Congestion Control Working Group M. Welzl Internet-Draft University of Oslo Updates: 4341, 5681, 9002, 9260, 9438 (if T. Henderson approved) University of Washington Intended status: Standards Track G. Fairhurst Expires: 6 February 2027 University of Aberdeen M. P. Tahiliani National Institute of Technology Karnataka 5 August 2026 Increase of the Congestion Window when the Sender Is Rate-Limited draft-ietf-ccwg-ratelimited-increase-08 Abstract This document specifies how transport protocols increase their congestion window when the sender is rate-limited, and updates RFCs 4341, 5681, 9002, 9260, and 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://ietf-wg- ccwg.github.io/draft-ietf-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 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/ietf-wg-ccwg/draft-ietf-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 6 February 2027 [Page 1] Internet-Draft Rate-Limited cwnd Increase August 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 6 February 2027. 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 . . . . . . . . . . . . . . . . . 3 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 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. Updates to RFCs 4341, 5681, 9002, 9260, and 9438 . . . . . . 7 4.1. RFC 4341: Profile for Datagram Congestion Control Protocol (DCCP) Congestion Control ID 2: TCP-like Congestion Control . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.2. RFC 5681: TCP Congestion Control . . . . . . . . . . . . 7 4.3. RFC 9002: QUIC Loss Detection and Congestion Control . . 7 4.4. RFC 9260: Stream Control Transmission Protocol . . . . . 8 4.5. RFC 9438: CUBIC for Fast and Long-Distance Networks . . . 8 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 Welzl, et al. Expires 6 February 2027 [Page 2] Internet-Draft Rate-Limited cwnd Increase August 2026 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 7.1. Normative References . . . . . . . . . . . . . . . . . . 9 7.2. Informative References . . . . . . . . . . . . . . . . . 10 Appendix A. An Example Using cwnd Represented in Bytes . . . . . 10 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 13 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 utilize 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. This document provides a uniform behavior in (Section 3), and specifies updates to RFCs 4341, 5681, 9002, 9260, and 9438 in (Section 4). 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 utilize the current cwnd. An appendix provides an example of how rate-limited increase can play out. 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]. Additionally, the following are defined: Welzl, et al. Expires 6 February 2027 [Page 3] Internet-Draft Rate-Limited cwnd Increase August 2026 * cwnd-limited: A flow that has sent the maximum number of segments permitted by the cwnd, where the application utilises the allowed sending rate (based on the definition for TCP in Section 4.5.3 of [RFC7661]). * rate-limited: A flow that does not consume more than one half of cwnd and hence operates in the non-validated phase. This includes periods when an application is either idle or chooses to send at a rate less than the maximum permitted by the cwnd (based on the definition for TCP in Section 3 of [RFC7661]). * 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 also updates the 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 6 February 2027 [Page 4] Internet-Draft Rate-Limited cwnd Increase August 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 can be 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 The working of Rate-Limited Increase can be illustrated 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 TCP segments (or QUIC packets), rather than bytes. In both cases, this assumes 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 afterward, 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 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 6 February 2027 [Page 5] Internet-Draft Rate-Limited cwnd Increase August 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). Welzl, et al. Expires 6 February 2027 [Page 6] Internet-Draft Rate-Limited cwnd Increase August 2026 Rate-based congestion control algorithms MUST ensure their maximum sustained rate is limited to a value that is not greater than would have been permitted by the Rate-Limited Increase method defined in Section 3. 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. Updates to RFCs 4341, 5681, 9002, 9260, and 9438 4.1. RFC 4341: Profile for Datagram Congestion Control Protocol (DCCP) Congestion Control ID 2: TCP-like Congestion Control According to [RFC4341], a DCCP CCID specifying TCP-like behavior is allowed to grow the cwnd without limit during an uncongested period when it sends at a rate unconstrained by the current cwnd. This document updates Section 5.1 of [RFC4341] by adding the text in Section 3 to specify how the cwnd is managed when the sender is rate- limited. 