TCP Maintenance Working Group M. Mathis Internet-Draft N. Dukkipati Obsoletes: 6937 (if approved) Y. Cheng Intended status: Standards Track Google, Inc. Expires: 26 August 2021 22 February 2021 Proportional Rate Reduction for TCP draft-ietf-tcpm-prr-rfc6937bis-01 Abstract This document updates the experimental Proportional Rate Reduction (PRR) algorithm, described RFC 6937, to standards track. PRR potentially replaces the Fast Recovery and Rate-Halving algorithms. All of these algorithms regulate the amount of data sent by TCP or other transport protocol during loss recovery. PRR accurately regulates the actual flight size through recovery such that at the end of recovery it will be as close as possible to the ssthresh, as determined by the congestion control algorithm. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 26 August 2021. Copyright Notice Copyright (c) 2021 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 (https://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 Mathis, et al. Expires 26 August 2021 [Page 1] Internet-Draft Proportional Rate Reduction February 2021 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 Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Document and WG Information . . . . . . . . . . . . . . . 3 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Changes From RFC 6937 . . . . . . . . . . . . . . . . . . . . 5 4. Relationships to other standards . . . . . . . . . . . . . . 6 5. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 7 6. Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 9 8. Properties . . . . . . . . . . . . . . . . . . . . . . . . . 12 9. Adapting PRR to other transport protocols . . . . . . . . . . 14 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 11. Security Considerations . . . . . . . . . . . . . . . . . . . 15 12. Normative References . . . . . . . . . . . . . . . . . . . . 15 13. Informative References . . . . . . . . . . . . . . . . . . . 15 Appendix A. Strong Packet Conservation Bound . . . . . . . . . . 17 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 1. Introduction This document updates the Proportional Rate Reduction (PRR) algorithm described in [RFC6937] from experimental to standards track. PRR accuracy regulates the amount of data sent during loss recovery, such that at the end of recovery the flight size will be as close as possible to the ssthresh, as determined by the congestion control algorithm. PRR has been deployed in at least 3 major operating systems covering the vast majority of today's web traffic. The only change from RFC 6937 is the introduction of a new heuristic that replaces a manual configuration parameter. There have been no changes to the behaviors of the algorithms or the previously published results. The new heuristic only changes behaviors in corner cases that were not relevant prior to the Lost Retransmission Detection (LRD) algorithm which was not implemented until after RFC 6937 was published. This document also includes additional discussion about integration into other congestion control and recovery algorithms. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] Mathis, et al. Expires 26 August 2021 [Page 2] Internet-Draft Proportional Rate Reduction February 2021 1.1. Document and WG Information Formatted: 2021-02-22 14:22:57-08:00 Please send all comments, questions and feedback to tcpm@ietf.org About revision 00: The introduction above was drawn from draft-mathis-tcpm-rfc6937bis- 00. All of the text below was copied verbatim from RFC 6937, to facilitate comparison between RFC 6937 and this document as it evolves. About revision 01: * Recast the RFC 6937 introduction as background * Made "Changes From RFC 6937" an explicit section * Made Relationships to other standards more explicit * Added a generalized safeACK heuristic * Provided hints for non TCP implementations * Added language about detecting ACK splitting, but have no advice on actions (yet) 2. Background This section is copied almost verbatim from the introduction to RFC 6937. 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. RFC 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. Mathis, et al. Expires 26 August 2021 [Page 3] Internet-Draft Proportional Rate Reduction February 2021 With RFC 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 was 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 in Appendix A; however, in real networks it does not perform as well as the algorithms described in RFC 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 than RFC 6675 (transmitting fewer segments or taking more time to transmit them), it outperforms it, due to the lower probability of additional losses during recovery. Mathis, et al. Expires 26 August 2021 [Page 4] Internet-Draft Proportional Rate Reduction February 2021 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 in RFC 6675 and used here, but it is distinct from the Strong Packet Conservation Bound as described in Appendix A, which is defined solely on the basis of data arriving at the receiver. 