Network Working Group P. Balasubramanian
Internet-Draft Confluent
Intended status: Standards Track Y. Huang
Expires: 26 January 2023 M. Olson
Microsoft
25 July 2022
HyStart++: Modified Slow Start for TCP
draft-ietf-tcpm-hystartplusplus-06
Abstract
This doument describes HyStart++, a simple modification to the slow
start phase of congestion control algorithms. Traditional slow start
can overshoot the ideal send rate in many cases, causing high packet
loss and poor performance. HyStart++ uses a delay increase heuristic
to find an exit point before possible overshoot. It also adds a
mitigation to prevent jitter from causing premature slow start exit.
Status of This Memo
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Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. HyStart++ Algorithm . . . . . . . . . . . . . . . . . . . . . 3
4.1. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 3
4.2. Algorithm Details . . . . . . . . . . . . . . . . . . . . 4
4.3. Tuning constants and other considerations . . . . . . . . 6
5. Deployments and Performance Evaluations . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
[RFC5681] describes the slow start congestion control algorithm for
TCP. The slow start algorithm is used when the congestion window
(cwnd) is less than the slow start threshold (ssthresh). During slow
start, in absence of packet loss signals, TCP increases cwnd
exponentially to probe the network capacity. This fast growth can
overshoot the ideal sending rate and cause significant packet loss
which cannot always be recovered efficiently.
HyStart++ uses delay increase as a signal to exit slow start before
potential packet loss occurs as a result of overshoot. This is one
of two algorithms specified in [HyStart]. After the slow start exit,
a novel Conservative Slow Start (CSS) phase is used to determine
whether the slow start exit was premature and to resume slow start.
This mitigation improves performance in presence of jitter.
HyStart++ reduces packet loss and retransmissions, and improves
goodput in lab measurements and real world deployments.
While this document describes Hystart++ for TCP, it can also be used
for other transport protocols which use slow start such as QUIC
[RFC9002].
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Definitions
We repeat here some definition from [RFC5681] to aid the reader.
SENDER MAXIMUM SEGMENT SIZE (SMSS): The SMSS is the size of the
largest segment that the sender can transmit. This value can be
based on the maximum transmission unit of the network, the path MTU
discovery [RFC1191, RFC4821] algorithm, RMSS (see next item), or
other factors. The size does not include the TCP/IP headers and
options.
RECEIVER MAXIMUM SEGMENT SIZE (RMSS): The RMSS is the size of the
largest segment the receiver is willing to accept. This is the value
specified in the MSS option sent by the receiver during connection
startup. Or, if the MSS option is not used, it is 536 bytes
[RFC1122]. The size does not include the TCP/IP headers and options.
RECEIVER WINDOW (rwnd): The most recently advertised receiver window.
CONGESTION WINDOW (cwnd): A TCP state variable that limits the amount
of data a TCP can send. At any given time, a TCP MUST NOT send data
with a sequence number higher than the sum of the highest
acknowledged sequence number and the minimum of cwnd and rwnd.
4. HyStart++ Algorithm
4.1. Summary
[HyStart] specifies two algorithms (a "Delay Increase" algorithm and
an "Inter-Packet Arrival" algorithm) to be run in parallel to detect
that the sending rate has reached capacity. In practice, the Inter-
Packet Arrival algorithm does not perform well and is not able to
detect congestion early, primarily due to ACK compression. The idea
of the Delay Increase algorithm is to look for spikes in RTT (round-
trip time), which suggest that the bottleneck buffer is filling up.
In HyStart++, a TCP sender uses traditional slow start and then uses
the "Delay Increase" algorithm to trigger an exit from slow start.
But instead of going straight from slow start to congestion
avoidance, the sender spends a number of RTTs in a Conservative Slow
Start (CSS) phase to determine whether the exit from slow start was
premature. During CSS, the congestion window is grown exponentially
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like in regular slow start, but with a smaller exponential base,
resulting in less aggressive growth. If the RTT reduces during CSS,
it's concluded that the RTT spike was not related to congestion
caused by the connection sending at a rate greater than the ideal
send rate, and the connection resumes slow start. If the RTT
inflation persists throughout CSS, the connection enters congestion
avoidance.
4.2. Algorithm Details
For the pseudocode, we assume that Appropriate Byte Counting (as
described in [RFC3465]) is in use and L is the cwnd increase limit as
discussed in RFC 3465.
lastRoundMinRTT and currentRoundMinRTT are initialized to infinity at
the initialization time
Hystart++ measures rounds using sequence numbers, as follows:
Define windowEnd as a sequence number initialized to SND.NXT
When windowEnd is ACKed, the current round ends and windowEnd is
set to SND.NXT
At the start of each round during standard slow start ([RFC5681]) and
CSS:
lastRoundMinRTT = currentRoundMinRTT
currentRoundMinRTT = infinity
rttSampleCount = 0
For each arriving ACK in slow start, where N is the number of
previously unacknowledged bytes acknowledged in the arriving ACK:
Update the cwnd
- cwnd = cwnd + min (N, L * SMSS)
Keep track of minimum observed RTT
- currentRoundMinRTT = min(currentRoundMinRTT, currRTT)
- where currRTT is the RTT sampled from the latest incoming ACK
- rttSampleCount += 1
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For rounds where at least N_RTT_SAMPLE RTT samples have been
obtained and currentRoundMinRTT and lastRoundMinRTT are valid,
check if delay increase triggers slow start exit
- if (rttSampleCount >= N_RTT_SAMPLE AND currentRoundMinRTT !=
infinity AND lastRoundMinRTT != infinity)
o RttThresh = clamp(MIN_RTT_THRESH, lastRoundMinRTT / 8,
MAX_RTT_THRESH)
o if (currentRoundMinRTT >= (lastRoundMinRTT + RttThresh))
+ cssBaselineMinRtt = currentRoundMinRTT
+ exit slow start and enter CSS
CSS lasts at most CSS_ROUNDS rounds. If the transition into CSS
happens in the middle of a round, that partial round counts towards
the limit.
