Convergence of Congestion Control from Retained State

Internet-Draft	Congestion Control Convergence	April 2024
Kuhn, et al.	Expires 24 October 2024	[Page]

Abstract

This document specifies a cautious method for IETF transports that enables fast startup of congestion control for a wide range of connections. It reuses a set of computed congestion control parameters that are based on previously observed path characteristics between the same pair of transport endpoints. These parameters are stored, allowing them to be later used to modify the congestion control behavior of a subsequent connection.¶

It describes assumptions and defines requirements for how a sender utilizes these parameters to provide opportunities for a connection to more rapidly get up to speed and rapidly utilize available capacity. It discusses how use of the method impacts the capacity at a shared network bottleneck and the safe response that is needed after any indication that the new rate is inappropriate.¶

1. Introduction

All Internet transports are required to either use a Congestion Control (CC) method, or to constrain their rate of transmission [RFC8085]. In 2010, a survey of alternative CC methods [RFC5783], noted that there are challenges when a CC method operates across an Internet path with a high and/or varying Bandwidth-Delay Product (BDP). This mechanism targets a solution for these challenges.¶

A CC method typically takes time to ramp-up the sending rate, called the "slow-start phase", informally known as the time to "Get up to speed". This slow-start phase defines a time in which a sender intentionally uses less capacity than might be available, with the intention to avoid or limit overshooting the available capacity for the path. The slow-start design can increase queuing (latency or jitter) and/or congestion packet loss for the flow. Any overshoot can have a detrimental effect on other flows sharing a common bottleneck. A sender can use a method to observe the rate of acknowledged data, and seek to avoid overshooting the bottleneck capacity (e.g., Hystart++ [RFC9406]). In the extreme case, an overshoot can result in persistent congestion with unwanted starvation of other flows [RFC8867] (i.e., preventing other flows from successfully sharing the capacity at a common bottleneck).¶

This document proposes a CC method that is expected to reduce the time to complete a transfer when the transfer sends significantly more data than allowed by the Initial congestion Window (IW), and where the BDP of the path is also significantly more than the IW. It introduces an alternative method to select initial CC parameters, that seek to more rapidly and safely grow the sending rate controlled by the congestion window (CWND). CC methods that are rate-based can make similar adjustments to their target sending rate.¶

This method is based on temporal sharing (sometimes known as caching) of a saved set of CC parameters that relate to previous observations of the same path. The parameters include: the saved_cwnd for the path and the minimum Round Trip Time (RTT). These parameters are stored and used to modify the CC behavior of a subsequent connection between the same endpoints.¶

When used with the QUIC transport, this provides transport services that resemble those currently available in TCP, using methods such as TCP Control Block (TCB) [RFC9040] caching.¶

1.1. Use of saved CC parameters by a Sender

CC parameters are used by Careful Resume for three functions:¶

Information about the utilised path capacity (saved_cwnd) to determine an appropriate set of CC parameters for re-using the path.¶
Information to characterize the saved path to confirm whether the current path is consistent with a saved path.¶
Information to check the validity of the saved CC parameters, including the time for which the parameters remain valid.¶

"Generally, implementations are advised to be cautious when using saved CC parameters on a new path", as stated in [RFC9000]. While this statement has been proposed in the context of QUIC standardization, this advice is appropriate for any IETF transport protocol. Care is therefore needed to assure safe use and to be robust to changes in traffic patterns, network routing, and link/node conditions. There are cases where using the saved parameters of a previous connection is not appropriate (e.g., Section 3.2).¶

1.2. Receiver Preference

Whilst a sender could take optimization decisions without considering the receiver's preference, there are cases where a receiver could have information that is not available at the sender, or might benefit from understanding that Careful Resume might be used. In these cases, a receiver could explicitly ask to enable or inhibit tuning of the CC when an application initiates a new session or resume an existing one. A receiver could also tune policies for using the connection (e.g., managing the receiver window or flow credit).¶

Examples where a receiver might request not to use Careful Resume include:¶

a receiver that can predict the pattern of traffic (e.g., insight into the volume of data to be sent, the expected length of a session, or the required maximum transfer rate);¶
a receiver with a local indication that a path/local interface has changed since the CC parameters were stored;¶
knowledge of the current hardware limitations at a receiver;¶
a receiver that can predict additional capacity will be needed for other concurrent or later flows (i.e., prefers to activate Careful Resume for a different connection).¶

A related document proposes an extension for QUIC that allows sender-generated CC parameters to be stored at the receiver [I-D.kuhn-quic-bdpframe-extension]. Transferring the information to a receiver releases the need for a sender to retain transport state for each receiver, and allows a receiver to express a preference for whether to use the method.¶

1.3. Examples of Scenarios of Interest

This section provides a set of examples where Careful Resume is expected to improve performance.¶

Either endpoint can assume the role of a sender or a receiver. Careful Resume also supports a bidirectional data transfer, where both endpoints simultaneously send data (e.g., remote execution of an application, or a bidirectional video conference call).¶

In one example, an application uses a series of connections over a path (i.e., resumes a connection to the same endpoint). Without a new method, each connection would need to individually discover appropriate CC parameters, whereas Careful Resume allows the flow to use a rate that is based on the previously observed CC parameters.¶

