Multicast [7]

Multicast allows a packet to be sent simultaneously to a configurable and manageable group of users. This function is very useful to support classes of broadcast

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such as TIS and FIS. Obviously PTT voice is an intrinsic capability of an analog half- duplex communication system. However, with the move toward integrated voice and data services over a single medium, multicast data distribution has become a key feature of most proposed PTT voice solutions of a packet data network.

Data broadcast services such as TIS and FIS are currently offered as Layer-2

broadcast services. This is a viable alternative but does introduce certain shortcomings. In particular, the broadcast ground station must be connected directly to an application that generates the data stream. Granted this direct connection may be remote (via a X.25 link) but the ground station cannot just be connected to a data network. The advantage of direct connection to a data network is that ground stations (such as VDL-B) could offer multiple logical channels to carry various services such as TIS-B, FIS-B, LAAS

broadcasts and potentially also handle some bi-directional data messaging. Such a configuration provides for a decoupling between the datalink infrastructure and the network services they support.

Table 8 summarizes key comparative aspects of the multicast architectures while the following sections describe these architectural aspects in further detail.

Table 9 Multicast Comparison

ACARS ATN/CLNP IP Near Term IP Far Term

Addresses Broadcast address NA Yes Yes Group Management No NA Yes Yes Protocols NA NA IGMP, BGMP, MLD, PIM-SM IGMP, BGMP, MLD, PIM-SM 3.7.1 ACARS

ACARS provides a message broadcast capability to all aircraft and provides features to allow specific multicast groups such as multicast to all of an airlines aircraft or all aircraft of a particular model. However, ACARS does not provide robust multicast group management features that would allow for the dynamic creation and management of multicast groups as would be required to accommodate many of the service needs.

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3.7.2 ATN/CLNP[12, 13, 14, 15, 16]

ATN/CLNP does not provide multicast services. An explanation provided for this omission13 _{was that ATN/CLNP was intended to only support guaranteed delivery}

services. Since receipt acknowledgement of multicast packets is an onerous problem, the delivery of multicast services was not pursued.

3.7.3 IP – Near Term [7]

In the near term, the multicast function can be achieved using the IP multicast protocols available for IPv6. Hosts wanting to join IP-multicast groups would need to support Multicast Listener Discovery (MLD) for IPv6 as defined in RFC 2710 for communication with their mobile router. A large selection of protocols is available for communication between routers as necessary to manage the addition and deletion and of nodes to the routing tree and the routing of packets through the multicast tree. Distance Vector Multicast Routing Protocols (DVMRP – RFC1075), Multicast Open Shortest Path First (MOSPF – RFC1584), Protocol Independent Multicast – Sparse Mode (PIM-SM RFC2362) and Border Gateway Multicast Protocol (BGMP) are potential protocol components to implement IP multicast within the MCNA. However, more research is required to investigate these design alternatives and assure compliance with FTI terrestrial IP network design.

3.7.4 IP - End State

In the end state, the IP solution for multicast should be the same as for the near term.

3.7.5 Layer-2 / Layer-3 Coordination and Efficiency

IP multicast is implemented as a means of restricting unnecessary use of system capacity to distribute common information to multiple users. A roughly comparable service could be achieved using a publish/subscribe server that receives a packet, replicates it and sends copies out to all of the subscribing hosts. Since the Air-Ground sub-networks are typically the most constrained resource, it is desirable to assure an efficient mapping between the network layer and A-G sub-network.

IP multicast uses a routing tree to distribute packets. In the case of PIM-SM, requests to join a multicast group are forward up the tree to the base, called the Rendezvous Point (RP) and multicast enabled routers along this tree add routes necessary to accommodate the request. The critical design aspect in the MCNA implementation of multicast is how the aircraft are treated with respect to the access point that they are attached. A terrestrial network typically has a separate link between each pair of connected routers. In wireless networks, a group of routers may be connected over a shared link to a single access router. Ideally the multicast enabled access router will treat a shared connection by

13

Aloke Roy, Honeywell, provided this explanation from his experience helping to develop the ATN/CLNP specifications.

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multiple subscribing aircraft as such and only broadcast (or layer-2 multicast) the packet once rather than forwarding the packet individually to each aircraft. This interface can become particularly complex with satellite systems or terrestrial base stations that have multiple beams. In this case, each beam should be treated as a separate connection and multicast packets should be forwarded to each beam that has active subscriptions to the applicable multicast group.

3.7.6 Coordination with FTI

Acknowledging that MCNA will need to interface with with ground networks

implemented by various FTI link service offerings, the MCNA multicast architecture will need to be compatible with IP multicast services provided by FTI if and when those services become available. This should not be a significant coordination effort given that IP multicast is fairly standard and the Layer-2/Layer-3 multicast interface for which we are concerned is implemented at the access routers which would typically be within the domain of MCNA rather than FTI. However, specific IP multicast protocols such as PIM-SM and BGMP should be coordinated. At this time our access to specifics about FTI is limited. However, given our knowledge of the transition plan for FTI, it is anticipated that IP multicast considerations will be addressed at later stage of that program.