a variable "recover",
   whose initial value is the initial send sequence number.

   1)  Three duplicate ACKs:
       When the third duplicate ACK is received and the sender is not
       already in the Fast Recovery procedure, check to see if the
       Cumulative Acknowledgement field covers more than "recover".  If
       so, go to Step 1A.  Otherwise, go to Step 1B.

   1A) Invoking Fast Retransmit:
       If so, then set ssthresh to no more than the value given in
       equation 1 below.  (This is equation 3 from [RFC2581]).

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

       In addition, record the highest sequence number transmitted in
       the variable "recover", and go to Step 2.

   1B) Not invoking Fast Retransmit:
       Do not enter the Fast Retransmit and Fast Recovery procedure.  In
       particular, do not change ssthresh, do not go to Step 2 to
       retransmit the "lost" segment, and do not execute Step 3 upon
       subsequent duplicate ACKs.

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

   3)  Fast Recovery:
       For each additional duplicate ACK received while in Fast
       Recovery, increment cwnd by SMSS.  This artificially inflates the
       congestion window in order to reflect the additional segment that
       has left the network.






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   4)  Fast Recovery, continued:
       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.

       Full acknowledgements:
       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 + SMSS) 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 is
       encouraged to take measures to avoid a possible burst of data, in
       case the amount of data outstanding in the network is much less
       than the new congestion window allows.  A simple mechanism is to
       limit the number of data packets that can be sent in response to
       a single acknowledgement; this is known as "maxburst_" in the NS
       simulator.  Exit the Fast Recovery procedure.

       Partial acknowledgements:
       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 by the
       cumulative acknowledgement field.  If the partial ACK
       acknowledges at least one SMSS of new data, then add back SMSS
       bytes to the congestion window.  As in Step 3, this artificially
       inflates the congestion window in order to reflect the additional
       segment that has left the network.  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.  Timer management is discussed in
       more detail in Section 4.




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   6)  Retransmit timeouts:
       After a retransmit timeout, record the highest sequence number
       transmitted in the variable "recover" and exit the Fast Recovery
       procedure if applicable.

   Step 1 specifies a check that the Cumulative Acknowledgement field
   covers more than "recover".  Because the acknowledgement field
   contains the sequence number that the sender next expects to receive,
   the acknowledgement "ack_number" covers more than "recover" when:

      ack_number - 1 > recover;

   i.e., at least one byte more of data is acknowledged beyond the
   highest byte that was outstanding when Fast Retransmit was last
   entered.

   Note that in Step 5, the congestion window is deflated after 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.

   This document does not specify the sender's response to duplicate
   ACKs when the Fast Retransmit/Fast Recovery algorithm is not invoked.
   This is addressed in other documents, such as those describing the
   Limited Transmit procedure [RFC3042].  This document also does not
   address issues of adjusting the duplicate acknowledgement threshold,
   but assumes the threshold specified in the IETF standards; the
   current standard is RFC 2581, which specifies a threshold of three
   duplicate acknowledgements.

   As a final note, we would observe that in the absence of the SACK
   option, the data sender is working from limited information.  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 in Response to Partial
    Acknowledgements

   One possible variant to the response to partial acknowledgements
   specified in Section 3 concerns when to reset the retransmit timer
   after a partial acknowledgement.  The algorithm in Section 3, Step 5,
   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



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   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.
   We note that the Impatient variant in Section 3 doesn't follow the
   recommended algorithm in RFC 2988 of restarting the retransmit timer
   after every packet transmission or retransmission [RFC2988, Step
   5.1].

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

   When N packets have been dropped from a window of data for a large
   value of N, the Slow-but-Steady variant can remain in Fast Recovery
   for N round-trip times, retransmitting one more dropped packet each
   round-trip time; for these scenarios, the Impatient variant gives a
   faster recovery and better performance.  The tests "ns test-suite-
   newreno.tcl impatient1" and "ns test-suite-newreno.tcl slow1" in the
   NS simulator illustrate such a scenario, where the Impatient variant
   performs better than the Slow-but-Steady variant.  The Impatient
   variant can be particularly important for TCP connections with large
   congestion windows, as illustrated by the tests "ns test-suite-
   newreno.tcl impatient4" and "ns test-suite-newreno.tcl slow4" in the
   NS simulator.