In another example, an application connects after a disruption had temporarily reduced the path capacity. When the endpoint returns to use a path with the original characteristics, using a rate that is based on the previously observed CC parameters.¶

There is particular benefit for any path with an RTT that is much larger than typical Internet paths. In a specific example, an application connected via a satellite access network [IJSCN] could require 9 seconds to complete a 5.3 MB transfer using standard CC, whereas a sender using Careful Resume could reduce this transfer time to 4 seconds. The time to complete a 1 MB transfer could similarly be reduced by 62 % [MAPRG111]. This benefit is also expected for other sizes of transfer and for different path characteristics when a path has a large BDP.¶

3. The Phases of CC using Resume

This section defines a series of phases that the CC algorithm moves through as a connection uses Careful Resume, as shown in Figure 1.¶


Observe ...> Connect -> Reconnaissance --------------------> Normal
(Normal)                 |                                    ^
                         v                                    |
                        Unvalidated --------------------------+
                         |      |                             |
                         |      +--> Validating --------------+
                         |               |                    |
                         |               |                    |
                         +---------------+--> Safe Retreat ---+

Figure 1: Key transistions between Phases in Careful Resume

Figure 1: Phases when a connection uses the Careful Resume. The Observe Phase is performed by an established connection as an action within the Normal Phase. Examples of the transitions between phases are provided in Appendix A.¶

3.1. Observe Phase

During a previous established connection, the CC parameters for the specific path to an endpoint are saved. This characterizes the path and determines the saved_cwnd. The saved_cwnd is a measure of the currently utilised capacity for the connection, measured as the number of bytes sent over a RTT. This could be computed by measuring the volume of data acknowledged in one RTT. The CC parameters also include the minimum RTT (saved_rtt) and the receiver Endpoint Token (saved_endpoint_token).¶

An implementation can store the CC parameters at the server (or could exchange this information with a receiver [I-D.kuhn-quic-bdpframe-extension]).¶

Observe Phase: The sender updates the stored CC parameters and/or sends the updated CC parameter information after each observation. If the measured CWND is less than four times the Initial Window (IW) (i.e., CWND less than IW*4), a sender can choose to not store and/or send CC parameters, because the additional actions associated with performing Careful Resume for a small CWND would not justify the use of the method.¶
Observe Phase (Sending CC Parameters): When sending the CC parameters to a receiver, these ought to be updated if there are significant changes in the saved CC parameters; The frequency of update SHOULD be less than one update for several RTTs [I-D.kuhn-quic-bdpframe-extension].¶

Implementation notes are provided in Section 4.1.¶

3.2. Reconnaissance Phase

A sender enters the Reconnaissance Phase after connection setup. In this phase, the CWND is initialised to the IW, and the sender transmits initial data. The CWND MAY be increased using normal CC as each acknowledgment confirms delivery of a packet (i.e., the CC is unchanged).¶

The phase seeks to determine if the path is consistent with a previously observed path (saved in the CC parameters). There are a set of conditions that need to be confirmed before the sender is permitted to enter the Unvalidated Phase:¶

Reconnaissance Phase (Endpoint change): If the current remote endpoint is not the same as a saved endpoint, the sender MUST enter the Normal Phase. If the Endpoint Token differs (i.e., the saved_endpoint_token is different from the current_endpoint_token), it is assumed to represent a different network path. The sender also enters the Normal Phase when there are no corresponding saved CC parameters.¶
Reconnaissance Phase (Lifetime of saved CC parameters): The CC parameters are temporal. If the LifeTime of the observed CC parameters is exceeded Section 4.3.1, the CC parameters are no longer used and sender enters the Normal Phase.¶
Reconnaissance Phase (Confirming the RTT): During this phase, a sender MUST record the minimum RTT for the current connection.¶
Reconnaissance Phase (Avoiding using Careful Resume): A receiver can use a method (e.g., [I-D.kuhn-quic-bdpframe-extension]) to request that the sender instead enters the Normal Phase.¶
Reconnaissance Phase (Detected congestion): If the sender detects congestion (e.g., packet loss or ECN-CE marking), the sender does not use the Careful Resume method and MUST enter the Normal Phase to respond to the detected congestion.¶
Reconnaissance Phase (Using saved_cwnd): Only one connection can use a specific set of saved CC parameters. If another connection has already started to use the saved_cwnd, the sender MUST enter the Normal Phase.¶
Reconnaissance Phase (Rate-limited sender): If the sender is rate-limited [RFC7661], it might send insufficient data to be able to validate transmission at the higher rate. Careful Resume is allowed to remain in the Reconnaissance Phase and to not transition to the Unvalidated Phase until the sender has more data ready to send in the transmission buffer than is permitted by the current CWND. In some implementations, the decision to enter the Unvalidated Phase could require coordination with the management of buffers in the interface to the higher layers.¶

When a sender confirms the path and it receives an acknowledgement for the initial data without reported congestion, it MAY then enter the Unvalidated Phase. This transition occurs when a sender has more data than permitted by the current CWND.¶

When a path is not confirmed, Careful Resume is not used and the sender enters the Normal Phase.¶

