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Transport Congestion Control Working Group This document specifies how transport protocols increase their congestion window when the sender is rate-limited. Such a limitation can be caused by the sending application not supplying data or by receiver flow control. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Congestion Control Working Group Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction A sender of a congestion controlled transport protocol becomes "rate-limited" when it does not have data to send even though the congestion control rules would allow it to transmit data. This could occur because the application has not provided sufficient data to fully utilise the congestion window (cwnd). It could also occur because the receiver has limited the flow control (e.g., by the advertised TCP receiver window (rwnd) or by the conection or stream flow credit in quic). Current RFCs specifying congestion control mechanisms diverge regarding the rules for increasing the cwnd when the sender is rate-limited. Congestion Window Validation (CWV) provides an experimental specification defining how to manage a cwnd that has become larger than the current flight size. In contrast, this present document concerns the increase in cwnd when a sender is rate limited. These two topics are distinct, but are related, because both describe the management of cwnd when the sender does not fully utilise the current cwnd. This document specifies a uniform rule that congestion control mechanisms MUST apply and provides a recommendation that congestion control implementations SHOULD follow. An appendix provides an overview of the divergence in current RFCs and some current implementations regarding cwnd increase when the sender is rate-limited.

Terminology This document uses the terms defined in .

Conventions and Definitions 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 when, and only when, they appear in all capitals, as shown here.

Increase rules Irrespective of the current state of a congestion control mechanism, senders of congestion controlled transport protocols:

1. MUST include a way to limit the growth of cwnd when FlightSize < cwnd.
2. SHOULD limit the growth of cwnd when FlightSize < cwnd with inc(maxFS).

In rule #2, "inc" is a function returning the maximum unconstrained increase that would result from the congestion control mechanism within one RTT, based on the "maxFS" parameter. For example, for Slow Start, as specified in , inc(maxFS)=2*maxFS, such that equation 2 in becomes: Similarly, with rule #2 applied to Congestion Avoidance, inc(maxFS)=1+maxFS, such that equation 3 in becomes: maxFS is the largest value of FlightSize since the last time that cwnd was decreased. If cwnd has never been decreased, maxFS is the maximum value of FlightSize since the start of the data transfer.

Discussion If the sending rate is less than permitted by cwnd for multiple RTTs, either by the sending application or by the receiver-advertised window, continuously increasing the cwnd would cause a mismatch between the cwnd and the capacity the path supports (i.e., over-estimating the capacity). Such unlimited growth in the cwnd is therefore disallowed by the first rule. However, in most common congestion control mechanisms, in the absence of an indication of congestion, a cwnd that has been fully utilized during an RTT is permitted to be increased during the immediately following RTT. Thus, such an increase is allowed by the second rule.

Security Considerations Transport protocols that provide authentication (including those using encryption), or are carried over protocols that provide authentication, can protect the congestion control mechanisms from network attack. While congestion control designs could result in unwanted competing traffic, they do not directly result in new security considerations.

IANA Considerations This document has no IANA actions.

References Normative References This document defines TCP's four intertwined congestion control algorithms: slow start, congestion avoidance, fast retransmit, and fast recovery. In addition, the document specifies how TCP should begin transmission after a relatively long idle period, as well as discussing various acknowledgment generation methods. This document obsoletes RFC 2581. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. CUBIC is a standard TCP congestion control algorithm that uses a cubic function instead of a linear congestion window increase function to improve scalability and stability over fast and long-distance networks. CUBIC has been adopted as the default TCP congestion control algorithm by the Linux, Windows, and Apple stacks. This document updates the specification of CUBIC to include algorithmic improvements based on these implementations and recent academic work. Based on the extensive deployment experience with CUBIC, this document also moves the specification to the Standards Track and obsoletes RFC 8312. This document also updates RFC 5681, to allow for CUBIC's occasionally more aggressive sending behavior. This document describes the Stream Control Transmission Protocol (SCTP) and obsoletes RFC 4960. It incorporates the specification of the chunk flags registry from RFC 6096 and the specification of the I bit of DATA chunks from RFC 7053. Therefore, RFCs 6096 and 7053 are also obsoleted by this document. In addition, RFCs 4460 and 8540, which describe errata for SCTP, are obsoleted by this document. SCTP was originally designed to transport Public Switched Telephone Network (PSTN) signaling messages over IP networks. It is also suited to be used for other applications, for example, WebRTC. SCTP is a reliable transport protocol operating on top of a connectionless packet network, such as IP. It offers the following services to its users: The design of SCTP includes appropriate congestion avoidance behavior and resistance to flooding and masquerade attacks. This document describes loss detection and congestion control mechanisms for QUIC. Informative References This document provides a mechanism to address issues that arise when TCP is used for traffic that exhibits periods where the sending rate is limited by the application rather than the congestion window. It provides an experimental update to TCP that allows a TCP sender to restart quickly following a rate-limited interval. This method is expected to benefit applications that send rate-limited traffic using TCP while also providing an appropriate response if congestion is experienced. This document also evaluates the Experimental specification of TCP Congestion Window Validation (CWV) defined in RFC 2861 and concludes that RFC 2861 sought to address important issues but failed to deliver a widely used solution. This document therefore reclassifies the status of RFC 2861 from Experimental to Historic. This document obsoletes RFC 2861.

