In order to associate traffic with its QoS requirements, protocols with support for QoS guarantees are typically connection oriented. An important step in connection establishment in such protocols is the admission control. The purpose of admission control is to calculate which network resources, such as buffer, bandwidth, and priority queues, are required in order to provide the QoS guarantees, and to determine if enough such resources are available. If this is the case, connection establishment reserves these resources for the new connection. When a client requests sending data from a source node to some destinations, an establishment message is typically sent out by the source, carrying the traffic parameters of the requested connection. This message follows the route of the connection, triggering an admission test in each node it traverses, and tentatively reserving resources in the node if all tests are passed. The resource reservation for the new connection must not endanger the guarantees given to the already established connections, so the admission test checks this condition with a local resource manager in each node. If the test fails, the connection request is denied. The establishment process is aborted by sending a reject message to the source. Otherwise, the establishment message is forwarded to the destinations.
The following approaches are a selection of current resource allocation schemes:
The process of adapting the resource allocation during the second pass of the Tenet Scheme is called resource relaxation. Effects of resource relaxation are extended end-to-end deadlines, relaxed jitter requirements, or decreased peak traffic rates or permissible burst sizes. This scheme can be applied to unicast efficiently, but is very cumbersome in multicast, because the amount of resources that can be relaxed in the nodes in one subtree depends on the resource requirements in another subtree, and vice versa. Thus, if relaxation is performed in the second pass, this can lead to low effective resource utilization. We therefore suggest a three-pass scheme that effectively relaxes resource requirements along the multicast tree.
This paper is structured as follows. In Section 2, we describe the issues related to providing real-time communication services in a multiparty environment. We describe connection establishment protocols and their interactions with schedulers at the nodes. (In this work, we focus on rate-controlled static priority scheduling at the nodes.) We will describe why traditional resource allocation protocols are not sufficient to support multicasting. In Section 3, we describe a novel three-pass establishment protocol that allows for resource relaxation on multicast connections. We compare the performance with existing establishment protocols in Section 4.