In this chapter has been shown that it is possible to set up a point-to-multipoint VCC in ATM. A specification for Classical IP and ARP over ATM has been discussed as well. But still, it is not trivial to use the point-to-multipoint connections of ATM for IP multicasting. IP multicasting brings along another complication in the form of the abstract multicast address that must be resolved to a group of ATM addresses or an ATM point-to-multipoint VCC (identified by the VPI/VCI pair).
LANE does support a multicast service, the same as Ethernet, but LANE also emulates some of the physical properties from Ethernet. Some specific IP applications (e.g. NFS) generate significantly larger IP packets than the MTU size of Ethernet. This causes overhead in the hosts, whose IP entities must split the IP packet into parts and re-assemble them on the other end. This increases the chance for corrupt packets at IP level, because of lost Ethernet frames. Analogous to physical Ethernet, the LANE multicast method is actually a broadcast on ATM level within the ELAN, where the LANE entity in the hosts simply filters out the local subscribed addresses. Unlike physical Ethernet, LANE must do the filtering in software, which causes processing overhead. In the next chapter, a multicast address resolution scheme between layer 3 and ATM is discussed and two possibilities for the forwarding of IP multicast transmissions within the LIS are compared.
For an implementation of an mIP/ATM system, an implementation of LANE might be a good place to start, because some of the entities in LANE appear to offer some of the same functions that an IP multicasting system would need (copying and forwarding of multicast packets).
IPv6 [DH95] has been explicitly left out of this discussion, even
though IPv6 is the future of the Internet and supports multicast and
anycast addressing by default. IPv6 is not yet generally used, so very
little is known about its operation. Furthermore, the attempts at
formulating an IPv6 over ATM specification are still
``work in
progress'',
and not progressing very quickly
.