Multicasting is the process that 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 link layer technology to support a variety of protocols, including IP. ATM based IP hosts and routers use a Multicast Address Resolution Server (MARS) to support IP multicast over the ATM Forum"s UNI 3.0/3.1 point to multipoint connection service. Clusters of endpoints share a MARS and use it to track and disseminate information identifying the nodes listed as receivers for given multicast groups. This allows endpoints to establish and manage point to multipoint VCs when transmitting to the group.
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 and to define specific endpoint behaviors for managing point to multipoint VCs to achieve multicasting of layer 3 packets. 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.
Protocol Structure
| Address family (2 bytes) | Protocol identification (7 bytes) | Reserved (3 bytes) | ||||
| Checksum (2 bytes) | Extensions offset (2 bytes) | Operation code (2 bytes) | Type & length of source ATM Number (1 byte) | Type & length of source ATM subaddress (1 byte) | ||
- 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.
Sponsor Source: MARS is defined by IETF (http://www.ietf.org) RFC2022
Reference: http://www.javvin.com/protocol/rfc2022.pdf: Support for Multicast over UNI 3.0/3.1 based ATM Networks