Internet DRAFT - draft-nishida-tcpm-standard-cc-analysis
draft-nishida-tcpm-standard-cc-analysis
TCPM Working Group Y. Nishida
Internet-Draft AWS
Intended status: Informational 5 July 2023
Expires: 6 January 2024
Analysis for the Differences Between Standard Congestion Control Schemes
draft-nishida-tcpm-standard-cc-analysis-02
Abstract
Reno-based congestion control has been referred as the standard
document from IETF for long time that describes congestion control
principle of the Internet. In the meantime, IETF recently has
published two new congestion control standards that use slightly
different schemes from the previous one. This document provides
analysis for the differences between these standards in order to
provide helpful information when an unified congestion control
principles for the Internet is standardized in the future.
Status of This Memo
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This Internet-Draft will expire on 6 January 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Analysis for differences between RFC9002 and RFC5681 . . . . 3
3.1. Initial Window . . . . . . . . . . . . . . . . . . . . . 3
3.2. Loss Window . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. SlowStart Threshold After Packet loss . . . . . . . . . . 4
3.4. Window Growth in Slow Start Period . . . . . . . . . . . 4
3.5. Loss Recovery Algorithm . . . . . . . . . . . . . . . . . 5
3.6. Loss Detection Schemes . . . . . . . . . . . . . . . . . 5
3.7. Minimal RTO Values . . . . . . . . . . . . . . . . . . . 6
3.8. Congestion Window Increment on new Acknowledgment
Reception . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Analysis for differences between CUBIC and RFC5681 . . . . . 6
4.1. Multiplicative Window Decrease Factor . . . . . . . . . . 6
4.2. Reno-friendly Model . . . . . . . . . . . . . . . . . . . 7
4.3. SlowStart Threshold After Packet loss . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix A: Deriving increase factor for CUBIC from AIMD model . 11
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
[RFC5681] specifies Reno-based congestion control and it has been
referred as the standard document from IETF that outlines congestion
control principle of the Internet. On the other hand, IETF recently
have published two new congestion control standards; [RFC9002] as
Reno-based congestion control for QUIC [RFC9000] and
[I-D.ietf-tcpm-rfc8312bis] as CUBIC congestion control for various
transport protocols. We believe all transport protocols should share
the same congestion control principle so that they can share network
resources mostly fairly. From this point, we believe the concepts
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described in these standards should not conflict each other. In our
study, the new standards mostly follow the principles described in
[RFC5681], however, there are certain differences in their schemes or
the constant values, which may create certain performance
differences.
This document provides a list of such differences as a result of our
study, but does not provide any evaluations nor analysis for the
performance impacts by them. Hence, some differences described in
the document might be proved to be negligible in further analysis.
Or, others may be considered to create distinct performance
differences so that they will need to be updated to avoid conflicts
between the standards. However, given that the scale of the
Internet, we think such evaluations will not be easy as it would
require large-scale and long-term analysis.
The aim of the document is to simply describe the differences in them
and discuss their potential impacts as a reference for further
analysis. We hope the document will be an useful resource when an
unified congestion control principles for the Internet is needed to
be standardized in the future.
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.
3. Analysis for differences between RFC9002 and RFC5681
We think there are the following differences between RFC9002 and
RFC5681.
3.1. Initial Window
RFC5681 specifies Initial Window to be at most 4 segments or 4380
bytes as specified in [RFC3390] while RFC9002 allows it to be up to
10 segments or 14720 bytes. [RFC6928] allows TCP connections to use
up to 10 segments or 14600 bytes for Initial Window, however RFC5681
does not adopt it as it is an experimental document. The difference
in the choice of Initial Window will have certain impacts on the
growth rate of congestion window.
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3.2. Loss Window
RFC5681 specifies Loss Window Size to be 1 segment while RFC9002 uses
2 segments for it. As Section 4.8 in RFC9002 describes, using 2
segments for Loss Window can reduce the chance of RTO and avoid
additional delays caused by delayed ack algorithm. However, this
could mean when an RFC5681 connection and an RFC9002 connection get
RTOs at the same time, the RFC9002 connection can recover congestion
window size more than 2 times faster than the RFC5681 connection.
3.3. SlowStart Threshold After Packet loss
In RFC9002, a sender sets the slow-start threshold to the half value
of the congestion window when packet loss is detected. However, in
RFC5681, it uses the half value of the flight size instead of
congestion window. As there can be some situations where there is a
significant difference between flightsize and congestion window, the
choice here will have considerable impacts.
Even more, RFC5681 implicitly disallows to use congestion window here
as it states:
" 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. "
We fathom this sentence stemmed from the fact that there were some
implementations which increment congestion window on every
acknowledgment even though receiver's window was fully subscribed in
the past. However, In RFC9002, it is prohibited to increase
congestion window when it is underutilized to prevent this situation
(Section 7.8). RFC9002 also allows to use other mechanisms to update
its congestion window during idle periods such as [RFC7661].
