Session Startup
RSVP is often set up between two clients in a point-to-point situation (such as a
phone call), or between multiple senders and multiple recipients (multicast). It is
even possible for RSVP to negotiate a multipoint to single-point transmission. In
any case, the RSVP session startup process reserves resources in a single direction
only.To have full-duplex QoS guarantees, it is necessary for the session startup
process to be performed twice, once in each direction. For example, in the case
of setting up a VoIP call between two telephone users, it would usually be necessary
to set up two reservations, one each way, to guarantee good QoS for both
callers. On the other hand, a video stream would necessitate only a one-way
reservation. Since we are talking about RSVP in an AVVID environment, however,
let us consider the reservations required for a videoconference between two
people. Since we know that the voice and video components have different
bandwidth requirements, there would obviously need to be a separation of the
reservations for voice and video. Consider that both pieces (voice and video)
would need to be bi-directional, and now we have the need for a total of four
reservations between any two given routers. If you take that example and apply it
to a four-point any-to-any videoconference, you have 4 x (4 - 1) = 12 reservations
being managed on each router.When using the formula A x (B - 1) = C,
where A = Bi-directional Flows, B = Total number of Routers, and C = Total
Reservations per router, it isn’t difficult to realize the challenges you will face
when trying to scale RSVP in a large-scale production environment.This is especially
true in hub and spoke environments where all videoconferences between
remote sites must pass through a hub router (which, in order to guarantee true
end-to-end QoS, must maintain reservations for each flow).
Now that we’ve explored some of the background information that you’ll
need to keep in mind when considering how many sessions will be started, let us
step through the process of an RSVP session startup. In Figure 8.1, we have two
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228 Chapter 8 • Advanced QoS for AVVID Environments
AVVID clients across a RSVP-enabled network (perhaps, a desktop-to-desktop
videoconference). At the top we have Client A, which we will designate as the
sender, and at the bottom we have Client B, which we will consider the receiver.
Thus, after the reservation is set up, the data, whether it is voice, video, or something
else, will flow from Client A to Client B in a downstream manner.