   One can also construct scenarios where the Slow-but-Steady variant
   gives better performance than the Impatient variant.  As an example,
   this occurs when only a small number of packets are dropped, the RTO
   is sufficiently small that the retransmit timer expires, and
   performance would have been better without a retransmit timeout.  The
   tests "ns test-suite-newreno.tcl impatient2" and "ns test-suite-
   newreno.tcl slow2" in the NS simulator illustrate such a scenario.

   The Slow-but-Steady variant can also achieve higher goodput than the
   Impatient variant, by avoiding unnecessary retransmissions.  This
   could be of special interest for cellular links, where every
   transmission costs battery power and money.  The tests "ns test-
   suite-newreno.tcl impatient3" and "ns test-suite-newreno.tcl slow3"
   in the NS simulator illustrate such a scenario.  The Slow-but-Steady
   variant can also be more robust to delay variation in the network,
   where a delay spike might force the Impatient variant into a timeout
   and go-back-N recovery.




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   Neither of the two variants discussed above are optimal.  Our
   recommendation is for the Impatient variant, as specified in Section
   3 of this document, because of the poor performance of the Slow-but-
   Steady variant for TCP connections with large congestion windows.

   One possibility for a more optimal algorithm would be one that
   recovered from multiple packet drops as quickly as does slow-start,
   while resetting the retransmit timers after each partial
   acknowledgement, as described in the section below.  We note,
   however, that there is a limitation to the potential performance in
   this case in the absence of the SACK option.

5.  Retransmissions after a Partial Acknowledgement

   One possible variant to the response to partial acknowledgements
   specified in Section 3 would be to retransmit more than one packet
   after each partial acknowledgement, and to reset the retransmit timer
   after each retransmission.  The algorithm specified in Section 3
   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, retransmitting two packets after each partial
   acknowledgement.  Such an approach would take less than N roundtrip
   times to recover from N losses [Hoe96].  However, in the absence of
   SACK, recovering as quickly as slow-start introduces the likelihood
   of unnecessarily retransmitting packets, and this could significantly
   complicate the recovery mechanisms.

   We note that the response to partial acknowledgements specified in
   Section 3 of this document and in RFC 2582 differs from the response
   in [FF96], even though both approaches only retransmit one packet in
   response to a partial acknowledgement.  Step 5 of Section 3 specifies
   that the TCP sender responds to a partial ACK by deflating the
   congestion window by the amount of new data acknowledged, adding back
   SMSS bytes if the partial ACK acknowledges at least SMSS bytes of new
   data, and sending a new segment if permitted by the new value of
   cwnd.  Thus, only one previously-sent packet is retransmitted in
   response to each partial acknowledgement, but additional new packets
   might be transmitted as well, depending on the amount of new data
   acknowledged by the partial acknowledgement.  In contrast, the
   variant of NewReno illustrated in [FF96] simply set the congestion
   window to ssthresh when a partial acknowledgement was received.  The
   approach in [FF96] is more conservative, and does not attempt to
   accurately track the actual number of outstanding packets after a
   partial acknowledgement is received.  While either of these
   approaches gives acceptable performance, the variant specified in
   Section 3 recovers more smoothly when multiple packets are dropped



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   from a window of data.  (The [FF96] behavior can be seen in the NS
   simulator by setting the variable "partial_window_deflation_" for
   "Agent/TCP/Newreno" to 0; the behavior specified in Section 3 is
   achieved by setting "partial_window_deflation_" to 1.)

6.  Avoiding Multiple Fast Retransmits

   This section describes the motivation for the sender's state variable
   "recover", and discusses possible heuristics for distinguishing
   between a retransmitted packet that was dropped, and three duplicate
   acknowledgements from the unnecessary retransmission of three
   packets.

   In the absence of the SACK option or timestamps, a duplicate
   acknowledgement carries no information to identify the data packet or
   packets at the TCP data receiver that triggered that duplicate
   acknowledgement.  In this case, 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 unnecessary retransmission of a data packet
   that had already been received at the TCP data receiver.  Because of
   this, with the Retransmit and Fast Recovery algorithms in Reno TCP,
   multiple segment losses from a single window of data can sometimes
   result in unnecessary multiple Fast Retransmits (and multiple
   reductions of the congestion window) [F94].

