Network Working Group                                       T. Henderson
Internet-Draft                                                    Boeing
Obsoletes: 3782  (if approved)                                  S. Floyd
Intended status: Standards Track                                    ICSI
Expires:  September 15,  October 20, 2011                                     A. Gurtov
                                                                    HIIT
                                                              Y. Nishida
                                                            WIDE Project
                                                          March 14,
                                                          April 20, 2011

       The NewReno Modification to TCP's Fast Recovery Algorithm
                   draft-ietf-tcpm-rfc3782-bis-01.txt
                   draft-ietf-tcpm-rfc3782-bis-02.txt

Abstract

   RFC 5681 [RFC5681] documents the following four intertwined TCP
   congestion control algorithms: Slow Start, Congestion Avoidance, Fast
   Retransmit, slow start, congestion avoidance, fast
   retransmit, and Fast Recovery. fast recovery.  RFC 5681 explicitly allows
   certain modifications of these algorithms, including modifications
   that use the TCP Selective Acknowledgement (SACK) option [RFC2883], (RFC 2883),
   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].

   The purpose of this revision from [RFC3782] is to make errata changes
   and to adopt a proposal from Yoshifumi Nishida to slightly increase
   the minimum window size after Fast Recovery from one to two segments,
   to improve performance when the receiver uses delayed acknowledgments. Hoe.  This document obsoletes RFC 3782.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on September 15, 2011.

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   the document authors.  All rights reserved.

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   than English.

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.  This document applies to 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 acknowledgments, the sender
   infers a packet loss, and retransmits the indicated packet.  After
   this, the data sender could receive additional duplicate
   acknowledgments, 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 acknowledgment 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
   acknowledgment for this packet will acknowledge all of the packets
   transmitted before Fast Retransmit was entered.  However, if there
   are multiple packet drops, then the acknowledgment for the
   retransmitted packet will acknowledge some but not all of the packets
   transmitted before the Fast Retransmit.  We call this acknowledgment
   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 5681 that
   incorporates a response to partial acknowledgments 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 in RFC 5681.  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 acknowledgments 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 acknowledgments, 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.  Previous versions of this RFC [RFC2582, RFC3782] provide
   simulation-based evidence of the possible performance gains.

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 [RFC5681].  FLIGHT SIZE is
   defined as in [RFC5681] as follows:

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

   This document defines an additional sender-side state variable
   called RECOVER:

      RECOVER:
         When in Fast Recovery, this variable records the send sequence
         number that must be acknowledged before the Fast Recovery
         procedure is declared to be over.

3.  The Fast Retransmit and Fast Recovery Algorithms in NewReno

3.1.  Protocol Overview

   The basic idea of these extensions to the Fast Retransmit and
   Fast Recovery algorithms described in Section 3.2 of [RFC5681]
   is as follows.  The TCP sender can infer, from the arrival of
   duplicate acknowledgments, whether multiple losses in the same
   window of data have most likely occurred, and avoid taking a
   retransmit timeout or making multiple congestion window reductions
   due to such an event.

   The standard implementation of the Fast Retransmit and Fast Recovery
   algorithms is given in [RFC5681].  This section specifies the basic
   NewReno algorithm.  Section 4 describes heuristics for processing
   duplicate acknowledgments after a retransmission timeout.  Sections
   5 and 6 provide some guidance to implementors based on experience
   with NewReno implementations.  Several appendices provide more
   background information and describe variations that an implementor
   may want to consider when tuning performance for certain network
   scenarios.

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

3.2.  Specification

   The NewReno algorithm procedures specified in this document extends the
   implementation in Section 3.2 of [RFC5681] by introducing a variable specified as
   "recover" whose initial value are followed
   with the following modifications.

   1)  Initialization of TCP protocol control block:
       When the TCP protocol control block is initialized, Recover is
       set to the initial send sequence number.
   This new variable

   2)  Three duplicate ACKs:
       When the third duplicate ACK is used by received, the TCP sender first
       checks the value of Recover to record see if the send sequence
   number that must be acknowledged before Cumulative Acknowledgment
       field covers more than Recover.  If so, the Fast Recovery
   procedure value of Recover is declared
       incremented to be over.  This variable is used below
   in step 1, in the response to a partial or new
   acknowledgment 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.

