The Benefits to Applications of using Explicit Congestion Notification (ECN)

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Replaced by RFC 8087, RFC 8087
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Network Working Group                                           M. Welzl
Internet-Draft                                        University of Oslo
Intended status: Experimental                               G. Fairhurst
Expires: August 18, 2014                          University of Aberdeen
                                                       February 14, 2014

 The Benefits to Applications of using Explicit Congestion Notification


   This document describes the potential benefits to applications when
   they 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.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 18, 2014.

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   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, a Congestion Experienced
   (CE)-marking in the IP header of a received packet.  Both of these
   are treated by transports as indications of (potential) congestion.
   ECN may also be enabled by other transports.  UDP applications may
   enable ECN when they are able to correctly process the ECN signals
   (e.g. ECN with RTP [RFC6679]).

   When an application enables the use of ECN, the transport layer sets
   the ECT(0) or ECT(1) codepoint in the IP header of packets it sends
   to indicate to routers that they may mark rather than drop packets in
   periods of congestion.  This marking is generally performed by Active
   Queue Management (AQM) [RFC2309] and may be the result of various AQM
   algorithms, where the exact combination of AQM/ECN algorithms is
   generally not known by the transport endpoints.

   ECN makes it possible for the network to signal congestion without
   packet loss.  This lets the network deliver some packets to an
   application that would otherwise have been dropped.  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 reduction from ECN-marking
   instead of dropping packets.

   This focus of this document is on usage of ECN, not its
   implementation in hosts, routers and other network devices.  Some of
   the benefits of ECN that are discussed rely upon routers marking
   packets at a lower level of congestion before they would otherwise
   drop packets from queue overflow.  Following a recommendation in
   [RFC3168], which says: "for a router, the CE codepoint of an ECN-
   Capable packet SHOULD only be set if the router would otherwise have
   dropped the packet as an indication of congestion to the end nodes",
   it has often been assumed that routers mark packets at the same level
   of congestion at which they would otherwise drop them (e.g. in
   [RFC2884]), but there are indications that this configuration is not
   ideal [KH13].

   Some of the benefits are only realised when the transport endpoint
   behaviour is also updated, this is discussed further in Section 4.

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