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Internet Engineering Task Force (IETF)                         M. MathisRequest for Comments: 6937                                  N. DukkipatiCategory: Experimental                                          Y. ChengISSN: 2070-1721                                             Google, Inc.                                                                May 2013Proportional Rate Reduction for TCPAbstract   This document describes an experimental Proportional Rate Reduction   (PRR) algorithm as an alternative to the widely deployed Fast   Recovery and Rate-Halving algorithms.  These algorithms determine the   amount of data sent by TCP during loss recovery.  PRR minimizes   excess window adjustments, and the actual window size at the end of   recovery will be as close as possible to the ssthresh, as determined   by the congestion control algorithm.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for examination, experimental implementation, and   evaluation.   This document defines an Experimental Protocol for the Internet   community.  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).  Not   all documents approved by the IESG are a candidate for any level of   Internet Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6937.Mathis, et al.                Experimental                      [Page 1]

RFC 6937               Proportional Rate Reduction              May 2013Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Table of Contents1. Introduction ....................................................22. Definitions .....................................................53. Algorithms ......................................................63.1. Examples ...................................................64. Properties ......................................................95. Measurements ...................................................116. Conclusion and Recommendations .................................127. Acknowledgements ...............................................138. Security Considerations ........................................139. References .....................................................139.1. Normative References ......................................139.2. Informative References ....................................14Appendix A. Strong Packet Conservation Bound ......................151.  Introduction   This document describes an experimental algorithm, PRR, to improve   the accuracy of the amount of data sent by TCP during loss recovery.   Standard congestion control [RFC5681] requires that TCP (and other   protocols) reduce their congestion window (cwnd) in response to   losses.  Fast Recovery, described in the same document, is the   reference algorithm for making this adjustment.  Its stated goal is   to recover TCP's self clock by relying on returning ACKs during   recovery to clock more data into the network.  Fast Recovery   typically adjusts the window by waiting for one half round-trip time   (RTT) of ACKs to pass before sending any data.  It is fragile because   it cannot compensate for the implicit window reduction caused by the   losses themselves.Mathis, et al.                Experimental                      [Page 2]

RFC 6937               Proportional Rate Reduction              May 2013RFC 6675 [RFC6675] makes Fast Recovery with Selective Acknowledgement   (SACK) [RFC2018] more accurate by computing "pipe", a sender side   estimate of the number of bytes still outstanding in the network.   WithRFC 6675, Fast Recovery is implemented by sending data as   necessary on each ACK to prevent pipe from falling below slow-start   threshold (ssthresh), the window size as determined by the congestion   control algorithm.  This protects Fast Recovery from timeouts in many   cases where there are heavy losses, although not if the entire second   half of the window of data or ACKs are lost.  However, a single ACK   carrying a SACK option that implies a large quantity of missing data   can cause a step discontinuity in the pipe estimator, which can cause   Fast Retransmit to send a burst of data.   The Rate-Halving algorithm sends data on alternate ACKs during   recovery, such that after 1 RTT the window has been halved.  Rate-   Halving is implemented in Linux after only being informally published   [RHweb], including an uncompleted document [RHID].  Rate-Halving also   does not adequately compensate for the implicit window reduction   caused by the losses and assumes a net 50% window reduction, which   was completely standard at the time it was written but not   appropriate for modern congestion control algorithms, such as CUBIC   [CUBIC], which reduce the window by less than 50%.  As a consequence,   Rate-Halving often allows the window to fall further than necessary,   reducing performance and increasing the risk of timeouts if there are   additional losses.   PRR avoids these excess window adjustments such that at the end of   recovery the actual window size will be as close as possible to   ssthresh, the window size as determined by the congestion control   algorithm.  It is patterned after Rate-Halving, but using the   fraction that is appropriate for the target window chosen by the   congestion control algorithm.  During PRR, one of two additional   Reduction Bound algorithms limits the total window reduction due to   all mechanisms, including transient application stalls and the losses   themselves.   We describe two slightly different Reduction Bound algorithms:   Conservative Reduction Bound (CRB), which is strictly packet   conserving; and a Slow Start Reduction Bound (SSRB), which is more   aggressive than CRB by, at most, 1 segment per ACK.  PRR-CRB meets   the Strong Packet Conservation Bound described inAppendix A;   however, in real networks it does not perform as well as the   algorithms described inRFC 6675, which prove to be more aggressive   in a significant number of cases.  SSRB offers a compromise by   allowing TCP to send 1 additional segment per ACK relative to CRB in   some situations.  Although SSRB is less aggressive thanRFC 6675Mathis, et al.                Experimental                      [Page 3]

