Multicasting is the process whereby a source host or protocol entity sends a packet to multiple destinations simultaneously using a single, local 'transmit' operation. ATM is being utilized as a new link layer technology to support a variety of protocols, including IP. The MARS protocol has two broad goals: to define a group address registration and membership distribution mechanism that allows UNI 3.0/3.1 based networks to support the multicast service of protocols such as IP and to define specific endpoint behaviors for managing point to multipoint VCs to achieve multicasting of layer 3 packets. The Multicast Address Resolution Server (MARS) is an extended analog of the ATM ARP Server. It acts as a registry, associating layer 3 multicast group identifiers with the ATM interfaces representing the group's members. MARS messages support the distribution of multicast group membership information between MARS and endpoints (hosts or routers). Endpoint address resolution entities query the MARS when a layer 3 address needs to be resolved to the set of ATM endpoints making up the group at any one time. Endpoints keep the MARS informed when they need to join or leave particular layer 3 groups. To provide for asynchronous notification of group membership changes, the MARS manages a point to multipoint VC out to all endpoints desiring multicast support. Each MARS manages a cluster of ATM-attached endpoints.
(Compliant with IETF RFC2022.)

The format of the header is shown in the following illustration:

Address family
Defines the type of link layer addresses being carried.

Protocol ID
Contains 2 subfields:
16 bits, protocol type
40 bits Optional SNAP extension to protocol type.

Reserved
This reserved field may be subdivided and assigned specific meanings for other control protocols indicated by the version number.

Checksum
This field carries a standard IP checksum calculated across the entire message.

Extension offset
This field identifies the existence and location of an optional supplementary parameters list.

Operation code
This field is divided into 2 sub fields: version and type. Version indicates the operation being performed, within the context of the control protocol version indicated by mar$op.version.

Type and length of ATM source number
Information regarding the source hardware address.

Type and length of ATM source subaddress
Information regarding the source hardware subaddress