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Network Working Group                                           S. FloydRequest for Comments: 2582                                         ACIRICategory: Experimental                                      T. Henderson                                                           U.C. Berkeley                                                              April 1999The NewReno Modification to TCP's Fast Recovery AlgorithmStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  It does not specify an Internet standard of any kind.   Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1999).  All Rights Reserved.AbstractRFC 2001 [RFC2001] documents the following four intertwined TCP   congestion control algorithms: Slow Start, Congestion Avoidance, Fast   Retransmit, and Fast Recovery.RFC 2581 [RFC2581] explicitly allows   certain modifications of these algorithms, including modifications   that use the TCP Selective Acknowledgement (SACK) option [MMFR96],   and modifications that respond to "partial acknowledgments" (ACKs   which cover new data, but not all the data outstanding when loss was   detected) in the absence of SACK.  This document describes a specific   algorithm for responding to partial acknowledgments, referred to as   NewReno.  This response to partial acknowledgments was first proposed   by Janey Hoe in [Hoe95].1. Introduction   For the typical implementation of the TCP Fast Recovery algorithm   described in [RFC2581] (first implemented in the 1990 BSD Reno   release, and referred to as the Reno algorithm in [FF96]), the TCP   data sender only retransmits a packet after a retransmit timeout has   occurred, or after three duplicate acknowledgements have arrived   triggering the Fast Retransmit algorithm.  A single retransmit   timeout might result in the retransmission of several data packets,   but each invocation of the Reno Fast Retransmit algorithm leads to   the retransmission of only a single data packet.Floyd & Henderson             Experimental                      [Page 1]

RFC 2582      NewReno Modification to TCP's Fast Recovery     April 1999   Problems can arise, therefore, when multiple packets have been   dropped from a single window of data and the Fast Retransmit and Fast   Recovery algorithms are invoked.  In this case, if the SACK option is   available, the TCP sender has the information to make intelligent   decisions about which packets to retransmit and which packets not to   retransmit during Fast Recovery.  This document applies only for TCP   connections that are unable to use the TCP Selective Acknowledgement   (SACK) option.   In the absence of SACK, there is little information available to the   TCP sender in making retransmission decisions during Fast Recovery.   From the three duplicate acknowledgements, the sender infers a packet   loss, and retransmits the indicated packet.  After this, the data   sender could receive additional duplicate acknowledgements, as the   data receiver acknowledges additional data packets that were already   in flight when the sender entered Fast Retransmit.   In the case of multiple packets dropped from a single window of data,   the first new information available to the sender comes when the   sender receives an acknowledgement for the retransmitted packet (that   is the packet retransmitted when Fast Retransmit was first entered).   If there had been a single packet drop, then the acknowledgement for   this packet will acknowledge all of the packets transmitted before   Fast Retransmit was entered (in the absence of reordering).  However,   when there were multiple packet drops, then the acknowledgement for   the retransmitted packet will acknowledge some but not all of the   packets transmitted before the Fast Retransmit.  We call this packet   a partial acknowledgment.   Along with several other suggestions, [Hoe95] suggested that during   Fast Recovery the TCP data sender respond to a partial acknowledgment   by inferring that the indicated packet has been lost, and   retransmitting that packet.  This document describes a modification   to the Fast Recovery algorithm in Reno TCP that incorporates a   response to partial acknowledgements received during Fast Recovery.   We call this modified Fast Recovery algorithm NewReno, because it is   a slight but significant variation of the basic Reno algorithm.  This   document does not discuss the other suggestions in [Hoe95] and   [Hoe96], such as a change to the ssthresh parameter during Slow-   Start, or the proposal to send a new packet for every two duplicate   acknowledgements during Fast Recovery.  The version of NewReno in   this document also draws on other discussions of NewReno in the   literature [LM97].   We do not claim that the NewReno version of Fast Recovery described   here is an optimal modification of Fast Recovery for responding to   partial acknowledgements, for TCPs that are unable to use SACK.   Based on our experiences with the NewReno modification in the NSFloyd & Henderson             Experimental                      [Page 2]

