Congestion Control Working Group M. Welzl Internet-Draft University of Oslo Updates: RFC5681, RFC9002, RFC9260, RFC9438 (if T. Henderson approved) University of Washington Intended status: Standards Track G. Fairhurst Expires: 21 August 2026 University of Aberdeen M. P. Tahiliani National Institute of Technology Karnataka 17 February 2026 Increase of the Congestion Window when the Sender Is Rate-Limited draft-ietf-ccwg-ratelimited-increase-03 Abstract This document specifies how transport protocols increase their congestion window when the sender is rate-limited, and updates RFC 5681, RFC 9002, RFC 9260, and RFC 9438. Such a limitation can be caused by the sending application not supplying data or by receiver flow control. About This Document This note is to be removed before publishing as an RFC. The latest revision of this draft can be found at https://mwelzl.github.io/draft-ccwg-ratelimited-increase/draft-ietf- ccwg-ratelimited-increase.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-ccwg- ratelimited-increase/. Discussion of this document takes place on the Congestion Control Working Group Working Group mailing list (mailto:ccwg@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/ccwg/. Subscribe at https://www.ietf.org/mailman/listinfo/ccwg/. Source for this draft and an issue tracker can be found at https://github.com/mwelzl/draft-ccwg-ratelimited-increase. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Welzl, et al. Expires 21 August 2026 [Page 1] Internet-Draft Rate-Limited cwnd Increase February 2026 Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 21 August 2026. Copyright Notice Copyright (c) 2026 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 3. Rate-Limited Increase . . . . . . . . . . . . . . . . . . . . 4 3.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1.1. Unconstrained sender . . . . . . . . . . . . . . . . 5 3.1.2. Sender constrained by Rate-Limited Increase . . . . . 6 3.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . 6 3.2.1. Rate-based congestion control . . . . . . . . . . . . 6 3.2.2. Pacing . . . . . . . . . . . . . . . . . . . . . . . 7 4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.1. Normative References . . . . . . . . . . . . . . . . . . 7 6.2. Informative References . . . . . . . . . . . . . . . . . 8 Appendix A. An Example Using cwnd Represented in Bytes . . . . . 8 Appendix B. The state of RFCs and implementations . . . . . . . 11 B.1. TCP ("Reno" congestion control) . . . . . . . . . . . . . 11 B.1.1. Specification . . . . . . . . . . . . . . . . . . . . 11 B.1.2. Implementation . . . . . . . . . . . . . . . . . . . 11 Welzl, et al. Expires 21 August 2026 [Page 2] Internet-Draft Rate-Limited cwnd Increase February 2026 B.1.3. Assessment . . . . . . . . . . . . . . . . . . . . . 12 B.2. CUBIC . . . . . . . . . . . . . . . . . . . . . . . . . . 12 B.2.1. Specification . . . . . . . . . . . . . . . . . . . . 12 B.2.2. Implementation . . . . . . . . . . . . . . . . . . . 12 B.2.3. Assessment . . . . . . . . . . . . . . . . . . . . . 12 B.3. The Stream Control Transmission Protocol (SCTP) . . . . . 12 B.3.1. Specification . . . . . . . . . . . . . . . . . . . . 12 B.3.2. Assessment . . . . . . . . . . . . . . . . . . . . . 13 B.4. The QUIC Transport Protocol . . . . . . . . . . . . . . . 13 B.4.1. Specification . . . . . . . . . . . . . . . . . . . . 13 B.4.2. Assessment . . . . . . . . . . . . . . . . . . . . . 13 B.5. The Datagram Congestion Control Protocol (DCCP) CCID2 . . 13 B.5.1. Specification . . . . . . . . . . . . . . . . . . . . 13 B.5.2. Assessment . . . . . . . . . . . . . . . . . . . . . 14 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 14 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 1. Introduction A sender of a congestion controlled transport protocol becomes "rate- limited" when it does not send any data 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 sender using flow control (e.g., by the advertised TCP receiver window (rwnd) or by the connection or stream flow credit in QUIC). Current RFCs specifying congestion control algorithms diverge regarding the rules for increasing the cwnd when the sender is rate-limited. Congestion Window Validation (CWV) [RFC7661] provides an experimental specification defining how to manage a cwnd that has become larger than the current flight size, and how to respond to detected congestion when this is the case. 