RFC 9937 | PRR | December 2025
Mathis, et al. | Standards Track | [Page]

Stream:
  Internet Engineering Task Force (IETF)
RFC:
  9937
Obsoletes:
  6937
Category:
  Standards Track
Published:
  December 2025
ISSN:
  2070-1721
Authors:
  M. Mathis
  N. Cardwell
  Google, Inc.
  Y. Cheng
  Google, Inc.
  N. Dukkipati
  Google, Inc.

# RFC 9937

# Proportional Rate Reduction (PRR)

## Abstract

This document specifies a Standards Track version of the Proportional Rate Reduction (PRR) algorithm that obsoletes the Experimental version described in RFC 6937. PRR regulates the amount of data sent by TCP or other transport protocols during fast 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.¶

## Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9937.¶

## Copyright Notice

Copyright (c) 2025 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 extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

## 1. Introduction

Van Jacobson's packet conservation principle [Jacobson88] defines a self clock process wherein N data segments delivered to the receiver generate acknowledgments that the data sender uses as the clock to trigger sending another N data segments into the network.¶

Congestion control algorithms like Reno [RFC5681] and CUBIC [RFC9438] are built on the conceptual foundation of this self clock process. They control the sending process of a transport protocol connection by using a congestion window ("cwnd") to limit "inflight", the volume of data that a connection estimates is in flight in the network at a given time. Furthermore, these algorithms require that transport protocol connections reduce their cwnd in response to packet losses. Fast recovery (see [RFC5681] and [RFC6675]) is the algorithm for making this cwnd reduction using feedback from acknowledgments. Its stated goal is to maintain a sender's self clock by relying on returning ACKs during recovery to clock more data into the network. Without Proportional Rate Reduction (PRR), fast recovery typically adjusts the window by waiting for a large fraction of a round-trip time (RTT) (one half round-trip time of ACKs for Reno [RFC5681] or 30% of a round-trip time for CUBIC [RFC9438]) to pass before sending any data.¶

[RFC6675] makes fast recovery with Selective Acknowledgment (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 allow pipe to rise to match ssthresh, the target window size for fast recovery, as determined by the congestion control algorithm. This protects fast recovery from timeouts in many cases where there are heavy losses. However, [RFC6675] has two significant drawbacks. First, because it makes a large multiplicative decrease in cwnd at the start of fast recovery, it can cause a timeout if the entire second half of the window of data or ACKs are lost. Second, 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 large burst of data.¶

PRR regulates the transmission process during fast recovery in a manner that avoids these excess window adjustments, such that transmissions progress smoothly, and at the end of recovery, the actual window size will be as close as possible to ssthresh.¶

PRR's approach is inspired by Van Jacobson's packet conservation principle. As much as possible, PRR relies on the self clock process and is only slightly affected by the accuracy of estimators, such as the estimate of the volume of in-flight data. This is what gives the algorithm its precision in the presence of events that cause uncertainty in other estimators.¶

When inflight is above ssthresh, PRR reduces inflight smoothly toward ssthresh by clocking out transmissions at a rate that is in proportion to both the delivered data and ssthresh.¶

When inflight is less than ssthresh, PRR adaptively chooses between one of two Reduction Bounds to limit the total window reduction due to all mechanisms, including transient application stalls and the losses themselves. As a baseline, to be cautious when there may be considerable congestion, PRR uses its Conservative Reduction Bound (CRB), which is strictly packet conserving. When recovery seems to be progressing well, PRR uses its Slow Start Reduction Bound (SSRB), which is more aggressive than PRR-CRB by at most one segment per ACK. PRR-CRB meets the Strong Packet Conservation Bound described in Appendix A; however, when used in real networks as the sole approach, it does not perform as well as the algorithm described in [RFC6675], which proves to be more aggressive in a significant number of cases. PRR-SSRB offers a compromise by allowing a connection to send one additional segment per ACK, relative to PRR-CRB, in some situations. Although PRR-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 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 inflight estimator used by PRR, 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.¶