RFC 6937               Proportional Rate Reduction              May 2013   (transmitting fewer segments or taking more time to transmit them),   it outperforms it, due to the lower probability of additional losses   during recovery.   The Strong Packet Conservation Bound on which PRR and both Reduction   Bounds are based is patterned after Van Jacobson's packet   conservation principle: segments delivered to the receiver are used   as the clock to trigger sending the same number of segments back into   the network.  As much as possible, PRR and the Reduction Bound   algorithms rely on this self clock process, and are only slightly   affected by the accuracy of other estimators, such as pipe [RFC6675]   and cwnd.  This is what gives the algorithms their precision in the   presence of events that cause uncertainty in other estimators.   The original definition of the packet conservation principle   [Jacobson88]  treated packets that are presumed to be lost (e.g.,   marked as candidates for retransmission) as having left the network.   This idea is reflected in the pipe estimator defined inRFC 6675 and   used here, but it is distinct from the Strong Packet Conservation   Bound as described inAppendix A, which is defined solely on the   basis of data arriving at the receiver.   We evaluated these and other algorithms in a large scale measurement   study presented in a companion paper [IMC11] and summarized inSection 5.  This measurement study was based onRFC 3517 [RFC3517],   which has since been superseded byRFC 6675.  Since there are slight   differences between the two specifications, and we were meticulous   about our implementation ofRFC 3517, we are not comfortable   unconditionally asserting that our measurement results apply toRFC6675, although we believe this to be the case.  We have instead   chosen to be pedantic about describing measurement results relative   toRFC 3517, on which they were actually based.  General discussions   of algorithms and their properties have been updated to refer toRFC6675.   We found that for authentic network traffic, PRR-SSRB outperforms   bothRFC 3517 and Linux Rate-Halving even though it is less   aggressive thanRFC 3517.  We believe that these results apply toRFC6675 as well.   The algorithms are described as modifications toRFC 5681 [RFC5681],   "TCP Congestion Control", using concepts drawn from the pipe   algorithm [RFC6675].  They are most accurate and more easily   implemented with SACK [RFC2018], but do not require SACK.Mathis, et al.                Experimental                      [Page 4]

RFC 6937               Proportional Rate Reduction              May 20132.  Definitions   The following terms, parameters, and state variables are used as they   are defined in earlier documents:RFC 793: snd.una (send unacknowledged)RFC 5681: duplicate ACK, FlightSize, Sender Maximum Segment Size      (SMSS)RFC 6675: covered (as in "covered sequence numbers")   Voluntary window reductions: choosing not to send data in response to   some ACKs, for the purpose of reducing the sending window size and   data rate   We define some additional variables:   SACKd: The total number of bytes that the scoreboard indicates have      been delivered to the receiver.  This can be computed by scanning      the scoreboard and counting the total number of bytes covered by      all SACK blocks.  If SACK is not in use, SACKd is not defined.   DeliveredData: The total number of bytes that the current ACK      indicates have been delivered to the receiver.  When not in      recovery, DeliveredData is the change in snd.una.  With SACK,      DeliveredData can be computed precisely as the change in snd.una,      plus the (signed) change in SACKd.  In recovery without SACK,      DeliveredData is estimated to be 1 SMSS on duplicate      acknowledgements, and on a subsequent partial or full ACK,      DeliveredData is estimated to be the change in snd.una, minus 1      SMSS for each preceding duplicate ACK.   Note that DeliveredData is robust; for TCP using SACK, DeliveredData   can be precisely computed anywhere in the network just by inspecting   the returning ACKs.  The consequence of missing ACKs is that later   ACKs will show a larger DeliveredData.  Furthermore, for any TCP   (with or without SACK), the sum of DeliveredData must agree with the   forward progress over the same time interval.   We introduce a local variable "sndcnt", which indicates exactly how   many bytes should be sent in response to each ACK.  Note that the   decision of which data to send (e.g., retransmit missing data or send   more new data) is out of scope for this document.Mathis, et al.                Experimental                      [Page 5]