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 the cwnd when a sender does not fully utilise the current cwnd. An appendix provides an example of how rate-limited increase can play out. RFC-Ed Note, please remove the following sentence prior to publication: Another appendix provides an overview of the divergence in current RFCs and some implementations regarding cwnd increase when the sender is rate-limited (the second appendix is to be removed before publication). Welzl, et al. Expires 21 August 2026 [Page 3] Internet-Draft Rate-Limited cwnd Increase February 2026 2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 2.1. Terminology This document uses the terms defined in Section 2 of [RFC5681] and Section 3 of [RFC7661]. Additionally, we define: * initcwnd: The initial value of the congestion window, also known as the "initial window" ("IW" in [RFC5681]). * maxFS: 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, and at least as large as initcwnd. 3. Rate-Limited Increase When FlightSize < cwnd, regardless of the current state of a congestion control algorithm, the following "Rate-Limited Increase" rules apply for senders using a congestion controlled transport protocol: The sender MUST initialise the maxFS parameter to initcwnd when the congestion control algorithm is started. Thereafter when the FlightSize is updated, the sender updates maxFS: maxFS = max(FlightSize, maxFS) Upon a reduction of cwnd (for any reason), maxFS MUST be reset to zero. This ensures that maxFS is reinitialized using the first FlightSize measurement taken after the cwnd reduction. The sender MUST cap cwnd to be no larger than limit(maxFS). The function limit() returns the maximum cwnd value the congestion control algorithm would yield by increasing for all ACKs that would be produced by successfully transmitting one window of size maxFS. For example, for Slow Start, as specified in [RFC5681], limit(maxFS)=2*maxFS, such that equation 2 in [RFC5681] becomes: cwnd_new = cwnd + min (N, SMSS) cwnd = min(cwnd_new, 2*maxFS) Welzl, et al. Expires 21 August 2026 [Page 4] Internet-Draft Rate-Limited cwnd Increase February 2026 where cwnd and SMSS follow their definitions in [RFC5681] and N is the number of previously unacknowledged bytes acknowledged in the incoming ACK. Similarly, with Rate-Limited Increase applied in Congestion Avoidance, limit(maxFS)=SMSS+maxFS, such that equation 3 in [RFC5681] becomes: cwnd_new = cwnd + SMSS*SMSS/cwnd cwnd = min(cwnd_new, SMSS+maxFS) where cwnd and SMSS follow their definitions in [RFC5681]. NOTE: This specification defines the current method used to increase the cwnd for a rate-limited sender. Without a way to reduce cwnd when the transport sender becomes rate-limited, maxFS can stay valid for a long time, possibly not reflecting the reality of the end-to- end Internet path in use. This is remedied by "Congestion Window Validation" in [RFC7661], which also defines a "pipeACK" variable that measures the recently acknowledged size of the network pipe when the sender was rate-limited. 3.1. Example We illustrate the working of Rate-Limited Increase by showing the increase of cwnd in two scenarios: when the growth of cwnd is unconstrained, and when the rate-limited sender is constrained by Rate-Limited Increase. For simplicity, this example accounts for the cwnd in segments, rather than bytes. In both cases, we assume the initial cwnd (initcwnd) = 10 segments, as defined for TCP in [RFC6928] and QUIC in [RFC9002], a single connection begins with Slow Start, the sender transmits a total of 14 segments but pauses after transmitting 10 segments and resumes the transmission for the remaining 4 segments afterwards, no packets are lost, and an ACK is sent for every packet. 