This document specifies several main changes from the earlier version of PRR in [RFC6937]. First, it introduces a new adaptive heuristic that replaces a manual configuration parameter that determined how conservative PRR was when inflight was less than ssthresh (whether to use PRR-CRB or PRR-SSRB). Second, the algorithm specifies behavior for non-SACK connections (connections that have not negotiated SACK [RFC2018] support via the "SACK-permitted" option). Third, the algorithm ensures a smooth sending process even when the sender has experienced high reordering and starts loss recovery after a large amount of sequence space has been SACKed. Finally, this document also includes additional discussion about the integration of PRR with congestion control and loss detection algorithms.¶

PRR has extensive deployment experience in multiple TCP implementations since the first widely deployed TCP PRR implementation in 2011 [First_TCP_PRR].¶

## 2. Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

## 3. Definitions

The following terms, parameters, and state variables are used as they are defined in earlier documents:¶

SND.UNA:
  The oldest unacknowledged sequence number. This is defined in Section 3.4 of [RFC9293].¶
SND.NXT:
  The next sequence number to be sent. This is defined in Section 3.4 of [RFC9293].¶
duplicate ACK:
  An acknowledgment is considered a "duplicate ACK" or "duplicate acknowledgment" when (a) the receiver of the ACK has outstanding data, (b) the incoming acknowledgment carries no data, (c) the SYN and FIN bits are both off, (d) the acknowledgment number is equal to SND.UNA, and (e) the advertised window in the incoming acknowledgment equals the advertised window in the last incoming acknowledgment. This is defined in Section 2 of [RFC5681].¶
FlightSize:
  The amount of data that has been sent but not yet cumulatively acknowledged. This is defined in Section 2 of [RFC5681].¶
Receiver Maximum Segment Size (RMSS):
  The RMSS is the size of the largest segment the receiver is willing to accept. This is the value specified in the MSS option sent by the receiver during connection startup (see Section 3.7.1 of [RFC9293]). Or if the MSS option is not used, it is the default of 536 bytes for IPv4 or 1220 bytes for IPv6 (see Section 3.7.1 of [RFC9293]). The size does not include the TCP/IP headers and options. The RMSS is defined in Section 2 of [RFC5681] and Section 3.8.6.3 of [RFC9293].¶
Sender Maximum Segment Size (SMSS):
  The SMSS is the size of the largest segment that the sender can transmit. This value can be based on the Maximum Transmission Unit (MTU) of the network, the path MTU discovery [RFC1191] [RFC8201] [RFC4821] algorithm, RMSS, or other factors. The size does not include the TCP/IP headers and options. This is defined in Section 2 of [RFC5681].¶
Receiver Window (rwnd):
  The most recently received advertised receiver window, in bytes. At any given time, a connection MUST NOT send data with a sequence number higher than the sum of SND.UNA and rwnd. This is defined in Section 2 of [RFC5681].¶
Congestion Window (cwnd):
  A state variable that limits the amount of data a connection can send. At any given time, a connection MUST NOT send data if inflight (see below) matches or exceeds cwnd. This is defined in Section 2 of [RFC5681].¶
Slow Start Threshold (ssthresh):
  The slow start threshold (ssthresh) state variable is used to determine whether the slow start or congestion avoidance algorithm is used to control data transmission. During fast recovery, ssthresh is the target window size for a fast recovery episode, as determined by the congestion control algorithm. This is defined in Section 3.1 of [RFC5681].¶

PRR defines additional variables and terms:¶

Delivered Data (DeliveredData):
  The data sender's best estimate of the total number of bytes that the current ACK indicates have been delivered to the receiver since the previously received ACK.¶
In-Flight Data (inflight):
  The data sender's best estimate of the number of unacknowledged bytes in flight in the network, i.e., bytes that were sent and neither lost nor received by the data receiver.¶
Recovery Flight Size (RecoverFS):
  The number of bytes the sender estimates might possibly be delivered over the course of the current PRR episode.¶
SafeACK:
  A local boolean variable indicating that the current ACK indicates the recovery is making good progress and the sender can send more aggressively, increasing inflight, if appropriate.¶
SndCnt:
  A local variable indicating exactly how many bytes should be sent in response to each ACK.¶
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.¶