RFC 6937               Proportional Rate Reduction              May 20133.  Algorithms   At the beginning of recovery, initialize PRR state.  This assumes a   modern congestion control algorithm, CongCtrlAlg(), that might set   ssthresh to something other than FlightSize/2:      ssthresh = CongCtrlAlg()  // Target cwnd after recovery      prr_delivered = 0         // Total bytes delivered during recovery      prr_out = 0               // Total bytes sent during recovery      RecoverFS = snd.nxt-snd.una // FlightSize at the start of recovery   On every ACK during recovery compute:      DeliveredData = change_in(snd.una) + change_in(SACKd)      prr_delivered += DeliveredData      pipe = (RFC 6675 pipe algorithm)      if (pipe > ssthresh) {         // Proportional Rate Reduction         sndcnt = CEIL(prr_delivered * ssthresh / RecoverFS) - prr_out      } else {         // Two versions of the Reduction Bound         if (conservative) {    // PRR-CRB           limit = prr_delivered - prr_out         } else {               // PRR-SSRB           limit = MAX(prr_delivered - prr_out, DeliveredData) + MSS         }         // Attempt to catch up, as permitted by limit         sndcnt = MIN(ssthresh - pipe, limit)      }   On any data transmission or retransmission:      prr_out += (data sent) // strictly less than or equal to sndcnt3.1.  Examples   We illustrate these algorithms by showing their different behaviors   for two scenarios: TCP experiencing either a single loss or a burst   of 15 consecutive losses.  In all cases we assume bulk data (no   application pauses), standard Additive Increase Multiplicative   Decrease (AIMD) congestion control, and cwnd = FlightSize = pipe = 20   segments, so ssthresh will be set to 10 at the beginning of recovery.   We also assume standard Fast Retransmit and Limited Transmit   [RFC3042], so TCP will send 2 new segments followed by 1 retransmit   in response to the first 3 duplicate ACKs following the losses.Mathis, et al.                Experimental                      [Page 6]

RFC 6937               Proportional Rate Reduction              May 2013   Each of the diagrams below shows the per ACK response to the first   round trip for the various recovery algorithms when the zeroth   segment is lost.  The top line indicates the transmitted segment   number triggering the ACKs, with an X for the lost segment.  "cwnd"   and "pipe" indicate the values of these algorithms after processing   each returning ACK.  "Sent" indicates how much 'N'ew or   'R'etransmitted data would be sent.  Note that the algorithms for   deciding which data to send are out of scope of this document.   When there is a single loss, PRR with either of the Reduction Bound   algorithms has the same behavior.  We show "RB", a flag indicating   which Reduction Bound subexpression ultimately determined the value   of sndcnt.  When there are minimal losses, "limit" (both algorithms)   will always be larger than ssthresh - pipe, so the sndcnt will be   ssthresh - pipe, indicated by "s" in the "RB" row.RFC 6675   ack#   X  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19   cwnd:    20 20 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11   pipe:    19 19 18 18 17 16 15 14 13 12 11 10 10 10 10 10 10 10 10   sent:     N  N  R                          N  N  N  N  N  N  N  N   Rate-Halving (Linux)   ack#   X  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19   cwnd:    20 20 19 18 18 17 17 16 16 15 15 14 14 13 13 12 12 11 11   pipe:    19 19 18 18 17 17 16 16 15 15 14 14 13 13 12 12 11 11 10   sent:     N  N  R     N     N     N     N     N     N     N     N   PRR   ack#   X  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19   pipe:    19 19 18 18 18 17 17 16 16 15 15 14 14 13 13 12 12 11 10   sent:     N  N  R     N     N     N     N     N     N        N  N   RB:                                                          s  s       Cwnd is not shown because PRR does not use it.   Key for RB   s: sndcnt = ssthresh - pipe                 // from ssthresh   b: sndcnt = prr_delivered - prr_out + SMSS  // from banked   d: sndcnt = DeliveredData + SMSS            // from DeliveredData   (Sometimes, more than one applies.)   Note that all 3 algorithms send the same total amount of data.RFC 6675 experiences a "half window of silence", while the   Rate-Halving and PRR spread the voluntary window reduction across an   entire RTT.Mathis, et al.                Experimental                      [Page 7]