4.2. RFC 5681: TCP Congestion Control [RFC5681] specifies no limit on cwnd growth in the standard TCP behavior when a TCP sender is unable to send at the maximum rate allowed by the cwnd. This document updates [RFC5681] by adding the text in Section 3 to specify how the cwnd is managed when the sender is rate-limited. 4.3. RFC 9002: QUIC Loss Detection and Congestion Control 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." This limits the cwnd growth in accordance with Rate-Limited Increase, but it is more conservative. Welzl, et al. Expires 6 February 2027 [Page 7] Internet-Draft Rate-Limited cwnd Increase August 2026 This document updates [RFC9002] by replacing the final sentence of the cited text with the text in Section 3 to specify how the cwnd is managed when the sender is rate-limited. 4.4. RFC 9260: Stream Control Transmission Protocol Section 7.2.1 of [RFC9260] states: "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." The quoted statement from [RFC9260] limits cwnd growth in accordance with Rate-Limited Increase, but it is more conservative. Section 7.2.1 of [RFC9260] only discusses Slow Start. Congestion Avoidance is discussed in Section 7.2.2 of [RFC9260]; however, this section does not contain a similar rule. It is therefore clear that the quoted statement from [RFC9260] only applies to Slow Start. This document updates [RFC9260] by replacing the final sentence of the quoted text with the text in Section 3 to specify how the cwnd is managed when the sender is rate-limited. Section 7.2.2 of [RFC9260] is also updated to add text in Section 3. This ensures that the update applies to both Slow Start and Congestion Avoidance. 4.5. RFC 9438: CUBIC for Fast and Long-Distance Networks Section 5.8 of [RFC9438] states: "Cubic doesn't increase cwnd when it's limited by the sending application or rwnd". This limits the cwnd growth in accordance with Rate-Limited Increase, but it is more conservative. This document updates [RFC9438] by replacing the quoted text with the text in Section 3 to specify how the cwnd is managed when the sender is rate-limited. Welzl, et al. Expires 6 February 2027 [Page 8] Internet-Draft Rate-Limited cwnd Increase August 2026 5. 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. 6. IANA Considerations This document requests no IANA action. 7. References 7.1. 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, . [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, . [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, . [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, . Welzl, et al. Expires 6 February 2027 [Page 9] Internet-Draft Rate-Limited cwnd Increase August 2026 [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, . 7.2. 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-06, 6 July 2026, . [RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis, "Increasing TCP's Initial Window", RFC 6928, DOI 10.17487/RFC6928, April 2013, . [RFC7661] Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating TCP to Support Rate-Limited Traffic", RFC 7661, DOI 10.17487/RFC7661, October 2015, . 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 (or segments in the case of TCP) 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. The SMSS (QUIC MPS)=1000. N is the number of previously unacknowledged bytes in a received acknowledgement. For simplicity, in this example the receiver sends an ACK for each received packet. The initial sender state is: Sender sequence number (seqno) = 0 SMSS = 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 received packet. Welzl, et al. Expires 6 February 2027 [Page 10] Internet-Draft Rate-Limited cwnd Increase August 2026 Round 1, the sender has 4000B to send in 4 packets (1000B); cwnd=10000 Send seqno=0; FS=1000; maxFS=10000 Send seqno=1000; FS=2000; maxFS=10000 Send seqno=2000; FS=3000; maxFS=10000 Send seqno=3000; FS=4000; maxFS=10000 Received 4 ACKs (each N=1000); maxFS=10000 cwnd_new += N; cwnd = min(cwnd_new, 2*maxFS) ACK for 1000 ACK’ed=1000; FS-=1000: cwnd+= 1000; cwnd=11000 ACK for 2000 ACK’ed=1000; FS-=1000: cwnd+= 1000; cwnd=12000 ACK for 3000 ACK’ed=1000; FS-=1000: cwnd+= 1000; cwnd=13000 ACK for 4000 ACK’ed=1000; FS-=1000: cwnd+= 1000; cwnd=14000 Note: This round maxFS was not increased and cwnd was increased. Round 2, the sender has 8000B to send in 8 packets (1000B), 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 8 ACKs (N=2000); maxFS=10000 cwnd_new += N; cwnd = min(cwnd_new, 2*maxFS) ACK