A multicast packet is a network packet meant to be received by more than one host, but not by all hosts. This functionality is obtained by assigning special hardware addresses to groups of hosts. Packets directed to one of the special addresses should be received by all the hosts in that group. In the case of Ethernet, a multicast address has the least significant bit of the first address octet set in the destination address, while every device board has that bit clear in its own hardware address.

The tricky part of dealing with host groups and hardware addresses is performed by applications and the kernel, and the interface driver doesn’t need to deal with these problems.

Transmission of multicast packets is a simple problem because they look exactly like any other packets. The interface transmits them over the communication medium without looking at the destination address. It’s the kernel that has to assign a correct hardware destination address; the hard_header device method, if defined, doesn’t need to look in the data it arranges.

The kernel handles the job of tracking which multicast addresses are of interest at any given time. The list can change frequently, since it is a function of the applications that are running at any given time and the user’s interest. It is the driver’s job to accept the list of interesting multicast addresses and deliver to the kernel any packets sent to those addresses. How the driver implements the multicast list is somewhat ...