TCP Alternative Backoff with ECN (ABE)
RFC 8511
Internet Engineering Task Force (IETF) N. Khademi
Request for Comments: 8511 M. Welzl
Category: Experimental University of Oslo
ISSN: 2070-1721 G. Armitage
Netflix
G. Fairhurst
University of Aberdeen
December 2018
TCP Alternative Backoff with ECN (ABE)
Abstract
Active Queue Management (AQM) mechanisms allow for burst tolerance
while enforcing short queues to minimise the time that packets spend
enqueued at a bottleneck. This can cause noticeable performance
degradation for TCP connections traversing such a bottleneck,
especially if there are only a few flows or their bandwidth-delay
product (BDP) is large. The reception of a Congestion Experienced
(CE) Explicit Congestion Notification (ECN) mark indicates that an
AQM mechanism is used at the bottleneck, and the bottleneck network
queue is therefore likely to be short. Feedback of this signal
allows the TCP sender-side ECN reaction in congestion avoidance to
reduce the Congestion Window (cwnd) by a smaller amount than the
congestion control algorithm's reaction to inferred packet loss.
Therefore, this specification defines an experimental change to the
TCP reaction specified in RFC 3168, as permitted by RFC 8311.
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 candidates for any level of
Internet Standard; see 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/rfc8511.
Khademi, et al. Experimental [Page 1]
RFC 8511 ABE December 2018
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Choice of ABE Multiplier . . . . . . . . . . . . . . . . 4
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Rationale for Using ECN to Vary the Degree of Backoff . . 6
4.2. An RTT-Based Response to Indicated Congestion . . . . . . 7
5. ABE Deployment Requirements . . . . . . . . . . . . . . . . . 7
6. ABE Experiment Goals . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
Khademi, et al. Experimental [Page 2]
RFC 8511 ABE December 2018
1. Introduction
Explicit Congestion Notification (ECN) [RFC3168] makes it possible
for an Active Queue Management (AQM) mechanism to signal the presence
of incipient congestion without necessarily incurring packet loss.
This lets the network deliver some packets to an application that
would have been dropped if the application or transport did not
support ECN. This packet loss reduction is the most obvious benefit
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