RFC 6937               Proportional Rate Reduction              May 2013   Next, we consider the same initial conditions when the first 15   packets (0-14) are lost.  During the remainder of the lossy RTT, only   5 ACKs are returned to the sender.  We examine each of these   algorithms in succession.RFC 6675   ack#   X  X  X  X  X  X  X  X  X  X  X  X  X  X  X 15 16 17 18 19   cwnd:                                              20 20 11 11 11   pipe:                                              19 19  4 10 10   sent:                                               N  N 7R  R  R   Rate-Halving (Linux)   ack#   X  X  X  X  X  X  X  X  X  X  X  X  X  X  X 15 16 17 18 19   cwnd:                                              20 20  5  5  5   pipe:                                              19 19  4  4  4   sent:                                               N  N  R  R  R   PRR-CRB   ack#   X  X  X  X  X  X  X  X  X  X  X  X  X  X  X 15 16 17 18 19   pipe:                                              19 19  4  4  4   sent:                                               N  N  R  R  R   RB:                                                       b  b  b   PRR-SSRB   ack#   X  X  X  X  X  X  X  X  X  X  X  X  X  X  X 15 16 17 18 19   pipe:                                              19 19  4  5  6   sent:                                               N  N 2R 2R 2R   RB:                                                      bd  d  d   In this specific situation,RFC 6675 is more aggressive because once   Fast Retransmit is triggered (on the ACK for segment 17), TCP   immediately retransmits sufficient data to bring pipe up to cwnd.   Our measurement data (seeSection 5) indicates thatRFC 6675   significantly outperforms Rate-Halving, PRR-CRB, and some other   similarly conservative algorithms that we tested, showing that it is   significantly common for the actual losses to exceed the window   reduction determined by the congestion control algorithm.   The Linux implementation of Rate-Halving includes an early version of   the Conservative Reduction Bound [RHweb].  In this situation, the 5   ACKs trigger exactly 1 transmission each (2 new data, 3 old data),   and cwnd is set to 5.  At a window size of 5, it takes 3 round trips   to retransmit all 15 lost segments.  Rate-Halving does not raise the   window at all during recovery, so when recovery finally completes,   TCP will slow start cwnd from 5 up to 10.  In this example, TCP   operates at half of the window chosen by the congestion control for   more than 3 RTTs, increasing the elapsed time and exposing it to   timeouts in the event that there are additional losses.Mathis, et al.                Experimental                      [Page 8]

