AMT Operation

AMT is used to create multicast tunnels dynamically between multicast-enabled networks across islands of unicast-only networks. To do this, several steps occur sequentially.

1. First the AMT relay (typically a router) advertises an anycast address prefix and route into the unicast routing infrastructure.
2. Next the AMT gateway (a host) sends out AMT relay discovery messages to the nearest AMT relay reachable across the unicast-only infrastructure. To reduce the possibility of replay attacks or dictionary attacks, the relay discovery messages contain a cryptographic nonce. A cryptographic nonce is a random number used only once.
3. The closest relay in the topology receives the AMT relay discovery message and returns the nonce from the discovery message in an AMT relay advertisement message. This enables the gateway to learn the relay's unique IP address. The AMT relay now has an address to use for all subsequent (S,G), entries it will join.
4. The AMT gateway sends an AMT request message to the AMT relay's unique IP address to begin the process of joining the (S,G).
5. The AMT relay sends an AMT membership query back to the gateway.
6. The AMT gateway receives the AMT query message and sends an AMT membership update message containing the IGMP join messages.
7. The AMT relay sends a join message toward the source to build a native multicast tree in the native multicast infrastructure.
8. As packets are received from the source, the AMT relay replicates the packets to all interfaces in the outgoing interface list, including the AMT tunnel. The multicast traffic is then encapsulated in unicast AMT multicast data messages.
9. To maintain state in the AMT relay, the AMT gateway sends periodic AMT membership updates.
10. After the tunnel is established, the AMT tunnel state is refreshed with each membership update message sent. The timeout for the refresh messages is 240 seconds.
11. When the AMT gateway leaves the group, the AMT relay can free resources associated with the tunnel.

Note the following operational details:

• The AMT relay creates an AMT pseudo interface (tunnel interface). AMT tunnel interfaces are implemented as generic UDP encapsulation (ud) logical interfaces. These logical interfaces have the identifier format ud-fpc/pic/port.unit.
• All multicast packets (data and control) are encapsulated in unicast packets. UDP encapsulation is used for all AMT control and data packets using the IANA reserved UDP port number (2268) for AMT.
• The AMT relay maintains a receiver list for each multicast session. The relay maintains the multicast state for each gateway that has joined a particular group or (S,G) pair.