Implementation requirements are provided in Section 4.2.¶

3.3. Unvalidated Phase

The Unvalidated Phase is designed to enable the CWND to more rapidly get up to speed.¶

The sender only enters this phase when the saved CC parameters are confirmed:¶

Unvalidated Phase (Confirming the path on entry): The calculation of a sending rate from a saved_cwnd is directly impacted by the RTT, therefore a significant change in the RTT is a strong indication that the previously observed CC parameters may not be valid for the current path. If the RTT measurement is not confirmed, i.e., the current_rtt is greater than or equal to (saved_rtt / 2) or the current_rtt is less than or equal to (saved_rtt x 10) (see Section 4.2.1), the sender MUST enter the Normal Phase (see trigger rtt_not_validated in Section 5).¶

When the RTT is confirmed:¶

Unvalidated Phase (Initialising PipeSize): The variable PipeSize is initialised to CWND on entry to the Unvalidated Phase. This records the value before the jump is applied.¶
Unvalidated Phase (Setting the jump_cwnd): To avoid starving other flows that could have either started or increased their used capacity after the Observation Phase, the jump_cwnd MUST be no more than half of the saved_cwnd. Hence, jump_cwnd is less than or equal to the (saved_cwnd/2). CWND = jump_cwnd.¶
Unvalidated Phase (Pacing trandmission): Transmission using an unvalidated CWND MUST use pacing.¶
Unvalidated Phase (Confirming the path during transmission) If a sender determines that the previous CC parameters are not valid (due to a detected path change), the Safe Retreat Phase is entered. (The sender has not yet received feedback for the jump in CWND, because less than an RTT has passed before the Unvalidated Phase was entered. Therefore, any detected congestion must have resulted from packets sent before the Unvalidated Phase.)¶
Unvalidated Phase (Receiving acknowledgements for reconnaissance packets): The variable PipeSize is increased by the amount of data that is acknowledged by each acknowledgment (in bytes). This indicates a previously unacknowledged packet has been succesfully sent over the path.¶
Unvalidated Phase (Receiving acknowledgements for an unvalidated packet): The sender enters the Validating Phase when an acknowledgement is received for the first packet number (or higher) that was sent in the Unvalidated Phase (see first_unvalidated_packet_acknowledged in Section 5).¶

Implementation notes are provided in Section 4.3.¶

3.4. Validating Phase

The Validating Phase checks that the packets sent in the Unvalidated Phase were received without inducing congestion. The CWND remains unvalidated and the sender typically remains in this phase for one RTT.¶

Validating Phase (Check flight_size on entry): On entry to the Validating Phase, if the flight_size is less equal to the PipeSize (the validated window), the CWND is reset to the PipeSize and the Normal Phase is then immediately entered. There is no need to validate the current data in flight.¶
Validating Phase (Limiting CWND on entry): On entry to the Validating Phase, the CWND is set to the flight_size. This corresponds to the capacity being validated (see trigger rate_limited in Section 5).¶
Validating Phase (Updating CWND): The CWND is updated using the normal rules for the current congestion controller, this typically allows CWND to be increased for each ACK received that indicates a packet has been successfully sent across the path.¶
Validating Phase (Receiving acknowledgements for unvalidated packets): The variable PipeSize is increased upon each acknowledgment that indicates a packet has been successfully sent over the path. This records the validated PipeSize in bytes.¶
Validating Phase (Congestion indication): If a sender determines that congestion was experienced (e.g., packet loss or ECN-CE marking), Careful Resume enters the Safe Retreat Phase (see trigger packet_loss and ECN_CE in Section 5).¶
Validating Phase (Receiving acknowledgement for all unvalidated packets): The sender enters the Normal Phase when an acknowledgement is received for the last packet number (or higher) that was sent in the Unvalidated Phase (see last_unvalidated_packet_acknowledged in Section 5).¶

3.5. Safe Retreat Phase

This phase is entered when the first loss/ECN-CE marking is detected for unvalidated packets. On entry to the Safe Retreat Phase, the sender MUST drain the path of any other unvalidated packets before entering the Normal Phase. (This trigger is the same as used by a QUIC sender to transition from Slow Start to Recovery [RFC9002].)¶

Safe Retreat Phase (Removing saved information): The set of saved CC parameters for the path are deleted, to prevent these from being used again by other flows. To avoid persistent overshoot, on entering the Normal Phase, the congestion control algorithm needs to itself determine the path capacity (e.g., using SS or bandwidth probing), which MUST NOT consider capacity information deduced from packets sent in the Unvalidated Phase.¶
Safe Retreat Phase (Re-initializing CC): On entry, the CWND MUST be reduced to no more than the (PipeSize/2). This avoids persistent starvation by allowing capacity for other flows to regain their share of the total capacity.¶
Note: The minimum CWND in QUIC is 2 packets (see: [RFC9002] section 4.8).¶
Safe Retreat Phase (QUIC recovery): When the CWND is reduced, a QUIC sender can immediately send a single packet prior to the reduction [RFC9002]. (This speeds up loss recovery if the data in the lost packet is retransmitted and is similar to TCP as described in Section 5 of [RFC6675].)¶
Safe Retreat Phase (Increasing CWND): The CWND MUST NOT be increased in the Safe Retreat Phase.¶
Safe Retreat Phase (Tracking PipeSize): The sender continues to update the PipeSize after processing each ACK. This value is used to reset the ssthresh when leaving this phase, it does not modify CWND.¶
Safe Retreat Phase (Receiving acknowledgement for all unvalidated packets): The sender enters Normal Phase when the last packet (or later) sent during the Unvalidated Phase has been acknowledged. The sender MUST set the ssthresh to no more than the PipeSize (see exit_recovery in Section 5).¶

