Proportional Rate Reduction for TCP

Abstract

This document updates the experimental Proportional Rate Reduction (PRR) algorithm, described RFC 6937, to standards track. PRR potentially replaces the Fast Recovery to 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 slow start threshold (ssthresh), as determined by the congestion control algorithm.¶

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 slow start threshold (ssthresh), as determined by the congestion control algorithm. PRR has been deployed in at least three major TCP implementations covering the vast majority of today's web traffic.¶

The main change from RFC 6937 is the introduction of a new heuristic that replaces a manual configuration parameter and non-SACK support. The new heuristic only changes behaviors in corner cases that were not relevant before the Lost Retransmission Detection (LRD) algorithm. LRD was not implemented until after RFC 6937 was published. This document also includes additional discussion about integration into other congestion control and lost detection 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]¶

2. Background

This section is copied almost verbatim from the introduction to [RFC6937].¶

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.¶

[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. With [RFC6675], Fast Recovery is implemented by sending data as necessary on each ACK to prevent pipe from falling below 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.¶

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 uses 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 [RFC6675], 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 [RFC6675] (transmitting fewer segments or taking more time to transmit them), it outperforms 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 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 [RFC6937] is the introduction of a new heuristic that uses good recovery progress (For TCP, when the latest ACK advances snd.una and does not indicate prior fast retransmit has been lost) to select which Reduction Bound. [RFC6937] left the choice of Reduction Bound to the discretion of the implementer but recommended to use 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 when tokens are depleted, 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 cause a data rate that is substantially larger than the token rate, causing 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 rounds. Both cases lead to prolonged recovery decimating application goodput.¶

Investigating these environments led to the development of a "safeACK" heuristic to dynamically switch between Reduction Bounds: by default conservatively use PRR-CRB and only switch to PRR-SSRB when ACKs indicate the recovery is making good progress (snd.una is advancing without any new losses).¶

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 essential are only common in the presence of heavy losses such as traffic policers [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.¶

Another subtle change is to force a fast retransmit upon the first ACK that triggers the recovery. Previously PRR may not allow a fast retransmit (i.e. sndcnt is 0) on the first ACK depending on the loss situation. Forcing a fast retransmit is important to keep the ACK clock and avoid potential RTO events. The forced fast retransmit only happens once during the entire recovery and still follows the packet conservation principles in PRR. This heuristic has been in the first widely deployed implementation.¶

Since [RFC6937] was written, PRR has also been adapted to perform multiplicative window reduction for non-loss based congestion control algorithms, such as for [RFC3168] style ECN. This is typically done by using some parts of the loss recovery state machine (in particular the RecoveryPoint from [RFC6675]) to invoke the PRR ACK processing for exactly one round trip worth of ACKs.¶

For [RFC6937] we published a companion paper [IMC11] in which we evaluated [RFC3517] 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 with SACK [RFC2018] but does not require SACK.¶

The SafeACK heuristic came about as a result of robust Lost Retransmission Detection under development in an early precursor to [RFC8985]. Without LRD, policers that cause very high loss rates are guaranteed to also cause retransmission timeouts because both [RFC5681] and [RFC6675] will send retransmissions above the policed rate. It is RECOMMENDED that PRR is implemented together with [RFC8985].¶

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.¶

PRR defines additional variables:¶

DeliveredData is the number of bytes newly delivered by the most recent ACK. With SACK, DeliveredData is the change in snd.una plus the bytes newly selectively acknowledged. Without SACK, DeliveredData is the change in snd.una for a partial ACK or 1 MSS worth of bytes for a DUPACK.¶

Note that without SACK, a poorly-behaved receiver that returns extraneous DUPACKs as depicted in [Savage99] can artificially inflate DeliveredData. As a mitigation, PRR disallows incrementing DeliveredData when the total bytes delivered exceeds the outstanding data upon recovery (i.e., RecoverFS).¶

safeACK: A local boolean variable indicating that the current ACK reported good progress. SafeACK is true only when the ACK has cumulatively acknowledged new data and the ACK does not indicate further losses. Both conditions indicate the recovery is making good progress and can send more aggressively.¶