for 5000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=15000 ACK for 6000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=16000 ACK for 7000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=17000 ACK for 8000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=18000 ACK for 9000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=19000 ACK for 10000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=20000 ACK for 11000 ACK’ed=1000; FS-=1000: cwnd+=0; cwnd=20000 ACK for 12000 ACK’ed=1000; FS-=1000: cwnd+=0; cwnd=20000 Note: This round maxFS was not increased and cwnd was limited to 2*maxFS. Round 3, the sender has 4000B to send in 4 packets (1000B), cwnd=20000 Welzl, et al. Expires 6 February 2027 [Page 11] Internet-Draft Rate-Limited cwnd Increase August 2026 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 (N=2000); maxFS=10000 cwnd_new += N; cwnd = min(cwnd_new, 2*maxFS) ACK for 13000 ACK’ed=1000; FS-=1000: cwnd+=0; cwnd=20000 ACK for 14000 ACK’ed=1000; FS-=1000: cwnd+=0; cwnd=20000 ACK for 15000 ACK’ed=1000; FS-=1000: cwnd+=0; cwnd=20000 ACK for 16000 ACK’ed=1000; FS-=1000: 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 (1000B), 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 (N=2000); maxFS=20000 cwnd_new += N; cwnd = min(cwnd_new, 2*maxFS) Welzl, et al. Expires 6 February 2027 [Page 12] Internet-Draft Rate-Limited cwnd Increase August 2026 ACK for 18000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=21000 ACK for 19000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=22000 ACK for 19000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=23000 ACK for 20000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=24000 ACK for 21000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=25000 ACK for 22000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=26000 ACK for 23000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=27000 ACK for 24000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=28000 ACK for 25000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=29000 ACK for 26000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=30000 ACK for 27000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=31000 ACK for 28000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=32000 ACK for 29000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=33000 ACK for 30000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=34000 ACK for 31000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=35000 ACK for 32000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=36000 ACK for 33000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=37000 ACK for 34000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=38000 ACK for 35000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=39000 ACK for 36000 ACK’ed=1000; FS-=1000: cwnd+=1000; cwnd=40000 Note: In this round, maxFS increased and therefore cwnd increased to 2*maxFS. Appendix B. 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. Welzl, et al. Expires 6 February 2027 [Page 13] Internet-Draft Rate-Limited cwnd Increase August 2026 * -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 - 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 complements RFC7661. - Checked/updated definitions. - Added an example with cwnd in bytes. Welzl, et al. Expires 6 February 2027 [Page 14] Internet-Draft Rate-Limited cwnd Increase August 2026 * draft-ietf-ccwg-ratelimited-increase-04 - Repeated definitions from RFC 7661 instead of just pointing at the RFC - Made RFC 7661 informational * draft-ietf-ccwg-ratelimited-increase-05 - Rephrased references to RFC 7661 - Nits * draft-ietf-ccwg-ratelimited-increase-06 - Changed some TCP-specific language so it applies to QUIC too - RFC 4341 added to list of updated RFCs in the abstract - Nits * draft-ietf-ccwg-ratelimited-increase-07 - Updated this list - Made RFC 4341 reference normative - Fixed Updates list to match boilerplate - Moved some of appendix B to the main text as a new section that (only) defines the RFC updates. - Updated description lines in appendix A * draft-ietf-ccwg-ratelimited-increase-08 - Addressed a comment from Eric Vyncke about the paragraph on rate-based cc. algs. Acknowledgments The authors would like to thank Neal Cardwell and Martin Duke for suggesting improvements to this document. Authors' Addresses Welzl, et al. Expires 6 February 2027 [Page 15] Internet-Draft Rate-Limited cwnd Increase August 2026 Michael Welzl University of Oslo PO Box 1080 Blindern 0316 Oslo Norway Email: michawe@ifi.uio.no URI: http://welzl.at/ Tom Henderson University of Washington 185 Stevens Way Seattle, WA 98195, United States Email: tomh@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 6 February 2027 [Page 16]