TCP/IP Tutorial and Technical Overview - IBM Redbooks

If the path message reaches the first RSVP router, the router stores the IP
address from the last hop field in the message, which is the address of the
sender. Then the router inserts its own IP address into the last hop field, sends
the path message to the next router, and the process repeats until the message
has reached the receiver. At the end of this process, each router will know the
address from the previous router and the path can be accessed backwards.
Figure 8-4 shows the process of the path definition.
Sender
Path
Router 1
Address of
Sender
Figure 8-4 RSVP path definition process
Routers that have received a path message are prepared to process resource
reservations for a flow. All packets that belongs to this flow will take the same
way through the routers (the way that was defined with the path messages).
The status in a system after sending the path messages is as follows: All
receivers know that a sender can provide a special QoS for a flow and all routers
know about the possible resource reservation for this flow.
Now if a receiver wants to reserve QoS for this flow, it sends a reservation (Resv)
message. The reservation message contains the QoS requested from this
receiver for a specific flow and is represented by the filterspec and flowspec that
form the flow descriptor. The receiver sends the Resv message to the last router
in the path with the address it received from the path message. Because every
RSVP-capable device knows the address of the previous device on the path,
reservation messages travel the path in reverse direction toward the sender and
298 TCP/IP Tutorial and Technical Overview
Router 2
Address of
Router 1
Router 3
Address of
Router 2
Router 4
Path
Router 5
Address of
Router 3
Path
Receiver
Address of
Router 5