Implementation requirements are provided in Section 4.5.¶

3.5.1. Loss Recovery after entering Safe Retreat

Unacknowledged packets that were sent in the Unvalidated Phase can be lost when there is congestion. Loss recovery commences using the reduced CWND that was set on entry to the Safe Retreat Phase.¶

NOTE: A TCP or SCTP sender is required to retransmit all lost data. For QUIC and DCCP, the need for loss recovery depends on the sender policy for retransmission.¶
NOTE: During loss recovery, a receiver can cumulatively acknowledge data that was previously sent in the Unvalidated Phase in addition to acknowledging successful retransmission of data. [RFC3465] describes how to appropriately account for such acknowledgments.¶
NOTE: On entry to the Safe Retreat Phase, the CWND can be significantly reduced, when there was multiple loss, recovery of all lost data could require multiple RTTs to complete.¶

The sender leaves the Safe Retreat Phase when the last packet number (or higher) sent in the Unvalidated Phase is acknowledged. If the last packet number is not cumulatively acknowledged, then additional packets might need to be retransmitted.¶

3.6. Normal Phase

In the Normal Phase, the sender transitions to using the normal CC method (e.g., in congestion avoidance if CWND is more than ssthresh). (Note that when the sender did not use the entire jump_cwnd the CWND was reduced on entering the Validating Phase.¶

Implementation requirements are provided in Section 4.6.¶

3.7. RTO Expiry while using Careful Resume

A sender that experiences a Retransmission Time Out (RTO) expiry ceases to use Careful Resume. The sender continues enters the Normal Phase.¶

NOTE: As in loss recovery, data sent in the Unvalidated Phase could be later acknowledged after an RTO event (see Section 3.5.1).¶

4. Congestion Control Guidelines and Requirements

This section provides guidance for implementation and use.¶

4.1. Determining the Current Path Capacity in the Observe Phase

There are various approaches to measuring the capacity used by a connection. Congestion controllers, such as Reno or CUBIC [RFC9438], can estimate the capacity by utilizing the CWND or flight_size. A different approach could estimate the same parameters for a rate-based congestion controller, such as BBR [I-D.cardwell-iccrg-bbr-congestion-control], or by observing the rate at which data is acknowledged by the remote endpoint.¶

Implementations are expected to include a LifeTime parameter in the CC parameters that can be used to remove old CC parameters when no longer needed, or the CC parameters are out of date.¶

Observe Phase: There are cases where the current CWND does not reflect the path capacity. At the end of slow start, the CWND can be significantly larger than needed to fully utilize the path (i.e., a CWND overshoot). It is inappropriate to use an overshoot in the CWND as a basis for estimating the capacity. In most cases, the CWND will converge to a stable value after several more RTTs. One mitigation could be to set the saved_cwnd based on the flight_size, or an averaged CWND.¶
Observe Phase (Rate-limited sender): When the sender is rate-limited, or in an RTT following a burst of transmission, a sender typically transmits much less data than allowed. Such observations could be discounted when estimating the saved_cwnd (e.g., when a previous observation recorded a higher value.)¶

4.2. Confirming the Path in the Reconnaissance Phase

The CC is not modified during the Reconnaissance Phase. A sender therefore needs to limit the initial data, sent in the first RTT of transmitted data, to not more than the IW [RFC9000]. This transmission using the IW is assumed to be a safe starting point for any path to avoid adding excessive load to a potentially congested path. (When used in a controlled network, additional information about local path characteristics could be known, which might be used to configure a non-standard IW.)¶

The method does not permit multiple concurrent reuse of the saved CC parameters. When multiple new concurrent connections are made to a server, each can have a valid endpoint_token, but the saved_cwnd can only be used for one new connection. This is designed to prevent a sender from performing multiple jumps in the cwnd, each individually based on the same saved_cwnd, and hence creating an excessive aggregate load at the bottleneck.¶

The method used to prevent re-use of the saved CC parameters will depend on the design of the server that is being used (e.g., if all connections from a given client IP arrive at the same server process, then the server process could use a hash table). A distributed system might be required when using some types of load balancing, to ensure this invariant when the load balancing hashes connections by 4-tuple and hence multiple connections from the same client device are served by different server processes.¶

In the Reconnaissance Phase a sender initiates a connection and starts sending initial data. This measures the current minimum RTT. If a decision is made to use Careful Resume, this is used to confirm the path.¶

4.2.1. Confirming the RTT

Path characteristics can change over time for many reasons, resulting in the previously observed CC parameters becoming irrelevant. The sender therefore compares the saved_RTT with each of a series of measured RTT samples.¶

