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rfc:rfc6582

Internet Engineering Task Force (IETF) T. Henderson Request for Comments: 6582 Boeing Obsoletes: 3782 S. Floyd Category: Standards Track ICSI ISSN: 2070-1721 A. Gurtov

                                                    University of Oulu
                                                            Y. Nishida
                                                          WIDE Project
                                                            April 2012
     The NewReno Modification to TCP's Fast Recovery Algorithm

Abstract

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

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6582.

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

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 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.

Henderson, et al. Standards Track [Page 2] RFC 6582 TCP NewReno April 2012

 This document applies to TCP connections that are unable to use the
 TCP Selective Acknowledgment (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 respond 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 behavior that has been
 historically referred to as Reno.  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
 network simulator known as ns-2 [NS] and with numerous
 implementations of NewReno, we believe that this modification
 improves the performance of the fast retransmit and fast recovery

Henderson, et al. Standards Track [Page 3] RFC 6582 TCP NewReno April 2012

 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

 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].
 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 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.

Henderson, et al. Standards Track [Page 4] RFC 6582 TCP NewReno April 2012

3.2. Specification

 The procedures specified in Section 3.2 of [RFC5681] are followed,
 with the modifications listed below.  Note that this specification
 avoids the use of the key words defined in RFC 2119 [RFC2119], since
 it mainly provides sender-side implementation guidance for
 performance improvement, and does not affect interoperability.
 1)  Initialization of TCP protocol control block:
     When the TCP protocol control block is initialized, recover is
     set to the initial send sequence number.
 2)  Three duplicate ACKs:
     When the third duplicate ACK is received, the TCP sender first
     checks the value of recover to see if the Cumulative
     Acknowledgment field covers more than recover.  If so, the value
     of recover is incremented to the value of the highest sequence
     number transmitted by the TCP so far.  The TCP then enters fast
     retransmit (step 2 of Section 3.2 of [RFC5681]).  If not, the TCP
     does not enter fast retransmit and does not reset ssthresh.
 3)  Response to newly acknowledged data:
     Step 6 of [RFC5681] specifies the response to the next ACK that
     acknowledges previously unacknowledged data.  When an ACK arrives
     that acknowledges new data, this ACK could be the acknowledgment
     elicited by the initial retransmission from fast retransmit, 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, 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 (1) is the amount of data presently
     outstanding.  This is 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.

Henderson, et al. Standards Track [Page 5] RFC 6582 TCP NewReno April 2012

     Partial acknowledgments:
     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 Acknowledgment field.  If the partial ACK acknowledges
     at least one SMSS of new data, then add back SMSS bytes to the
     congestion window.  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 step 4 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.
 4)  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 2 above specifies a check that the Cumulative Acknowledgment
 field covers more than recover.  Because the acknowledgment field
 contains the sequence number that the sender next expects to receive,
 the acknowledgment "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 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.

Henderson, et al. Standards Track [Page 6] RFC 6582 TCP NewReno April 2012

 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 [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 sender that
 implements the algorithm specified in Section 3.2 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.

Henderson, et al. Standards Track [Page 7] RFC 6582 TCP NewReno April 2012

 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 check is performed if, in step 2 of Section 3.2, the
 Cumulative Acknowledgment field does not cover more than recover.
 2*)  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 (enter fast retransmit).
      Otherwise, duplicate ACKs likely result from unnecessary
      retransmissions (do not enter fast retransmit).
 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.
 [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.

Henderson, et al. Standards Track [Page 8] RFC 6582 TCP NewReno April 2012

4.2. Timestamp Heuristic

 If this heuristic is used, the sender stores the timestamp of the
 last acknowledged segment.  In addition, the last sentence of step 2
 in Section 3.2 of this document is replaced as follows:
 2**) 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 (enter fast
      retransmit).  Otherwise, duplicate ACKs likely result from
      unnecessary retransmissions (do not enter fast retransmit).
 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 the Eifel
 detection algorithm [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 [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 3.2, step 3 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

Henderson, et al. Standards Track [Page 9] RFC 6582 TCP NewReno April 2012

 is to limit the number of data packets that can be sent in response
 to a single acknowledgment.  (This is known as "maxburst_" in ns-2
 [NS].)  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 state variable recover may also need to be updated in
 order to continue to permit possible entry into fast recovery
 (Section 3.2, step 2).  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 state variable recover.  Entry into fast recovery is only
 possible when the Cumulative Acknowledgment field covers more than
 the state variable recover.
 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, the sender should
 follow the guidance in Section 4.  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 ns-2 [NS].
 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 3.2, step 3, 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 3.2, step 3), cwnd should not be updated until a further
 event occurs (e.g., arrival of an ack, or timeout) after this
 adjustment.

Henderson, et al. Standards Track [Page 10] RFC 6582 TCP NewReno April 2012

7. Security Considerations

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

8. 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 in a number of scenarios discussed in previous versions of
 this RFC ([RFC2582] [RFC3782]).
 A number of options for the basic algorithms presented in Section 3
 are also referenced in Appendix A of this document.  These include
 the handling of the retransmission timer, the response to partial
 acknowledgments, and whether or not the sender must maintain a state
 variable called 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 variant of NewReno is implemented.