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 the PRR state. This assumes a modern congestion control algorithm, CongCtrlAlg(), that might set ssthresh to something other than FlightSize/2:¶

   ssthresh = CongCtrlAlg()      // Target flight size in recovery
   prr_delivered = 0             // Total bytes delivered in recovery
   prr_out = 0                   // Total bytes sent in recovery
   RecoverFS = snd.nxt - snd.una // FlightSize right before recovery

On every ACK starting or during the recovery:
   DeliveredData = bytes newly cumulatively acknowledged
   if (SACK is used) {
      DeliveredData += bytes newly selectively acknowledged
   } else if (ACK is a DUPACK and prr_delivered < RecoverFS) {
      DeliveredData += MSS
   }
   if (DeliveredData is 0)
      Return


   prr_delivered += DeliveredData
   pipe = (RFC 6675 pipe algorithm)
   safeACK = (snd.una advances and no further loss indicated)
   if (pipe > ssthresh) {
      // Proportional Rate Reduction
      sndcnt = CEIL(prr_delivered * ssthresh / RecoverFS) - prr_out
   } else {
      // PRR-CRB by default
      sndcnt = MAX(prr_delivered - prr_out, DeliveredData)
      if (safeACK) {
         // PRR-SSRB when recovery is in good progress
         sndcnt +=MSS
      }
      // Attempt to catch up, as permitted
      sndcnt = MIN(ssthresh - pipe, sndcnt)
   }


   if (prr_out is 0 AND sndcnt is 0) {
      // Force a fast retransmit upon entering recovery
      sndcnt = MSS
   }
   cwnd = pipe + sndcnt

On any data transmission or retransmission:
   prr_out += (data sent)

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 [RFC5681], 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 but before further (re)transmission. "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.¶

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




PRR
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 10
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

Note that both algorithms send the same total amount of data. RFC 6675 experiences a "half window of silence" while PRR spreads 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.¶

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




PRR
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

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 PRR, 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.¶

Under such heavy losses, PRR uses the PRR-CRB to follow the packet conservation principle. 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. PRR-CRB's conservative window reduction causes it to take excessively long to recover the losses and exposes it to additional timeouts.¶

While not shown in the figure above, once the fast retransmits sent upon ACK#17 deliver and solicit further ACKs that increment the snd.una, PRR enters PRR-SSRB and increases the window by exactly 1 segment per ACK until pipe rises to ssthresh during recovery. On heavy losses when cwnd is large, PRR-SSRB recovers the losses exponentially faster than PRR-CRB. 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 severe loss events, where the total losses are smaller than the difference between FlightSize and ssthresh, PRR-CRB and PRR-SSRB are not invoked since PRR stays in the proportional rate reduction mode.¶

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.¶

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.¶

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 when the ACK indicates the recovery is in good progress without further losses. 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. In one major implementation (Linux TCP), PRR has been the default fast recovery algorithm for its default and supported congestion control modules.¶

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 initial implementation.¶

Neal Cardwell and Mark Allman improved the document through their insightful reviews.¶

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, <https://www.rfc-editor.org/info/rfc793>.

[RFC2018]

Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, DOI 10.17487/RFC2018, October 1996, <https://www.rfc-editor.org/info/rfc2018>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC5681]

Allman, M., Paxson, V., and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, <https://www.rfc-editor.org/info/rfc5681>.

[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, <https://www.rfc-editor.org/info/rfc6675>.

[RFC8985]

Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "The RACK-TLP Loss Detection Algorithm for TCP", RFC 8985, DOI 10.17487/RFC8985, February 2021, <https://www.rfc-editor.org/info/rfc8985>.

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.

[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, <https://www.rfc-editor.org/info/rfc3042>.

[RFC3168]

Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001, <https://www.rfc-editor.org/info/rfc3168>.

[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, <https://www.rfc-editor.org/info/rfc3517>.

[RFC6937]

Mathis, M., Dukkipati, N., and Y. Cheng, "Proportional Rate Reduction for TCP", RFC 6937, DOI 10.17487/RFC6937, May 2013, <https://www.rfc-editor.org/info/rfc6937>.

[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/>.

[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.