2.1 RSVP Messages

       Previous       Incoming           Outgoing             Next
       Hops           Interfaces         Interfaces           Hops

       _____             _____________________                _____
      |     | data -->  |                     |  data -->    |     |
      |  A  |-----------| a                 c |--------------|  C  |
      |_____| Path -->  |                     |  Path -->    |_____|
              <-- Resv  |                     |  <-- Resv     _____
       _____            |       ROUTER        |           |  |     |
      |     |  |        |                     |           |--|  D  |
      |  B  |--| data-->|                     |  data --> |  |_____|
      |_____|  |--------| b                 d |-----------|
               | Path-->|                     |  Path --> |   _____
       _____   | <--Resv|_____________________|  <-- Resv |  |     |
      |     |  |                                          |--|  D' |
      |  B' |--|                                          |  |_____|
      |_____|  |                                          |

                         Figure 9: Router Using RSVP

Figure 9 illustrates RSVP's model of a router node. Each data flow arrives from a "previous hop" through a corresponding "incoming interface" and departs through one or more "outgoing interface"(s). The same interface may act in both the incoming and outgoing roles for different data flows in the same session. Multiple previous hops and/or next hops may be reached through a given physical interface; for example, the figure implies that D and D' are connected to (d) with a broadcast LAN.

There are two fundamental RSVP message types: Resv and Path.

Each receiver host sends RSVP reservation request (Resv) messages upstream towards the senders. These messages must follow exactly the reverse of the path(s) the data packets will use, upstream to all the sender hosts included in the sender selection. They create and maintain "reservation state" in each node along the path(s). Resv messages must finally be delivered to the sender hosts themselves, so that the hosts can set up appropriate traffic control parameters for the first hop. The processing of Resv messages was discussed previously in Section 1.2.

Each RSVP sender host transmits RSVP "Path" messages downstream along the uni-/multicast routes provided by the routing protocol(s), following the paths of the data. These Path messages store "path state" in each node along the way. This path state includes at least the unicast IP address of the previous hop node, which is used to route the Resv messages hop-by-hop in the reverse direction. (In the future, some routing protocols may supply reverse path forwarding information directly, replacing the reverse-routing function of path state).

A Path message contains the following information in addition to the previous hop address:

• Sender Template

  A Path message is required to carry a Sender Template, which describes the format of data packets that the sender will originate. This template is in the form of a filter spec that could be used to select this sender's packets from others in the same session on the same link.

  Sender Templates have exactly the same expressive power and format as filter specs that appear in Resv messages. Therefore a Sender Template may specify only the sender IP address and optionally the UDP/TCP sender port, and it assumes the protocol Id specified for the session.

• Sender Tspec

  A Path message is required to carry a Sender Tspec, which defines the traffic characteristics of the data flow that the sender will generate. This Tspec is used by traffic control to prevent over-reservation, and perhaps unnecessary Admission Control failures.

• Adspec

  A Path message may carry a package of OPWA advertising information, known as an "Adspec". An Adspec received in a Path message is passed to the local traffic control, which returns an updated Adspec; the updated version is then forwarded in Path messages sent downstream.

Path messages are sent with the same source and destination addresses as the data, so that they will be routed correctly through non-RSVP clouds (see Section 2.9). On the other hand, Resv messages are sent hop-by-hop; each RSVP-speaking node forwards a Resv message to the unicast address of a previous RSVP hop.