2.10 Host Model

Before a session can be created, the session identification (DestAddress, ProtocolId [, DstPort]) must be assigned and communicated to all the senders and receivers by some out-of-band mechanism. When an RSVP session is being set up, the following events happen at the end systems.

H1

A receiver joins the multicast group specified by DestAddress, using IGMP.

H2

A potential sender starts sending RSVP Path messages to the DestAddress.

H3

A receiver application receives a Path message.

H4

A receiver starts sending appropriate Resv messages, specifying the desired flow descriptors.

H5

A sender application receives a Resv message.

H6

A sender starts sending data packets.

There are several synchronization considerations.

• H1 and H2 may happen in either order.

• Suppose that a new sender starts sending data (H6) but there are no multicast routes because no receivers have joined the group (H1). Then the data will be dropped at some router node (which node depends upon the routing protocol) until receivers(s) appear.

• Suppose that a new sender starts sending Path messages (H2) and data (H6) simultaneously, and there are receivers but no Resv messages have reached the sender yet (e.g., because its Path messages have not yet propagated to the receiver(s)). Then the initial data may arrive at receivers without the desired QoS. The sender could mitigate this problem by awaiting arrival of the first Resv message (H5); however, receivers that are farther away may not have reservations in place yet.

• If a receiver starts sending Resv messages (H4) before receiving any Path messages (H3), RSVP will return error messages to the receiver.

  The receiver may simply choose to ignore such error messages, or it may avoid them by waiting for Path messages before sending Resv messages.

A specific application program interface (API) for RSVP is not defined in this protocol spec, as it may be host system dependent. However, Section 3.11.1 discusses the general requirements and outlines a generic interface.