## 1 Introduction
### Multicast Model  
The original wireline multicast protocols were based on a stringent multicast model. According to this model, a source could send data packets to multicast group without being part of the group, a sender should not need to know the address and identity of the receivers, and any receiver or source could join and leave the group dynamically. The multicast model defines the expectations applications will have to the distribution mechanism and it therefore imposes restrictions on the design space of the QoS multicast protocols. A protocol that does not adhere to the model can have a simpler design and potentially be more efficient. However, the applications may be more complex or costly to develop. The applications must either be tailor-made for the particular protocol, or an additional adaptation layer must be developed.  
The QoS requirement for multicast may change the semantics of multicast when there are multiple sources per group. In a non-QoS setting, all receivers in a multicast group receive information from all sources as long as the network is not partitioned. Adding QoS implies that potentially there may not be enough resources on a logical link. The admission to these bottleneck links will depend on the sequence sources and receivers joining the group. If we assume admission control is used, it is then not given that all receivers will get information from the same set of sources. Translated into a video conference setting, there is a risk that the participants do not have the same view of the other participants. A shared distribution mechanism has the same potential problem for inconsistent admission control.  
For shared trees, resources can be reserved either as shared or source specific. If the resources are shared by all sources, there is no difference between the semantics for QoS and non-QoS. The admission control is then whether a receiver or source can use the path to the tree. The reservation will not be performed in the rest of the tree. The result is that a source is either connected to all receivers or to none. Likewise, a receiver is connected either to all sources or to none. This is equivalent to whether a path exists in the non-QoS case. However, a shared resource reservation between multiple sources has a limited applicability for offering QoS. The exception is when the application imposes a strict ordering of when the sources use the resources, like voice in a conference.  
To preserve the multicast semantics, admission control should be limited to whether a source or receivers should be allowed to join with given QoS requirements or to join without any requirements. To our knowledge the interaction between admission control/resource reservation and the group semantics has been discussed only to a limited degree.  
There is a close relationship among the semantics of the admission control, adherence to the multicast model, the type of resource reservation/QoS offered, and the complexity of the protocols. The lack of adherence to the multicast model also indicates the potential need for special design of multicast enabled applications. As shown in Table 2 (see Appendix), many of the protocols do not appear to be designed in accordance with the multicast model.