If the current RTT sample is less than a half of the saved_RTT, this is regarded as too small, and is an indicator of a path change. (This factor of two arises, because the rate should not exceed the observed rate when the saved_cwnd was measured, because the jump_cwnd is calculated as half the measured saved_cwnd.)¶

A current RTT larger than that at the time the saved_cwnd was measured results in a proportionally lower resumed rate, because the transmission using the CR method is paced based on the current RTT (i.e., the larger RTT samples reduces the paced sending rate in the Unvalidated Phase - and hence is still safe).¶

Also note that when the RTT is incorrectly measured as larger than the actual path RTT, the sender will receive an acknowledgment for an unvalidated packet before it might have completed the Unvalidated Phase, this acknowledgment resets the CWND to reflect the flight_size, and the sender enters the Validating Phase.¶

an RTT sample more than ten times the saved_RTT is indicative of a path change. (The value of ten was chosen to accommodate both increases in latency from buffering on a path, and any variation between RTT samples).¶

A sender in Reconnaissance Phase reverts to the Normal Phase if congestion is detected. Some transport protocols implement methods that infer potential congestion from an increase in the RTT. In the Reconnaissance Phase, this indication occurs earlier than congestion which is reported by loss or by ECN marking. Designs need to consider if this is a suitable trigger to revert to the Normal Phase.¶

4.3. Safety Requirements for the Unvalidated Phase

This section defines the safety requirements for using saved CC parameters to tentatively update the CWND. These safety requirements mitigate the risk of adding excessive congestion to an already congested path.¶

Unvalidated Phase (Jump): A connection must not directly use the previously saved_cwnd to directly initialize a new flow causing it to resume sending at the same rate. The jump_cwnd must be no more than half the previously saved_cwnd.¶

4.3.1. Lifetime of CC Parameters

The long-term use of the previously observed parameters is not appropriate, a lifetime therefore needs to be specified during which the saved CC parameters can be safely re-used.¶

[RFC9040] provides guidance on the implementation of TCP Control Block Interdependence, but does not specify how long a saved parameter can safely be reused.¶

[RFC7661] specifies a method for managing an unvalidated CWND. This states: "After a fixed period of time (the non-validated period (NVP)), the sender adjusts the cwnd (Section 4.4.3). The NVP SHOULD NOT exceed five minutes." Section 5 of [RFC7661] discusses the rationale for choosing that period. However, RFC 7661 targets rate-limited connections using normal CC. The method described in the present specification includes additional mechanisms to avoid and mitigate the effects of overshoot, and therefore this can be used to justify a longer lifetime of the saved_cwnd using the Careful Resume method.¶

4.3.2. Pacing in the Unvalidated Phase

The sender must avoid sending a burst of packets greater than IW as a result of a step-increase in the CWND. (This is consistent with [RFC8085], [RFC9000]). Pacing sent packets as a function of the current RTT, rather than the saved_RTT, provides an additional safety during the Unvalidated Phase. Other sender mitigations have also been suggested to avoid line-rate bursts (e.g., [I-D.hughes-restart]).¶

Pacing places a limitation on the minimum acceptable current_RTT to avoid sending at a rate higher than was previously observed.¶

The following example provides a relevant pacing rhythm using the RTT and the saved_cwnd. The Inter-packet Transmission Time (ITT) is determined by using the current Maximum Message Size (MMS), the saved_cwnd and the RTT. A safety margin can avoid sending more than a recommended maximum (max_jump):¶

jump_cwnd = min(max_jump,saved_cwnd/2)¶
ITT = (current_RTT x MMS)/jump_cwnd¶

This follows the idea presented in [RFC4782], [I-D.irtf-iccrg-sallantin-initial-spreading] and [CONEXT15].¶

4.3.3. Exit from the Unvalidated Phase because of Variable Network Conditions

Careful Resume has been designed to be robust to changes in network conditions due to variations in the forwarding path, such as reconfiguration of equipment, or changes in the link conditions. This is mitigated by path confirmation.¶
Careful Resume has been designed to be robust to changes in network traffic, including the arrival of new flows that compete for capacity at a shared bottleneck. This is mitigated by jumping to no more than a half of the saved_cwnd and by using pacing.¶
Careful Resume has been designed to prevent unduly suppressing flows that used the capacity since the available capacity was measured. This is further mitigated by bounding the duration of the Unvalidated Phase (and the following Validating Phase).¶

4.4. Safety Requirements for the Validating Phase

When a sender completes the Unvalidated Phase, either by sending a jump_cwnd of data or after one RTT, it ceases to use the unvalidated CWND. That is, CWND is reset to the flight_size, and the sender awaits reception of the acknowledgments to validate the use of this capacity. New packets are sent when previously sent data is newly acknowledged. The purpose is to trigger a entry to the Safe Retreat Phase when the capacity is not validated.¶

Note that the CWND is increased during the Validating Phase, based on received ACKs. This allows new data to be sent, but this does not have any final impact on the CWND if congestion is detected.¶

4.5. Safety Requirements for the Safe Retreat Phase

This section defines the safety requirements after congestion has been detected during the Unvalidated Phase.¶

The Safe Retreat Phase sets a safe CWND value to drain unvalidated packets from the path when a packet loss is detected or acknowledgments indicate sent packets were ECN CE-marked.¶