   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 Acknowledgment 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 4 from [RFC5681]).

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

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

   1B) Not invoking Fast Retransmit:
       Do not enter by the TCP so far.  The TCP then enters Fast Retransmit and Fast Recovery procedure.  In
       particular, do not change ssthresh, do not go to Step
       (step 2 to
       retransmit of Section 3.2 of [RFC5681]).  If not, the "lost" segment, and do TCP does not execute Step 3 upon
       subsequent duplicate ACKs.

   2)  Entering Fast Retransmit:
       Retransmit the lost segment
       enter fast retransmit and set cwnd does not reset ssthresh.

   3)  Response to ssthresh plus
       3*SMSS.  This artificially "inflates" the congestion window by the
       number newly acknowledged data:
       Step 6 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 [RFC5681] specifies the congestion window in order response to reflect the additional segment next ACK that has left the network.

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

   5)
       acknowledges previously unacknowledged data.  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.  There are two cases.

       Full acknowledgments:
       If this ACK acknowledges all of the data up to and including
       "recover",
       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, max(FlightSize, SMSS) + SMSS) or
       (2) ssthresh, where ssthresh is the value set when Fast Retransmit
       was entered, and where FlightSize in step 1; this (1) 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
       presently outstanding.  This is
       exited.) termed "deflating" the window.
       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 acknowledgment.  Exit the Fast Recovery procedure.

       Partial acknowledgments:
       If this ACK does *not* acknowledge all of the data up to and
       including "recover", 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 acknowledgment 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  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 Step 4 above). of
       Section 3.2 of [RFC5681].

       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.

   6)

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

   Step 1 2 above specifies a check that the Cumulative Acknowledgment
   field covers more than "recover". Recover.  Because the acknowledgment field
   contains the sequence number that the sender next expects to receive,
   the acknowledgment "ack_number" covers more than "recover" Recover when:

      ack_number - 1 > recover; 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, 3 above, the congestion window is deflated after
   a partial acknowledgment is received.  The congestion window was
   likely to have been inflated considerably when the partial
   acknowledgment was received.  In addition, depending on the original
   pattern of packet losses, the partial acknowledgment 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 acknowledgment
   threshold, but assumes the threshold specified in the IETF standards;
   the current standard is RFC 5681, [RFC5681], which specifies a threshold of three
   duplicate acknowledgments.

   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.  Handling Duplicate Acknowledgments After A Timeout

   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
   acknowledgments that do not cover more than "recover".  In this
   case, the duplicate acknowledgments 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 acknowledgments 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 acknowledgments
   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 acknowledgment field, that allow the
   sender to distinguish, in some cases, between three duplicate
   acknowledgments following a retransmitted packet that was dropped,
   and three duplicate acknowledgments 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 acknowledgments do not cover more than
   "recover".

   For example, when three duplicate acknowledgments are caused by the
   unnecessary retransmission of three packets, this is likely to be
   accompanied by the cumulative acknowledgment 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 acknowledgment is the timestamp of
   the most recent data packet that advanced the cumulative
   acknowledgment field [RFC1323].  If timestamps are used, and the
   sender stores the timestamp of the last acknowledged segment, then
   the timestamp echoed by duplicate acknowledgments can be used to
   distinguish between a retransmitted packet that was dropped and
   three duplicate acknowledgments from the unnecessary
   retransmission of three packets.

4.1.  ACK Heuristic

   If the ACK-based heuristic is used, then following the advancement of
   the cumulative acknowledgment field, the sender stores the value of
   the previous cumulative acknowledgment 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 Acknowledgment 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.

   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.
   RFC 5681
   [RFC5681] 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.

4.2.  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 Acknowledgment 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).

   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].