For each arriving ACK in CSS, where N is the number of previously
unacknowledged bytes acknowledged in the arriving ACK:
Update the cwnd
- cwnd = cwnd + (min (N, L * SMSS) / CSS_GROWTH_DIVISOR)
Keep track of minimum observed RTT
- currentRoundMinRTT = min(currentRoundMinRTT, currRTT)
- where currRTT is the sampled RTT from the incoming ACK
- rttSampleCount += 1
For CSS rounds where at least N_RTT_SAMPLE RTT samples have been
obtained, check if current round's minRTT drops below baseline
indicating that HyStart exit was spurious.
- if (currentRoundMinRTT < cssBaselineMinRtt)
o cssBaselineMinRtt = infinity
o resume slow start including HyStart++
If CSS_ROUNDS rounds are complete, enter congestion avoidance.
* ssthresh = cwnd
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If loss or ECN-marking is observed anytime during standard slow start
or CSS, enter congestion avoidance.
* ssthresh = cwnd
4.3. Tuning constants and other considerations
It is RECOMMENDED that a HyStart++ implementation use the following
constants:
* MIN_RTT_THRESH = 4 msec
* MAX_RTT_THRESH = 16 msec
* N_RTT_SAMPLE = 8
* CSS_GROWTH_DIVISOR = 4
* CSS_ROUNDS = 5
These constants have been determined with lab measurements and real
world deployments. An implementation MAY tune them for different
network characteristics.
The delay increase sensitivity is determined by MIN_RTT_THRESH and
MAX_RTT_THRESH. Smaller values of MIN_RTT_THRESH may cause spurious
exits from slow start. Larger values of MAX_RTT_THRESH may result in
slow start not exiting until loss is encountered for connections on
large RTT paths.
A TCP implementation is required to take at least one RTT sample each
round. Using lower values of N_RTT_SAMPLE will lower the accuracy of
the measured RTT for the round; higher values will improve accuracy
at the cost of more processing.
The minimum value of CSS_GROWTH_DIVISOR MUST be at least 2. A value
of 1 results in the same aggressive behavior as regular slow start.
Values larger than 4 will cause the algorithm to be less aggressive
and maybe less performant.
Smaller values of CSS_ROUNDS may miss detecting jitter and larger
values may limit performance.
An implementation SHOULD use HyStart++ only for the initial slow
start (when ssthresh is at its initial value of arbitrarily high per
[RFC5681]) and fall back to using traditional slow start for the
remainder of the connection lifetime. This is acceptable because
subsequent slow starts will use the discovered ssthresh value to exit
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slow start and avoid the overshoot problem. An implementation MAY
use HyStart++ to grow the restart window ([RFC5681]) after a long
idle period.
In application limited scenarios, the amount of data in flight could
fall below the BDP and result in smaller RTT samples which can
trigger an exit back to slow start. It is expected that a connection
might oscillate between CSS and slow start in such scenarios. But
this behavior will neither result in a connection prematurely
entering congestion avoidance nor cause overshooting compared to slow
start.
5. Deployments and Performance Evaluations
As of the time of writing, HyStart++ as described in draft versions
01 through 04 was default enabled for all TCP connections in the
Windows operating system for over three years with an actual L = 8.
The original Hystart has been default-enabled for all TCP connections
in the Linux operating system using the default congestion control
module CUBIC ([RFC8312]) for a decade with an infinite L.
In lab measurements with Windows TCP, HyStart++ shows both goodput
improvements as well as reductions in packet loss and retransmissions
compared to traditional slow start. For example, across a variety of
tests on a 100 Mbps link with a bottleneck buffer size of bandwidth-
delay product, HyStart++ reduces bytes retransmitted by 50% and
retransmission timeouts by 36%.
In an A/B test where we compare HyStart++ draft 01 to traditional
slow start across a large Windows device population, out of 52
billion TCP connections, 0.7% of connections move from 1 RTO to 0
RTOs and another 0.7% connections move from 2 RTOs to 1 RTO with
HyStart++. This test did not focus on send heavy connections and the
impact on send heavy connections is likely much higher. We plan to
conduct more such production experiments to gather more data in the
future.
6. Security Considerations
HyStart++ enhances slow start and inherits the general security
considerations discussed in [RFC5681].
7. IANA Considerations
This document has no actions for IANA.
8. References
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8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", DOI 10.17487/RFC2119, BCP 14,
RFC 2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", DOI 10.17487/RFC3465, RFC 3465, February
2003, <https://www.rfc-editor.org/info/rfc3465>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", DOI 10.17487/RFC5681, RFC 5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
8.2. Informative References
[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,
<https://pdfs.semanticscholar.org/25e9/
ef3f03315782c7f1cbcd31b587857adae7d1.pdf>.
[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC8312, February 2018,
<https://www.rfc-editor.org/info/rfc8312>.
[RFC9002] Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
and Congestion Control", DOI 10.17487/RFC9002, RFC 9002,
May 2021, <https://www.rfc-editor.org/info/rfc9002>.
Authors' Addresses
Praveen Balasubramanian
Confluent
899 West Evelyn Ave
Mountain View, CA 94041
United States of America
Email: pravb.ietf@gmail.com
Yi Huang
Microsoft
One Microsoft Way
Redmond, WA 94052
United States of America
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Phone: +1 425 703 0447
Email: huanyi@microsoft.com
Matt Olson
Microsoft
Phone: +1 425 538 8598
Email: maolson@microsoft.com
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