RFC 6937               Proportional Rate Reduction              May 2013   PRR-CRB implements a Conservative Reduction Bound.  Since the total   losses bring pipe below ssthresh, data is sent such that the total   data transmitted, prr_out, follows the total data delivered to the   receiver as reported by returning ACKs.  Transmission is controlled   by the sending limit, which is set to prr_delivered - prr_out.  This   is indicated by the RB:b tagging in the figure.  In this case,   PRR-CRB is exposed to exactly the same problems as Rate-Halving; the   excess window reduction causes it to take excessively long to recover   the losses and exposes it to additional timeouts.   PRR-SSRB increases the window by exactly 1 segment per ACK until pipe   rises to ssthresh during recovery.  This is accomplished by setting   limit to one greater than the data reported to have been delivered to   the receiver on this ACK, implementing slow start during recovery,   and indicated by RB:d tagging in the figure.  Although increasing the   window during recovery seems to be ill advised, it is important to   remember that this is actually less aggressive than permitted byRFC5681, which sends the same quantity of additional data as a single   burst in response to the ACK that triggered Fast Retransmit.   For less extreme events, where the total losses are smaller than the   difference between FlightSize and ssthresh, PRR-CRB and PRR-SSRB have   identical behaviors.4.  Properties   The following properties are common to both PRR-CRB and PRR-SSRB,   except as noted:   PRR maintains TCP's ACK clocking across most recovery events,   including burst losses.RFC 6675 can send large unclocked bursts   following burst losses.   Normally, PRR will spread voluntary window reductions out evenly   across a full RTT.  This has the potential to generally reduce the   burstiness of Internet traffic, and could be considered to be a type   of soft pacing.  Hypothetically, any pacing increases the probability   that different flows are interleaved, reducing the opportunity for   ACK compression and other phenomena that increase traffic burstiness.   However, these effects have not been quantified.   If there are minimal losses, PRR will converge to exactly the target   window chosen by the congestion control algorithm.  Note that as TCP   approaches the end of recovery, prr_delivered will approach RecoverFS   and sndcnt will be computed such that prr_out approaches ssthresh.Mathis, et al.                Experimental                      [Page 9]

RFC 6937               Proportional Rate Reduction              May 2013   Implicit window reductions, due to multiple isolated losses during   recovery, cause later voluntary reductions to be skipped.  For small   numbers of losses, the window size ends at exactly the window chosen   by the congestion control algorithm.   For burst losses, earlier voluntary window reductions can be undone   by sending extra segments in response to ACKs arriving later during   recovery.  Note that as long as some voluntary window reductions are   not undone, the final value for pipe will be the same as ssthresh,   the target cwnd value chosen by the congestion control algorithm.   PRR with either Reduction Bound improves the situation when there are   application stalls, e.g., when the sending application does not queue   data for transmission quickly enough or the receiver stops advancing   rwnd (receiver window).  When there is an application stall early   during recovery, prr_out will fall behind the sum of the   transmissions permitted by sndcnt.  The missed opportunities to send   due to stalls are treated like banked voluntary window reductions;   specifically, they cause prr_delivered - prr_out to be significantly   positive.  If the application catches up while TCP is still in   recovery, TCP will send a partial window burst to catch up to exactly   where it would have been had the application never stalled.  Although   this burst might be viewed as being hard on the network, this is   exactly what happens every time there is a partial RTT application   stall while not in recovery.  We have made the partial RTT stall   behavior uniform in all states.  Changing this behavior is out of   scope for this document.   PRR with Reduction Bound is less sensitive to errors in the pipe   estimator.  While in recovery, pipe is intrinsically an estimator,   using incomplete information to estimate if un-SACKed segments are   actually lost or merely out of order in the network.  Under some   conditions, pipe can have significant errors; for example, pipe is   underestimated when a burst of reordered data is prematurely assumed   to be lost and marked for retransmission.  If the transmissions are   regulated directly by pipe as they are withRFC 6675, a step   discontinuity in the pipe estimator causes a burst of data, which   cannot be retracted once the pipe estimator is corrected a few ACKs   later.  For PRR, pipe merely determines which algorithm, PRR or the   Reduction Bound, is used to compute sndcnt from DeliveredData.  While   pipe is underestimated, the algorithms are different by at most 1   segment per ACK.  Once pipe is updated, they converge to the same   final window at the end of recovery.   Under all conditions and sequences of events during recovery, PRR-CRB   strictly bounds the data transmitted to be equal to or less than the   amount of data delivered to the receiver.  We claim that this Strong   Packet Conservation Bound is the most aggressive algorithm that doesMathis, et al.                Experimental                     [Page 10]