5.  Implementation Issues for the Data Receiver

   [RFC5681] 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 acknowledgment when they send a partial acknowledgment,
   but instead wait first for their delayed acknowledgment timer to
   expire [C98].  As [C98] notes, this severely limits the potential
   benefit of NewReno by delaying the receipt of the partial
   acknowledgment at the data sender.  Echoing RFC 5681, [RFC5681], our
   recommendation is that the data receiver send an immediate
   acknowledgment for an out-of-order segment, even when that
   out-of-order segment fills a hole in the buffer.

6.  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
   not reliable because it can be reset upon window updates and
   out-of-order acknowledgments.

   When updating the Cumulative Acknowledgment field outside of
   Fast Recovery, the "recover" state variable may also need to be
   updated in order to continue to permit possible entry into Fast
   Recovery (Section 3, step 1).  This issue arises when an update
   of the Cumulative Acknowledgment field results in a sequence
   wraparound that affects the ordering between the Cumulative
   Acknowledgment field and the "recover" state variable.  Entry
   into Fast Recovery is only possible when the Cumulative
   Acknowledgment field covers more than the "recover" state variable.

   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
   acknowledgments.  When three or more duplicate acknowledgments are
   received, the Cumulative Acknowledgment 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.

   It has been observed that some TCP implementations enter a slow start
   or congestion avoidance window updating algorithm immediately after
   the cwnd is set by the equation found in (Section 3, step 5), even
   without a new external event generating the cwnd change.  Note that
   after cwnd is set based on the procedure for exiting Fast Recovery
   (Section 3, step 5), cwnd SHOULD NOT be updated until a further
   event occurs (e.g., arrival of an ack, or timeout) after this
   adjustment.

7.  Security Considerations

   RFC 5681

   [RFC5681] discusses general security considerations concerning TCP
   congestion control.  This document describes a specific algorithm
   that conforms with the congestion control requirements of RFC 5681, [RFC5681],
   and so those considerations apply to this algorithm, too.  There are
   no known additional security concerns for this specific algorithm.

8.  IANA Considerations

   This document has no actions for IANA.

9.  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 5681) (RFC5681) in a number of scenarios discussed herein. in
   previous versions of this RFC ([RFC2582], [RFC3782]).

   A number of options to the basic algorithm presented in Section 3 are
   also described referenced in appendices Appendix A to this document.  These include the
   handling of the retransmission timer (Appendix A), timer, the response to partial acknowledgments (Appendix B),
   acknowledgments, and whether or not the sender
   maintains must maintain a state
   variable called "recover" (Appendix C). Recover.  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.

10.  Acknowledgments

   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.  Jeffrey
   Hsu provided clarifications on the handling of the recover variable
   that were applied to RFC 3782 as errata, and now are in Section 8
   of this document.  Yoshifumi Nishida contributed a modification
   to the fast recovery algorithm to account for the case in which
   flightsize is 0 when the TCP sender leaves fast recovery, and the
   TCP receiver uses delayed acknowledgments.  Alexander Zimmermann
   provided several suggestions to improve the clarity of the document.

11.  References
11.1.  Normative References

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

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

   [RFC5681] Allman, M., Paxson, V. and  E. Blanton, "TCP Congestion
             Control", RFC 5681, September 2009.

11.2.  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".

   [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".

   [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.

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

   [RFC2883] Floyd, S., J. Mahdavi, M. Mathis, and M. Podolsky, "The
             Selective Acknowledgment (SACK) Option for TCP, RFC 2883, July 2000.

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

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

   [RFC3782] Floyd, S., T. Henderson, and A. Gurtov, "The NewReno
             Modification to TCP's Fast Recovery Algorithm", RFC 3782, April 2004.

Appendix A.  Resetting the Retransmit Timer in Response to Partial
              Acknowledgments

             One possible variant to the response to partial acknowledgments
             specified in Section 3 concerns when to reset the retransmit timer
             after a partial acknowledgment.  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
             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 (step 5.1 of
             [RFC2988]).

             In contrast, the NewReno simulations in [FF96] illustrate the
             algorithm described above with the modification that the retransmit
             timer is reset after each partial acknowledgment.  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 Impatient variant can be particularly important for TCP
             connections with large congestion windows.

             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 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
             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.