9. 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 the precursor RFCs 2582 and 3782.  Jeffrey Hsu provided
 clarifications on the handling of the variable recover; these
 clarifications were applied to RFC 3782 via an erratum and are
 incorporated into the text of Section 6 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.

Henderson, et al. Standards Track [Page 11] RFC 6582 TCP NewReno April 2012

10. References

10.1. Normative References

 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
           Control", RFC 5681, September 2009.

10.2. Informative References

 [C98]     Cardwell, N., "delayed ACKs for retransmitted packets:
           ouch!".  November 1998, Email to the tcpimpl mailing list,
           archived at
           <http://groups.yahoo.com/group/tcp-impl/message/1428>.
 [F94]     Floyd, S., "TCP and Successive Fast Retransmits", Technical
           report, May 1995.
           <ftp://ftp.ee.lbl.gov/papers/fastretrans.ps>.
 [FF96]    Fall, K. and S. Floyd, "Simulation-based Comparisons of
           Tahoe, Reno and SACK TCP", Computer Communication Review,
           July 1996.  <ftp://ftp.ee.lbl.gov/papers/sacks.ps.Z>.
 [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.  <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, July 28, 2003.  <http://www.ietf.org/mail-archive/
           web/tsvwg/current/msg04334.html>.
 [Hen98]   Henderson, T., "Re: NewReno and the 2001 Revision",
           September 1998.  Email to the tcpimpl mailing list,
           archived at
           <http://groups.yahoo.com/group/tcp-impl/message/1321>.
 [Hoe95]   Hoe, J., "Startup Dynamics of TCP's Congestion Control and
           Avoidance Schemes", Master's Thesis, MIT, June 1995.
 [Hoe96]   Hoe, J., "Improving the Start-up Behavior of a Congestion
           Control Scheme for TCP", ACM SIGCOMM, August 1996.
           <http://ccr.sigcomm.org/archive/1996/conf/hoe.pdf>.

Henderson, et al. Standards Track [Page 12] RFC 6582 TCP NewReno April 2012

 [LM97]    Lin, D. and R. Morris, "Dynamics of Random Early
           Detection", SIGCOMM 97, October 1997.
 [NS]      "The Network Simulator version 2 (ns-2)",
           <http://www.isi.edu/nsnam/ns/>.
 [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., Mahdavi, J., Mathis, M., and M. Podolsky, "An
           Extension to the Selective Acknowledgement (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., Henderson, T., and A. Gurtov, "The NewReno
           Modification to TCP's Fast Recovery Algorithm", RFC 3782,
           April 2004.

Henderson, et al. Standards Track [Page 13] RFC 6582 TCP NewReno April 2012

Appendix A. Additional Information

 Previous versions of this RFC ([RFC2582] [RFC3782]) contained
 additional informative material on the following subjects, and may be
 consulted by readers who may want more information about possible
 variants to the algorithms and who may want references to specific
 [NS] simulations that provide NewReno test cases.
 Section 4 of [RFC3782] discusses some alternative behaviors for
 resetting the 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
 motivation for the sender's state variable recover.
 Section 9 of [RFC3782] introduces some NS simulation test suites for
 NewReno.  In addition, references to simulation results can be found
 throughout [RFC3782].
 Section 10 of [RFC3782] provides a comparison of Reno and
 NewReno TCP.
 Section 11 of [RFC3782] lists changes relative to [RFC2582].

Appendix 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, the
 first option carries a risk of performance degradation: With the
 first option, if FlightSize is zero, the result will be 1 SMSS.  This
 means TCP can transmit only 1 segment at that moment, which can cause
 a delay in ACK transmission at the receiver due to a delayed ACK
 algorithm.
 The FlightSize on Full ACK reception can be zero in some situations.
 A typical example is where the 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 the window size
 is not small, loss of ACK packets or a receive buffer shortage during
 fast recovery can also increase the possibility of falling into this
 situation.

Henderson, et al. Standards Track [Page 14] RFC 6582 TCP NewReno April 2012

 The proposed fix in this document, which sets cwnd to at least 2*SMSS
 if the implementation uses option 1 in the Full ACK case
 (Section 3.2, step 3, option 1), ensures that the sender TCP
 transmits at least two segments on Full ACK reception.
 In addition, an erratum was reported for RFC 3782 (an editorial
 clarification to Section 8); this erratum has been addressed in
 Section 6 of this document.
 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 [RFC3782] were removed, and instead
 Appendix A of this document was added to back-reference this
 informative material.  A few references that have no citation in the
 main body of the document have been removed.

Henderson, et al. Standards Track [Page 15] RFC 6582 TCP NewReno April 2012

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
 University of Oulu
 Centre for Wireless Communications CWC
 P.O. Box 4500
 FI-90014 University of Oulu
 Finland
 EMail: gurtov@ee.oulu.fi
 Yoshifumi Nishida
 WIDE Project
 Endo 5322
 Fujisawa, Kanagawa  252-8520
 Japan
 EMail: nishida@wide.ad.jp

Henderson, et al. Standards Track [Page 16]

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