## 2 Mechanisms Specific to QoS and Multicast in Ad Hoc Networks
### Mechanisms for Resource Reservation/Release  
To achieve QoS, resources must be reserved, either explicitly or implicitly. In implicit resource reservation, the system is viewed as a black box and the number of flows into the system is restricted to those that can achieve the target QoS. There are no resources associated with a particular flow. When a new source or receiver is to be admitted into the system, end-to-end probing is used to determine whether the resulting QoS is acceptable. M-CAMP [30] is an example of a protocol using implicit reservation. With explicit resource reservation, each node associates resources to a particular source or a multicast group. Transmission bandwidth is the limiting resource and therefore the resource reserved. Most of the protocols use explicit reservation. As mentioned in the description part of the routing methods, many protocols explore alternative paths. If the admission control is performed at intermediate nodes, resource reservation must also be done in conjunction with the admission control. In both cases, there is a likelihood that an allocated reservation is not being used. It may be on a path that is not selected, or a node downstream might reject the join request. Many protocols therefore divide the reservation into stages, like \"possible\", \"likely\", and a final commit stage when it is determined that packets will actually flow through the node [15, 16, 30, 42]. This ensures that resources are not unnecessarily blocked. The protocol QoS-ODMPR [15] uses three states (explored, registered, and reserved) for resource reservation. The path discovery process reserves \"explored\" resources. The response back from the receivers changes the state of the selected forwarding nodes to \"registered\". Only when traffic actual flows through the node, the reservation state changes to \"reserved\". Similarly, E-QMR [16] uses three states called free, allocate, and reserved. It is a policy issue if the sum of the resource reservation in the various stages can be more than the total resources. If most reservations are not committed, oversubscription could be beneficial to avoid unnecessary blocking. The final commit stage ensures that the sum of reserved resources is within acceptable limits. To our knowledge, this policy consideration has not been explored. The location of the resource reservation is a design choice that depends on the underlying MAC protocol. With a shared access, a node's transmissions will affect the available capacity at neighboring nodes. Two neighboring nodes may not have the same interference patterns and will therefore not necessarily have the same available bandwidth. It is therefore not given that free resources at a node also are free at the neighboring nodes. The protocols only perform reservation at the nodes in the forwarding path. However, the resource estimation may consider the impact of reservation at other nodes, for example in Hu [17]. However, this will not reflect the impact of reserving resources at the node on flows forwarded from other nodes. As discussed in the Introduction section, there are two types of reservation for shared tree/mesh, namely a shared reservation, which is common for all sources, and a source specific reservation. A shared resource reservation reflects applications where only one source is active. A typical case is voice conferences. One protocol, ODQMM [38], offers both types and with different protocol functions. Reservation for the shared resources only travels to the existing distribution tree, while source specific travels to all nodes in the distribution tree. Several of the protocols do not make this distinction and one must infer the type based on how far the reservation travels in the distribution tree/mesh. Figure 1(a) illustrates the classification of resource reservation/release mechanisms of QoS enable multicast protocols in ad hoc networks. Figure 1(b) illustrates the types of reservation/release mechanisms for shared tree/mesh.