             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
             acknowledgment, 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.

Appendix B.  Retransmissions after a Partial Acknowledgment

             One possible variant to the response to partial acknowledgments
             specified in Section 3 would be to retransmit more than one packet
             after each partial acknowledgment, and to reset the retransmit timer
             after each retransmission.  The algorithm specified in Section 3
             retransmits a single packet after each partial acknowledgment.  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
             acknowledgment.  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 acknowledgments 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 acknowledgment.  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 acknowledgment, but additional new packets
             might be transmitted as well, depending on the amount of new data
             acknowledged by the partial acknowledgment.  In contrast, the
             variant of NewReno illustrated in [FF96] simply set the congestion
             window to ssthresh when a partial acknowledgment 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 acknowledgment is received.  While either of these
             approaches gives acceptable performance, the variant specified in
             Section 3 recovers more smoothly when multiple packets are dropped
             from a window of data.

Appendix C.  Avoiding Multiple Fast Retransmits

             This appendix describes the motivation for the sender's state
             variable "recover".

             In the absence of the SACK option or timestamps, a duplicate
             acknowledgment carries no information to identify the data packet or
             packets at the TCP data receiver that triggered that duplicate
             acknowledgment.  In this case, the TCP data sender is unable to
             distinguish between a duplicate acknowledgment that results from a
             lost or delayed data packet, and a duplicate acknowledgment 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.
             RFC 2582 also defined Careful and Less Careful variants of the NewReno
             algorithm, and recommended the Careful variant.

             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".

Appendix D.  Simulations

             This section provides pointers to simulation scripts available in
             the NS simulator that reproduce behavior described above.

             In Section 3, a simple mechanism is described 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.

             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
             acknowledgment.  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.

             Regarding the handling of duplicate acknowledgments after a timeout,
             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 (Section 4) 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".

             Examples of applying the timestamp heuristic (Section 4) are in
             validation tests "./test-all-newreno newreno_rto_loss_tsh" and
             "./test-all-newreno newreno_rto_dup_tsh".

             Section 6 described a problem involving possible spurious timeouts,
             and mentions that this bug existed in the NS simulator.
             This bug in the NS simulator was fixed in July 2003,
             with the variable "exitFastRetrans_".

             Regarding the Slow-but-Steady and Impatient variants described
             in Appendix A, The tests "ns
             test-suite-newreno.tcl impatient1" and "ns test-suite-newreno.tcl
             slow1" in the NS simulator illustrate a scenario in which 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.  The tests
             "ns test-suite-newreno.tcl impatient2" and
             "ns test-suite-newreno.tcl slow2" in the NS simulator illustrate
             scenarios in which the Slow-but-Steady variant outperforms the Impatient
             variant.  The tests "ns test-suite-newreno.tcl impatient3" and
             "ns test-suite-newreno.tcl slow3" in the NS simulator illustrate
             scenarios in which the Slow-but-Steady variants avoid unnecessary
             retransmissions.

             Appendix B describes different policies for partial window deflation.
             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.

             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
             An NS validation test "./test-all-newreno newreno5_noBF" in
             directory "tcl/test" of the NS simulator illustartes the default
             variant of NewReno TCP that doesn't use the variable "recover";
             this gives performance similar to that on page 8 of [F03].

Appendix E.  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
             acknowledgments.  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].

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

             There are two separate scenarios in which the TCP sender could
             receive three duplicate acknowledgments 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 acknowledgments
             acknowledging "recover".  The second scenario would be that the
             sender unnecessarily retransmitted three packets below "recover", and
             that these three packets triggered three duplicate acknowledgments
             acknowledging "recover".  In the absence of SACK, the TCP sender is
             unable to distinguish between these two scenarios.

             For the Careful variant  Additional Information

   Previous versions of Fast Retransmit, this RFC ([RFC2582], [RFC3782]) contained
   additional informative material on the data sender would
             have following subjects, and
   may be consulted by readers who may want more information about
   possible variants 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 algorithm and who may want references
   to Fast Retransmit, the data
             sender would Fast Retransmit as desired in specific [NS] simulations that provide NewReno test cases.

