multicast notes...

multicast notes

  DVMRP (distance vector mulitcast routing protocol)

     based on rip; generates a routing table with the multicast group it has knowledge of with corresponding distances between router and destination. uses reverse path flooding, sending a copy of a received packet out all interfaces except the one upon which it was received

PIM Dense Mode (PIM-DM) uses dense multicast routing. It implicitly builds shortest-path trees by flooding multicast traffic domain wide, and then pruning back branches of the tree where no receivers are present. PIM-DM is straightforward to implement but generally has poor scaling properties. The first multicast routing protocol, DVMRP used dense-mode multicast routing.^[1] See the PIM Internet Standard RFC 3973

PIM Sparse Mode (PIM-SM) explicitly builds unidirectional shared trees rooted at a rendezvous point (RP) per group, and optionally creates shortest-path trees per source. PIM-SM generally scales fairly well for wide-area usage. See the PIM Internet Standard RFC 4601.

Of these, PIM-SM has the widest deployment.  PIM-SM is commonly used in IPTV systems for routing multicast streams between VLANs, Subnets or local area networks.

Why PIM-SM? For large networks with bandwidth constraints; also the flexibility that comes with source and destination multicasting occurring anywhere in the network. PIM-SM is considered the best choice for a new multicast implementation

Sparse mode

See rfc4601 http://www.rfc-editor.org/rfc/rfc4601.txt

Protocol Independent Multicast - Sparse-Mode (PIM-SM) is a protocol for efficiently routing Internet Protocol (IP) packets to multicast groups that may span wide-area and inter-domain internets. The protocol is named protocol-independent because it is not dependent on any particular unicast routing protocol for topology discovery, and sparse-mode because it is suitable for groups where a very low percentage of the nodes (and their routers) will subscribe to the multicast session. Unlike earlier dense-mode multicast routing protocols such as DVMRP and dense multicast routing which flooded packets across the network and then pruned off branches where there were no receivers, PIM-SM explicitly constructs a tree from each sender to the receivers in the multicast group.

Multicast clients

A router receives explicit Join/Prune messages from those neighboring routers that have downstream group members.

• In order to join a multicast group, G, a host conveys its membership information through the Internet Group Management Protocol (IGMP).
• The router then forwards data packets addressed to a multicast group G to only those interfaces on which explicit joins have been received.
• A Designated Router (DR) sends periodic Join/Prune messages toward a group-specific Rendezvous Point (RP) for each group for which it has active members.
• Note that one router will be automatically or statically designated as the rendezvous point (RP), and all routers must explicitly join through the RP.
• Each router along the path toward the RP builds a wild card (any-source) state for the group and sends Join/Prune messages on toward the RP.
• The term route entry is used to refer to the state maintained in a router to represent the distribution tree.
• A route entry may include such fields as:
• source address
• the group address
• the incoming interface from which packets are accepted
• the list of outgoing interfaces to which packets are sent
• timers, flag bits, etc.
• The wild card route entry's incoming interface points toward the RP
• The outgoing interfaces point to the neighboring downstream routers that have sent Join/Prune messages toward the RP as well as the directly connected hosts which have requested membership to group G.
• This state creates a shared, RP-centered, distribution tree that reaches all group members.

Multicast sources

• When a data source first sends to a group, its Designated Router (DR) unicasts Register messages to the Rendezvous Point (RP) with the source's data packets encapsulated within.
• If the data rate is high, the RP can send source-specific Join/Prune messages back towards the source and the source's data packets will follow the resulting forwarding state and travel un-encapsulated to the RP.
• Whether they arrive encapsulated or natively, the RP forwards the source's de-capsulated data packets down the RP-centered distribution tree toward group members.
• If the data rate warrants it, routers with local receivers can join a source-specific, shortest path, distribution tree, and prune this source's packets off the shared RP-centered tree.
• For low data rate sources, neither the RP, nor last-hop routers need join a source-specific shortest path tree and data packets can be delivered via the shared RP-tree.

Once the other routers which need to receive those group packets have subscribed, the RP will unsubscribe to that multicast group, unless it also needs to forward packets to another router or node. Additionally, the routers will use reverse-path forwarding to ensure that there are no loops for packet forwarding among routers that wish to receive multicast packets.

Deployment

To support IP multicast, the sending and receiving nodes, intermediate routers and the network infrastructure between them must be multicast-enabled. In deploying IP multicast as an end-to-end solution, you will need to consider the following four areas:

Addressing

You must have an IP multicast address to communicate with a group of receivers rather than a single receiver, and you must have a mechanism for mapping this address onto MAC layer multicast addresses where they exist. End node hosts must have network interface cards (NICs) that efficiently filter for LAN data link layer addresses which are mapped back to the network layer IP multicast addresses. IP address space is divided into four sections-Classes A, B, C and D. The first three classes are used for unicast traffic. Class D addresses are reserved for multicast traffic and are allocated dynamically. (See IP Group Addressing below.)

Dynamic Host Registration

The end node host must have software supporting Internet Group Management Protocol (IGMP-defined in RFC 2236) to communicate requests to join a multicast group and receive multicast traffic. IGMP specifies how the host should inform the network that it is a member of a particular multicast group.

Multicast Routing

The network must be able to build packet distribution trees that allow sources to send packets to all receivers. These trees ensure that only one copy of a packet exists on any given network. There are several standards for routing IP multicast traffic. The Cisco-recommended solution is Protocol Independent Multicast (PIM), a multicast protocol that can be used with all unicast IP routing protocols.

Multicast Applications

End node hosts must have IP multicast application software such as video conferencing and must be able to support IP multicast transmission and reception in the TCP/IP protocol stack.