3.4. Window Growth in Slow Start Period
In RFC9002, when a sender is in slow start, the congestion window
increases by the number of bytes acknowledged on each acknowledgment
segment arrival. On the other hand, RFC5681 increases congestion
window by at most 1 full segment. RFC5681 mentions RFC3465 [RFC3465]
which uses similar method to RFC9002, however RFC5681 does not
recommend to use it. In addition, RFC3465 defines the limited
factor: L which controls the aggressiveness of the algorithm.
RFC3465 recommends to use L=2. This means it can allow to increase
congestion window by at most 2 full segments. This algorithm will be
more conservative than RFC9002 in the presence of stretch acks that
is not uncommon these days.
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3.5. Loss Recovery Algorithm
The gist of the loss recovery algorithm in RFC5681 is to retransmit
all lost segments found in the previous round trip and once all of
them have been acknowledged, it migrates to Congestion Avoidance from
Recovery period. The detailed algorithms are specified in [RFC6582]
and [RFC6675].
On the other hand, RFC9002 specifies the ends of Recovery period as
when one of any packets sent during the Recovery period is
acknowledged. This means RFC9002 can end Recovery period even not
all lost segments in the previous round trip has been successfully
retransmitted. Moreover, it can end Recovery period even if some
segments have been lost during Recovery period as long as one or more
packets have been acknowledged.
Although we think this behavior will not lead to a congestion
collapse, it looks more aggressive than RFC5681. For example, when
there is a congestion where some but not all segments have been lost
during several round trips, RFC5681 reduces congestion window by half
every round trip (as long as retransmission schemes work
successfully, otherwise it will be timed out). On the other hand,
RFC9002 will repeat Recovery period and Congestion Avoidance Period
in turn, which reduces congestion window by half every other round
trip.
Another aspect of the loss recovery in RFC9002 is persistent
congestion that is equivalent to TCP's RTO. In RFC9002, data sender
establishes persistent congestion only when all sent packets are lost
for a long enough duration. This period is equivalent to the
duration for an RTO and two TLPs[RFC8985] in TCP. This will mean
RFC9002 reduces congestion window to minimal value only when there is
a extreme severe congestion. On the other hand, RFC5681 has more
chances for RTOs as it gets RTOs when fast retransmission/fast
recovery scheme doesn't work due to insufficient number of
acknowledgments.
3.6. Loss Detection Schemes
In RFC9002, in addition to acknowledgment-based loss detection scheme
which is also defined in RFC5681, it specifies another loss detection
scheme similar to RACK-TLP[RFC8985]. Although RACK-TCP is a standard
document, RFC5681 has no description for it.
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3.7. Minimal RTO Values
RFC9002 follows most of the parts of [RFC6298] which defines the
standard algorithm of retransmission timer computations and
managements for TCP. However, it does not follow 1 sec for minimal
RTO values in RFC6298 as it does not specify minimal RTO. This might
not be a major problem because it is known that various TCP
implementations already adopt lower values for minimal RTO. In
addition, QUIC has more explicit mechanism to identify spurious RTOs
than TCP, hence we believe there is no risk for large-scale network
issues in this. However, the impacts for not having minimum RTO is
still an important research topic for the performance and efficiency
of transport protocols.
3.8. Congestion Window Increment on new Acknowledgment Reception
In RFC5861, a TCP connection increments congestion window by at most
1 SMSS bytes upon a new ACK receipt during slow start. This means
even though a new ACK acknowledges more than 1 SMSS, it only
increases 1 SMSS per ACK. This logic is overridden by [RFC9406] when
hystart++ is used in the connection. If hystart++ is enabled,
congestion window can be increased by the amount of acknowledged in
an ACK packet during slow-start as long as some packet pacing
mechanisms are used in the connection. However, the increase of
congestion window per ACK is limited to 8 SMSS if there is no pacing.
On the other hand, in RFC9002, with or without pacing mechanisms, a
QUIC connection can increment congestion window by the amount of
acknowledged in a new ACK during slow-start.
The difference of these logics may affect the performance in the
presence of stretch ACKs.
4. Analysis for differences between CUBIC and RFC5681
We think the following points in the CUBIC specification can cause
differences behavior from RFC5681.
4.1. Multiplicative Window Decrease Factor
The CUBIC specification [I-D.ietf-tcpm-rfc8312bis] uses 0.7 for
Multiplicative Window Decrease factor while RFC5681 uses 0.5. We
think the rationale for using 0.5 in RFC5681 is derived from the
following sentences in [Jac88], hence we presume using 0.7 instead
will not be too aggressive to lead to a congestion collapse.
However, it can still be more aggressive than RFC5681 which may cause
unfair resource sharing.
