RSVP defines a "session" to be a data flow with a particular destination and transport-layer protocol. RSVP treats each session independently, and this document often omits the implied qualification "for the same session".
An RSVP session is defined by the triple: (DestAddress, ProtocolId [, DstPort]). Here DestAddress, the IP destination address of the data packets, may be a unicast or multicast address. ProtocolId is the IP protocol ID. The optional DstPort parameter is a "generalized destination port", i.e., some further demultiplexing point in the transport or application protocol layer. DstPort could be defined by a UDP/TCP destination port field, by an equivalent field in another transport protocol, or by some application-specific information.
Although the RSVP protocol is designed to be easily extensible for greater generality, the basic protocol documented here supports only UDP/TCP ports as generalized ports. Note that it is not strictly necessary to include DstPort in the session definition when DestAddress is multicast, since different sessions can always have different multicast addresses. However, DstPort is necessary to allow more than one unicast session addressed to the same receiver host.
Figure 2 illustrates the flow of data packets in a single RSVP session, assuming multicast data distribution. The arrows indicate data flowing from senders S1 and S2 to receivers R1, R2, and R3, and the cloud represents the distribution mesh created by multicast routing. Multicast distribution forwards a copy of each data packet from a sender Si to every receiver Rj; a unicast distribution session has a single receiver R. Each sender Si may be running in a unique Internet host, or a single host may contain multiple senders distinguished by "generalized source ports".
Senders Receivers
_____________________
( ) ===> R1
S1 ===> ( Multicast )
( ) ===> R2
( distribution )
S2 ===> ( )
( by Internet ) ===> R3
(_____________________)
Figure 2: Multicast Distribution Session
For unicast transmission, there will be a single destination host but there may be multiple senders; RSVP can set up reservations for multipoint-to-single-point transmission.