Static Internet multicast
M. Ohta, Jon Crowcroft · 1998
The current IP Multicast [1] model appears to achieve a level of simplicity by extending the IP unicast addressing model (historically the classful A,B, and C net numbers) from the mask and longest match schemes of CIDR[2], with a new classful address space, class D. The routing systems have been also built in a deceptively simple way in one of three manners - either broadcast and prune (DVMRP[3], Dense Mode PIM[17]), destination list based tree computation (MOSPF[4]) or single centered trees (current sparse mode PIM and CBT[16]). The multicast service creates the illusion of a spectrum that one can "tune in to", as an application writer. Due to this view, many have seen the multicast pilot service, the Mbone, as a worldwide Ethernet, where simple distributed algorithms can be used to allocate "wavelengths" and advertise them through "broadcast" on a channel (the session directory), associated with a spectrum. These three pieces of the picture have tempted people to construct a distributed architecture for a number of next level services that cannot work at more than a modest scale, since they ignore the basic spirit of location independence for senders and receivers of IP packets, whether unicast or multicast. The problem is that many of these services are attempting to group activities at source, when it is only at join time that user grouping becomes apparent (if you like, multicast usage is a good example of very late binding). These services include Address Allocation and Session Creation, Advertisement and Discovery. This memo proposes approaches to solve some current multicast problems rather statically with DNS[13] and URL based approach, and avoid the misguided pitfalls of trying to use address allocation to implement traffic aggregation for different sources or aggregation of multicast route policy control through control of such aggregated sources. Note that a minor level of aggregation occurs in applications which source cumulative layered data (e.g. audio/video/game data[18] or layered multimedia conferencing tools[5] such as vic/rat and ric[6]) - this memo is orthogonal to such an approach, which in any case ony results in a small constant factor reduction in state. A lot of the IP multicast additional pieces of baggage are associated with the multimedia conferencing on Mbone - however, the commercial internet use of multicast includes many other applications - for these, SDR may not be the best directory model.