The Safe Retreat reaction differs from a traditional reaction to detected congestion, because the jump_cwnd can result in a significantly higher rate than would be allowed by the slow-start mechanism. This could aggressively feed a congested bottleneck, resulting in overshoot where a disproportionate number of packets from existing flows are displaced from the buffer at the congested bottleneck. For this reason, a sender needs to react to detected congestion by reducing CWND significantly below the saved_cwnd.¶

Note: Proportional Rate Reduction (PRR) [RFC6937] assumes that it is safe to reduce the rate gradually when in congestion avoidance. PRR is therefore not appropriate when there might be significant overshoot in the use of the capacity, which can be the case when the Safe Retreat Phase is entered.¶

Acknowledgements for unvalidated packets are tracked to measure the maximum capacity, called the PipeSize (The first unvalidated packet can be determined by recording the sequence number of the first packet sent in this phase.) This PipeSize is not a safe measure of the currently available share of the capacity whenever there was also a significant overshoot at the bottleneck, but may be used to reset ssthresh.¶

4.6. Returning to Normal Congestion Control

After using Careful Resume, the CC controller returns to the Normal Phase. The implementation details for different transports depend on the design of the transport.¶

In the Normal Phase, a sender can enter the Observation Phase to perform observation of the path.¶

{XXX-Editor note: A future revision should discuss updating the saved parameters, whether used or not, after reaching normal operation for use the next time even if that update is to just refresh the expiration time.}¶

4.7. Limitations from Transport Protocols

A sender is limited by any rate-limitation of the transport protocol that is used.¶

For QUIC this includes flow control mechanisms or preventing amplification attacks. In particular, a QUIC receiver might need to issue proactive MAX_DATA frames to increase the flow control limits of a connection that is started when using Careful Resume to gain the expected benefit.¶
A TCP sender is limited by the receiver window (rwnd). Unless configured at a receiver, the rwnd constrains the rate of increase for a connection and reduces the benefit of Careful Resume.¶

9. References

9.1. Normative References

[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>.
[RFC8085]: Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[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>.
[RFC8610]: Birkholz, H., Vigano, C., and C. Bormann, "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[RFC8801]: Pfister, P., Vyncke, É., Pauly, T., Schinazi, D., and W. Shao, "Discovering Provisioning Domain Names and Data", RFC 8801, DOI 10.17487/RFC8801, July 2020, <https://www.rfc-editor.org/info/rfc8801>.
[RFC9000]: Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, May 2021, <https://www.rfc-editor.org/info/rfc9000>.
[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>.

9.2. Informative References

[CONEXT15]: Li, Q., Dong, M., and P B. Godfrey, "Halfback: Running Short Flows Quickly and Safely", ACM CoNEXT , 2015.
[I-D.cardwell-iccrg-bbr-congestion-control]: Cardwell, N., Cheng, Y., Yeganeh, S. H., Swett, I., and V. Jacobson, "BBR Congestion Control", Work in Progress, Internet-Draft, draft-cardwell-iccrg-bbr-congestion-control-02, 7 March 2022, <https://datatracker.ietf.org/doc/html/draft-cardwell-iccrg-bbr-congestion-control-02>.
[I-D.hughes-restart]: Hughes, A., Touch, J., and J. Heidemann, "Issues in TCP Slow-Start Restart After Idle", , Work in Progress, Internet-Draft, draft-hughes-restart-00, December 2001, <https://www.ietf.org/archive/id/draft-hughes-restart-00.txt>.
[I-D.ietf-quic-qlog-quic-events]: Marx, R., Niccolini, L., Seemann, M., and L. Pardue, "QUIC event definitions for qlog", Work in Progress, Internet-Draft, draft-ietf-quic-qlog-quic-events-07, 4 March 2024, <https://datatracker.ietf.org/doc/html/draft-ietf-quic-qlog-quic-events-07>.
[I-D.irtf-iccrg-sallantin-initial-spreading]: Sallantin, R., Baudoin, C., Arnal, F., Dubois, E., Chaput, E., and A. Beylot, "Safe increase of the TCP's Initial Window Using Initial Spreading", Work in Progress, Internet-Draft, draft-irtf-iccrg-sallantin-initial-spreading-00, 15 January 2014, <https://datatracker.ietf.org/doc/html/draft-irtf-iccrg-sallantin-initial-spreading-00>.
[I-D.kuhn-quic-bdpframe-extension]: Kuhn, N., Emile, S., Fairhurst, G., Secchi, R., and C. Huitema, "Signalling CC Parameters for Careful Resume using QUIC", Work in Progress, Internet-Draft, draft-kuhn-quic-bdpframe-extension-05, 4 March 2024, <https://datatracker.ietf.org/doc/html/draft-kuhn-quic-bdpframe-extension-05>.
[IJSCN]: Thomas, L., Dubois, E., Kuhn, N., and E. Lochin, "Google QUIC performance over a public SATCOM access", International Journal of Satellite Communications and Networking 10.1002/sat.1301, 2019.
[MAPRG111]: Kuhn, N., Stephan, E., Fairhurst, G., Jones, T., and C. Huitema, "Feedback from using QUIC's 0-RTT-BDP extension over SATCOM public access", IETF 111 - MAPRG meeting , 2022.
[RFC3465]: Allman, M., "TCP Congestion Control with Appropriate Byte Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February 2003, <https://www.rfc-editor.org/info/rfc3465>.
[RFC4782]: Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-Start for TCP and IP", RFC 4782, DOI 10.17487/RFC4782, January 2007, <https://www.rfc-editor.org/info/rfc4782>.
[RFC5783]: Welzl, M. and W. Eddy, "Congestion Control in the RFC Series", RFC 5783, DOI 10.17487/RFC5783, February 2010, <https://www.rfc-editor.org/info/rfc5783>.
[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>.
[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>.
[RFC7661]: Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating TCP to Support Rate-Limited Traffic", RFC 7661, DOI 10.17487/RFC7661, October 2015, <https://www.rfc-editor.org/info/rfc7661>.
[RFC8867]: Sarker, Z., Singh, V., Zhu, X., and M. Ramalho, "Test Cases for Evaluating Congestion Control for Interactive Real-Time Media", RFC 8867, DOI 10.17487/RFC8867, January 2021, <https://www.rfc-editor.org/info/rfc8867>.
[RFC9002]: Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection and Congestion Control", RFC 9002, DOI 10.17487/RFC9002, May 2021, <https://www.rfc-editor.org/info/rfc9002>.
[RFC9040]: Touch, J., Welzl, M., and S. Islam, "TCP Control Block Interdependence", RFC 9040, DOI 10.17487/RFC9040, July 2021, <https://www.rfc-editor.org/info/rfc9040>.
[RFC9406]: Balasubramanian, P., Huang, Y., and M. Olson, "HyStart++: Modified Slow Start for TCP", RFC 9406, DOI 10.17487/RFC9406, May 2023, <https://www.rfc-editor.org/info/rfc9406>.