RFC 2582      NewReno Modification to TCP's Fast Recovery     April 1999   simulator [NS], we believe that this modification improves the   performance of the Fast Retransmit and Fast Recovery algorithms in a   wide variety of scenarios, and we are simply documenting it for the   benefit of the IETF community.  We encourage the use of this   modification to Fast Recovery, and we further encourage feedback   about operational experiences with this or related modifications.2. Definitions   This document assumes that the reader is familiar with the terms   MAXIMUM SEGMENT SIZE (MSS), CONGESTION WINDOW (cwnd), and FLIGHT SIZE   (FlightSize) defined in [RFC2581].  FLIGHT SIZE is defined as in   [RFC2581] as follows:      FLIGHT SIZE:         The amount of data that has been sent but not yet acknowledged.3. The Fast Retransmit and Fast Recovery algorithms in NewReno   The standard implementation of the Fast Retransmit and Fast Recovery   algorithms is given in [RFC2581].  The NewReno modification of these   algorithms is given below.  This NewReno modification differs from   the implementation in [RFC2581] only in the introduction of the   variable "recover" in step 1, and in the response to a partial or new   acknowledgement in step 5.  The modification defines a "Fast Recovery   procedure" that begins when three duplicate ACKs are received and   ends when either a retransmission timeout occurs or an ACK arrives   that acknowledges all of the data up to and including the data that   was outstanding when the Fast Recovery procedure began.   1.  When the third duplicate ACK is received and the sender is not       already in the Fast Recovery procedure, set ssthresh to no more       than the value given in equation 1 below.  (This is equation 3       from [RFC2581]).         ssthresh = max (FlightSize / 2, 2*MSS)           (1)       Record the highest sequence number transmitted in the variable       "recover".   2.  Retransmit the lost segment and set cwnd to ssthresh plus 3*MSS.       This artificially "inflates" the congestion window by the number       of segments (three) that have left the network and which the       receiver has buffered.   3.  For each additional duplicate ACK received, increment cwnd by       MSS.  This artificially inflates the congestion window in order       to reflect the additional segment that has left the network.Floyd & Henderson             Experimental                      [Page 3]

RFC 2582      NewReno Modification to TCP's Fast Recovery     April 1999   4.  Transmit a segment, if allowed by the new value of cwnd and the       receiver's advertised window.   5.  When an ACK arrives that acknowledges new data, this ACK could be       the acknowledgment elicited by the retransmission from step 2, or       elicited by a later retransmission.       If this ACK acknowledges all of the data up to and including       "recover", then the ACK acknowledges all the intermediate       segments sent between the original transmission of the lost       segment and the receipt of the third duplicate ACK.  Set cwnd to       either (1) min (ssthresh, FlightSize + MSS); or (2) ssthresh,       where ssthresh is the value set in step 1; this is termed       "deflating" the window.  (We note that "FlightSize" in step 1       referred to the amount of data outstanding in step 1, when Fast       Recovery was entered, while "FlightSize" in step 5 refers to the       amount of data outstanding in step 5, when Fast Recovery is       exited.) If the second option is selected, the implementation       should take measures to avoid a possible burst of data, in case       the amount of data outstanding in the network was much less than       the new congestion window allows [HTH98].  Exit the Fast Recovery       procedure.       If this ACK does *not* acknowledge all of the data up to and       including "recover", then this is a partial ACK.  In this case,       retransmit the first unacknowledged segment.  Deflate the       congestion window by the amount of new data acknowledged, then       add back one MSS and send a new segment if permitted by the new       value of cwnd.  This "partial window deflation" attempts to       ensure that, when Fast Recovery eventually ends, approximately       ssthresh amount of data will be outstanding in the network.  Do       not exit the Fast Recovery procedure (i.e., if any duplicate ACKs       subsequently arrive, execute Steps 3 and 4 above).       For the first partial ACK that arrives during Fast Recovery, also       reset the retransmit timer.   Note that in Step 5, the congestion window is deflated when a partial   acknowledgement is received.  The congestion window was likely to   have been inflated considerably when the partial acknowledgement was   received.  In addition, depending on the original pattern of packet   losses, the partial acknowledgement might acknowledge nearly a window   of data.  In this case, if the congestion window was not deflated,   the data sender might be able to send nearly a window of data back-   to-back.   There are several possible variants to the simple response to partialFloyd & Henderson             Experimental                      [Page 4]

