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IETF RFC 9406
Last modified on Monday, May 15th, 2023
 Permanent link to RFC 9406
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Internet Engineering Task Force (IETF) P. Balasubramanian
Request for Comments: 9406 Confluent
Category: Standards Track Y. Huang
ISSN: 2070-1721 M. Olson
Microsoft
May 2023
HyStart++: Modified Slow Start for TCP
 Abstract
This document describes HyStart++, a simple modification to the slow
start phase of congestion control algorithms. Slow start can
overshoot the ideal send rate in many cases, causing high packet loss
and poor performance. HyStart++ uses increase in round-trip delay as
a 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
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 9406.
Copyright Notice
Copyright (c) 2023 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
2. Terminology
3. Definitions
4. HyStart++ Algorithm
4.1. Summary
4.2. Algorithm Details
4.3. Tuning Constants and Other Considerations
5. Deployments and Performance Evaluations
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
[RFC 5681] 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 the absence of packet loss signals, TCP increases the cwnd
exponentially to probe the network capacity. This fast growth can
overshoot the ideal sending rate and cause significant packet loss
that cannot always be recovered efficiently.
HyStart++ builds upon Hybrid Start (HyStart), originally described in
[HyStart]. HyStart++ uses increase in round-trip delay 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] for
finding a safe exit point for slow start. After the slow start exit,
a new 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 the 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 that use slow start, such as QUIC
[RFC 9002] or the Stream Control Transmission Protocol (SCTP)
[RFC 9260].
2. Terminology
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 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
3. Definitions
To aid the reader, we repeat some definitions from [RFC 5681]:
SENDER MAXIMUM SEGMENT SIZE (SMSS): 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
algorithm [RFC 1191] [RFC 4821], RMSS (see next item), or other
factors. The size does not include the TCP/IP headers and
options.
RECEIVER MAXIMUM SEGMENT SIZE (RMSS): The size of the largest
segment that 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
[RFC 1122]. 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 the 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 standard 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 in a fashion
similar to 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
The following pseudocode uses a limit, L, to control the
aggressiveness of the cwnd increase during both standard slow start
and CSS. While an arriving ACK may newly acknowledge an arbitrary
number of bytes, the HyStart++ algorithm limits the number of those
bytes applied to increase the cwnd to L*SMSS bytes.
lastRoundMinRTT and currentRoundMinRTT are initialized to infinity at
the initialization time. currRTT is the RTT sampled from the latest
incoming ACK and initialized to infinity.
lastRoundMinRTT = infinity
currentRoundMinRTT = infinity
currRTT = infinity
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 [RFC 5681] and
CSS, initialize the variables used to compute the last round's and
current round's minimum RTT:
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 the minimum observed RTT:
currentRoundMinRTT = min(currentRoundMinRTT, currRTT)
rttSampleCount += 1
For rounds where at least N_RTT_SAMPLE RTT samples have been obtained
and currentRoundMinRTT and lastRoundMinRTT are valid, check to see if
delay increase triggers slow start exit:
if ((rttSampleCount >= N_RTT_SAMPLE) AND
(currentRoundMinRTT != infinity) AND
(lastRoundMinRTT != infinity))
RttThresh = max(MIN_RTT_THRESH,
min(lastRoundMinRTT / MIN_RTT_DIVISOR, MAX_RTT_THRESH))
if (currentRoundMinRTT >= (lastRoundMinRTT + RttThresh))
cssBaselineMinRtt = currentRoundMinRTT
exit slow start and enter CSS
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 the minimum observed RTT:
currentRoundMinRTT = min(currentRoundMinRTT, currRTT)
rttSampleCount += 1
For CSS rounds where at least N_RTT_SAMPLE RTT samples have been
obtained, check to see if the current round's minRTT drops below
baseline (cssBaselineMinRtt) indicating that slow start exit was
spurious:
if (currentRoundMinRTT < cssBaselineMinRtt)
cssBaselineMinRtt = infinity
resume slow start including HyStart++
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.
If CSS_ROUNDS rounds are complete, enter congestion avoidance by
setting the ssthresh to the current cwnd.
ssthresh = cwnd
If loss or Explicit Congestion Notification (ECN) marking is observed
at any time during standard slow start or CSS, enter congestion
avoidance by setting the ssthresh to the current cwnd.
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
MIN_RTT_DIVISOR = 8
N_RTT_SAMPLE = 8
CSS_GROWTH_DIVISOR = 4
CSS_ROUNDS = 5
L = infinity if paced, L = 8 if non-paced
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.
MIN_RTT_DIVISOR is a fraction of RTT to compute the delay threshold.
A smaller value would mean a larger threshold and thus less
sensitivity to delay increase, and vice versa.