3. Changes From RFC 6937 The largest change since RFC 6937 [RFC6937] is the introduction of a new heuristic that uses good recovery progress (For TCP, snd.una advances and no additional segments are marked as lost) to select which Reduction Bound. RFC 6937 left the choice of Reduction Bound to the discretion of the implementer but recommended to use BBR-SSRB by default. For all of the environments explored in earlier PRR research, the new heuristic is consistent with the old recommendation. The paper "An Internet-Wide Analysis of Traffic Policing" [Flach2016policing] uncovered a crucial situation, not previously explored, where both Reduction Bounds perform very poorly, but for different reasons. Under many configurations, token bucket traffic policers [token_bucket] can suddenly start discarding a large fraction of the traffic, without any warning to the end systems. The transport congestion control has no opportunity to measure the token rate, and sets ssthresh based on the previously observed path performance. This value for ssthresh may result in a data rate that is substantially larger than the token rate, causing persistent high loss. Under these conditions, both reduction bounds perform very poorly. PRR-CRB is too timid, sometimes causing very long recovery times at smaller than necessary windows, and PRR-SSRB is too aggressive, often causing many retransmissions to be lost multiple times. Mathis, et al. Expires 26 August 2021 [Page 5] Internet-Draft Proportional Rate Reduction February 2021 Investigating these environments led to the development of a "safeACK" heuristic to dynamically switch between Reduction Bounds: use PRR-SSRB for ACKs reporting that the recovery is making good progress (snd.una is advancing without any new losses) and PRR-CRB otherwise This heuristic is only invoked where application-limited behavior, losses or other events cause the flight size to fall below ssthresh. The extreme loss rates that make the heuristic important are only common in the presence of token bucket policers, which are pathologically wasteful and inefficient [Flach2016policing]. In these environments the heuristic serves to salvage a bad situation and any reasonable implementation of the heuristic performs far better than either bound by itself. The heuristic has no effect whatsoever in congestion events where there are no lost retransmissions, including all of the examples described below and in RFC 6937. Since RFC 6937 was written, PRR has also been adapted to perform multiplicative window reduction for non-loss based congestion control algorithms, such as for RFC 3168 style ECN. This is typically done by using some parts of the loss recovery state machine (in particular the RecoveryPoint from RFC 6675) to invoke the PRR ACK processing for exactly one round trip worth of ACKs. For RFC 6937 we published a companion paper [IMC11] in which we evaluated Fast Retransmit, Rate-Halving and various experimental PRR versions in a large scale measurement study. Today, the legacy algorithms used in that study have already faded from the code base, making such comparisons impossible without recreating historical algorithms. Readers interested in the measurement study should review section 5 of RFC 6937 and the IMC paper [IMC11]. 4. Relationships to other standards PRR is described as modifications to "TCP Congestion Control" [RFC5681], and "A Conservative Loss Recovery Algorithm Based on Selective Acknowledgment (SACK) for TCP" [RFC6675]. It is most accurate and more easily implemented with SACK [RFC2018], but does not require SACK. Mathis, et al. Expires 26 August 2021 [Page 6] Internet-Draft Proportional Rate Reduction February 2021 The SafeACK heuristic came about as a consequence of robust Lost Retransmission Detection under development in an early precursor to [RACK]. Without LRD, policers that cause very high loss rates are guaranteed to also cause retransmission timeouts because both RFC 5681 and RFC 6675 will send retransmissions above the policed rate. PRR and the SafeACK heuristic were already well in place before the RACK algorithm was fully matured. Note that there is no experience implementing or testing RACK without PRR. For this reason it is recommended that PRR is implemented with RACK. 5. 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. 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. If this calculation results in a negative DeliveredData the data sender can infer that the receiver is using a ACK splitting attack [and do what? @@@@] Note that DeliveredData is robust; for TCP using SACK, DeliveredData can be precisely computed anywhere along the return path by inspecting the returning ACKs. The consequence of missing ACKs is Mathis, et al. Expires 26 August 2021 [Page 7] Internet-Draft Proportional Rate Reduction February 2021 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. safeACK: A local variable indicating that the current ACK reported good progress -- snd.una advanced with no additional segments newly marked lost. sndcnt: A local variable indicating 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. 6. 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 Figure 1 On every ACK during recovery compute: DeliveredData = change_in(snd.una) + change_in(SACKd) prr_delivered += DeliveredData pipe = (RFC 6675 pipe algorithm) safeACK = (snd.una advances with no new losses) if (pipe > ssthresh) { // Proportional Rate Reduction sndcnt = CEIL(prr_delivered * ssthresh / RecoverFS) - prr_out } else { // Two version of the reduction bound if (safeACK) { // PRR+SSRB limit = MAX(prr_delivered - prr_out, DeliveredData) + MSS } else { // PRR+CRB limit = prr_delivered - prr_out } // Attempt to catch up, as permitted by limit sndcnt = MIN(ssthresh - pipe, limit) } Figure 2 Mathis, et al. Expires 26 August 2021 [Page 8] Internet-Draft Proportional Rate Reduction February 2021 On any data transmission or retransmission: prr_out += (data sent) // strictly less than or equal to sndcnt Figure 3 7. 