RFC 6937               Proportional Rate Reduction              May 2013   not lead to additional forced losses in some environments.  It has   the property that if there is a standing queue at a bottleneck with   no cross traffic, the queue will maintain exactly constant length for   the duration of the recovery, except for +1/-1 fluctuation due to   differences in packet arrival and exit times.  SeeAppendix A for a   detailed discussion of this property.   Although the Strong Packet Conservation Bound is very appealing for a   number of reasons, our measurements summarized inSection 5   demonstrate that it is less aggressive and does not perform as well   asRFC 6675, which permits bursts of data when there are bursts of   losses.  PRR-SSRB is a compromise that permits TCP to send 1 extra   segment per ACK as compared to the Packet Conserving Bound.  From the   perspective of a strict Packet Conserving Bound, PRR-SSRB does indeed   open the window during recovery; however, it is significantly less   aggressive thanRFC 6675 in the presence of burst losses.5.  Measurements   In a companion IMC11 paper [IMC11], we describe some measurements   comparing the various strategies for reducing the window during   recovery.  The experiments were performed on servers carrying Google   production traffic and are briefly summarized here.   The various window reduction algorithms and extensive instrumentation   were all implemented in Linux 2.6.  We used the uniform set of   algorithms present in the base Linux implementation, including CUBIC   [CUBIC], Limited Transmit [RFC3042], threshold transmit (Section 3.1   in [FACK]) (this algorithm was not present inRFC 3517, but a similar   algorithm has been added toRFC 6675), and lost retransmission   detection algorithms.  We confirmed that the behaviors of Rate-   Halving (the Linux default),RFC 3517, and PRR were authentic to   their respective specifications and that performance and features   were comparable to the kernels in production use.  All of the   different window reduction algorithms were all present in a common   kernel and could be selected with a sysctl, such that we had an   absolutely uniform baseline for comparing them.   Our experiments included an additional algorithm, PRR with an   unlimited bound (PRR-UB), which sends ssthresh-pipe bursts when pipe   falls below ssthresh.  This behavior parallelsRFC 3517.   An important detail of this configuration is that CUBIC only reduces   the window by 30%, as opposed to the 50% reduction used by   traditional congestion control algorithms.  This accentuates the   tendency forRFC 3517 and PRR-UB to send a burst at the point when   Fast Retransmit gets triggered because pipe is likely to already be   below ssthresh.  Precisely this condition was observed for 32% of theMathis, et al.                Experimental                     [Page 11]

RFC 6937               Proportional Rate Reduction              May 2013   recovery events: pipe fell below ssthresh before Fast Retransmit was   triggered, thus the various PRR algorithms started in the Reduction   Bound phase, andRFC 3517 sent bursts of segments with the Fast   Retransmit.   In the companion paper, we observe that PRR-SSRB spends the least   time in recovery of all the algorithms tested, largely because it   experiences fewer timeouts once it is already in recovery.RFC 3517 experiences 29% more detected lost retransmissions and 2.6%   more timeouts (presumably due to undetected lost retransmissions)   than PRR-SSRB.  These results are representative of PRR-UB and other   algorithms that send bursts when pipe falls below ssthresh.   Rate-Halving experiences 5% more timeouts and significantly smaller   final cwnd values at the end of recovery.  The smaller cwnd sometimes   causes the recovery itself to take extra round trips.  These results   are representative of PRR-CRB and other algorithms that implement   strict packet conservation during recovery.6.  Conclusion and Recommendations   Although the Strong Packet Conservation Bound used in PRR-CRB is very   appealing for a number of reasons, our measurements show that it is   less aggressive and does not perform as well asRFC 3517 (and by   implicationRFC 6675), which permits bursts of data when there are   bursts of losses.RFC 3517 andRFC 6675 are conservative in the   original sense of Van Jacobson's packet conservation principle, which   included the assumption that presumed lost segments have indeed left   the network.  PRR-CRB makes no such assumption, following instead a   Strong Packet Conservation Bound in which only packets that have   actually arrived at the receiver are considered to have left the   network.  PRR-SSRB is a compromise that permits TCP to send 1 extra   segment per ACK relative to the Strong Packet Conservation Bound, to   partially compensate for excess losses.   From the perspective of the Strong Packet Conservation Bound,   PRR-SSRB does indeed open the window during recovery; however, it is   significantly less aggressive thanRFC 3517 (andRFC 6675) in the   presence of burst losses.  Even so, it often outperformsRFC 3517   (and presumablyRFC 6675) because it avoids some of the self-   inflicted losses caused by bursts.   At this time, we see no reason not to test and deploy PRR-SSRB on a   large scale.  Implementers worried about any potential impact of   raising the window during recovery may want to optionally support   PRR-CRB (which is actually simpler to implement) for comparisonMathis, et al.                Experimental                     [Page 12]

