An Expedited Forwarding PHB
RFC 2598

Document Type RFC - Proposed Standard (June 1999; Errata)
Obsoleted by RFC 3246
Authors Kathleen Nichols  , Van Jacobson  , Kedernath Poduri 
Last updated 2013-03-02
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Network Working Group                                        V. Jacobson
Request for Comments: 2598                                    K. Nichols
Category: Standards Track                                  Cisco Systems
                                                               K. Poduri
                                                            Bay Networks
                                                               June 1999

                      An Expedited Forwarding PHB

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.


   The definition of PHBs (per-hop forwarding behaviors) is a critical
   part of the work of the Diffserv Working Group.  This document
   describes a PHB called Expedited Forwarding. We show the generality
   of this PHB by noting that it can be produced by more than one
   mechanism and give an example of its use to produce at least one
   service, a Virtual Leased Line.  A recommended codepoint for this PHB
   is given.

   A pdf version of this document is available at

1.  Introduction

   Network nodes that implement the differentiated services enhancements
   to IP use a codepoint in the IP header to select a per-hop behavior
   (PHB) as the specific forwarding treatment for that packet [RFC2474,
   RFC2475].  This memo describes a particular PHB called expedited
   forwarding (EF). The EF PHB can be used to build a low loss, low
   latency, low jitter, assured bandwidth, end-to-end service through DS
   domains.  Such a service appears to the endpoints like a point-to-
   point connection or a "virtual leased line".  This service has also
   been described as Premium service [2BIT].

Jacobson, et al.            Standards Track                     [Page 1]
RFC 2598              An Expedited Forwarding PHB              June 1999

   Loss, latency and jitter are all due to the queues traffic
   experiences while transiting the network.  Therefore providing low
   loss, latency and jitter for some traffic aggregate means ensuring
   that the aggregate sees no (or very small) queues. Queues arise when
   (short-term) traffic arrival rate exceeds departure rate at some
   node.  Thus a service that ensures no queues for some aggregate is
   equivalent to bounding rates such that, at every transit node, the
   aggregate's maximum arrival rate is less than that aggregate's
   minimum departure rate.

   Creating such a service has two parts:

      1) Configuring nodes so that the aggregate has a well-defined
         minimum departure rate. ("Well-defined" means independent of
         the dynamic state of the node.  In particular, independent of
         the intensity of other traffic at the node.)

      2) Conditioning the aggregate (via policing and shaping) so that
         its arrival rate at any node is always less than that node's
         configured minimum departure rate.

   The EF PHB provides the first part of the service.  The network
   boundary traffic conditioners described in [RFC2475] provide the
   second part.

   The EF PHB is not a mandatory part of the Differentiated Services
   architecture, i.e., a node is not required to implement the EF PHB in
   order to be considered DS-compliant.  However, when a DS-compliant
   node claims to implement the EF PHB, the implementation must conform
   to the specification given in this document.

   The next sections describe the EF PHB in detail and give examples of
   how it might be implemented.  The keywords "MUST", "MUST NOT",
   "REQUIRED", "SHOULD", "SHOULD NOT", and "MAY" that appear in this
   document are to be interpreted as described in [Bradner97].

2. Description of EF per-hop behavior

   The EF PHB is defined as a forwarding treatment for a particular
   diffserv aggregate where the departure rate of the aggregate's
   packets from any diffserv node must equal or exceed a configurable
   rate.  The EF traffic SHOULD receive this rate independent of the
   intensity of any other traffic attempting to transit the node.  It
   SHOULD average at least the configured rate when measured over any
   time interval equal to or longer than the time it takes to send an
   output link MTU sized packet at the configured rate.  (Behavior at
   time scales shorter than a packet time at the configured rate is

Jacobson, et al.            Standards Track                     [Page 2]
RFC 2598              An Expedited Forwarding PHB              June 1999

   deliberately not specified.) The configured minimum rate MUST be
   settable by a network administrator (using whatever mechanism the
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