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" We usually run our nets with ρ <= 0.5 so it's probable that
there are now exactly two conversations sharing the bandwidth.
I.e., you should reduce your window by half because the bandwidth
available to you has been reduced by half. And, if there are more
than two conversations sharing the bandwidth, halving your window
is conservative "
In order to compensate the aggressiveness by using the aggressive
decrease factor, CUBIC uses "Reno-friendly model" which employs
slower window growth rate in low BDP environments. We will discuss
the validity of the model in the following section, however, even if
the model is valid, CUBIC can be more aggressive than RFC5681 in some
situations. For example, when there are a congestion that can last
several round trips, CUBIC reduces congestion window by 30% every
round trip while RFC5681 reduces it by half. In this situation, the
window decrease rate for CUBIC will be mostly the half of RFC5681's.
In addition, during Recovery period, CUBIC transmits data with 70% of
the previous congestion window size while RFC5681 uses 50% of it.
Another example is that congestion window size can be much larger
than network capacity during slow start and CUBIC's high decrease
factor may have more impacts than RFC5681. For example, let's say
network capacity is 100Mbps and a TCP sender's congestion window size
at a certain round trip allows to transfer data at 99Mbps, which is
lower than the capacity. If this sender is in slow-start, the
congestion window size may glow to transfer data at 198Mbps in the
next round trip and can cause many packet losses. In case of
RFC5681, congestion window will be reduced by half in the following
round trip and transfer rate will be 99Mbps. However, in case of
CUBIC, the transfer rate will be 138.6Mbps which still exceeds the
network capacity. This would mean CUBIC can saturate network for two
round trips in this example while RFC5681 does only for one round
trip. However, this might be a pathological example since many
recent transport stacks support pacing mechanism and [HyStart] or
[I-D.ietf-tcpm-hystartplusplus] to mitigate the overshooting during
slow-start.
4.2. Reno-friendly Model
CUBIC employs Reno-friendly model which is designed to be fair to
RFC5681 in low BDP environments. The model in CUBIC is derived from
[FHP00] and it is based on AIMD congestion control as same as
RFC5681, but adopts different increase factor α and multiplicative
factor β. In RFC5681, α is 1.0 and β is 0.5. This means RFC5681
increases congestion window by 1 segment per acknowledgment when a
transport protocol is in congestion avoidance and reduces congestion
window by half when packet losses are detected. In the meantime, in
{I-D.ietf-tcpm-rfc8312bis}}, α is around 0.529 and β is 0.7. Hence,
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CUBIC reduces congestion window less than RFC5681 at packet loss, but
at the same time, it reduces the window growth rate so that the
performance of CUBIC and RFC5681 will be mostly the same.
We explain the rationale behind the values for α and β for CUBIC in
Appendix "Appendix A: Deriving increase factor for CUBIC from AIMD
model", but the important point for using these values is that it is
based on the following two presumptions.
* CUBIC connection(s) and Reno connection(s) are sharing the same
network resource
* When packet losses happen, both CUBIC connection and Reno
connection lost packets at the same time.
Although the first presumption can be considered as a common
situation, the second presumption will not be a common one as there
should be various patterns in packet losses. Moreover, in this
model, CUBIC increases congestion window by 0.529 segments per
acknowledgment which means CUBIC transmits a segment upon the arrival
of every two acknowledgments because a transport protocol usually
does not send a segment until it has at least one full segment space
available in congestion window. This makes harder to establish the
second presumption even more.
This might be a relatively minor point that do not have significant
impacts on overall performance of the model, however, more detailed
analysis with realistic packet dynamics will be desirable. A recent
report on this point shows that the model in CUBIC looks mostly fair
to RFC5681 in low BDP environments [AIMD-friendliness].
4.3. SlowStart Threshold After Packet loss
In CUBIC, a sender sets the slow-start threshold to the half value of
the FlightSize just like specified in RFC5681. However, it is not
prohibited to use congestion widow instead. In addition, CUBIC
mentions employing [RFC7661] as a more sophisticated approach.
5. Security Considerations
TODO Security
6. IANA Considerations
This document has no IANA actions.
7. References
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7.1. Normative References
[I-D.ietf-tcpm-rfc8312bis]
Xu, L., Ha, S., Rhee, I., Goel, V., and L. Eggert, "CUBIC
for Fast and Long-Distance Networks", Work in Progress,
Internet-Draft, draft-ietf-tcpm-rfc8312bis-15, 31 January
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
tcpm-rfc8312bis-15>.
[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>.
[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>.
7.2. Informative References
[AIMD-friendliness]
Briscoe, B. and O. Albisser, "Friendliness between AIMD
Algorithms", RFC Editor, please replace this URL with the
permanent arXiv one , 8 August 2022,
<https://raw.githubusercontent.com/bbriscoe/cubic-
reno/main/creno_tr.pdf>.
