Internet DRAFT - draft-balasubramanian-tcpm-hystartplusplus
draft-balasubramanian-tcpm-hystartplusplus
Network Working Group P. Balasubramanian
Internet-Draft Y. Huang
Intended status: Informational M. Olson
Expires: October 28, 2020 Microsoft
April 26, 2020
HyStart++: Modified Slow Start for TCP
draft-balasubramanian-tcpm-hystartplusplus-03
Abstract
This informational memo describes HyStart++, a simple modification to
the slow start phase of TCP congestion control algorithms. 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. This memo also describes the details of
the current implementation in the Windows operating system.
Status of This Memo
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This Internet-Draft will expire on October 28, 2020.
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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. Constants used and tuning . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
[RFC0793] and [RFC5681] describe the slow start mechanism 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. 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 ms or 300ms) 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 for further congestion window increases until the first
packet loss occurs.
This document describes HyStart++ as implemented in the Microsoft
Windows operating system. HyStart++ is widely deployed on the public
Internet. Precise documentation of running code enables follow-up
IETF Experimental or Standards Track RFCs. It also enables other
implementations and sharing of results for various workloads.
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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
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.
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 the first packet loss occurs. 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 slow start. LSS helps avoid massive packet losses and
subsequent time spent in loss recovery or retransmission timeout.
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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)
RttThresh = clamp(MIN_RTT_THRESH, lastRoundMinRTT / 8,
MAX_RTT_THRESH)
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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. Constants used and tuning
The Windows operating system implementation of HyStart++ uses the
following constants:
LOW_CWND = 16
MIN_RTT_THRESH = 4 msec
MAX_RTT_THRESH = 16 msec
LSS_DIVISOR = 0.25
N_RTT_SAMPLE = 8
An implementation MAY experiment with these constants and tune them
for different network characteristics. Windows operating system
implementation uses the same values for all connections. The maximum
value of LSS_DIVISOR SHOULD NOT exceed 0.5 which is the value
recommended in [RFC3742].
An implementation MAY choose to use HyStart++ for all slow starts
including the ones post a retransmission timeout, or a long idle
period. The Windows operating system implementation uses HyStart++
only for the initial slow start and uses traditional slow start for
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subsequent ones. This is acceptable because subsequent slow starts
will use the discovered ssthresh value to exit slow start.
5. Security Considerations
HyStart++ enhances slow start and inherits the general security
considerations discussed in [RFC5681].
6. IANA Considerations
This document has no actions for IANA.
7. Acknowledgements
Neal Cardwell suggested the idea for using the maximum of cwnd value
computed by LSS and congestion avoidance after exiting slow start.
8. References
8.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[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>.
8.2. Informative References
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[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>.
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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