## 4. Changes Relative to 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 that a prior fast retransmit has been lost) to select the Reduction Bound (PRR-CRB or PRR-SSRB). [RFC6937] left the choice of Reduction Bound to the discretion of the implementer but recommended to use PRR-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 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 replenishment 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 for multiple rounds. Both cases lead to prolonged recovery, decimating application latency and/or 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 detecting any new losses). The SafeACK heuristic was experimented with in Google's Content Delivery Network (CDN) [Flach2016policing] and implemented in Linux TCP since 2015.¶

This SafeACK heuristic is only invoked where losses, application-limited behavior, or other events cause the current estimate of in-flight data to fall below ssthresh. The high 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 performs better than either bound by itself.¶

Another PRR algorithm change improves the sending process when the sender enters recovery after a large portion of sequence space has been SACKed. This scenario could happen when the sender has previously detected reordering, for example, by using [RFC8985]. In the previous version of PRR, RecoverFS did not properly account for sequence ranges SACKed before entering fast recovery, which caused PRR to initially send too slowly. With the change, PRR properly accounts for sequence ranges SACKed before entering fast recovery.¶

Yet another 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 in fast recovery, depending on the loss situation. Forcing a fast retransmit is important to maintain the ACK clock and avoid potential retransmission timeout (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 implemented since the first widely deployed TCP PRR implementation in 2011 [First_TCP_PRR].¶

In another change, upon exiting recovery, a data sender sets cwnd to ssthresh. This is important for robust performance. Without setting cwnd to ssthresh at the end of recovery and with application-limited sender behavior and some loss patterns, cwnd could end fast recovery well below ssthresh, leading to bad performance. The performance could, in some cases, be worse than [RFC6675] recovery, which simply sets cwnd to ssthresh at the start of recovery. This behavior of setting cwnd to ssthresh at the end of recovery has been implemented since the first widely deployed TCP PRR implementation in 2011 [First_TCP_PRR] and is similar to [RFC6675], which specifies setting cwnd to ssthresh at the start of recovery.¶

Since [RFC6937] was written, PRR has also been adapted to perform multiplicative window reduction for non-loss-based congestion control algorithms, such as for Explicit Congestion Notification (ECN) as specified in [RFC3168]. This can be 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. However, there can be interactions between using PRR and approaches to Active Queue Management (AQM) and ECN; guidance on the development and assessment of congestion control mechanisms is provided in [RFC9743].¶

## 5. Relationships to Other Standards

PRR MAY be used in conjunction with any congestion control algorithm that intends to make a multiplicative decrease in its sending rate over approximately the time scale of one round-trip time, as long as the current volume of in-flight data is limited by a congestion window (cwnd) and the target volume of in-flight data during that reduction is a fixed value given by ssthresh. In particular, PRR is applicable to both Reno [RFC5681] and CUBIC [RFC9438] congestion control. PRR is described as a modification to "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.¶

PRR can be used in conjunction with a wide array of loss detection algorithms. This is because PRR does not have any dependencies on the details of how a loss detection algorithm estimates which packets have been delivered and which packets have been lost. Upon the reception of each ACK, PRR simply needs the loss detection algorithm to communicate how many packets have been marked as lost and how many packets have been marked as delivered. Thus, PRR MAY be used in conjunction with the loss detection algorithms specified or described in the following documents: Reno [RFC5681], NewReno [RFC6582], SACK [RFC6675], Forward Acknowledgment (FACK) [FACK], and Recent Acknowledgment Tail Loss Probe (RACK-TLP) [RFC8985]. Because of the performance properties of RACK-TLP, including resilience to tail loss, reordering, and lost retransmissions, it is RECOMMENDED that PRR is implemented together with RACK-TLP loss recovery [RFC8985].¶

The SafeACK heuristic came about as a result of robust Lost Retransmission Detection under development in an early precursor to [RFC8985]. Without Lost Retransmission Detection, policers that cause very high loss rates are at very high risk of causing retransmission timeouts because Reno [RFC5681], CUBIC [RFC9438], and [RFC6675] can send retransmissions significantly above the policed rate.¶

