Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 328
Path Messages

When a sender transmits a Path message, it will be received by routers along the path. A router inserts its own IP address as the message’s last hop. As the Path message is propagated through the network, each router notes the previous router’s address and then inserts its own IP address before forwarding the Path message on. Having each router note the last router’s IP address, for a flow, allows a router that receives a reservation request to know how to forward that request back in the direction of the sender. This ensures that the receivers will take the correct path for a particular flow. Why? Most network designs have more than one path and a receiver may make a reservation in a path that the sender did not specify.

Path messages can be sent at any time and routers maintain the path state in what is known as a soft state. Routers maintain the path information only for a certain period of time, after which they will delete the state. This allows for dynamic flexibility in the path. A new path (via topology changes) may be set up that renders the old path obsolete. A router may fail in the path and no alternate path is available; therefore, the path information is obsolete and needs to be deleted.