9.1 Overview

The DiffServ architecture provides QoS by dividing traffic into different categories, marking each packet with a code point that indicates its category, and scheduling packets according accordingly. The DiffServ module in ns can support four classes of traffic, each of which has three dropping precedences allowing differential treatment of traffic within a single class. Packets in a single class of traffic are enqueued into one corresponding physical RED queue, which contains three virtual queues (one for each drop precedence).

Different RED parameters can be configured for virtual queues, causing packets from one virtual queue to be dropped more frequently than packets from another. A packet with a lower dropping precedence is given better treatment in times of congestion because it is assigned a code point that corresponds to a virtual queue with relatively lenient RED parameters.

The DiffServ module in ns has three major components:

Policy:

Policy is specified by network administrator about the level of service a class of traffic should receive in the network.

Edge router:

Edge router marks packets with a code point according to the policy specified.

Core router:

Core router examines packets' code point marking and forwarding them accordingly.

DiffServ attempts to restrict complexity to only the edge routers.