3.1.1. Unconstrained sender Initially, cwnd = initcwnd. Therefore, using initcwnd = 10 segments, the sender transmits 10 segments and pauses. Since the sender is in the Slow Start phase, the arrival of an each ACK for the 10 sent segments increases the cwnd by 1 segment, resulting in the cwnd increasing to 20 segments. Subsequently, after the pause, the sender transmits 4 segments and pauses again. As a consequence, the arrival of 4 ACKs results in cwnd further increasing to 24 segments, even though the sender is rate-limited (i.e., has never sent more than 10 segments per round-trip time (RTT)). Welzl, et al. Expires 21 August 2026 [Page 5] Internet-Draft Rate-Limited cwnd Increase February 2026 3.1.2. Sender constrained by Rate-Limited Increase Initially, cwnd = initcwnd. Therefore, using initcwnd = 10 segments, the sender transmits 10 segments and pauses; note that FlightSize and maxFS are both 10 segments at this point. Since the sender is in the Slow Start phase, the arrival of each ACK for the 10 sent segments increases the cwnd by 1 segment, resulting in the cwnd increasing to 20 segments. Subsequently, when the sender resumes and transmits 4 new segments, Rate-Limited Increase constrains the growth of the cwnd because FlightSize < cwnd and therefore this caps the cwnd to be no larger than limit(maxFS) = 2 X maxFS = 2 X 10 segments = 20 segments. 3.2. Discussion If the sending rate is less than the rate permitted by the cwnd for multiple RTTs, limited either by the sending application or by the receiver-advertised window, a continuous increase in the cwnd would cause a mismatch between the cwnd and the capacity that the path supports (i.e., over-estimating the capacity). Such unlimited growth in the cwnd is therefore disallowed. However, in most common congestion control algorithms, in the absence of an indication of congestion, a cwnd that has been fully utilized during an RTT (where a sender was cwnd-limited) permits the cwnd to be increased during the immediately following RTT. This increase is allowed by Rate-Limited Increase. 3.2.1. Rate-based congestion control The present document updates congestion control specifications that use a cwnd to limit the number of unacknowledged bytes (or packets) that a sender is allowed to emit. Use of a cwnd variable to control sending rate is not the only mechanism available and not the only mechanism that is used in practice. Congestion control algorithms can also constrain data transmission by explicitly calculating the sending rate over some time interval, by "pacing" packets (injecting pauses in between their transmission) or via combinations of the above (e.g., BBR combines these three methods [I-D.ietf-ccwg-bbr]). The guiding principle behind Rate-Limited Increase applies to all congestion control algorithms: in the absence of a congestion indication, a sender is allowed to increase its rate from the amount of data that it has transmitted during the previous RTT (this holds irrespective of whether the sender is rate-limited or not). Therefore, congestion control algorithms SHOULD implement a behavior that is equivalent to Rate-Limited Increase, irrespective of whether they use a cwnd variable or not. Welzl, et al. Expires 21 August 2026 [Page 6] Internet-Draft Rate-Limited cwnd Increase February 2026 3.2.2. Pacing Pacing mechanisms seek to avoid the negative impacts associated with "bursts" (flights of packets transmitted back-to-back). Rate-Limited Increase introduces a limit using "maxFS", which is based on the number of bytes in flight during a previous RTT; thus, as long as the number of bytes in flight per RTT is unaffected by pacing, Rate- Limited Increase does not constrain the use of pacing mechanisms. 4. Security Considerations While congestion control designs could result in unwanted competing traffic, they do not directly result in new security considerations. The security considerations are the same as for other congestion control methods. Such methods rely on the receiver appropriately acknowledging receipt of data. The ability of an on-path or off-path attacker to influence congestion control depends upon the security properties of the transport protocol being used. Transport protocols that provide authentication (including those using encryption), or are carried over protocols that provide authentication, can protect their congestion control algorithm from network attack. This is orthogonal to the specification of congestion control rules. 5. IANA Considerations This document requests no IANA action. 