Proportional Rate Reduction for TCP
RFC 6937
Internet Engineering Task Force (IETF) M. Mathis
Request for Comments: 6937 N. Dukkipati
Category: Experimental Y. Cheng
ISSN: 2070-1721 Google, Inc.
May 2013
Proportional Rate Reduction for TCP
Abstract
This document describes an experimental Proportional Rate Reduction
(PRR) algorithm as an alternative to the widely deployed Fast
Recovery and Rate-Halving algorithms. These algorithms determine the
amount of data sent by TCP during loss recovery. PRR minimizes
excess window adjustments, and the actual window size at the end of
recovery will be as close as possible to the ssthresh, as determined
by the congestion control algorithm.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. 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). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6937.
Mathis, et al. Experimental [Page 1]
RFC 6937 Proportional Rate Reduction May 2013
Copyright Notice
Copyright (c) 2013 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
(http://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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
2. Definitions .....................................................5
3. Algorithms ......................................................6
3.1. Examples ...................................................6
4. Properties ......................................................9
5. Measurements ...................................................11
6. Conclusion and Recommendations .................................12
7. Acknowledgements ...............................................13
8. Security Considerations ........................................13
9. References .....................................................13
9.1. Normative References ......................................13
9.2. Informative References ....................................14
Appendix A. Strong Packet Conservation Bound ......................15
1. Introduction
This document describes an experimental algorithm, PRR, to improve
the accuracy of the amount of data sent by TCP during loss recovery.
Standard congestion control [RFC5681] requires that TCP (and other
protocols) reduce their congestion window (cwnd) in response to
losses. Fast Recovery, described in the same document, is the
reference algorithm for making this adjustment. Its stated goal is
to recover TCP's self clock by relying on returning ACKs during
recovery to clock more data into the network. Fast Recovery
typically adjusts the window by waiting for one half round-trip time
(RTT) of ACKs to pass before sending any data. It is fragile because
it cannot compensate for the implicit window reduction caused by the
losses themselves.
Mathis, et al. Experimental [Page 2]
RFC 6937 Proportional Rate Reduction May 2013
RFC 6675 [RFC6675] makes Fast Recovery with Selective Acknowledgement
(SACK) [RFC2018] more accurate by computing "pipe", a sender side
estimate of the number of bytes still outstanding in the network.
With RFC 6675, Fast Recovery is implemented by sending data as
necessary on each ACK to prevent pipe from falling below slow-start
threshold (ssthresh), the window size as determined by the congestion
control algorithm. This protects Fast Recovery from timeouts in many
cases where there are heavy losses, although not if the entire second
Show full document text