Support for Multicasting Is Optional

Support for multicasting is defined as follows:

• A switch may or may nor implement the Multicast Forwarding Table. If it does not, then the switch actions are those defined in “When the Table Lookup Fails” on page 677.

• A router may or may not support multicast packet routing. How this is determined is outside the scope of the specification.

• A HCA may or may not support multicasting. This can be determined by performing a Query HCA verb call. The following parameters related to multicasting are returned:

  - The maximum number of multicast groups supported by the HCA. This is zero if the HCA does not support UD multicast.

  - The maximum number of QPs that can be attached to multicast groups for this HCA. This value is zero if the HCA does not support IBA unreliable multicast.

  - The maximum number of QPs per multicast group supported by the HCA. This is zero if the HCA does not support UD multicast.

  - Ability of the HCA to support raw packet multicast.

• TCAs may or may not support multicast operations.

If the CI supports UD multicast, it must support at least one multicast group.

Relative to CAs, the specification contains the following statement:

“It is expected that any implementation of the Unreliable Datagram transport service will trivially support the generation of multicast packets.”

The author believes that this refers to the fact that a multicast message (i.e., a message with a multicast destination address) can be sent from any UD QP (even one that is not a member of a multicast group).