The state of RFCs and implementations This section is provided as input for IETF discussion, and should be removed before publication.

TCP ("Reno" congestion control)

Specification does not contain a rule to limit the growth of cwnd when the sender is rate-limited. This statement (page 8) gives an impression that such cwnd growth might be expected:

• Implementation Note: An easy mistake to make is to simply use cwnd, rather than FlightSize, which in some implementations may incidentally increase well beyond rwnd.

also suggests there is no increase limitation in the standard TCP behavior (which changes), on page 4:

• Standard TCP does not impose additional restrictions on the growth of the congestion window when a TCP sender is unable to send at the maximum rate allowed by the cwnd. In this case, the rate-limited sender may grow a cwnd far beyond that corresponding to the current transmit rate, resulting in a value that does not reflect current information about the state of the network path the flow is using.

Implementation

• ns-2 allows cwnd to grow when it is rate-limited by rwnd. (Rate-limited by the sending application: not tested.)
• ns-3 allows cwnd to grow when it is rate-limited by either an application or the rwnd.
• In Congestion Avoidance, Linux only allows cwnd to grow when the sender is unconstrained. Before kernel version 3.16, this also applied to Slow Start. The check for "unconstrained" is done by checking if FlightSize is greater or equal to cwnd. Since kernel version 3.16, which was published in August 2014, in Slow Start, the increase implements rule #2 in in the tcp_is_cwnd_limited function in tcp.h.

Assessment Linux implements a limit to cwnd growth in accordance with rule #1 in ; in Slow Start, this limit follows rule #2, while in Congestion Avoidance, it is more conservative than rule #2. The specification and the ns-2 and ns-3 implementations are in conflict with rules #1 and #2 in .

CUBIC

Specification says:

• Cubic doesn't increase cwnd when it's limited by the sending application or rwnd.

Implementation The description of Linux described in also applies to Cubic.

Assessment Both the specification and the Linux implementation limit cwnd growth in accordance with rule #1 in ; in Congestion Avoidance, this limit is more conservative than rule #2 in , and in Slow Start, it implements rule #2 in .

SCTP

Specification says:

• When cwnd is less than or equal to ssthresh, an SCTP endpoint MUST use the slow-start algorithm to increase cwnd only if the current congestion window is being fully utilized and the data sender is not in Fast Recovery. Only when these two conditions are met can the cwnd be increased; otherwise, the cwnd MUST NOT be increased.

Assessment The quoted statement from prescribes the same cwnd growth limitation that is also specified for Cubic and implemented for both Reno and Cubic in Linux. It is in accordance with rule #1 in , and more conservative than rule #2 in . is specifically limited to Slow Start. Congestion Avoidance is discussed in , but this section neither contains a similar rule nor does it refer back to the rule that limits the growth of cwnd in Section 7.2.1. It is thus implicitly clear that the quoted rule only applies to Slow Start, whereas the rules in apply to both Slow Start and Congestion Avoidance.

QUIC

Specification states:

• When bytes in flight is smaller than the congestion window and sending is not pacing limited, the congestion window is underutilized. This can happen due to insufficient application data or flow control limits. When this occurs, the congestion window SHOULD NOT be increased in either slow start or congestion avoidance.

• A sender that paces packets might delay sending packets and not fully utilize the congestion window due to this delay. A sender SHOULD NOT consider itself application limited if it would have fully utilized the congestion window without pacing delay.

Assessment With the exception of pacing, this specification conservatively limits the growth in cwnd, similar to Cubic and SCTP. The exception for pacing in the second paragraph requires the application to notify the transport layer that it paces packets. Pacing is typically done with delays below an RTT; thus, rule #2 in should cover this case without the need for such a notification from the application.

Others {XXX - Other protocols and mechanisms in RFCs include: TFRC; various multicast and multipath mechanisms; the RMCAT mechanisms for real-time media. Other protocol specs containing congestion control include: DCCP, MP-DCCP, MPTCP, RTP extensions for CC. This can get huge... how many / which of these should we discuss? XXX}