11. Multicast Considerations
11. Multicast Considerations
A multicast group address, as defined in the original Internet architecture, is an identifier of a grouping of topologically independent receiver host locations. The address encoding itself does not determine the location of the receiver(s). The multicast routing protocol, and the network-based state the protocol creates, determine where the receivers are located.
In the context of LISP, a multicast group address is both an EID and a Routing Locator. Therefore, no specific semantic or action needs to be taken for a destination address, as it would appear in an IP header. Therefore, a group address that appears in an inner IP header built by a source host will be used as the destination EID. The outer IP header (the destination Routing Locator address), prepended by a LISP router, will use the same group address as the destination Routing Locator.
Having said that, only the source EID and source Routing Locator need to be dealt with. Therefore, an ITR merely needs to put its own IP address in the source 'Routing Locator' field when prepending the outer IP header. This source Routing Locator address, like any other Routing Locator address, MUST be globally routable.
Therefore, an EID-to-RLOC mapping does not need to be performed by an ITR when a received data packet is a multicast data packet or when processing a source-specific Join (either by IGMPv3 or PIM). But the source Routing Locator is decided by the multicast routing protocol in a receiver site. That is, an EID-to-RLOC translation is done at control time.
Another approach is to have the ITR not encapsulate a multicast packet and allow the packet built by the host to flow into the core even if the source address is allocated out of the EID namespace. If the RPF-Vector TLV [RFC5496] is used by PIM in the core, then core
routers can RPF to the ITR (the locator address, which is injected into core routing) rather than the host source address (the EID address, which is not injected into core routing).
To avoid any EID-based multicast state in the network core, the first approach is chosen for LISP-Multicast. Details for LISP-Multicast and interworking with non-LISP sites are described in [RFC6831] and [RFC6832].