## 6. Algorithm

### 6.1. Initialization Steps

At the beginning of a congestion control response episode initiated by the congestion control algorithm, a data sender using PRR MUST initialize the PRR state.¶

The timing of the start of a congestion control response episode is entirely up to the congestion control algorithm, and (for example) could correspond to the start of a fast recovery episode, or a once-per-round-trip reduction when lost retransmits or lost original transmissions are detected after fast recovery is already in progress.¶

The PRR initialization allows a congestion control algorithm, CongCtrlAlg(), that might set ssthresh to something other than FlightSize/2 (including, e.g., CUBIC [RFC9438]).¶

A key step of PRR initialization is computing Recovery Flight Size (RecoverFS), the number of bytes the data sender estimates might possibly be delivered over the course of the PRR episode. This can be thought of as the sum of the following values at the start of the episode: inflight, the bytes cumulatively acknowledged in the ACK triggering recovery, the bytes SACKed in the ACK triggering recovery, and the bytes between SND.UNA and SND.NXT that have been marked lost. The RecoverFS includes losses because losses are marked using heuristics, so some packets previously marked as lost may ultimately be delivered (without being retransmitted) during recovery. PRR uses RecoverFS to compute a smooth sending rate. Upon entering fast recovery, PRR initializes RecoverFS, and RecoverFS remains constant during a given fast recovery episode.¶

The full sequence of PRR algorithm initialization steps is as follows:¶

       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
       // Bytes SACKed before entering recovery will not be
       // marked as delivered during recovery:
       RecoverFS -= (bytes SACKed in scoreboard)
       // Include the (common) case of selectively ACKed bytes:
       RecoverFS += (bytes newly SACKed)
       // Include the (rare) case of cumulatively ACKed bytes:
       RecoverFS += (bytes newly cumulatively acknowledged)

¶

### 6.2. Per-ACK Steps

On every ACK starting or during fast recovery, excluding the ACK that concludes a PRR episode, PRR executes the following steps.¶

First, the sender computes DeliveredData, the data sender's best estimate of the total number of bytes that the current ACK indicates have been delivered to the receiver since the previously received ACK. With SACK, DeliveredData can be computed precisely as the change in SND.UNA, plus the signed change in quantity of data marked SACKed in the scoreboard. Thus, in the special case when there are no SACKed sequence ranges in the scoreboard before or after the ACK, DeliveredData is the change in SND.UNA. In recovery without SACK, DeliveredData is estimated to be 1 SMSS on each received duplicate ACK (i.e., SND.UNA did not change). When SND.UNA advances (i.e., a full or partial ACK), DeliveredData is the change in SND.UNA, minus 1 SMSS for each preceding duplicate ACK. Note that without SACK, a poorly behaved receiver that returns extraneous duplicate ACKs (as described in [Savage99]) could attempt to artificially inflate DeliveredData. As a mitigation, if not using SACK, then PRR disallows incrementing DeliveredData when the total bytes delivered in a PRR episode would exceed the estimated data outstanding upon entering recovery (RecoverFS).¶

Next, the sender computes inflight, the data sender's best estimate of the number of bytes that are in flight in the network. To calculate inflight, connections with SACK enabled and using [RFC6675] loss detection MAY use the "pipe" algorithm as specified in [RFC6675]. SACK-enabled connections using RACK-TLP loss detection [RFC8985] or other loss detection algorithms MUST calculate inflight by starting with SND.NXT - SND.UNA, subtracting out bytes SACKed in the scoreboard, subtracting out bytes marked lost in the scoreboard, and adding bytes in the scoreboard that have been retransmitted since they were last marked lost. For non-SACK-enabled connections, instead of subtracting out bytes SACKed in the SACK scoreboard, senders MUST subtract out: min(RecoverFS, 1 SMSS for each preceding duplicate ACK in the fast recovery episode); the min() with RecoverFS is to protect against misbehaving receivers [Savage99].¶

