AMT Applications

Transit service providers have a challenge in the Internet because many local service providers are not multicast-enabled. The challenge is how to entice content owners to transmit video and other multicast traffic across their backbones. The cost model for the content owners might be prohibitively high if they have to pay for unicast streams for the majority of their subscribers.

Until more local providers are multicast-enabled, there is a transition strategy proposed by the Internet Engineering Task Force (IETF) and implemented in open source software. This strategy is called Automatic IP Multicast Without Explicit Tunnels (AMT). AMT involves setting up relays at peering points in multicast networks that can be reached from gateways installed on hosts connected to unicast networks.

Without AMT, when a user who is connected to a unicast-only network wants to receive multicast content, the content owner can allow the user to join through unicast. However, the content owner incurs an added cost because the owner needs extra bandwidth to support the unicast subscribers.

AMT allows any host to receive multicast. On the client end is an AMT gateway that is a single host. Once the gateway has located an AMT relay, which might be a host but is more typically a router, the gateway periodically sends Internet Group Management Protocol (IGMP) messages over a dynamically created UDP tunnel to the relay. AMT relays and gateways cooperate to transmit multicast traffic sourced within the multicast network to end-user sites. AMT relays receive the traffic natively and unicast-encapsulate it to gateways. This allows anyone on the Internet to create a dynamic tunnel to download multicast data streams.

With AMT, a multicast-enabled service provider can offer multicast services to a content owner. When a customer of the unicast-only local provider wants to receive the content and subscribes using an AMT join, the multicast-enabled transit provider can then efficiently transport the content to the unicast-only local provider, which sends it on to the end user.

AMT is an excellent way for transit service providers (who can get access to the content, but do not have many end users) to provide multicast service to content owners, where it would not otherwise be economically feasible. It is also a useful transition strategy for local service providers who do not yet have multicast support on all downstream equipment.

AMT is also useful for connecting two multicast-enabled service providers that are separated by a unicast-only service provider.

Similarly, AMT can be used by local service providers whose networks are multicast-enabled to tunnel multicast traffic over legacy edge devices such as digital subscriber line access multiplexers (DSLAMs) that have limited multicast capabilities.

• A three-way handshake is used to join groups from unicast receivers to prevent spoofing and denial-of-service (DoS) attacks.
• An AMT relay acting as a replication server joins the multicast group and translates multicast traffic into multiple unicast streams.
• The discovery mechanism uses anycast, enabling the discovery of the relay that is closest to the gateway in the network topology.
• An AMT gateway acting as a client is a host that joins the multicast group.
• Tunnel count limits on relays can limit bandwidth usage and avoid degradation of service.

AMT is described in detail in Internet draft draft-ietf-mboned-auto-multicast-10.txt, Automatic IP Multicast Without Explicit Tunnels (AMT).