   With the Fast Retransmit and Fast Recovery algorithms in Reno 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 TCP unless some explicit
   mechanism is added to avoid this, such as the use of the "recover"
   variable.  (This modification is called "bugfix" in [F98], and is
   illustrated on pages 7 and 9 of that document.  Unnecessary Fast
   Retransmits for Reno without "bugfix" is illustrated on page 6 of
   [F98].)

   Section 3 of [RFC2582] defined a default variant of NewReno TCP that
   did not use the variable "recover", and did not check if duplicate
   ACKs cover the variable "recover" before invoking Fast Retransmit.
   With this default variant from RFC 2582, the problem of multiple Fast
   Retransmits from a single window of data can occur after a Retransmit
   Timeout (as in page 8 of [F98]) or in scenarios with reordering (as
   in the validation test "./test-all-newreno newreno5_noBF" in
   directory "tcl/test" of the NS simulator.  This gives performance
   similar to that on page 8 of [F03].)  RFC 2582 also defined Careful
   and Less Careful variants of the NewReno algorithm, and recommended
   the Careful variant.



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   The algorithm specified in Section 3 of this document corresponds to
   the Careful variant of NewReno TCP from RFC 2582, and eliminates the
   problem of multiple Fast Retransmits.  This algorithm uses the
   variable "recover", whose initial value is the initial send sequence
   number.  After each retransmit timeout, the highest sequence number
   transmitted so far is recorded in the variable "recover".

   If, after a retransmit timeout, the TCP data sender retransmits three
   consecutive packets that have already been received by the data
   receiver, then the TCP data sender will receive three duplicate
   acknowledgements that do not cover more than "recover".  In this
   case, the duplicate acknowledgements are not an indication of a new
   instance of congestion.  They are simply an indication that the
   sender has unnecessarily retransmitted at least three packets.

   However, when a retransmitted packet is itself dropped, the sender
   can also receive three duplicate acknowledgements that do not cover
   more than "recover".  In this case, the sender would have been better
   off if it had initiated Fast Retransmit.  For a TCP that implements
   the algorithm specified in Section 3 of this document, the sender
   does not infer a packet drop from duplicate acknowledgements in this
   scenario.  As always, the retransmit timer is the backup mechanism
   for inferring packet loss in this case.

   There are several heuristics, based on timestamps or on the amount of
   advancement of the cumulative acknowledgement field, that allow the
   sender to distinguish, in some cases, between three duplicate
   acknowledgements following a retransmitted packet that was dropped,
   and three duplicate acknowledgements from the unnecessary
   retransmission of three packets [Gur03, GF04].  The TCP sender MAY
   use such a heuristic to decide to invoke a Fast Retransmit in some
   cases, even when the three duplicate acknowledgements do not cover
   more than "recover".

   For example, when three duplicate acknowledgements are caused by the
   unnecessary retransmission of three packets, this is likely to be
   accompanied by the cumulative acknowledgement field advancing by at
   least four segments.  Similarly, a heuristic based on timestamps uses
   the fact that when there is a hole in the sequence space, the
   timestamp echoed in the duplicate acknowledgement is the timestamp of
   the most recent data packet that advanced the cumulative
   acknowledgement field [RFC1323].  If timestamps are used, and the
   sender stores the timestamp of the last acknowledged segment, then
   the timestamp echoed by duplicate acknowledgements can be used to
   distinguish between a retransmitted packet that was dropped and three
   duplicate acknowledgements from the unnecessary retransmission of
   three packets.  The heuristics are illustrated in the NS simulator in
   the validation test "./test-all-newreno".



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6.1.  ACK Heuristic

   If the ACK-based heuristic is used, then following the advancement of
   the cumulative acknowledgement field, the sender stores the value of
   the previous cumulative acknowledgement as prev_highest_ack, and
   stores the latest cumulative ACK as highest_ack.  In addition, the
   following step is performed if Step 1 in Section 3 fails, before
   proceeding to Step 1B.