Next, the sender computes 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. For example, an ACK triggering "rescue" retransmission (Section 4 of [RFC6675], NextSeg() condition 4) may indicate further losses. Both conditions indicate the recovery is making good progress and the sender can send more aggressively, increasing inflight, if appropriate.¶

Finally, the sender uses DeliveredData, inflight, SafeACK, and other PRR state to compute SndCnt, a local variable indicating exactly how many bytes should be sent in response to each ACK, and then uses SndCnt to update cwnd.¶

The full sequence of per-ACK PRR algorithm steps is as follows:¶

       if (DeliveredData is 0)
          Return

       prr_delivered += DeliveredData
       inflight = (estimated volume of in-flight data)
       SafeACK = (SND.UNA advances and no further loss indicated)
       if (inflight > ssthresh) {
          // Proportional Rate Reduction
          // This uses integer division, rounding up:
          #define DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
          out = DIV_ROUND_UP(prr_delivered * ssthresh, RecoverFS)
          SndCnt = out - prr_out
       } else {
          // PRR-CRB by default
          SndCnt = MAX(prr_delivered - prr_out, DeliveredData)
          if (SafeACK) {
             // PRR-SSRB when recovery is making good progress
             SndCnt += SMSS
          }
          // Attempt to catch up, as permitted
          SndCnt = MIN(ssthresh - inflight, SndCnt)
       }

       if (prr_out is 0 AND SndCnt is 0) {
          // Force a fast retransmit upon entering recovery
          SndCnt = SMSS
       }
       cwnd = inflight + SndCnt

¶

After the sender computes SndCnt and uses it to update cwnd, the sender transmits more data. 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.¶

### 6.3. Per-Transmit Steps

On any data transmission or retransmission, PRR executes the following:¶

       prr_out += (data sent)

¶

### 6.4. Completion Steps

A PRR episode ends upon either completing fast recovery or before initiating a new PRR episode due to a new congestion control response episode.¶

On the completion of a PRR episode, PRR executes the following:¶

       cwnd = ssthresh

¶

Note that this step that sets cwnd to ssthresh can potentially, in some scenarios, allow a burst of back-to-back segments into the network.¶

It is RECOMMENDED that implementations use pacing to reduce the burstiness of data traffic. This recommendation is consistent with current practice to mitigate bursts (e.g., [PACING]), including pacing transmission bursts after restarting from idle.¶

## 7. Properties

The following properties are common to both PRR-CRB and PRR-SSRB, except as noted:¶

PRR attempts to maintain the sender's ACK clocking across recovery events, including burst losses. By contrast, [RFC6675] 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 the sender 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, and there is no application stall, the final value for inflight will be the same as ssthresh.¶

PRR using 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 its receive window. When there is an application stall early during recovery, prr_out will fall behind the sum of transmissions allowed 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 the sender is still in recovery, the sender will send a partial window burst to grow inflight to catch up to exactly where it would have been had the application never stalled. Although such a burst could negatively impact the given flow or other sharing flows, this is exactly what happens every time there is a partial-RTT application stall while not in recovery. PRR makes 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 inflight estimator. While in recovery, inflight 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, inflight can have significant errors; for example, inflight 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 inflight as they are with [RFC6675], a step discontinuity in the inflight estimator causes a burst of data, which cannot be retracted once the inflight estimator is corrected a few ACKs later. For PRR dynamics, inflight merely determines which algorithm, PRR or the Reduction Bound, is used to compute SndCnt from DeliveredData. While inflight is underestimated, the algorithms are different by at most 1 segment per ACK. Once inflight 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. 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, earlier measurements (in Section 6 of [RFC6675]) demonstrate that it is less aggressive and does not perform as well as [RFC6675], which permits bursts of data when there are bursts of losses. PRR-SSRB is a compromise that permits a sender to send one 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 [RFC6675] in the presence of burst losses.¶

## 8. Examples

This section illustrates the PRR and [RFC6675] algorithms by showing their different behaviors for two example scenarios: a connection experiencing either a single loss or a burst of 15 consecutive losses. All cases use bulk data transfers (no application pauses), Reno congestion control [RFC5681], and cwnd = FlightSize = inflight = 20 segments, so ssthresh will be set to 10 at the beginning of recovery. The scenarios use standard Fast Retransmit [RFC5681] and Limited Transmit [RFC3042], so the sender will send two new segments followed by one retransmit in response to the first three duplicate ACKs following the losses.¶