RFC 2582      NewReno Modification to TCP's Fast Recovery     April 1999   acknowledgements described above.  First, there is a question of when   to reset the retransmit timer after a partial acknowledgement.  This   is discussed further inSection 4 below.   There is a related question of how many packets to retransmit after   each partial acknowledgement.  The algorithm described above   retransmits a single packet after each partial acknowledgement.  This   is the most conservative alternative, in that it is the least likely   to result in an unnecessarily-retransmitted packet.  A variant that   would recover faster from a window with many packet drops would be to   effectively Slow-Start, requiring less than N roundtrip times to   recover from N losses [Hoe96].  With this slightly-more-aggressive   response to partial acknowledgements, it would be advantageous to   reset the retransmit timer after each retransmission.  Because we   have not experimented with this variant in our simulator, we do not   discuss this variant further in this document.   A third question involves avoiding multiple Fast Retransmits caused   by the retransmission of packets already received by the receiver.   This is discussed inSection 5 below.  Avoiding multiple Fast   Retransmits is particularly important if more aggressive responses to   partial acknowledgements are implemented, because in this case the   sender is more likely to retransmit packets already received by the   receiver.   As a final note, we would observe that in the absence of the SACK   option, the data sender is working from limited information.  One   could spend a great deal of time considering exactly which variant of   Fast Recovery is optimal for which scenario in this case.  When the   issue of recovery from multiple dropped packets from a single window   of data is of particular importance, the best alternative would be to   use the SACK option.4. Resetting the retransmit timer.   The algorithm inSection 3 resets the retransmit timer only after the   first partial ACK.  In this case, if a large number of packets were   dropped from a window of data, the TCP data sender's retransmit timer   will ultimately expire, and the TCP data sender will invoke Slow-   Start.  (This is illustrated on page 12 of [F98].)  We call this the   Impatient variant of NewReno.   In contrast, the NewReno simulations in [FF96] illustrate the   algorithm described above, with the modification that the retransmit   timer is reset after each partial acknowledgement.  We call this the   Slow-but-Steady variant of NewReno.  In this case, for a window with   a large number of packet drops, the TCP data sender retransmits at   most one packet per roundtrip time.  (This behavior is illustrated inFloyd & Henderson             Experimental                      [Page 5]

RFC 2582      NewReno Modification to TCP's Fast Recovery     April 1999   the New-Reno TCP simulation of Figure 5 in [FF96], and on page 11 of   [F98].)   For TCP implementations where the Retransmission Timeout Value (RTO)   is generally not much larger than the round-trip time (RTT), the   Impatient variant can result in a retransmit timeout even in a   scenario with a small number of packet drops.  For TCP   implementations where the Retransmission Timeout Value (RTO) is   usually considerably larger than the round-trip time (RTT), the Slow-   but-Steady variant can remain in Fast Recovery for a long time when   multiple packets have been dropped from a window of data.  Neither of   these variants are optimal; one possibility for a more optimal   algorithm might be one that recovered more quickly from multiple   packet drops, and combined this with the Slow-but-Steady variant in   terms of resetting the retransmit timers.  We note, however, that   there is a limitation to the potential performance in this case in   the absence of the SACK option.5. Avoiding Multiple Fast Retransmits   In the absence of the SACK option, a duplicate acknowledgement   carries no information to identify the data packet or packets at the   TCP data receiver that triggered that duplicate acknowledgement.  The   TCP data sender is unable to distinguish between a duplicate   acknowledgement that results from a lost or delayed data packet, and   a duplicate acknowledgement that results from the sender's   retransmission of a data packet that had already been received at the   TCP data receiver.  Because of this, multiple segment losses from a   single window of data can sometimes result in unnecessary multiple   Fast Retransmits (and multiple reductions of the congestion window)   [Flo94].   With the Fast Retransmit and Fast Recovery algorithms in Reno or   NewReno TCP, the performance problems caused by multiple Fast   Retransmits are relatively minor (compared to the potential problems   with Tahoe TCP, which does not implement Fast Recovery).   Nevertheless, unnecessary Fast Retransmits can occur with Reno or   NewReno TCP, particularly if a Retransmit Timeout occurs during Fast   Recovery.  (This is illustrated for Reno on page 6 of [F98], and for   NewReno on page 8 of [F98].)  With NewReno, the data sender remains   in Fast Recovery until either a Retransmit Timeout, or until all of   the data outstanding when Fast Retransmit was entered has been   acknowledged.  Thus with NewReno, the problem of multiple Fast   Retransmits from a single window of data can only occur after a   Retransmit Timeout.   The following modification to the algorithms inSection 3 eliminates   the problem of multiple Fast Retransmits.  (This modification isFloyd & Henderson             Experimental                      [Page 6]

