## 2 Mechanisms Specific to QoS and Multicast in Ad Hoc Networks
### Mechanisms for Resource Reservation/Release
If there are multiple sources in a multicast group, source specific resource reservation and thereby admission control may result in that the receivers are not connected to the same set of sources. From an application viewpoint this may not be acceptable. The third alternative is represented by most of the mesh protocols, for example QAMNet [11]. They have source specific reservation along the primary paths, additional forwarders schedule the packets without any guarantees. Thereby they preserve the connection semantics of the non-QoS multicast, all sources will connect to the same subset of receivers. However, the quality may vary. This could be generalized into protocols without admission control, but with resource reservation. All sources and receivers are admitted. If there are enough resources on the selected path, the necessary resources are reserved. Otherwise the source or receivers are connected as best effort [16]. Reservation implies state. Traditionally, state is classified into soft sate and hard state. Hard states is maintained through explicit set up and tear down, while soft state is maintained with explicit and periodic set up and timer based tear down. Most of the protocols in the survey use soft state, since this is well suited for the dynamic topology and the dynamic membership of multicast groups. CQMRP [12] and AQM [29] use hard state for reserving resource. In CQMRP [12], a QoS-error notification message is sent out when the link break occurs. The node will release the reserved resources for the multicast group at the time it receives a QoS-error message. Also, during connection  
Figure 1: (a, left) Classification of resource reservation/release mechanisms of QoS enable multicast protocols in ad hoc networks; (b, right) Classification of resource reservation/release mechanisms for shared tree/mesh. termination, the reserved resources are released by sending explicit resource release message. In AQM [29], a session is started and closed by a session initiator with the initiation message and the termination message, respectively. Additionally, a node informs its forwarders on the multicast graph when leaving a session by sending a leave message. Upon receiving the session termination message sent by the initiator, the forwarder frees the allocated resource. Reservation mechanisms can also be classified into three types: per-flow reservation (IntServ), per-class reservation (DiffServ), and hybrid reservation (IntServ over DiffServ). In the per-flow reservation method, resources are reserved for certain flows or sources. It follows the IntServ model. Most of protocols use this kind of reservation, e.g., [12, 15, 16, 30, 38, 42]. The DiffServ model is used by protocols in [11, 43]. With this technique, there is no flow reservation and the reservation is implicit. The hybrid reservation used in [16, 37] combines some features from both IntServ and DiffServ. In these protocols, the bandwidth is partitioned into fix reservation for accepted sources and shared reservation for other sources. The forwarding node provides IntServ and reserves bandwidth for every source that has been accepted. It provides DiffServ when it receives data packets from rejected sources or best-effort traffic and it has available bandwidth.