Appendix A. Notes on the Careful Resume Phases

The table below is provided to illustrate the operation of Careful Resume. This table is informative, please refer to the body of the document for the normative specification. The description is based on a Normal CC that uses Reno or CUBIC. The PipeSize tracks the validated CWND.¶

+------+---------+---------+------------+-----------+------------+
|Phase |Normal   |Recon.   |Unvalidated |Validating |Safe Retreat|
+------+---------+---------+------------+-----------+------------+
|      |Observe  |Confirm  |Send faster |Validate   |Drain path; |
|      |CC params|path     |using saved |new CWND;  |Update PS   |
|      |         |         |            |Update PS  |            |
+------+---------+---------+------------+-----------+------------+
|On    |    -    |CWND=IW  |PS=CWND;    |If (FS>PS) |CWND=(PS/2) |
|entry:|         |         |CWND        |{CWND=FS}  |            |
|      |         |         |=jump_cwnd  |else       |            |
|      |         |         |            |{CWND=PS;  |            |
|      |         |         |            |enter      |
|      |         |         |            |Normal}    |            |
+------+---------+---------+------------+-----------+------------+
|CWND: |When in  |CWND     |CWND is not |CWND can   |CWND is not |
|      |observe, |increases|increased   |increase   |increased   |
|      |measure  |using    |            |using      |            |
|      |sav_cwnd |normal CC|            |normal CC  |            |
|      |         |methods  |            |methods    |            |
+------+---------+---------+------------+-----------+------------+
|PS:   |    -    |    -    |              PS+=ACked              |
+------+---------+---------+------------+-----------+------------+
|RTT:  |Measure  |Measure  |      -     |     -     |      -     |
|      |saved_rtt|current  |            |           |            |
|      |         |_rtt     |            |           |            |
+------+---------+---------+------------+-----------+------------+
|If    |Normal   |Normal   |          Enter         |      -     |
|loss  |CC method|CC method|          Safe          |            |
|or    |         |CR is not|          Retreat       |            |
|ECNCE:|         |allowed  |                        |            |
+------+---------+---------+------------+-----------+------------+
|Next  |Observe  |If (     |If (FS=CWND |If (ACK    |If (ACK     |
|Phase:|(as      |FS=CWND  |or >1 RTT   |>= last    |>= last     |
|      |needed)  |and CR   |has passed  |unvalidated|unvalidated |
|      |         |confirmed|or ACK      |packet),   |packet),    |
|      |         |), enter |>= last     |enter      |{ssthresh=PS|
|      |         |Unvalidat|unvalidated |Normal     |and enter   |
|      |         |-ing else|packet),    |           |Normal}     |
|      |         |enter    |enter       |           |            |
|      |         |Normal   |Validating  |           |            |
+------+---------+---------+------------+-----------+------------+

Figure 3: Illustration of the operation of Careful Resume

The following abbreviations are used SS = slow start FS= flight_size; PS = PipeSize. The PipeSize is set to the CWND on entry to the Unvalidated Phase. This tracks the validated portion of the CWND and is updated as each additional packet is acknowledged.¶

Note: For an implementation that keeps track of transmitted data in terms of packets: In the Unvalidated Phase, the first unvalidated packet corresponds to the highest sent packet recorded on entry to this phase. In the Validating and Safe Retreat Phases, corresponds to the last unvalidated packet is also the highest sent packet recorded on entry to this phase.¶