6. References 6.1. 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, . [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, . [RFC7661] Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating TCP to Support Rate-Limited Traffic", RFC 7661, DOI 10.17487/RFC7661, October 2015, . Welzl, et al. Expires 21 August 2026 [Page 7] Internet-Draft Rate-Limited cwnd Increase February 2026 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9002] Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection and Congestion Control", RFC 9002, DOI 10.17487/RFC9002, May 2021, . [RFC9260] Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260, June 2022, . [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, . 6.2. Informative References [I-D.ietf-ccwg-bbr] Cardwell, N., Swett, I., and J. Beshay, "BBR Congestion Control", Work in Progress, Internet-Draft, draft-ietf- ccwg-bbr-04, 20 October 2025, . [RFC2861] Handley, M., Padhye, J., and S. Floyd, "TCP Congestion Window Validation", RFC 2861, DOI 10.17487/RFC2861, June 2000, . [RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion Control Protocol (DCCP) Congestion Control ID 2: TCP-like Congestion Control", RFC 4341, DOI 10.17487/RFC4341, March 2006, . [RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis, "Increasing TCP's Initial Window", RFC 6928, DOI 10.17487/RFC6928, April 2013, . Appendix A. An Example Using cwnd Represented in Bytes The following informative example is provided for a sender that maintains the cwnd in bytes. 36 packets are sent in this example over four rounds of transmission. This shows the initial growth of the cwnd by a rate-limited sender, followed by a transmission that uses the full available cwnd. Welzl, et al. Expires 21 August 2026 [Page 8] Internet-Draft Rate-Limited cwnd Increase February 2026 The initial sender state is: Sender sequence number (seqno) = 0 MSS = 1000 bytes cwnd = 10000 bytes (initcwnd) maxFS = 10000 bytes (initcwnd) FlightSize (FS) = 0 bytes ssthresh is infinity, i.e. the congestion control algorithm is in slow start. The network path’s bandwidth-delay product is such that, throughout this example, all packets in each round are sent before an ACK is received for the first packet in a round. One ACK is generated for each 2*MSS received bytes. Round 1, the sender has 4000B to send in 4 packets: MSS=1000, cwnd=10000 Send seqno=0; FS=1000; maxFS=10000 Send seqno=1000; FS=1000; maxFS=10000 Send seqno=2000; FS=2000; maxFS=10000 Send seqno=3000; FS=3000; maxFS=10000 Received 2 ACKs; maxFS=10000, if (cwnd<2*maxFS) {cwnd +=ACK’ed} ACK for 2000 ACK’ed=2000 : cwnd+= 2000; cwnd=12000 ACK for 4000 ACK’ed=2000 : cwnd+= 2000; cwnd=14000 Note: This round maxFS was not increased and cwnd was increased. Round 2, the sender has 8000B to send in 8 packets: MSS=1000, cwnd=14000 Send seqno=4000; FS=1000; maxFS=10000 Send seqno=5000; FS=2000; maxFS=10000 Send seqno=6000; FS=3000; maxFS=10000 Send seqno=7000; FS=4000; maxFS=10000 Send seqno=8000; FS=5000; maxFS=10000 Send seqno=9000; FS=6000; maxFS=10000 Send seqno=10000; FS=7000; maxFS=10000 Send seqno=11000; FS=8000; maxFS=10000 Received 4 ACKs; maxFS=10000, if (cwnd<2*maxFS) {cwnd +=ACK’ed} ACK for 6000 ACK’ed=2000 : cwnd+=2000; cwnd=16000 ACK for 8000 ACK’ed=2000 : cwnd+=2000; cwnd=18000 ACK for 10000 ACK’ed=2000 : cwnd+=2000; cwnd=20000 ACK for 12000 ACK’ed=2000 : cwnd+=0; cwnd=20000 Welzl, et al. Expires 21 August 2026 [Page 9] Internet-Draft Rate-Limited cwnd Increase February 2026 Note: This round maxFS was not increased and cwnd was increased to 2*maxFS. Round 3, the sender has 4000B to send in 4 packets: MSS=1000, cwnd=20000 Send seqno=12000; FS=1000; maxFS=10000 Send seqno=13000; FS=2000; maxFS=10000 Send seqno=14000; FS=3000; maxFS=10000 Send seqno=15000; FS=4000; maxFS=10000 Received 2 ACKs; maxFS=10000, if (cwnd<2*maxFS) {cwnd +=ACK’ed} ACK for 14000 ACK’ed=2000 : cwnd+=0; cwnd=20000 ACK for 16000 ACK’ed=2000 : cwnd+=0; cwnd=20000 Note: This round maxFS was not increased and cwnd was not increased. Round 4, the sender has 20000B to send in 20 packets: MSS=1000, cwnd=20000 Send seqno=16000; FS= 1000; maxFS=10000 Send seqno=17000; FS= 2000; maxFS=10000 Send seqno=18000; FS= 3000; maxFS=10000 Send seqno=19000; FS= 