RFC 6937               Proportional Rate Reduction              May 2013   studies.  Furthermore, there is one minor detail of PRR that can be   improved by replacing pipe by total_pipe, as defined by Laminar TCP   [Laminar].   One final comment about terminology: we expect that common usage will   drop "Slow Start Reduction Bound" from the algorithm name.  This   document needed to be pedantic about having distinct names for PRR   and every variant of the Reduction Bound.  However, we do not   anticipate any future exploration of the alternative Reduction   Bounds.7.  Acknowledgements   This document is based in part on previous incomplete work by Matt   Mathis, Jeff Semke, and Jamshid Mahdavi [RHID] and influenced by   several discussions with John Heffner.   Monia Ghobadi and Sivasankar Radhakrishnan helped analyze the   experiments.   Ilpo Jarvinen reviewed the code.   Mark Allman improved the document through his insightful review.8.  Security Considerations   PRR does not change the risk profile for TCP.   Implementers that change PRR from counting bytes to segments have to   be cautious about the effects of ACK splitting attacks [Savage99],   where the receiver acknowledges partial segments for the purpose of   confusing the sender's congestion accounting.9.  References9.1.  Normative References   [RFC0793]    Postel, J., "Transmission Control Protocol", STD 7,RFC 793, September 1981.   [RFC2018]    Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP                Selective Acknowledgment Options",RFC 2018, October                1996.   [RFC5681]    Allman, M., Paxson, V., and E. Blanton, "TCP Congestion                Control",RFC 5681, September 2009.Mathis, et al.                Experimental                     [Page 13]

RFC 6937               Proportional Rate Reduction              May 2013   [RFC6675]    Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo,                M., and Y. Nishida, "A Conservative Loss Recovery                Algorithm Based on Selective Acknowledgment (SACK) for                TCP",RFC 6675, August 2012.9.2.  Informative References   [RFC3042]    Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing                TCP's Loss Recovery Using Limited Transmit",RFC 3042,                January 2001.   [RFC3517]    Blanton, E., Allman, M., Fall, K., and L. Wang, "A                Conservative Selective Acknowledgment (SACK)-based Loss                Recovery Algorithm for TCP",RFC 3517, April 2003.   [IMC11]      Dukkipati, N., Mathis, M., Cheng, Y., and M. Ghobadi,                "Proportional Rate Reduction for TCP", Proceedings of                the 11th ACM SIGCOMM Conference on Internet Measurement                2011, Berlin, Germany, November 2011.   [FACK]       Mathis, M. and J. Mahdavi, "Forward Acknowledgment:                Refining TCP Congestion Control", ACM SIGCOMM SIGCOMM96,                August 1996.   [RHID]       Mathis, M., Semke, J., and J. Mahdavi, "The Rate-Halving                Algorithm for TCP Congestion Control", Work in Progress,                August 1999.   [RHweb]      Mathis, M. and J. Mahdavi, "TCP Rate-Halving with                Bounding Parameters", Web publication, December 1997,                <http://www.psc.edu/networking/papers/FACKnotes/current/>.   [CUBIC]      Rhee, I. and L. Xu, "CUBIC: A new TCP-friendly high-                speed TCP variant", PFLDnet 2005, February 2005.   [Jacobson88] Jacobson, V., "Congestion Avoidance and Control",                SIGCOMM Comput. Commun. Rev. 18(4), August 1988.   [Savage99]   Savage, S., Cardwell, N., Wetherall, D., and T.                Anderson, "TCP congestion control with a misbehaving                receiver", SIGCOMM Comput. Commun. Rev. 29(5), October                1999.   [Laminar]    Mathis, M., "Laminar TCP and the case for refactoring                TCP congestion control", Work in Progress, July 2012.Mathis, et al.                Experimental                     [Page 14]

