RFC 2597 (rfc2597) - Page 2 of 11

Assured Forwarding PHB Group

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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [Bradner].




Heinanen                    Standards Track