   1*)  If the Cumulative Acknowledgement field didn't cover more than
        "recover", check to see if the congestion window is greater than
        SMSS bytes and the difference between highest_ack and
        prev_highest_ack is at most 4*SMSS bytes.  If true, duplicate
        ACKs indicate a lost segment (proceed to Step 1A in Section 3).
        Otherwise, duplicate ACKs likely result from unnecessary
        retransmissions (proceed to Step 1B in Section 3).

   The congestion window check serves to protect against fast retransmit
   immediately after a retransmit timeout, similar to the
   "exitFastRetrans_" variable in NS.  Examples of applying the ACK
   heuristic are in validation tests "./test-all-newreno
   newreno_rto_loss_ack" and "./test-all-newreno newreno_rto_dup_ack" in
   directory "tcl/test" of the NS simulator.

   If several ACKs are lost, the sender can see a jump in the cumulative
   ACK of more than three segments, and the heuristic can fail.  A
   validation test for this scenario is "./test-all-newreno
   newreno_rto_loss_ackf".  RFC 2581 recommends that a receiver should
   send duplicate ACKs for every out-of-order data packet, such as a
   data packet received during Fast Recovery.  The ACK heuristic is more
   likely to fail if the receiver does not follow this advice, because
   then a smaller number of ACK losses are needed to produce a
   sufficient jump in the cumulative ACK.

6.2.  Timestamp Heuristic

   If this heuristic is used, the sender stores the timestamp of the
   last acknowledged segment.  In addition, the second paragraph of step
   1 in Section 3 is replaced as follows:

   1**) If the Cumulative Acknowledgement field didn't cover more than
        "recover", check to see if the echoed timestamp in the last
        non-duplicate acknowledgment equals the stored timestamp.  If
        true, duplicate ACKs indicate a lost segment (proceed to Step 1A
        in Section 3).  Otherwise, duplicate ACKs likely result from
        unnecessary retransmissions (proceed to Step 1B in Section 3).





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   Examples of applying the timestamp heuristic are in validation tests
   "./test-all-newreno newreno_rto_loss_tsh" and "./test-all-newreno
   newreno_rto_dup_tsh".  The timestamp heuristic works correctly, both
   when the receiver echoes timestamps as specified by [RFC1323], and by
   its revision attempts.  However, if the receiver arbitrarily echoes
   timestamps, the heuristic can fail.  The heuristic can also fail if a
   timeout was spurious and returning ACKs are not from retransmitted
   segments.  This can be prevented by detection algorithms such as
   [RFC3522].

7.  Implementation Issues for the Data Receiver

   [RFC2581] specifies that "Out-of-order data segments SHOULD be
   acknowledged immediately, in order to accelerate loss recovery."
   Neal Cardwell has noted that some data receivers do not send an
   immediate acknowledgement when they send a partial acknowledgment,
   but instead wait first for their delayed acknowledgement timer to
   expire [C98].  As [C98] notes, this severely limits the potential
   benefit of NewReno by delaying the receipt of the partial
   acknowledgement at the data sender.  Echoing RFC 2581, our
   recommendation is that the data receiver send an immediate
   acknowledgement for an out-of-order segment, even when that out-of-
   order segment fills a hole in the buffer.

8.  Implementation Issues for the Data Sender

   In Section 3, Step 5 above, it is noted that implementations should
   take measures to avoid a possible burst of data when leaving Fast
   Recovery, in case the amount of new data that the sender is eligible
   to send due to the new value of the congestion window is large.  This
   can arise during NewReno when ACKs are lost or treated as pure window
   updates, thereby causing the sender to underestimate the number of
   new segments that can be sent during the recovery procedure.
   Specifically, bursts can occur when the FlightSize is much less than
   the new congestion window when exiting from Fast Recovery.  One
   simple mechanism to avoid a burst of data when leaving Fast Recovery
   is to limit the number of data packets that can be sent in response
   to a single acknowledgment.  (This is known as "maxburst_" in the ns
   simulator.)  Other possible mechanisms for avoiding bursts include
   rate-based pacing, or setting the slow-start threshold to the
   resultant congestion window and then resetting the congestion window
   to FlightSize.  A recommendation on the general mechanism to avoid
   excessively bursty sending patterns is outside the scope of this
   document.

   An implementation may want to use a separate flag to record whether
   or not it is presently in the Fast Recovery procedure.  The use of
   the value of the duplicate acknowledgment counter for this purpose is



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   not reliable because it can be reset upon window updates and out-of-
   order acknowledgments.