RFC 2582      NewReno Modification to TCP's Fast Recovery     April 1999   called "bugfix" in [F98], and is illustrated on pages 7 and 9.)  This   modification uses a new variable "send_high", whose initial value is   the initial send sequence number.  After each retransmit timeout, the   highest sequence numbers transmitted so far is recorded in the   variable "send_high".   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 acknowledge "send_high".  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.   We note that if the TCP data sender receives three duplicate   acknowledgements that do not acknowledge "send_high", the sender does   not know whether these duplicate acknowledgements resulted from a new   packet drop or not.  For a TCP that implements the bugfix described   in this section for avoiding multiple fast retransmits, the sender   does not infer a packet drop from duplicate acknowledgements in these   circumstances.  As always, the retransmit timer is the backup   mechanism for inferring packet loss in this case.   The modification to Fast Retransmit for avoiding multiple Fast   Retransmits replaces Step 1 inSection 3 with Step 1A below.  In   addition, the modification adds Step 6 below:   1A. When the third duplicate ACK is received and the sender is not       already in the Fast Recovery procedure, check to see if those       duplicate ACKs cover more than "send_high".  If they do, then set       ssthresh to no more than the value given in equation 1, record       the the highest sequence number transmitted in the variable       "recover", and go to Step 2.  If the duplicate ACKs don't cover       "send_high", then do nothing.  That is, do not enter the Fast       Retransmit and Fast Recovery procedure, 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.   Steps 2-5 are the same as those steps inSection 3 above.   6.  After a retransmit timeout, record the highest sequence number       transmitted in the variable "send_high" and exit the Fast       Recovery procedure if applicable.   Step 1A above, in checking whether the duplicate ACKs cover *more*   than "send_high", is the Careful variant of this algorithm.  Another   possible variant would be to require simply that the three duplicateFloyd & Henderson             Experimental                      [Page 7]

RFC 2582      NewReno Modification to TCP's Fast Recovery     April 1999   acknowledgements *cover* "send_high" before initiating another Fast   Retransmit.  We call this the Less Careful variant to Fast   Retransmit.   There are two separate scenarios in which the TCP sender could   receive three duplicate acknowledgements acknowledging "send_high"   but no more than "send_high".  One scenario would be that the data   sender transmitted four packets with sequence numbers higher than   "send_high", that the first packet was dropped in the network, and   the following three packets triggered three duplicate   acknowledgements acknowledging "send_high".  The second scenario   would be that the sender unnecessarily retransmitted three packets   below "send_high", and that these three packets triggered three   duplicate acknowledgements acknowledging "send_high".  In the absence   of SACK, the TCP sender in 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.  The NS   simulator has implemented the Less Careful variant of NewReno, and   the TCP implementation in Sun's Solaris 7 implements the Careful   variant.  This document recommends the Careful variant given in Step   1A above.6. Implementation issues for the data receiver.   [RFC2001] specifies that "Out-of-order data segments SHOULD be   acknowledged immediately, in order to trigger the fast retransmit   algorithm." Neal Cardwell has noted [C98] 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.  As [C98] notes, this severely   limits the potential benefit from NewReno by delaying the receipt of   the partial acknowledgement at the data sender.  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.7. 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 partialFloyd & Henderson             Experimental                      [Page 8]