[FHP00] Floyd, S., Handley, M., and J. Padhye, "A Comparison of
Equation-Based and AIMD Congestion Control", May 2000,
<https://www.icir.org/tfrc/aimd.pdf>.
[HyStart] Ha, S. and I. Ree, "Hybrid Slow Start for High-Bandwidth
and Long-Distance Networks", DOI 10.1145/1851182.1851192,
International Workshop on Protocols for Fast Long-Distance
Networks , 2008.
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[I-D.ietf-tcpm-hystartplusplus]
Balasubramanian, P., Huang, Y., and M. Olson, "HyStart++:
Modified Slow Start for TCP", Work in Progress, Internet-
Draft, draft-ietf-tcpm-hystartplusplus-14, 27 February
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
tcpm-hystartplusplus-14>.
[Jac88] Jacobson, V., "Congestion Avoidance and Control", Computer
Communication Review, vol. 18, no. 4, pp. 314-329 , August
1988, <ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z.>.
[RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
Initial Window", RFC 3390, DOI 10.17487/RFC3390, October
2002, <https://www.rfc-editor.org/rfc/rfc3390>.
[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
2003, <https://www.rfc-editor.org/rfc/rfc3465>.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/rfc/rfc6298>.
[RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
NewReno Modification to TCP's Fast Recovery Algorithm",
RFC 6582, DOI 10.17487/RFC6582, April 2012,
<https://www.rfc-editor.org/rfc/rfc6582>.
[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
and Y. Nishida, "A Conservative Loss Recovery Algorithm
Based on Selective Acknowledgment (SACK) for TCP",
RFC 6675, DOI 10.17487/RFC6675, August 2012,
<https://www.rfc-editor.org/rfc/rfc6675>.
[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>.
[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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[RFC8985] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "The
RACK-TLP Loss Detection Algorithm for TCP", RFC 8985,
DOI 10.17487/RFC8985, February 2021,
<https://www.rfc-editor.org/rfc/rfc8985>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
[RFC9406] Balasubramanian, P., Huang, Y., and M. Olson, "HyStart++:
Modified Slow Start for TCP", RFC 9406,
DOI 10.17487/RFC9406, May 2023,
<https://www.rfc-editor.org/rfc/rfc9406>.
Acknowledgments
add people in tcpm-wg community.
Appendix A: Deriving increase factor for CUBIC from AIMD model
This section describes how increase factor: α used in CUBIC is
determined from AIMD congestion control model. We define AIMD(α, β)
as AIMD congestion control that uses an increase parameter α an a
decrease parameter β. Hence, AIMD(1, 0.5) represents congestion
control described in RFC5681 while CUBIC can be expressed as AIMD(α,
β) where β=0.7. We also define Wmax as the congestion window size
that can fully utilize network capacity.
At first, it is clear that β=0.7 is more aggressive than β=0.5 when
there is no other traffic. As cwnd growth is linear, if there's
enough long time for the data transfer, the average congestion window
for β =0.5 will be (1.0 + 0.5)/2 = 0.75 Wmax and it will be (1.0 +
0.7)/2 = 0.85 Wmax for β=0.7. Hence, it is obvious that this model
does not aim for the cases where there's no other traffic.
The choice of (α, β) for CUBIC is designed to be fair only when it
competes with AIMD(1.0, 0.5). Here, we define W1 as the max window
size for AIMD(1.0, 0.5) and W2 as the max window size for CUBIC's
AIMD(α=X, β =0.7) when they compete each other. In this situation,
AIMD(1.0, 0.5) model oscillates between 0.5 W1 and 1.0 W1 in a
congestion epoch. CUBIC's AIMD (α=X, β =0.7) model oscillates
between 0.7 W2 and 1.0 W2 in the same congestion epoch. When these
two models have the same loss ratio, it should satisfy the following
equation (1).
(1.0 + 0.5) W1 = (1.0 + 0.7) W2 (1)
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Also, in one congestion epoch, (α=1.0, β =0.5) increases congestion
window by 0.5 W1 while (α=X, β =0.7) increases 0.3 W2. The length of
congestion epoch for AIMD(1.0, 0.5) can be expressed as 0.5 W1/1.0
and it will be expressed as 0.3 W2 /X for AIMD(α=X, β =0.7). Because
AIMD(1.0), AIMD(α=X, β =0.7) should have the same congestion epoch
when they compete equaly, it should satisfy the following equation
(2).
0.5 W1 / 1.0 = 0.3 W2 / X (2)
From equations (1) and (2), we get X=0.529.
Contributors
The contents in this documents are the individual contributions from
the authors and do not relate to the authors' positions at their
affiliations.
Author's Address
Yoshifumi Nishida
Amazon Web Services
440 Terry Ave N
Seattle, WA 98109
United States of America
Email: nsd.ietf@gmail.com
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