5.2.2 Per-Hop Behaviour

After conditioning and classification, flow is subjected to forwarding behavior based on its DSCP value. This forwarding behavior is implemented using queuing, scheduling and dropping mechanisms. As the name suggests, this is done on a per-hop basis.

PHB is selected at a node by a mapping of the DSCP of the received packet, by using a DSCPÃ PHB map. This mapping may be 1Ã 1 or NÃ 1. There are two standard PHB groups: Assured Forwarding (AF) PHB group and Expedited Forwarding (EF) PHB. Also, to preserve partial backward compatibility with the IP precedence bits, Class Selector PHB has also been defined, and these are of the form xxx000.

AF PHB group:

The AF PHB group is further classified into 4 classes, each with 3 drop precedence (DP) levels. Class AF1 has the highest priority and resources available to it, and AF4 has relatively the lowest. Within each class AFx, the subclass with DP 1 will have the lowest dropping probability while the one with DP 3 will have the highest.

It is important to note that a DS node is not allowed to reorder the packets belonging to a particular AF class, irrespective of the DP of the packet. This service is important from the point of view of certain reordering sensitive protocols like IPSec and L2TP [9].

It is recommended that the AF implementation should attempt to minimize long term congestion while allowing short-term congestion within each class.

EF PHB:

EF PHB provides low loss, low jitter, low delay service.

The recommended PHB to DSCP mapping is as follows:

Table 51 DSCP values [10]