RSVP Operation

The operation of RSVP is shown in Figure 14-12. Because the routed path from sender to receiver may be asymmetric, such as when the path for traffic from Host A to Host B is via R1, R2, R3, and the return path is via R3, R1, senders must prime the routers to expect reservation requests for a particular flow. This is accomplished by using path messages from the sender to the first hop router toward the receiver.

Figure 14-12. RSVP Operation

Figure 14-12. RSVP Operation

The first hop router inserts its own address as the path message's last hop, and forwards it to the next hop router. This "last hop" field tells the next hop router where to forward a reservation message for this particular flow.

This hop-by-hop processing of path messfages continues until the receiver is reached. At this point, if the receiver sends a reservation message toward the sender, each router knows how to forward the reservation message back to the sender so that it flows through each router in the path from sender to receiver. The ip rsvp neighbor command can be used for neighboring routers from which the local router will accept reservations.

If an error occurs in the processing of path or reservation messages, a path-error or reservation-error message is generated and is routed hop-by-hop toward the sender or receiver, respectively. Each error message includes objects sufficient to uniquely identify the path or reservation message causing the error, and it always includes the ERROR-SPEC object. There are several possible errors that can occur:

• Admission failureReservation could not be granted due to unavailable resources.

• Administrative rejectionPolicy forbids reservation.

• No Path information for Reservation message.

• No Sender information for Reservation message.

• Conflicting reservation style The style does not match the existing state.

• Unknown reservation style.

• Conflicting destination ports Zero and non-zero destination port fields have appeared for the same session.

• Conflicting sender portsZero and non-zero source port fields have appeared for the same session.

• Service preemptedHard state already exists.

• Unknown object class.

• Unknown object type.

• Traffic Control ErrorMalformed requests have been issued.

• Traffic Control System Error.

• RSVP System ErrorImplementation-dependent debugging messages are present.

Paths and reservations have an associated refresh period, which is generally randomized within a range to avoid congestion issues associated with synchronization of control messages. If this period expires without a refresh of the reservation state, the reservation is expired. However, to liberate resources in a more timely manner, a reservation TEARDOWN message is used to remove the reservation even before its soft-state expires.

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