HyStart++: Modified Slow Start for TCP
draft-ietf-tcpm-hystartplusplus-00
Network Working Group P. Balasubramanian
Internet-Draft Y. Huang
Intended status: Standards Track M. Olson
Expires: January 14, 2021 Microsoft
July 13, 2020
HyStart++: Modified Slow Start for TCP
draft-ietf-tcpm-hystartplusplus-00
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
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