Assured Forwarding PHB Group
RFC 2597

Document Type RFC - Proposed Standard (June 1999; Errata)
Updated by RFC 3260
Authors Walter Weiss  , Juha Heinanen  , Fred Baker  , John Wroclawski 
Last updated 2013-03-02
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Network Working Group                                        J. Heinanen
Request for Comments: 2597                                 Telia Finland
Category: Standards Track                                       F. Baker
                                                           Cisco Systems
                                                                W. Weiss
                                                     Lucent Technologies
                                                           J. Wroclawski
                                                                 MIT LCS
                                                               June 1999

                      Assured Forwarding PHB Group

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.


   This document defines a general use Differentiated Services (DS)
   [Blake] Per-Hop-Behavior (PHB) Group called Assured Forwarding (AF).
   The AF PHB group provides delivery of IP packets in four
   independently forwarded AF classes.  Within each AF class, an IP
   packet can be assigned one of three different levels of drop
   precedence.  A DS node does not reorder IP packets of the same
   microflow if they belong to the same AF class.

1. Purpose and Overview

   There is a demand to provide assured forwarding of IP packets over
   the Internet.  In a typical application, a company uses the Internet
   to interconnect its geographically distributed sites and wants an
   assurance that IP packets within this intranet are forwarded with
   high probability as long as the aggregate traffic from each site does
   not exceed the subscribed information rate (profile).  It is
   desirable that a site may exceed the subscribed profile with the
   understanding that the excess traffic is not delivered with as high
   probability as the traffic that is within the profile.  It is also

Heinanen                    Standards Track                     [Page 1]
RFC 2597              Assured Forwarding PHB Group             June 1999

   important that the network does not reorder packets that belong to
   the same microflow, as defined in [Nichols], no matter if they are in
   or out of the profile.

   Assured Forwarding (AF) PHB group is a means for a provider DS domain
   to offer different levels of forwarding assurances for IP packets
   received from a customer DS domain.  Four AF classes are defined,
   where each AF class is in each DS node allocated a certain amount of
   forwarding resources (buffer space and bandwidth). IP packets that
   wish to use the services provided by the AF PHB group are assigned by
   the customer or the provider DS domain into one or more of these AF
   classes according to the services that the customer has subscribed
   to. Further background about this capability and some ways to use it
   may be found in [Clark].

   Within each AF class IP packets are marked (again by the customer or
   the provider DS domain) with one of three possible drop precedence
   values.  In case of congestion, the drop precedence of a packet
   determines the relative importance of the packet within the AF class.
   A congested DS node tries to protect packets with a lower drop
   precedence value from being lost by preferably discarding packets
   with a higher drop precedence value.

   In a DS node, the level of forwarding assurance of an IP packet thus
   depends on (1) how much forwarding resources has been allocated to
   the AF class that the packet belongs to, (2) what is the current load
   of the AF class, and, in case of congestion within the class, (3)
   what is the drop precedence of the packet.

   For example, if traffic conditioning actions at the ingress of the
   provider DS domain make sure that an AF class in the DS nodes is only
   moderately loaded by packets with the lowest drop precedence value
   and is not overloaded by packets with the two lowest drop precedence
   values, then the AF class can offer a high level of forwarding
   assurance for packets that are within the subscribed profile (i.e.,
   marked with the lowest drop precedence value) and offer up to two
   lower levels of forwarding assurance for the excess traffic.

   This document describes the AF PHB group. An otherwise DS-compliant
   node is not required to implement this PHB group in order to be
   considered DS-compliant, but when a DS-compliant node is said to
   implement an AF PHB group, it must conform to the specification in
   this document.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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