   Section 4 of [RFC3782] discusses some alternative behaviors for
   resetting the first scenario, and would
             unnecessarily Fast Retransmit in retransmit timer after a partial acknowledgment.

   Section 5 of [RFC3782] discusses some alternative behaviors for
   performing retransmission after a partial acknowledgment.

   Section 6 of [RFC3782] describes more information about the second scenario.  This document
             only specifies
   motivation for the Careful variant in sender's state variable Recover.

   Section 3.  Unnecessary Fast
             Retransmits with the Less Careful variant in scenarios with
             reordering are illustrated in page 8 9 of [F03].

             The document also specifies two heuristics that the TCP sender MAY
             use to decide [RFC3782] introduces some NS simulation test
   suites for NewReno.  In addition, references to invoke Fast Retransmit even when the three duplicate
             acknowledgments do not cover more than "recover".  These heuristics,
             an ACK-based heuristic and simulation
   results can be found throughout [RFC3782].

   Section 10 of [RFC3782] provides a timestamp heuristic, are described in
             Sections 6.1 comparison of Reno and 6.2 respectively.
   NewReno TCP.

   Section 11 of [RFC3782] listed changes relative to [RFC3782].

Appendix G. B.  Changes Relative to RFC 3782

   In [RFC3782], the cwnd after Full ACK reception will be set to
   (1) min (ssthresh, FlightSize + SMSS) or (2) ssthresh.  However,
   there is a risk in the first logic which results in performance
   degradation.  With the first logic, if FlightSize is zero, the result
   will be 1 SMSS. This means TCP can transmit only 1 segment at this
   moment, which can cause delay in ACK transmission at receiver due to
   delayed ACK algorithm.

   The FlightSize on Full ACK reception can be zero in some situations.
   A typical example is where sending window size during fast recovery is
   small. In this case, the retransmitted packet and new data packets can
   be transmitted within a short interval.  If all these packets
   successfully arrive, the receiver may generate a Full ACK that
   acknowledges all outstanding data.  Even if window size is not small,
   loss of ACK packets or receive buffer shortage during fast recovery can
   also increase the possibility to fall into this situation.

   The proposed fix in this document ensures that sender TCP transmits at
   least two segments on Full ACK reception.

   In addition, errata for RFC3782 (editorial clarification to Section 8
   of RFC2582, which is now Section 6 of this document) has been applied.

   The specification text (Section 3.2 herein) was rewritten to more
   closely track Section 3.2 of [RFC5681].

   Sections 4, 5, and 9-11 of RFC2582 [RFC3782] were relocated to appendices removed, and instead Appendix
   A of this document since they are non-normative and provide background
             information and references was added to simulation results. back-reference this informative
   material.

Appendix H. C.  Document Revision History
   To be removed upon publication

   +----------+--------------------------------------------------+
   | Revision | Comments                                         |
   +----------+--------------------------------------------------+
   | draft-00 | RFC3782 errata applied, and changes applied from |
   |          | draft-nishida-newreno-modification-02            |
   +----------+--------------------------------------------------+
   | draft-01 | Non-normative sections moved to appendices,      |
   |          | editorial clarifications applied as suggested    |
   |          | by Alexander Zimmermann.                         |
   +----------+--------------------------------------------------+
   | draft-02 | Better align specification text with RFC5681.    |
   |          | Replace informative appendices by a new appendix |
   |          | that just provides back-references to earlier    |
   |          | NewReno RFCs.                                    |
   +----------+--------------------------------------------------+

Authors' Addresses

   Tom Henderson
   The Boeing Company

   EMail: thomas.r.henderson@boeing.com

   Sally Floyd
   International Computer Science Institute

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

   Andrei Gurtov
   HIIT
   Helsinki Institute for Information Technology
   P.O. Box 19215
   00076 Aalto
   Finland

   EMail: gurtov@hiit.fi

   Yoshifumi Nishida
   WIDE Project
   Endo 5322
   Fujisawa, Kanagawa  252-8520
   Japan

   Email: nishida@wide.ad.jp