ust traverse backwards towards the sender installing state in the router. Figure 3-21 shows an RSVP message flow. The use of soft states also prevents the network from retaining unneces- sary state information when routes change. If the route towards the flow destination changes as a consequence of the routing protocol operation, all of the datagrams will take a new route. Periodic PATH messages will also take the new route. Routers in the old route will not receive any more PATH messages, and therefore no RESV message will be received either. At that point, the state stored in those routers will time out and be deleted. Differentiated Services (DiffServ) We have seen that RSVP and the integrated services architecture provide different treatment for different flows in the routers. Routers filter packets based on the information received in RSVP messages. For instance, as long as flows are defined by the destination address and destination port num- ber of the packets, routers have to examine the destination address and Sender Router1 Router2 Receiver Figure 3-21 RSVP message flow. (1) PATH: Sender (2) PATH: (3) PATH: Sender Sender Router 1 Router 1 The information received Router 2 in the PATH message is used to route the RESV (4) RESV: message towards the Router 2 (5) RESV: sender Router 1 (1) RESV: Router 1 Sender Sender Sender Packets of the flow Packets of the flow Packets of the flow Packets of the flow Packets of the flow Packets of the flow Chapter 3 port number of every datagram and map it to state information. The differ- entiated services (DiffServ) architecture [RFC 2475] simplifies the task by defining several traffic classes with different priority levels. Packets are tagged at the edge of the network with the required priority level. Routers in the network take their cue from these tags; each tag is associated with a particular way of handling packets (referred to as Per-Hop Behavior [PHB]), and the router only has to read the tag and look up its PHB. Examples of standard PHBs include expedited forw