4000; maxFS=10000 Send seqno=20000; FS= 5000; maxFS=10000 Send seqno=21000; FS= 6000; maxFS=10000 Send seqno=22000; FS= 7000; maxFS=10000 Send seqno=23000; FS= 8000; maxFS=10000 Send seqno=24000; FS= 9000; maxFS=10000 Send seqno=25000; FS=10000; maxFS=10000 Send seqno=26000; FS=11000; maxFS=11000 Send seqno=27000; FS=12000; maxFS=12000 Send seqno=28000; FS=13000; maxFS=13000 Send seqno=29000; FS=14000; maxFS=14000 Send seqno=30000; FS=15000; maxFS=15000 Send seqno=31000; FS=16000; maxFS=16000 Send seqno=32000; FS=17000; maxFS=17000 Send seqno=33000; FS=18000; maxFS=18000 Send seqno=34000; FS=19000; maxFS=19000 Send seqno=35000; FS=20000; maxFS=20000 Received 10 ACKs; maxFS=20000, if (cwnd<2*maxFS) {cwnd +=ACK’ed} Welzl, et al. Expires 21 August 2026 [Page 10] Internet-Draft Rate-Limited cwnd Increase February 2026 ACK for 18000 ACK’ed=2000 : cwnd+=2000; cwnd=22000 ACK for 20000 ACK’ed=2000 : cwnd+=2000; cwnd=24000 ACK for 22000 ACK’ed=2000 : cwnd+=2000; cwnd=26000 ACK for 24000 ACK’ed=2000 : cwnd+=2000; cwnd=28000 ACK for 26000 ACK’ed=2000 : cwnd+=2000; cwnd=30000 ACK for 28000 ACK’ed=2000 : cwnd+=2000; cwnd=32000 ACK for 30000 ACK’ed=2000 : cwnd+=2000; cwnd=34000 ACK for 32000 ACK’ed=2000 : cwnd+=2000; cwnd=36000 ACK for 34000 ACK’ed=2000 : cwnd+=2000; cwnd=38000 ACK for 36000 ACK’ed=2000 : cwnd+=2000; cwnd=40000 Note: In this round, maxFS was increased and cwnd was increased to 2*maxFS. Appendix B. The state of RFCs and implementations RFC-Ed Note: This section is provided as input for IETF discussion, and should be removed before publication. B.1. TCP ("Reno" congestion control) B.1.1. Specification [RFC7661] suggests there is no increase limitation in the standard TCP behavior (which [RFC7661] 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. B.1.2. Implementation * ns-2 allows cwnd to grow when it is rate-limited by rwnd. (Rate- limited by the sending application: not tested.) * Until release 3.42, ns-3 allowed cwnd to grow when rate-limited, either due to an application or rwnd limit. Since release 3.42, ns-3 TCP models conform to Rate-Limited Increase, following the current Linux TCP approach in this regard (see next bullet). * In Congestion Avoidance, Linux only allows the cwnd to grow when the sender is unconstrained. Before kernel version 3.16, this also applied to Slow Start. The check for "unconstrained" is perfomed by checking if FlightSize is greater or equal to cwnd. Welzl, et al. Expires 21 August 2026 [Page 11] Internet-Draft Rate-Limited cwnd Increase February 2026 Since kernel version 3.16, which was published in August 2014, in Slow Start, the increase implements Rate-Limited Increase in the tcp_is_cwnd_limited function in tcp.h. B.1.3. Assessment Linux implements a limit to cwnd growth in accordance with Rate- Limited Increase; in Slow Start, this limit follows the rule's upper limit, while in Congestion Avoidance, it is more conservative than Rate-Limited Increase. The specification and the ns-2 and (older) ns-3 implementations are in conflict with Rate-Limited Increase. B.2. CUBIC B.2.1. Specification Section 5.8 of [RFC9438] says: Cubic doesn't increase cwnd when it's limited by the sending application or rwnd. B.2.2. Implementation The description of Linux described in Appendix B.1.2 also applies to Cubic. B.2.3. Assessment Both the specification and the Linux implementation limit the cwnd growth in accordance with Rate-Limited Increase; in Congestion Avoidance, this limit is more conservative than Rate-Limited Increase, and in Slow Start, it implements the "maxFS" upper limit of Rate-Limited Increase. B.3. The Stream Control Transmission Protocol (SCTP) B.3.1. Specification Section 7.2.1 of [RFC9260] 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. Welzl, et al. Expires 21 August 2026 [Page 12] Internet-Draft Rate-Limited cwnd Increase February 2026 B.3.2. Assessment The quoted statement from [RFC9260] 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 Rate-Limited Increase, and more conservative. Section 7.2.1 of [RFC9260] is specifically limited to Slow Start. Congestion Avoidance is discussed in Section 7.2.2 of [RFC9260] However, 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 Rate-Limited Increase applies to both