Each of the diagrams below shows the per ACK response to the first round trip for the two recovery algorithms when the zeroth segment is lost. The top line ("ack#") indicates the transmitted segment number triggering the ACKs, with an X for the lost segment. The "cwnd" and "inflight" lines indicate the values of cwnd and inflight, respectively, for these algorithms after processing each returning ACK but before further (re)transmission. The "sent" line 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
    a X  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22
    c   20 20 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
    i   19 19 18 18 17 16 15 14 13 12 11 10  9  9  9  9  9  9  9  9  9  9
    s    N  N  R                             N  N  N  N  N  N  N  N  N  N

    PRR
    a X  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22
    c   20 20 19 18 18 17 17 16 16 15 15 14 14 13 13 12 12 11 11 10 10 10
    i   19 19 18 18 17 17 16 16 15 15 14 14 13 13 12 12 11 11 10 10  9  9
    s    N  N  R     N     N     N     N     N     N     N     N     N  N

    a: ack#;  c: cwnd;  i: inflight;  s: sent

Figure 1

In this first example, ACK#1 through ACK#19 contain SACKs for the original flight of data, ACK#20 and ACK#21 carry SACKs for the limited transmits triggered by the first and second SACKed segments, and ACK#22 carries the full cumulative ACK covering all data up through the limited transmits. ACK#22 completes the fast recovery episode and thus completes the PRR episode.¶

Note that both algorithms send the same total amount of data, and both algorithms complete the fast recovery episode with a cwnd matching the ssthresh of 20. [RFC6675] experiences a "half window of silence" while PRR spreads the voluntary window reduction across an entire RTT.¶

Next, consider an example scenario with the same initial conditions, except that the first 15 packets (0-14) are lost. During the remainder of the lossy round trip, only 5 ACKs are returned to the sender. The following examines each of these algorithms in succession.¶

    RFC 6675
    a X  X  X  X  X  X  X  X  X  X  X  X  X  X  X  15 16 17 18 19
    c                                              20 20 10 10 10
    i                                              19 19  4  9  9
    s                                               N  N 6R  R  R

    PRR
    a X  X  X  X  X  X  X  X  X  X  X  X  X  X  X  15 16 17 18 19
    c                                              20 20  5  5  5
    i                                              19 19  4  4  4
    s                                               N  N  R  R  R

    a: ack#;  c: cwnd;  i: inflight;  s: sent

Figure 2

In this specific situation, [RFC6675] is more aggressive because once Fast Retransmit is triggered (on the ACK for segment 17), the sender immediately retransmits sufficient data to bring inflight up to cwnd. Earlier measurements (in Section 6 of [RFC6675]) indicate that [RFC6675] significantly outperforms the [RFC6937] version of PRR using only PRR-CRB and some other similarly conservative algorithms that were tested, showing that it is significantly common for the actual losses to exceed the cwnd reduction determined by the congestion control algorithm.¶

Under such heavy losses, during the first round trip of fast recovery, PRR uses the PRR-CRB to follow the packet conservation principle. Since the total losses bring inflight 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.¶

While not shown in the figure above, once the fast retransmits sent starting at ACK#17 are delivered and elicit ACKs that increment the SND.UNA, PRR enters PRR-SSRB and increases the window by exactly 1 segment per ACK until inflight 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 [RFC6675], 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.¶

## 9. Adapting PRR to Other Transport Protocols

The main PRR algorithm and reductions bounds can be adapted to any transport that can support [RFC6675]. In one major implementation (Linux TCP), PRR has been the fast recovery algorithm for its default and supported congestion control modules since its introduction in 2011 [First_TCP_PRR].¶

The SafeACK heuristic can be generalized as any ACK of a retransmission that does not cause some other segment to be marked for retransmission.¶