RFC 6937               Proportional Rate Reduction              May 2013Appendix A.  Strong Packet Conservation Bound   PRR-CRB is based on a conservative, philosophically pure, and   aesthetically appealing Strong Packet Conservation Bound, described   here.  Although inspired by Van Jacobson's packet conservation   principle [Jacobson88], it differs in how it treats segments that are   missing and presumed lost.  Under all conditions and sequences of   events during recovery, PRR-CRB strictly bounds the data transmitted   to be equal to or less than the amount of data delivered to the   receiver.  Note that the effects of presumed losses are included in   the pipe calculation, but do not affect the outcome of PRR-CRB, once   pipe has fallen below ssthresh.   We claim that this Strong Packet Conservation Bound is the most   aggressive algorithm that does not lead to additional forced losses   in some environments.  It has the property that if there is a   standing queue at a bottleneck that is carrying no other traffic, the   queue will maintain exactly constant length for the entire duration   of the recovery, except for +1/-1 fluctuation due to differences in   packet arrival and exit times.  Any less aggressive algorithm will   result in a declining queue at the bottleneck.  Any more aggressive   algorithm will result in an increasing queue or additional losses if   it is a full drop tail queue.   We demonstrate this property with a little thought experiment:   Imagine a network path that has insignificant delays in both   directions, except for the processing time and queue at a single   bottleneck in the forward path.  By insignificant delay, we mean when   a packet is "served" at the head of the bottleneck queue, the   following events happen in much less than one bottleneck packet time:   the packet arrives at the receiver; the receiver sends an ACK that   arrives at the sender; the sender processes the ACK and sends some   data; the data is queued at the bottleneck.   If sndcnt is set to DeliveredData and nothing else is inhibiting   sending data, then clearly the data arriving at the bottleneck queue   will exactly replace the data that was served at the head of the   queue, so the queue will have a constant length.  If queue is drop   tail and full, then the queue will stay exactly full.  Losses or   reordering on the ACK path only cause wider fluctuations in the queue   size, but do not raise its peak size, independent of whether the data   is in order or out of order (including loss recovery from an earlier   RTT).  Any more aggressive algorithm that sends additional data will   overflow the drop tail queue and cause loss.  Any less aggressive   algorithm will under-fill the queue.  Therefore, setting sndcnt to   DeliveredData is the most aggressive algorithm that does not cause   forced losses in this simple network.  Relaxing the assumptionsMathis, et al.                Experimental                     [Page 15]

RFC 6937               Proportional Rate Reduction              May 2013   (e.g., making delays more authentic and adding more flows, delayed   ACKs, etc.) is likely to increase the fine grained fluctuations in   queue size but does not change its basic behavior.   Note that the congestion control algorithm implements a broader   notion of optimal that includes appropriately sharing the network.   Typical congestion control algorithms are likely to reduce the data   sent relative to the Packet Conserving Bound implemented by PRR,   bringing TCP's actual window down to ssthresh.Authors' Addresses   Matt Mathis   Google, Inc.   1600 Amphitheatre Parkway   Mountain View, California  94043   USA   EMail: mattmathis@google.com   Nandita Dukkipati   Google, Inc.   1600 Amphitheatre Parkway   Mountain View, California  94043   USA   EMail: nanditad@google.com   Yuchung Cheng   Google, Inc.   1600 Amphitheatre Parkway   Mountain View, California  94043   USA   EMail: ycheng@google.comMathis, et al.                Experimental                     [Page 16]