RFC 2582      NewReno Modification to TCP's Fast Recovery     April 1999   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.   The tests "../../ns test-suite-newreno.tcl newreno1_B0" and "../../ns   test-suite-newreno.tcl newreno1_B" show the Slow-but-Steady and the   Impatient variants of NewReno, respectively.8. Conclusions   Our recommendation is that TCP implementations include the NewReno   modification to the Fast Recovery algorithm given inSection 3, along   with the modification for avoiding multiple Fast Retransmits given inSection 5.  The NewReno modification given inSection 3 can be   important even 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.  The NewReno modification   given inSection 3 implements the Impatient rather than the Slow-but-   Steady variant of NewReno.   While this document mentions several possible variations to the   NewReno algorithm, we have not explored all of these possible   variations, and therefore are unable to make recommendations about   some of them.  Our belief is that the differences between any two   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 invocation without SACK; it is   less important exactly which variant of NewReno is implemented.9. Acknowledgements   Many thanks to Anil Agarwal, Mark Allman, Vern Paxson, Kacheong Poon,   and Bernie Volz for detailed feedback on this document.10. References   [C98]         Neal Cardwell, "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]         Sally Floyd.  Revisions toRFC 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".   [FF96]        Kevin Fall and Sally Floyd.  Simulation-basedFloyd & Henderson             Experimental                      [Page 9]

RFC 2582      NewReno Modification to TCP's Fast Recovery     April 1999                 Comparisons of Tahoe, Reno and SACK TCP.  Computer                 Communication Review, July 1996.  URL                 "ftp://ftp.ee.lbl.gov/papers/sacks.ps.Z".   [Flo94]       S. Floyd, TCP and Successive Fast Retransmits.                 Technical report, October 1994.  URL                 "ftp://ftp.ee.lbl.gov/papers/fastretrans.ps".   [Hen98]       Tom Henderson, 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]       J. Hoe, Startup Dynamics of TCP's Congestion Control                 and Avoidance Schemes. Master's Thesis, MIT, 1995.  URL                 "http://ana-www.lcs.mit.edu/anaweb/ps-papers/hoe-thesis.ps".   [Hoe96]       J. Hoe, "Improving the Start-up Behavior of a                 Congestion Control Scheme for TCP",  In ACM SIGCOMM,                 August 1996.  URL                 "http://www.acm.org/sigcomm/sigcomm96/program.html".   [HTH98]       Hughes, A., Touch, J.  and J. Heidemann, "Issues in TCP                 Slow-Start Restart After Idle", Work in Progress, March                 1998.   [LM97]        Dong Lin and Robert Morris, "Dynamics of Random Early                 Detection", SIGCOMM 97, September 1997.  URL                 "http://www.acm.org/sigcomm/sigcomm97/program.html".   [MMFR96]      Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP                 Selective Acknowledgement Options",RFC 2018, October                 1996.   [NS]          The UCB/LBNL/VINT Network Simulator (NS). URL                 "http://www-mash.cs.berkeley.edu/ns/".   [RFC2001]     Stevens, W., "TCP Slow Start, Congestion Avoidance,                 Fast Retransmit, and Fast Recovery Algorithms",RFC2001, January 1997.   [RFC2581]     Stevens, W., Allman, M. and V. Paxson, "TCP Congestion                 Control",RFC 2581, April 1999.Floyd & Henderson             Experimental                     [Page 10]

RFC 2582      NewReno Modification to TCP's Fast Recovery     April 199911. Security ConsiderationsRFC 2581 discusses general security considerations concerning TCP   congestion control.  This document describes a specific algorithm   that conforms with the congestion control requirements ofRFC 2581,   and so those considerations apply to this algorithm, too.  There are   no known additional security concerns for this specific algorithm.12. AUTHORS' ADDRESSES   Sally Floyd   AT&T Center for Internet Research at ICSI (ACIRI)   Phone: +1 (510) 642-4274 x189   EMail: floyd@acm.org   URL:http://www.aciri.org/floyd/   Tom Henderson   University of California at Berkeley   Phone: +1 (510) 642-8919   EMail: tomh@cs.berkeley.edu   URL:http://www.cs.berkeley.edu/~tomh/Floyd & Henderson             Experimental                     [Page 11]

RFC 2582      NewReno Modification to TCP's Fast Recovery     April 199913.  Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Floyd & Henderson             Experimental                     [Page 12]