The Benefits of using Explicit Congestion Notification (ECN)

The information below is for an old version of the document
Document Type Active Internet-Draft (aqm WG)
Last updated 2015-04-02
Replaces draft-welzl-ecn-benefits
Stream IETF
Intended RFC status (None)
Formats plain text pdf html bibtex
Stream WG state WG Document
Document shepherd No shepherd assigned
IESG IESG state I-D Exists
Consensus Boilerplate Unknown
Telechat date
Responsible AD (None)
Send notices to (None)
Network Working Group                                       G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Informational                                  M. Welzl
Expires: October 4, 2015                              University of Oslo
                                                          April 02, 2015

      The Benefits of using Explicit Congestion Notification (ECN)


   This document describes the potential benefits when applications
   enable Explicit Congestion Notification (ECN).  It outlines the
   principal gains in terms of increased throughput, reduced delay and
   other benefits when ECN is used over network paths that include
   equipment that supports ECN-marking.  It also identifies some
   potential problems that might occur when ECN is used.  The document
   does not propose new algorithms that may be able to use ECN or
   describe the details of implementation of ECN in endpoint devices,
   routers and other network devices.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 4, 2015.

Copyright Notice

   Copyright (c) 2015 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
   ( in effect on the date of
   publication of this document.  Please review these documents

Fairhurst & Welzl        Expires October 4, 2015                [Page 1]
Internet-Draft               Benefits of ECN                  April 2015

   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.

1.  Introduction

   Internet Transports (such as TCP and SCTP) have two ways to detect
   congestion: the loss of a packet and, if Explicit Congestion
   Notification (ECN) [RFC3168] is enabled, by reception of a packet
   with a Congestion Experienced (CE)-marking in the IP header.  Both of
   these are treated by transports as indications of (potential)
   congestion.  ECN may also be enabled by other transports: UDP
   applications that provide congestion control may enable ECN when they
   are able to correctly process the ECN signals [RFC5405] (e.g., ECN
   with RTP [RFC6679]).

   Active Queue Management (AQM) is a class of techniques that can be
   used by network devices to manage the size of queues that build in
   network buffers.  A network device (router, middlebox, or other
   device that forwards packets through the network) that does not
   support AQM, typically uses a drop-tail policy to drop excess IP
   packets when its queue becomes full.  The discard of packets serves
   as a signal to the end-to-end transport that there may be congestion
   on the network path being used.  This triggers a congestion control
   reaction to reduce the maximum rate permitted by the sending

   When an application uses a transport that enables the use of ECN, the
   transport layer sets the ECT(0) or ECT(1) codepoint in the IP header
   of packets that it sends.  This indicates to network devices that
   they may mark, rather than drop, packets as the network queue builds.
   This can allow a network device to signal at a point before a
   transport experiences congestion loss or additional queuing delay.
   The marking is generally performed as the result of various AQM
   algorithms, where the exact combination of AQM/ECN algorithms does
   not need to be known by the transport endpoints.

   Since ECN makes it possible for the network to signal the presence of
   incipient congestion (network queueing) without incurring packet
   loss, it lets the network deliver some packets to an application that
   would otherwise have been dropped if the application or transport did
   not support ECN.  This packet loss reduction is the most obvious
   benefit of ECN, but it is often relatively modest.  However, enabling
   ECN can also result in a number of beneficial side-effects, some of
   which may be much more significant than the immediate packet loss
Show full document text