## 10. Measurement Studies

For [RFC6937], a companion paper [IMC11] evaluated [RFC3517] and various experimental PRR versions in a large-scale measurement study. At the time of publication, the legacy algorithms used in that study are no longer present in the code base used in that study, making such comparisons difficult without recreating historical algorithms. Readers interested in the measurement study should review Section 5 of [RFC6937] and the IMC paper [IMC11].¶

## 11. Operational Considerations

### 11.1. Incremental Deployment

PRR is incrementally deployable, because it utilizes only existing transport protocol mechanisms for data delivery acknowledgment and the detection of lost data. PRR only requires changes to the transport protocol implementation at the data sender; it does not require any changes at data receivers or in networks. This allows data senders using PRR to work correctly with any existing data receivers or networks. PRR does not require any changes to or assistance from routers, switches, or other devices in the network.¶

### 11.2. Fairness

PRR is designed to maintain the fairness properties of the congestion control algorithm with which it is deployed. PRR only operates during a congestion control response episode, such as fast recovery or when there is a step reduction in the cwnd from the TCP ECN reaction defined in [RFC3168], and only makes short-term, per-acknowledgment decisions to smoothly regulate the volume of in-flight data during an episode such that at the end of the episode it will be as close as possible to the slow start threshold (ssthresh), as determined by the congestion control algorithm. PRR does not modify the congestion control cwnd increase or decrease mechanisms outside of congestion control response episodes.¶

### 11.3. Protecting the Network Against Excessive Queuing and Packet Loss

Over long time scales, PRR is designed to maintain the queuing and packet loss properties of the congestion control algorithm with which it is deployed. As noted above, PRR only operates during a congestion control response episode, such as fast recovery or response to ECN, and only makes short-term, per-acknowledgment decisions to smoothly regulate the volume of in-flight data during an episode such that at the end of the episode it will be as close as possible to the slow start threshold (ssthresh), as determined by the congestion control algorithm.¶

Over short time scales, PRR is designed to cause lower packet loss rates than preceding approaches like [RFC6675]. At a high level, PRR is inspired by the packet conservation principle, and as much as possible, PRR relies on the self clock process. By contrast, with [RFC6675], 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 large burst of data that is much larger than the volume of delivered data. PRR avoids such bursts by basing transmission decisions on the volume of delivered data rather than the volume of lost data. Furthermore, as noted above, PRR-SSRB is less aggressive than [RFC6675] (transmitting fewer segments or taking more time to transmit them), and it outperforms due to the lower probability of additional losses during recovery.¶

## 12. IANA Considerations

This document has no IANA actions.¶

## 13. Security Considerations

PRR does not change the risk profile for transport protocols.¶

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.¶

## 14. References

### 14.1. Normative References

[RFC1191]
  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990, <https://www.rfc-editor.org/info/rfc1191>.
[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>.
[RFC4821]
  Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, <https://www.rfc-editor.org/info/rfc4821>.
[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>.
[RFC6582]
  Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The NewReno Modification to TCP's Fast Recovery Algorithm", RFC 6582, DOI 10.17487/RFC6582, April 2012, <https://www.rfc-editor.org/info/rfc6582>.
[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>.
[RFC8174]
  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8201]
  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., "Path MTU Discovery for IP version 6", STD 87, RFC 8201, DOI 10.17487/RFC8201, July 2017, <https://www.rfc-editor.org/info/rfc8201>.
[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>.
[RFC9293]
  Eddy, W., Ed., "Transmission Control Protocol (TCP)", STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022, <https://www.rfc-editor.org/info/rfc9293>.
[RFC9438]
  Xu, L., Ha, S., Rhee, I., Goel, V., and L. Eggert, Ed., "CUBIC for Fast and Long-Distance Networks", RFC 9438, DOI 10.17487/RFC9438, August 2023, <https://www.rfc-editor.org/info/rfc9438>.