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. 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. We are including the Linux Rate_Halving implementation to illustrate the state-of-the-art at the time, even though this algorithm is no longer supported. 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. Mathis, et al. Expires 26 August 2021 [Page 9] Internet-Draft Proportional Rate Reduction February 2021 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 (Historical 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.) Figure 4 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. 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. Mathis, et al. Expires 26 August 2021 [Page 10] Internet-Draft Proportional Rate Reduction February 2021 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 (Historical 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 Figure 5 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 earlier measurements [RFC 6937 section 6] indicates that RFC 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 included an early version of the Conservative Reduction Bound [RHweb]. With this algorithm, 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 Mathis, et al. Expires 26 August 2021 [Page 11] Internet-Draft Proportional Rate Reduction February 2021 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. 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 by RFC 5681, 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. 8. 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. Mathis, et al. Expires 26 August 2021 [Page 12] Internet-Draft Proportional Rate Reduction February 2021 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. 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 with RFC 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. Mathis, et al. Expires 26 August 2021 [Page 13] Internet-Draft Proportional Rate Reduction February 2021 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 does 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. See Appendix A for a detailed discussion of this property. Although the Strong Packet Conservation Bound is very appealing for a number of reasons, our earlier measurements [RFC 6937 section 6] demonstrate that it is less aggressive and does not perform as well as RFC 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 than RFC 6675 in the presence of burst losses. 9. Adapting PRR to other transport protocols The main PRR algorithm and reductions bounds can be adapted to any transport that can support RFC 6675. The safeACK heuristic can be generalized as any ACK of a retransmission that does not cause some other segment to be marked for retransmission. That is, PRR_SSRB is safe on any ACK that reduces the total number of pending and outstanding retransmissions. 10. 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. Neal Cardwell for reviewing and testing the patch. Mathis, et al. Expires 26 August 2021 [Page 14] Internet-Draft Proportional Rate Reduction February 2021 11. 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. 12. Normative References [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, . [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, DOI 10.17487/RFC2018, October 1996, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, . [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, DOI 10.17487/RFC6675, August 2012, . 13. Informative References [CUBIC] Rhee, I. and L. Xu, "CUBIC: A new TCP-friendly high-speed TCP variant", PFLDnet 2005, February 2005. [FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgment: Refining TCP Congestion Control", ACM SIGCOMM SIGCOMM96, August 1996. Mathis, et al. Expires 26 August 2021 [Page 15] Internet-Draft Proportional Rate Reduction February 2021 [Flach2016policing] Flach, T., Papageorge, P., Terzis, A., Pedrosa, L., Cheng, Y., Al Karim, T., Katz-Bassett, E., and R. Govindan, "An Internet-Wide Analysis of Traffic Policing", ACM SIGCOMM SIGCOMM2016, August 2016. [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. [Jacobson88] Jacobson, V., "Congestion Avoidance and Control", SIGCOMM Comput. Commun. Rev. 18(4), August 1988. [Laminar] Mathis, M., "Laminar TCP and the case for refactoring TCP congestion control", Work in Progress, 16 July 2012. [RFC3042] Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing TCP's Loss Recovery Using Limited Transmit", RFC 3042, DOI 10.17487/RFC3042, 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, DOI 10.17487/RFC3517, April 2003, . [RFC6937] Mathis, M., Dukkipati, N., and Y. Cheng, "Proportional Rate Reduction for TCP", RFC 6937, DOI 10.17487/RFC6937, May 2013, . [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, . [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. Mathis, et al. Expires 26 August 2021 [Page 16] Internet-Draft Proportional Rate Reduction February 2021 Appendix 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 the 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 assumptions Mathis, et al. Expires 26 August 2021 [Page 17] Internet-Draft Proportional Rate Reduction February 2021 (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 United States of America Email: mattmathis@google.com Nandita Dukkipati Google, Inc. 1600 Amphitheatre Parkway Mountain View, California 94043 United States of America Email: nanditad@google.com Yuchung Cheng Google, Inc. 1600 Amphitheatre Parkway Mountain View, California 94043 United States of America Email: ycheng@google.com Mathis, et al. Expires 26 August 2021 [Page 18]