Assured Forwarding in Differentiated Services

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8.6.4.1 Assured Forwarding in Differentiated Services

The Assured Forwarding standard for Per-Hop Behavior Differentiated Services is defined in RFC 2597. In AF, two basic properties define how each packet will be forwarded. The standard defines four Classes and three different values for Drop Precedence. The Class value is essentially a forwarding priority. Packets with the same Class value are all queued together. The standard requires that the packets of individual conversations be forwarded in the same order that they are received, as long as they are all of the same Class. The most common way to implement AF is to give a separate queue to each Class. This allows the network to ensure that flows from different Classes do not interfere with one another. It also permits higher-priority Classes to receive more bandwidth from the network by increasing the amount of data taken from the more important queues each time the router takes packets from them. In addition to the four Classes, AF defines three different types of Drop Precedence. This number simply tells the router which packets to drop first in case of congestion. When the Class queue fills up and the router needs to start dropping packets, the ones with lower Drop Precedence values are protected. The router should scan through the queue and drop the packets with the highest Drop Precedence values first. If dropping the packets does not alleviate the congestion problem, then the router should drop all of the next- highest Drop Precedence packets before dropping the ones with the lowest Drop Precedence values. In this way, AF can give important data streams better treatment as they pass through the network. Note, however, that AF does not necessarily guarantee a particular fraction of the total bandwidth for any one Class. It also doesnt give guaranteed end-to-end performance characteristics for specific data flows. Furthermore, it does not have the ability to give direct control over parameters such as jitter or bandwidth. It is merely a method for providing Preferential Delivery.

8.6.4.2 Expedited Forwarding in Differentiated Services