In this chapter, we describe a three-pass resource allocation protocol for multiparty real-time communications. To simplify the following discussion, we assume that all nodes use RCSP schedulers, and that the local admission control mechanisms are designed accordingly. Although the following discussion uses RCSP as a basis, it remains valid for other scheduling algorithms as well.
The three-pass establishment process consists of a request pass, followed by a reply pass, and then by a relaxation pass. During the request pass, an establish request message is sent along the - previously determined - multicast route from the sender to the receivers of the multiparty connection. Upon receipt of the request message, every node tentatively allocates as many local resources (bandwidth, buffer space, scheduling priority) as possible. The node then forwards the request message down the multicast tree. If the node is a branch node, each descendant is forwarded a copy of the request message.
When the request message reaches a receiver node, the performance requirements specified in the request message are compared in the final acceptance test with the ones provided by the connection, and an appropriate reply message is returned to the sender in the second pass, the reply pass. What needs to be done when a receiver rejects a connection establishment request depends on the establishment semantics (all-or-none establishment, partial establishment, or variations thereof). In the following description, we therefore assume that all receivers anccept the establishment request. In particular, if the performance requirements are met, an establishment-accept message is generated and returned; otherwise an establishment-reject message. During the reply pass, the establishment reply messages generated at the receiver nodes travel back to the sender. Each node in the multicast tree collects the reply messages from each descendant and forwards a single reply to its parent. The content of this message depends on the result of the establishment request in the subtree and on the semantics underlying the establishment request.
When the reply message reaches the sender, the relaxation pass is initiated, and the resources are relaxed from source to destinations. At the end of this pass, each node has allocated the minimum amount of resources necessary to satisfy the performance requirements of the connection. If appropriate priorities are assigned to establishment messages, data transmission can be started immediately after starting the relaxation pass. This reduces the establishment overhead, and has the three-pass mechanism perform as if it consisted only of two passes. In the following, we describe in detail each pass of a channel establishment. To explain this protocol, we will use the example of the multiparty connection in Figure 2.
Figure 2: Example of a Multiparty Connection.