The remaining subsections provide informative examples of use. Although the QLOG variables are expressed in bytes, to simplify the description, these examples are described in term of packet numbers.¶

A.1. Example with No Loss

In the first example of using the Careful Resume method, the sender starts by sending IW (10) packets in the Reconnaissance Phase, and then continues in a subsequent RTT to send more packets until the sender becomes CWND-limited (i.e., flight_size = CWND).¶

The sender then confirms the RTT and other conditions for using Careful Resume. In this example, this is confirmed when the sender has 29 packets in flight. The sender enters the Unvalidated Phase. This confirmation could have happened earlier if data was available to send. The sender initialises the PipeSize to the CWND (the same as the flight_size, i.e., 29 packets) and sets the CWND to 150 packets (based upon a previously observed saved_cwnd of 300 packets).¶

It now sends 121 unvalidated packets. This is the unused portion of the CWND. Each time a packet is sent, the sender checks whether 1 RTT has passed since entering the Unvalidated Phase (otherwise, the Validating Phase is entered). This check triggers only for cases where the sender is rate-limited, see the following example.¶

The PipeSize will increase after each ACK is received.¶

When the first unvalidated packet is acknowledged (packet number 30) the sender enters the Validating Phase. This transition would also occur if the flight_size increases to equal CWND. During this phase, the CWND can be increased for each ACK for an unvalidated packet, because this indicates that the packet was indeed validated.¶

When an ACK is received for the last packet sent in the Unvalidated Phase, the sender completes using Careful Resume. It enters the Normal Phase. If CWND is less than ssthresh, a Reno or CUBIC sender in the Normal Phase is permitted to use slow start to grow the CWND towards the ssthresh, and will then enter congestion avoidance.¶

A.2. Example with No Loss, Rate-Limited

A rate-limited sender will not fully utilize the available CWND when using Careful Resume, and CWND is therefore reset on entry to the Validating Phase, as described below.¶

The sender in this example starts by sending IW packets (10) in the Reconnaissance Phase, and sends as described in the first example, transitioning to the Unvalidated Phase. This sets the CWND to 150 packets, and the PipeSize is set to the flight_size (i.e., 29 packets).¶

The sender then becomes rate-limited because it only sends 50 unvalidated packets.¶

After ~1 RTT (detected by using local timestamps or by receiving an ACK for the first unvalidated packet), the sender will still not have fully used the CWND. It then enters the Validating Phase and resets the CWND to the current flight_size, (i.e., 50 packets). During this phase, the CWND can be increased for each ACK that validates reception of a packet. The PipeSize also increases with each ACK received, to reflect the discovered capacity.¶

When an ACK is received for the last packet sent in the Unvalidated Phase, the sender has completed using Careful Resume. It enters the Normal Phase. If CWND is less than ssthresh, a Reno or CUBIC sender in the Normal Phase is permitted to use slow start to grow the CWND towards the ssthresh, and will then enter congestion avoidance.¶

A.3. Example with Loss detected in the Reconnaissance Phase

When a packet is lost in the Reconnaissance Phase, the sender enters the Normal Phase and recovers this using normal method. There is no change to the CC method, because the sender has discovered a potential capacity limit and is not allowed to use Careful Resume.¶

A.4. Example with Loss detected in the Validating Phase

As in the first Example the sender enters the Unvalidated Phase it sets the CWND to 150 packets with the PipeSize initialized to the flight_size (i.e., 29 packets).¶

The sender now sends 121 unvalidated packets (the remaining unused CWND). This example considers the case when one of the unvalidated packet is lost, which we choose to be packet 64 (the 35th packet in the Unvalidated Phase).¶

Acknowledgements confirm the first 34 unvalidated packets are received without loss. The PipeSize at this point is equal to 29 + 34 = 63 packets.¶

The loss is then detected (by receiving three ACKs that do not cover packet number 35), the sender then enters the Safe Retreat Phase because the window was not validated. The PipeSize at this point is equal to 29 + 34 = 66 packets. Assuming IW=10. The CWND is reset to max(10,ps/2) = max(10,66/2) = 33 packets. This CWND is used during the Safe Retreat Phase, because congestion was detected and the sender still does not know if the remaining unvalidated packets will be successfully acknowledged. A conservative CWND calculation ensures the sender drains the path after this potentially severe congestion event. There is no further increase in CWND in this phase.¶

The sender continues to receives ACKs for the remaining 86 (121-35) unvalidated packets (recall that the 35th unvalidated packet was lost and had packet number 29+35=64). The PipeSize continues to further increase as each ACK acknowledges new data, because this tracks the size of the pipe discovered by the unvalidated packets. Although this cannot be used to safety initialise CWND (because was measured when the sender aggressively created overload), the estimated PipeSize (which, in this case, is 121-1 = 120 packets) can be used to set the ssthresh on exit from Safe Retreat, since it indicates an upper limit to the current capacity.¶

At the point where all packets sent in the Unvalidated Phase have been either acknowledged or declared lost, the sender enters the Normal Phase, but first updates ssthresh. Because CWND will now be less than ssthresh, a sender in the Normal Phase is permitted to use slow start to grow the CWND towards the ssthresh, after which it will enter congestion avoidance.¶