While all TCP implementations are REQUIRED to take at least one RTT
sample each round, implementations of HyStart++ are RECOMMENDED to
take at least N_RTT_SAMPLE RTT samples. 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.
Packet pacing [ASA00] is a possible mechanism to avoid large bursts
and their associated harm. A paced TCP implementation SHOULD use L =
infinity. Burst concerns are mitigated by pacing, and this setting
allows for optimal cwnd growth on modern networks.
For TCP implementations that pace to mitigate burst concerns, L
values smaller than infinity may suffer performance problems due to
slow cwnd growth in high-speed networks. For non-paced TCP
implementations, L values smaller than 8 may suffer performance
problems due to slow cwnd growth in high-speed networks; L values
larger than 8 may cause an increase in burstiness and thereby loss
rates, and result in poor performance.
An implementation SHOULD use HyStart++ only for the initial slow
start (when the ssthresh is at its initial value of arbitrarily high
per [RFC 5681]) and fall back to using standard slow start for the
remainder of the connection lifetime. This is acceptable because
subsequent slow starts will use the discovered ssthresh value to exit
slow start and avoid the overshoot problem. An implementation MAY
use HyStart++ to grow the restart window [RFC 5681] after a long idle
period.
In application-limited scenarios, the amount of data in flight could
fall below the bandwidth-delay product (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
At the time of this writing, HyStart++ as described in this document
has been default enabled for all TCP connections in the Windows
operating system for over two years with pacing disabled and an
actual L = 8.
In lab measurements with Windows TCP, HyStart++ shows goodput
improvements as well as reductions in packet loss and retransmissions
compared to standard 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 (RTOs) by 36%.
In an A/B test where we compared an implementation of HyStart++
(based on an earlier draft version of this document) to standard 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% of 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 [RFC 5681].
An attacker can cause HyStart++ to exit slow start prematurely and
impair the performance of a TCP connection by, for example, dropping
data packets or their acknowledgments.
The ACK division attack outlined in [SCWA99] does not affect
HyStart++ because the congestion window increase in HyStart++ is
based on the number of bytes newly acknowledged in each arriving ACK
rather than by a particular constant on each arriving ACK.
7. IANA Considerations
This document has no IANA actions.
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC 5681, September 2009,
<https://www.rfc-editor.org/info/RFC 5681>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
8.2. Informative References
[ASA00] Aggarwal, A., Savage, S., and T. Anderson, "Understanding
the performance of TCP pacing", Proceedings IEEE INFOCOM
2000, DOI 10.1109/INFCOM.2000.832483, March 2000,
<https://doi.org/10.1109/INFCOM.2000.832483>.
[HyStart] Ha, S. and I. Rhee, "Taming the elephants: New TCP slow
start", Computer Networks vol. 55, no. 9, pp. 2092-2110,
DOI 10.1016/j.comnet.2011.01.014, June 2011,
<https://doi.org/10.1016/j.comnet.2011.01.014>.
[RFC 1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC 1122, October 1989,
<https://www.rfc-editor.org/info/RFC 1122>.
[RFC 1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC 1191, November 1990,
<https://www.rfc-editor.org/info/RFC 1191>.
[RFC 4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC 4821, March 2007,
<https://www.rfc-editor.org/info/RFC 4821>.
[RFC 9002] Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
and Congestion Control", RFC 9002, DOI 10.17487/RFC 9002,
May 2021, <https://www.rfc-editor.org/info/RFC 9002>.
[RFC 9260] Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
Transmission Protocol", RFC 9260, DOI 10.17487/RFC 9260,
June 2022, <https://www.rfc-editor.org/info/RFC 9260>.
[SCWA99] Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
"TCP congestion control with a misbehaving receiver", ACM
SIGCOMM Computer Communication Review, vol. 29, issue 5,
pp. 71-78, DOI 10.1145/505696.505704, October 1999,
<https://doi.org/10.1145/505696.505704>.
Acknowledgments
During the discussions of this work on the TCPM mailing list and in
working group meetings, helpful comments, critiques, and reviews were
received from (listed alphabetically by last name) Mark Allman, Bob
Briscoe, Neal Cardwell, Yuchung Cheng, Junho Choi, Martin Duke, Reese
Enghardt, Christian Huitema, Ilpo Järvinen, Yoshifumi Nishida,
Randall Stewart, and Michael Tüxen.
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 98052
United States of America
Phone: +1 425 703 0447
Email: huanyi@microsoft.com
Matt Olson
Microsoft
One Microsoft Way
Redmond, WA 98052
United States of America
Phone: +1 425 538 8598
Email: maolson@microsoft.com
RFC TOTAL SIZE: 18917 bytes
PUBLICATION DATE: Monday, May 15th, 2023
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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