Network Working Group P. Balasubramanian
Internet-Draft Y. Huang
Intended status: Standards Track M. Olson
Expires: July 10, 2021 Microsoft
January 6, 2021
HyStart++: Modified Slow Start for TCP
draft-ietf-tcpm-hystartplusplus-01
Abstract
This doument describes HyStart++, a simple modification to the slow
start phase of TCP congestion control algorithms. Traditional slow
start can cause overshotting of the ideal send rate and cause large
packet loss within a round-trip time which results in poor
performance. HyStart++ combines the use of one variant of HyStart
and Limited Slow Start (LSS) to prevent overshooting of the ideal
sending rate, while also mitigating poor performance which can result
from false positives when HyStart is used alone.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. HyStart++ Algorithm . . . . . . . . . . . . . . . . . . . . . 3
4.1. Use of HyStart Delay Increase and Limited Slow Start . . 3
4.2. Algorithm Details . . . . . . . . . . . . . . . . . . . . 4
4.3. Tuning constants . . . . . . . . . . . . . . . . . . . . 5
5. Deployments and Performance Evaluations . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 7
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 sender increases cwnd
exponentially to probe the network capacity. Such a fast growth can
lead to overshooting the ideal sending rate and cause significant
packet loss. This is counter-productive for the TCP flow itself, and
also impacts the rest of the traffic sharing the bottleneck link.
TCP has several mechanisms for loss recovery, but they are only
effective for moderate loss. When these techniques are unable to
recover lost packets, a last-resort retransmission timeout (RTO) is
used to trigger packet recovery. In most operating systems, the
minimum RTO is set to a large value (200 msec or 300 msec) to prevent
spurious timeouts. This results in a long idle time which
drastically impairs flow completion times.
HyStart++ adds delay increase as a signal to exit slow start before
any packet loss occurs. This is one of two algorithms specified in
[HyStart]. After the HyStart delay algorithm finds an exit point,
LSS is used in conjunction with congestion avoidance for further
congestion window increases until the first packet loss is detected.
HyStart++ reduces packet loss and retransmissions, and improves
goodput in lab measurements as well as real world deployments.
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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. Use of HyStart Delay Increase and Limited Slow Start
[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 RTT spikes, which
suggest that the bottleneck buffer is filling up.
After the HyStart "Delay Increase" algorithm triggers an exit from
slow start, LSS (described in [RFC3742]) is used to increase Cwnd
until congestion is observed. LSS is used because the HyStart exit
is often premature as a result of RTT fluctuations or transient queue
buildup. LSS grows the cwnd fast but much slower than traditional
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slow start. LSS helps avoid massive packet losses and subsequent
time spent in loss recovery or retransmission timeout.
4.2. Algorithm Details
We assume that Appropriate Byte Counting (as described in [RFC3465])
is in use and L is the cwnd increase limit. The choice of value of L
is up to the implementation.
A round is chosen to be approximately the Round-Trip Time (RTT).
Round can be approximated using sequence numbers as follows:
Define windowEnd as a sequence number initialize to SND.UNA
When windowEnd is ACKed, the current round ends and windowEnd is
set to SND.NXT
At the start of each round during slow start:
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 and
w:
Update the cwnd
cwnd = cwnd + min (N, L * SMSS)
Keep track of minimum observed RTT
currentRoundMinRTT = min(currentRoundMinRTT, currRTT)
where currRTT is the measured RTT based on the incoming ACK
rttSampleCount += 1
For rounds where cwnd is at or higher than LOW_CWND and
N_RTT_SAMPLE RTT samples have been obtained, check if delay
increase triggers slow start exit
if (cwnd >= (LOW_CWND * SMSS) AND rttSampleCount >=
N_RTT_SAMPLE)
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RttThresh = clamp(MIN_RTT_THRESH, lastRoundMinRTT / 8,
MAX_RTT_THRESH)
if (currentRoundMinRTT >= (lastRoundMinRTT + RttThresh))
ssthresh = cwnd
exit slow start and enter LSS
For each arriving ACK in LSS, where N is the number of previously
unacknowledged bytes acknowledged in the arriving ACK:
K = cwnd / (LSS_DIVISOR * ssthresh)
cwnd = max(cwnd + (min (N, L * SMSS) / K), CA_cwnd())
CA_cwnd() denotes the cwnd that a congestion control algorithm would
have increased to if congestion avoidance started instead of LSS.
LSS grows cwnd very fast but for long-lived flows in high BDP
networks, the congestion avoidance algorithm could increase cwnd much
faster. For example, CUBIC congestion avoidance [RFC8312] in convex
region can ramp up cwnd rapidly. Taking the max can help improve
performance when exiting slow start prematurely.
HyStart++ ends when congestion is observed.
4.3. Tuning constants
It is RECOMMENDED that a HyStart++ implementation use the following
constants:
LOW_CWND = 16
MIN_RTT_THRESH = 4 msec
MAX_RTT_THRESH = 16 msec
LSS_DIVISOR = 0.25
N_RTT_SAMPLE = 8
These constants have been determined with lab measurements and real
world deployments. An implementation MAY tune them for different
network characteristics.
Using smaller values of LOW_CWND will cause the algorithm to kick in
before the last round RTT can be measured, particularly if the
implementation uses an initial cwnd of 10 MSS. Higher values will
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delay the detection of delay increase and reduce the ability of
HyStart++ to prevent overshoot problems.
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 maximum value of LSS_DIVISOR SHOULD NOT exceed 0.5, which is the
value recommended in [RFC3742]. Otherwise the cwnd growth could
again become too aggressive and cause ideal send rate overshoot.
Smaller values will cause the algorithm to be less aggressive and may
leave some cwnd growth on the table.
An implementation SHOULD use HyStart++ only for the initial slow
start 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 slow
start. An implementation MAY use HyStart++ to grow the restart
window ([RFC5681]) after a long idle period.
5. Deployments and Performance Evaluations
As of the time of writing, HyStart++ has been default enabled for all
TCP connections in Windows for two years. The original Hystart has
been default-enabled for all TCP connections in Linux TCP for a
decade.
In lab measurements with Windows TCP, HyStart++ shows both goodput
improvements as well as reductions in packet loss and
retransmissions. 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 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.
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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. Acknowledgements
Neal Cardwell suggested the idea of using the maximum of cwnd value
computed by LSS and congestion avoidance after exiting slow start.
9. References
9.1. 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/info/rfc2119>.
[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
2003, <https://www.rfc-editor.org/info/rfc3465>.
[RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large
Congestion Windows", RFC 3742, DOI 10.17487/RFC3742, March
2004, <https://www.rfc-editor.org/info/rfc3742>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
9.2. 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>.
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Authors' Addresses
Praveen Balasubramanian
Microsoft
One Microsoft Way
Redmond, WA 98052
USA
Phone: +1 425 538 2782
Email: pravb@microsoft.com
Yi Huang
Microsoft
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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