   When not in Fast Recovery, the value of the state variable "recover"
   should be pulled along with the value of the state variable for
   acknowledgments (typically, "snd_una") so that, when large amounts of
   data have been sent and acked, the sequence space does not wrap and
   falsely indicate that Fast Recovery should not be entered (Section 3,
   step 1, last paragraph).

   It is important for the sender to respond correctly to duplicate ACKs
   received when the sender is no longer in Fast Recovery (e.g., because
   of a Retransmit Timeout).  The Limited Transmit procedure [RFC3042]
   describes possible responses to the first and second duplicate
   acknowledgements.  When three or more duplicate acknowledgements are
   received, the Cumulative Acknowledgement field doesn't cover more
   than "recover", and a new Fast Recovery is not invoked, it is
   important that the sender not execute the Fast Recovery steps (3) and
   (4) in Section 3.  Otherwise, the sender could end up in a chain of
   spurious timeouts.  We mention this only because several NewReno
   implementations had this bug, including the implementation in the NS
   simulator.  (This bug in the NS simulator was fixed in July 2003,
   with the variable "exitFastRetrans_".)

9.  Simulations

   Simulations with NewReno are illustrated with the validation test
   "tcl/test/test-all-newreno" in the NS simulator.  The command
   "../../ns test-suite-newreno.tcl reno" shows a simulation with Reno
   TCP, illustrating the data sender's lack of response to a partial
   acknowledgement.  In contrast, the command "../../ns test-suite-
   newreno.tcl newreno_B" shows a simulation with the same scenario
   using the NewReno algorithms described in this paper.

10.  Comparisons between Reno and NewReno TCP

   As we stated in the introduction, we believe that the NewReno
   modification described in this document improves the performance of
   the Fast Retransmit and Fast Recovery algorithms of Reno TCP in a
   wide variety of scenarios.  This has been discussed in some depth in
   [FF96], which illustrates Reno TCP's poor performance when multiple
   packets are dropped from a window of data and also illustrates
   NewReno TCP's good performance in that scenario.

   We do, however, know of one scenario where Reno TCP gives better
   performance than NewReno TCP, that we describe here for the sake of
   completeness.  Consider a scenario with no packet loss, but with
   sufficient reordering so that the TCP sender receives three duplicate



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   acknowledgements.  This will trigger the Fast Retransmit and Fast
   Recovery algorithms.  With Reno TCP or with Sack TCP, this will
   result in the unnecessary retransmission of a single packet, combined
   with a halving of the congestion window (shown on pages 4 and 6 of
   [F03]).  With NewReno TCP, however, this reordering will also result
   in the unnecessary retransmission of an entire window of data (shown
   on page 5 of [F03]).

   While Reno TCP performs better than NewReno TCP in the presence of
   reordering, NewReno's superior performance in the presence of
   multiple packet drops generally outweighs its less optimal
   performance in the presence of reordering.  (Sack TCP is the
   preferred solution, with good performance in both scenarios.)  This
   document recommends the Fast Retransmit and Fast Recovery algorithms
   of NewReno TCP instead of those of Reno TCP for those TCP connections
   that do not support SACK.  We would also note that NewReno's Fast
   Retransmit and Fast Recovery mechanisms are widely deployed in TCP
   implementations in the Internet today, as documented in [PF01].  For
   example, tests of TCP implementations in several thousand web servers
   in 2001 showed that for those TCP connections where the web browser
   was not SACK-capable, more web servers used the Fast Retransmit and
   Fast Recovery algorithms of NewReno than those of Reno or Tahoe TCP
   [PF01].

11.  Changes Relative to RFC 2582

   The purpose of this document is to advance the NewReno's Fast
   Retransmit and Fast Recovery algorithms in RFC 2582 to Standards
   Track.

   The main change in this document relative to RFC 2582 is to specify
   the Careful variant of NewReno's Fast Retransmit and Fast Recovery
   algorithms.  The base algorithm described in RFC 2582 did not attempt
   to avoid unnecessary multiple Fast Retransmits that can occur after a
   timeout (described in more detail in the section above).  However,
   RFC 2582 also defined "Careful" and "Less Careful" variants that
   avoid these unnecessary Fast Retransmits, and recommended the Careful
   variant.  This document specifies the previously-named "Careful"
   variant as the basic version of NewReno.  As described below, this
   algorithm uses a variable "recover", whose initial value is the send
   sequence number.

