RSVP Session Startup, Resv Messages
Client A
(Sender)
Client B
(Receiver)
Resv
Resv
Resv
In both the Path and Resv messages, the upstream hop is usually referred
to as the previous hop, and the downstream hop is called the next hop.
This terminology is derived from the reference point of the data moving
in a downstream direction, from sender to receiver.
If the reservation is declined, an error message is sent to the receiver, Client
B, and the Resv packet is not forwarded. Only when Client A receives a Resv
packet does it know that it can start sending data and guarantee a particular QoS
to the downstream receiver, Client B.
You may think it is odd for the entire RSVP process to begin with the
sender building the Path message to the receiver.This might be analogous to a
television network deciding it is time for you to watch your favorite show and
automatically turning on the TV. However, there is usually some kind of non-
RSVP request originating from the receiver to set up this flow. Some good
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Figure 8.2 RSVP Session Startup, Resv Messages
Client A
(Sender)
Client B
(Receiver)
Resv
Resv
Resv
Resv
230 Chapter 8 • Advanced QoS for AVVID Environments
examples of this type of request might be an H.323 conversation between IP
telephony applications, or an IGMP request to join a multicast group to watch a
video clip.
NOTE
Though it is necessary for the sender to first transmit the Path message
before the receiver can transmit the Resv message, RSVP is still considered
receiver-oriented. That is, the receiver of the data flow initiates and
maintains the actual resource reservation used for that flow.