RFC 2597 (rfc2597) - Page 2 of 11
Assured Forwarding PHB Group
Alternative Format: Original Text Document
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