   The algorithm specified in Section 3 checks whether the
   acknowledgement field of a partial acknowledgement covers *more* than
   "recover", as defined in Section 3.  Another possible variant would
   be to simply require that the acknowledgement field covers *more than
   or equal to* "recover" before initiating another Fast Retransmit.  We
   called this the Less Careful variant in RFC 2582.



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   There are two separate scenarios in which the TCP sender could
   receive three duplicate acknowledgements acknowledging "recover" but
   no more than "recover".  One scenario would be that the data sender
   transmitted four packets with sequence numbers higher than "recover",
   that the first packet was dropped in the network, and the following
   three packets triggered three duplicate acknowledgements
   acknowledging "recover".  The second scenario would be that the
   sender unnecessarily retransmitted three packets below "recover", and
   that these three packets triggered three duplicate acknowledgements
   acknowledging "recover".  In the absence of SACK, the TCP sender is
   unable to distinguish between these two scenarios.

   For the Careful variant of Fast Retransmit, the data sender would
   have to wait for a retransmit timeout in the first scenario, but
   would not have an unnecessary Fast Retransmit in the second scenario.
   For the Less Careful variant to Fast Retransmit, the data sender
   would Fast Retransmit as desired in the first scenario, and would
   unnecessarily Fast Retransmit in the second scenario.  This document
   only specifies the Careful variant in Section 3.  Unnecessary Fast
   Retransmits with the Less Careful variant in scenarios with
   reordering are illustrated in page 8 of [F03].

   The document also specifies two heuristics that the TCP sender MAY
   use to decide to invoke Fast Retransmit even when the three duplicate
   acknowledgements do not cover more than "recover".  These heuristics,
   an ACK-based heuristic and a timestamp heuristic, are described in
   Sections 6.1 and 6.2 respectively.

12.  Conclusions

   This document specifies the NewReno Fast Retransmit and Fast Recovery
   algorithms for TCP.  This NewReno modification to TCP can even be
   important for TCP implementations that support the SACK option,
   because the SACK option can only be used for TCP connections when
   both TCP end-nodes support the SACK option.  NewReno performs better
   than Reno (RFC 2581) in a number of scenarios discussed herein.

   A number of options to the basic algorithm presented in Section 3 are
   also described.  These include the handling of the retransmission
   timer (Section 4), the response to partial acknowledgments (Section
   5), and the value of the congestion window when leaving Fast Recovery
   (section 3, step 5).  Our belief is that the differences between
   these variants of NewReno are small compared to the differences
   between Reno and NewReno.  That is, the important thing is to
   implement NewReno instead of Reno, for a TCP connection without SACK;
   it is less important exactly which of the variants of NewReno is
   implemented.




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13.  Security Considerations

   RFC 2581 discusses general security considerations concerning TCP
   congestion control.  This document describes a specific algorithm
   that conforms with the congestion control requirements of RFC 2581,
   and so those considerations apply to this algorithm, too.  There are
   no known additional security concerns for this specific algorithm.

14.  Acknowledgements

   Many thanks to Anil Agarwal, Mark Allman, Armando Caro, Jeffrey Hsu,
   Vern Paxson, Kacheong Poon, Keyur Shah, and Bernie Volz for detailed
   feedback on this document or on its precursor, RFC 2582.

15.  References

15.1.  Normative References

   [RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
             Selective Acknowledgement Options", RFC 2018, October 1996.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2581] Allman, M., Paxson, V. and  W. Stevens, "TCP Congestion
             Control", RFC 2581, April 1999.

   [RFC2582] Floyd, S. and T. Henderson, "The NewReno Modification to
             TCP's Fast Recovery Algorithm", RFC 2582, April 1999.

   [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
             Timer", RFC 2988, November 2000.

   [RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's
             Loss Recovery Using Limited Transmit", RFC 3042, January
             2001.

