Increase of the Congestion Window when the Sender Is Rate-Limited
draft-ietf-ccwg-ratelimited-increase-03
| Document | Type | Active Internet-Draft (ccwg WG) | |
|---|---|---|---|
| Authors | Michael Welzl , Tom Henderson , Gorry Fairhurst , Mohit P. Tahiliani | ||
| Last updated | 2026-02-17 | ||
| Replaces | draft-welzl-ccwg-ratelimited-increase | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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draft-ietf-ccwg-ratelimited-increase-03
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.
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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
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Please review these documents carefully, as they describe your rights
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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
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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).
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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)
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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)).
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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.
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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,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/rfc/rfc5681>.
[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/rfc/rfc7661>.
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[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/rfc/rfc8174>.
[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/rfc/rfc9002>.
[RFC9260] Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260,
June 2022, <https://www.rfc-editor.org/rfc/rfc9260>.
[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/rfc/rfc9438>.
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,
<https://datatracker.ietf.org/doc/html/draft-ietf-ccwg-
bbr-04>.
[RFC2861] Handley, M., Padhye, J., and S. Floyd, "TCP Congestion
Window Validation", RFC 2861, DOI 10.17487/RFC2861, June
2000, <https://www.rfc-editor.org/rfc/rfc2861>.
[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, <https://www.rfc-editor.org/rfc/rfc4341>.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928,
DOI 10.17487/RFC6928, April 2013,
<https://www.rfc-editor.org/rfc/rfc6928>.
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.
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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
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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}
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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.
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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.
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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.
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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
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- 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/
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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/
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