Increase of the Congestion Window when the Sender Is Rate-Limited
draft-welzl-ccwg-ratelimited-increase-01
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Michael Welzl , Tom Henderson , Gorry Fairhurst | ||
| Last updated | 2024-02-22 | ||
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draft-welzl-ccwg-ratelimited-increase-01
Congestion Control Working Group M. Welzl
Internet-Draft University of Oslo
Updates: RFC5681, RFC9002, RFC9260, RFC9438 (if T. Henderson
approved)
Intended status: Standards Track G. Fairhurst
Expires: 25 August 2024 University of Aberdeen
22 February 2024
Increase of the Congestion Window when the Sender Is Rate-Limited
draft-welzl-ccwg-ratelimited-increase-01
Abstract
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
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-constrained-increase/draft-welzl-
ccwg-constrained-increase.html. Status information for this document
may be found at https://datatracker.ietf.org/doc/draft-welzl-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-constrained-increase.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 25 August 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Increase rules . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Discussion . . . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. The state of RFCs and implementations . . . . . . . 6
A.1. TCP ("Reno" congestion control) . . . . . . . . . . . . . 6
A.1.1. Specification . . . . . . . . . . . . . . . . . . . . 6
A.1.2. Implementation . . . . . . . . . . . . . . . . . . . 6
A.1.3. Assessment . . . . . . . . . . . . . . . . . . . . . 7
A.2. CUBIC . . . . . . . . . . . . . . . . . . . . . . . . . . 7
A.2.1. Specification . . . . . . . . . . . . . . . . . . . . 7
A.2.2. Implementation . . . . . . . . . . . . . . . . . . . 7
A.2.3. Assessment . . . . . . . . . . . . . . . . . . . . . 7
A.3. SCTP . . . . . . . . . . . . . . . . . . . . . . . . . . 7
A.3.1. Specification . . . . . . . . . . . . . . . . . . . . 7
A.3.2. Assessment . . . . . . . . . . . . . . . . . . . . . 7
A.4. QUIC . . . . . . . . . . . . . . . . . . . . . . . . . . 8
A.4.1. Specification . . . . . . . . . . . . . . . . . . . . 8
A.4.2. Assessment . . . . . . . . . . . . . . . . . . . . . 8
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A.5. DCCP CCID2 . . . . . . . . . . . . . . . . . . . . . . . 8
A.5.1. Specification . . . . . . . . . . . . . . . . . . . . 8
A.5.2. Assessment . . . . . . . . . . . . . . . . . . . . . 9
A.6. Other Transports . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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 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) [RFC7661] 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 the 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.
1.1. Terminology
This document uses the terms defined in Section 2 of [RFC5681].
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.
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3. Increase rules
Irrespective of the current state of a congestion control mechanism,
senders using a congestion controlled transport protocol:
1. MUST include a limit to 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 that returns 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 [RFC5681],
inc(maxFS)=2*maxFS, such that equation 2 in [RFC5681] becomes:
cwnd_new = cwnd + min (N, SMSS)
cwnd = min(cwnd_new, 2*maxFS)
Similarly, with rule #2 applied to Congestion Avoidance,
inc(maxFS)=1+maxFS, such that equation 3 in [RFC5681] becomes:
cwnd_new = cwnd + SMSS*SMSS/cwnd
cwnd = min(cwnd_new, 1+maxFS)
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.
3.1. Discussion
If the sending rate is less than permitted by cwnd for multiple RTTs,
limited 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 that 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.
4. Security Considerations
While congestion control designs could result in unwanted competing
traffic, they do not directly result in new security considerations.
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Transport protocols that provide authentication (including those
using encryption), or are carried over protocols that provide
authentication, can protect their congestion control mechanisms from
network attack. This is orthogonal to the 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>.
[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>.
[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>.
[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
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[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>.
Appendix A. The state of RFCs and implementations
This section is provided as input for IETF discussion, and should be
removed before publication.
A.1. TCP ("Reno" congestion control)
A.1.1. Specification
[RFC5681] 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.
[RFC7661] also 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.
A.1.2. 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 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.
Since kernel version 3.16, which was published in August 2014, in
Slow Start, the increase implements rule #2 in Section 3 in the
tcp_is_cwnd_limited function in tcp.h.
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A.1.3. Assessment
Linux implements a limit to cwnd growth in accordance with rule #1 in
Section 3; 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 Section 3.
A.2. CUBIC
A.2.1. Specification
Section 5.8 of [RFC9438] says:
Cubic doesn't increase cwnd when it's limited by the sending
application or rwnd.
A.2.2. Implementation
The description of Linux described in Appendix A.1.2 also applies to
Cubic.
A.2.3. Assessment
Both the specification and the Linux implementation limit the cwnd
growth in accordance with rule #1 in Section 3; in Congestion
Avoidance, this limit is more conservative than rule #2 in Section 3,
and in Slow Start, it implements rule #2 in Section 3.
A.3. SCTP
A.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.
A.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 rule #1 in
Section 3, and more conservative than rule #2 in Section 3.
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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 the rules in Section 3 apply to both
Slow Start and Congestion Avoidance.
A.4. QUIC
A.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.
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.
A.4.2. 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 that when pacing is
enabled, it is specifically taken into account. Pacing could occur
over various timescales, but is typically done with delays below an
RTT; thus, rule #2 in Section 3 should cover this case.
A.5. DCCP CCID2
A.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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A.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 CCID2
!RFC4341, is updated by this document to specify the management of
the cwnd during an application-limited period.
A.6. Other Transports
{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: MPTCP, RTP extensions for CC. A DCCP Congestion Control ID
(CCID) specifing TFRC-like behaviour (including CCID3 !RFC4341),
needs to follow the recommendations for TFRC.
This can get huge... how many / which of these should we discuss?
XXX}
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 is might be within the QUIC stack. --
Adds explicit mention of DCCP/CCID2. -- Adds this change log.
Authors' Addresses
Michael Welzl
University of Oslo
PO Box 1080 Blindern
0316 Oslo
Norway
Email: michawe@ifi.uio.no
URI: http://welzl.at/
Tom Henderson
Mercer Island, WA,
United States
Email: tomh@tomh.org
URI: https://www.tomh.org/
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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/
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