15.2.  Informative References

   [C98]     Cardwell, N., "delayed ACKs for retransmitted packets:
             ouch!".  November 1998,  Email to the tcpimpl mailing list,
             Message-ID "Pine.LNX.4.02A.9811021421340.26785-
             100000@sake.cs.washington.edu", archived at "http://tcp-
             impl.lerc.nasa.gov/tcp-impl".







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   [F98]     Floyd, S., Revisions to RFC 2001, "Presentation to the
             TCPIMPL Working Group", August 1998.  URLs
             "ftp://ftp.ee.lbl.gov/talks/sf-tcpimpl-aug98.ps" and
             "ftp://ftp.ee.lbl.gov/talks/sf-tcpimpl-aug98.pdf".

   [F03]     Floyd, S., "Moving NewReno from Experimental to Proposed
             Standard?  Presentation to the TSVWG Working Group", March
             2003.  URLs "http://www.icir.org/floyd/talks/newreno-
             Mar03.ps" and "http://www.icir.org/floyd/talks/newreno-
             Mar03.pdf".

   [FF96]    Fall, K. and S. Floyd, "Simulation-based Comparisons of
             Tahoe, Reno and SACK TCP", Computer Communication Review,
             July 1996.  URL "ftp://ftp.ee.lbl.gov/papers/sacks.ps.Z".

   [F94]     Floyd, S., "TCP and Successive Fast Retransmits", Technical
             report, October 1994.  URL
             "ftp://ftp.ee.lbl.gov/papers/fastretrans.ps".

   [GF04]    Gurtov, A. and S. Floyd, "Resolving Acknowledgment
             Ambiguity in non-SACK TCP", Next Generation Teletraffic and
             Wired/Wireless Advanced Networking (NEW2AN'04), February
             2004.  URL "http://www.cs.helsinki.fi/u/gurtov/papers/
             heuristics.html".

   [Gur03]   Gurtov, A., "[Tsvwg] resolving the problem of unnecessary
             fast retransmits in go-back-N", email to the tsvwg mailing
             list, message ID <3F25B467.9020609@cs.helsinki.fi>, July
             28, 2003.  URL "http://www1.ietf.org/mail-archive/working-
             groups/tsvwg/current/msg04334.html".

   [Hen98]   Henderson, T., Re: NewReno and the 2001 Revision. September
             1998.  Email to the tcpimpl mailing list, Message ID
             "Pine.BSI.3.95.980923224136.26134A-
             100000@raptor.CS.Berkeley.EDU", archived at "http://tcp-
             impl.lerc.nasa.gov/tcp-impl".

   [Hoe95]   Hoe, J., "Startup Dynamics of TCP's Congestion Control and
             Avoidance Schemes", Master's Thesis, MIT, 1995.

   [Hoe96]   Hoe, J., "Improving the Start-up Behavior of a Congestion
             Control Scheme for TCP", ACM SIGCOMM, August 1996.  URL
             "http://www.acm.org/sigcomm/sigcomm96/program.html".

   [LM97]    Lin, D. and R. Morris, "Dynamics of Random Early
             Detection", SIGCOMM 97, September 1997.  URL
             "http://www.acm.org/sigcomm/sigcomm97/program.html".




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   [NS]      The Network Simulator (NS). URL
             "http://www.isi.edu/nsnam/ns/".

   [PF01]    Padhye, J. and S. Floyd, "Identifying the TCP Behavior of
             Web Servers", June 2001, SIGCOMM 2001.

   [RFC1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for
             High Performance", RFC 1323, May 1992.

   [RFC3517] Blanton, E., Allman, M., Fall, K. and L. Wang, "A
             Conservative Selective Acknowledgment (SACK)-based Loss
             Recovery Algorithm for TCP", RFC 3517, April 2003.

   [RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
             TCP", RFC 3522, April 2003.

Authors' Addresses

   Sally Floyd
   International Computer Science Institute

   Phone: +1 (510) 666-2989
   EMail: floyd@acm.org
   URL: http://www.icir.org/floyd/


   Tom Henderson
   The Boeing Company

   EMail: thomas.r.henderson@boeing.com


   Andrei Gurtov
   TeliaSonera

   EMail: andrei.gurtov@teliasonera.com















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Full Copyright Statement

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   to the rights, licenses and restrictions contained in BCP 78, and
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Acknowledgement

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