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Each source that implements RSVP will transmit Path messages
along the route that the data will follow. At each node along the
path, the path state is stored. The path state is used to route the
reservation messages. A fundamental component of the path state is
the IP address of the previous hop. In Figure 10-11, the previous
hop for router B is router A, as shown. The path message contains
other required components and possibly optional components for the
establishment of the path state. The two required components are the
Sender Template and the Sender Tspec. Sender Template contains a
description of the structure of the packets that the source sends in
the form of a filter spec. This implies that the sender template
will contain the IP address of the source and possibly the UDP port
the source is using. The Sender Tspec defines the characteristics of
the traffic the source will originate in order to prevent
over-reservation. An optional component of a path message is the
Adspec. An Adspec carries One Pass With Advertising (OPWA) information. As the Path message travels
towards the receiver, information is collected at each node so the
receiver is able to predict the end-to-end service. This information
is referred to as an advertisement, hence, the name Adspec. When the
path message arrives at a node, the Adspec is passed to the local
traffic control module. The local traffic control module updates the
Adspec which is sent in a path message to the next downstream
node. |
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