### 14.2. Informative References

[FACK]
  Mathis, M. and J. Mahdavi, "Forward Acknowledgment: Refining TCP Congestion Control", SIGCOMM '96: Conference Proceedings on Applications, Technologies, Architectures, and Protocols for Computer Communications, pp. 281-291, DOI 10.1145/248156.248181, August 1996, <https://dl.acm.org/doi/10.1145/248156.248181>.
[First_TCP_PRR]
  "Proportional Rate Reduction for TCP.", commit a262f0cdf1f2916ea918dc329492abb5323d9a6c, August 2011, <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a262f0cdf1f2916ea918dc329492abb5323d9a6c>.
[Flach2016policing]
  Flach, T., Papageorge, P., Terzis, A., Pedrosa, L., Cheng, Y., Karim, T., Katz-Bassett, E., and R. Govindan, "An Internet-Wide Analysis of Traffic Policing", SIGCOMM '16: Proceedings of the 2016 ACM SIGCOMM Conference, pp. 468-482, DOI 10.1145/2934872.2934873, August 2016, <https://doi.org/10.1145/2934872.2934873>.
[Hoe96Startup]
  Hoe, J., "Improving the Start-up Behavior of a Congestion Control Scheme for TCP", SIGCOMM '96: Conference Proceedings on Applications, Technologies, Architectures, and Protocols for Computer Communications, pp. 270-280, DOI 10.1145/248156.248180, August 1996, <https://doi.org/10.1145/248156.248180>.
[IMC11]
  Dukkipati, N., Mathis, M., Cheng, Y., and M. Ghobadi, "Proportional Rate Reduction for TCP", IMC '11: Proceedings of the 2011 ACM SIGCOMM Conference on Internet Measurement Conference, pp. 155-170, DOI 10.1145/2068816.2068832, November 2011, <https://doi.org/10.1145/2068816.2068832>.
[Jacobson88]
  Jacobson, V., "Congestion Avoidance and Control", Symposium proceedings on Communications architectures and protocols (SIGCOMM '88), pp. 314-329, DOI 10.1145/52325.52356, August 1988, <https://doi.org/10.1145/52325.52356>.
[PACING]
  Welzl, M., Eddy, W., Goel, V., and M. Tüxen, "Pacing in Transport Protocols", Work in Progress, Internet-Draft, draft-welzl-iccrg-pacing-03, 7 July 2025, <https://datatracker.ietf.org/doc/html/draft-welzl-iccrg-pacing-03>.
[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>.
[RFC9743]
  Duke, M., Ed. and G. Fairhurst, Ed., "Specifying New Congestion Control Algorithms", BCP 133, RFC 9743, DOI 10.17487/RFC9743, March 2025, <https://www.rfc-editor.org/info/rfc9743>.
[Savage99]
  Savage, S., Cardwell, N., Wetherall, D., and T. Anderson, "TCP Congestion Control with a Misbehaving Receiver", ACM SIGCOMM Computer Communication Review, vol. 29, no. 5, pp. 71-78, DOI 10.1145/505696.505704, October 1999, <https://doi.org/10.1145/505696.505704>.
[TCP-RH]
  Mathis, M., Mahdavi, J., and J. Semke, "The Rate-Halving Algorithm for TCP Congestion Control", Work in Progress, Internet-Draft, draft-mathis-tcp-ratehalving-00, 30 August 1999, <https://datatracker.ietf.org/doc/html/draft-mathis-tcp-ratehalving-00>.

## 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 inflight calculation but do not affect the outcome of PRR-CRB once inflight has fallen below ssthresh.¶

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.¶

This property is demonstrated with a 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. In particular, 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 the 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 (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.¶

## Acknowledgments

This document is based in part on previous work by Janey C. Hoe (see "Recovery from Multiple Packet Losses", Section 3.2 of [Hoe96Startup]), Matt Mathis, Jeff Semke, and Jamshid Mahdavi [TCP-RH] and influenced by several discussions with John Heffner.¶

Monia Ghobadi and Sivasankar Radhakrishnan helped analyze the experiments. Ilpo Jarvinen reviewed the initial implementation. Mark Allman, Richard Scheffenegger, Markku Kojo, Mirja Kuehlewind, Gorry Fairhurst, Russ Housley, Paul Aitken, Daniele Ceccarelli, and Mohamed Boucadair improved the document through their insightful reviews and suggestions.¶

## Authors' Addresses