RSVP operates in a two-pass manner. A transmitting source will advertise its content by sending RSVP path
messages through a multicast tree, indicating the bandwidth required for the content, the soft-state timeout interval,
and information about the upstream path ot the sender. Each receiver sends an RSVP reservation message
upstream into the multicast tree. This reservation message specifies the rate at which the receiver would llike to receive
the data from the source. When the reservation message reaches a router, the router adjusts its packet scheduler to
accommodate the reservation. It then sends a reservation upstream. The amount of bandwidth reserved upstream from
the router depends on the bandwidth reserved downstream.
Call Admission
The amount of bandwidth on a link that a router reserves should not exceed the link's capacity. Thus whenever a
router receives a new reservation message, it must first determine if its downstream links on the multicast tree can accommodate
the reservation. This admission test is performed whenever a router receives a reservation message.
If the admission test fails, the router rejects the reservation an;d returns an error message to the appropriate receiver(s).
RSVP does not define the admission test, but it assumes that the routers perform such a test and that RSVP can interact
with the test.
|