Slow Start and Congestion Avoidance. B.4. The QUIC Transport Protocol B.4.1. Specification Section 7.8 of [RFC9002] 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. B.4.2. Assessment With the exception of pacing, this specification conservatively limits the growth in cwnd, similar to Cubic and SCTP. It is in accordance with Rate-Limited Increase, and more conservative. B.5. The Datagram Congestion Control Protocol (DCCP) CCID2 B.5.1. Specification Section 5.1 of [RFC4341] states: >There are currently no standards governing TCP's use of the congestion window during an application- limited period. In particular, it is possible for TCP's congestion window to grow quite large during a long uncongested period when the sender is application limited, sending at a low rate. [RFC2861] essentially suggests that TCP's congestion window not be increased during application-limited periods when the congestion window is not being fully utilized. Welzl, et al. Expires 21 August 2026 [Page 13] Internet-Draft Rate-Limited cwnd Increase February 2026 B.5.2. Assessment A DCCP Congestion Control ID (CCID) specifing TCP-like behaviour ought to follow the method specified in this document. The current guidance relates only to [RFC2861]. The text in Section 5.1 of [RFC4341] is updated by this document to specify the management of the cwnd when the sender is rate-limited. Appendix C. Change Log * -00 was the first individual submission for feedback by CCWG. * -01 includes editorial improvements - Removes application interaction with QUIC pacing, since pacing might be within the QUIC stack. - Adds explicit mention of DCCP/CCID2. - Adds this change log. * -02 addresses comments from IETF-119 - Discusses rate-based controls and pacing. - Trims the list of possible RFCs to update. - Some editorial fixes: "congestion control algorithm" instead of "mechanism" for consistency with RFC5033.bis; earlier definition of maxFS; explicit mention of RFCs to update in abstract. * -03 addresses comments from IETF-120 - Introduces a third rule, with MAY, that avoids having an unvalidated long-lived maxFS (using pipeACK from RFC 7661). - Changes "inc" to "limit" and adapts the wording of rule 2 to make it clearer (thanks to Neal Cardwell). - Appendix: updates ns-3 in line with the recent implementation. - Appendix: makes the RFC 9002 text clearer and shorter. * draft-ietf-ccwg-ratelimited-increase-00 - adds Mohit Tahiliani as a co-author Welzl, et al. Expires 21 August 2026 [Page 14] Internet-Draft Rate-Limited cwnd Increase February 2026 - refines the "rule" text (shorter, clearer) - adds an example * draft-ietf-ccwg-ratelimited-increase-01 - Clarified what we mean with an RTT - rephrased example regarding initcwnd, citing RFCs 6928 and 9002 - removed the too vague rule 1 and made rule 2 (now rule 1) a MUST * draft-ietf-ccwg-ratelimited-increase-02 - Improved the last sentence of section 3.1.2. - Removed a confusing and unnecessary sentence about pacing (as suggested at IETF-123). * draft-ietf-ccwg-ratelimited-increase-03 - The editors checked rule 2, and found that rule 1 was sufficient, and did not depend on the ordering of rules in newCWV (RFC7661), hence rule 2 was finally removed. - Cleaned language and improved text explaining how this compliments RFC7661. - Checked/updated definitions. - Added an example with cwnd in bytes. Acknowledgments The authors would like to thank Neal Cardwell and Martin Duke for suggesting improvements to this document. Authors' Addresses Michael Welzl University of Oslo PO Box 1080 Blindern 0316 Oslo Norway Email: michawe@ifi.uio.no URI: http://welzl.at/ Welzl, et al. Expires 21 August 2026 [Page 15] Internet-Draft Rate-Limited cwnd Increase February 2026 Tom Henderson University of Washington 185 Stevens Way Seattle, WA 98195, United States Email: tomh@tomh.org URI: https://www.tomh.org/ Godred Fairhurst University of Aberdeen Fraser Noble Building Aberdeen, AB24 3UE United Kingdom Email: gorry@erg.abdn.ac.uk URI: https://www.erg.abdn.ac.uk/ Mohit P. Tahiliani National Institute of Technology Karnataka P. O. Srinivasnagar, Surathkal Mangalore, Karnataka - 575025 India Email: tahiliani@nitk.edu.in URI: https://tahiliani.in/ Welzl, et al. Expires 21 August 2026 [Page 16]