ICS 351: Today's plan

• multicasting in general
• IP multicast addresses
• IP multicasting
• IGMP
• PIM

Multicasting

• if a single sender needs to reach a group of receivers
• or if multiple senders need to reach a group of receivers
• or if multiple computers need to reach each other
• in general, a sender should send each packet (stream) exactly once
• intermediate systems (routers) should duplicate the packet (stream) once for each outgoing link
• overall, this forms a multicast tree
• the tree can be built starting from the sender towards the receivers (leaves), along the route that unicast packets would take, or
• the tree can be built by each receiver along the reverse path towards the sender, until a router that already carries the multicast data is found. This is reverse path forwarding
• for multicast with multiple senders, the tree can be seen as a spanning tree with no real root, and a standard flooding algorithm can be used to distribute the data throughout this overlay network
• alternately, the tree can have a designated root, a rendezvous point or RP

IP multicast addresses

• IP multicast addresses are in class D, beginning with 224 through 239
  1. the first byte for class A addresses is 0 through 127
  2. the first byte for class B addresses is 128 through 191
  3. the first byte for class C addresses is 192 through 223
  4. the first byte for class D addresses is 224 through 239
  5. the first byte for class E addresses is 240 through 255
• for example, 224.0.0.9 for RIP packets, 224.0.0.5 for OSPF packets

IP Multicasting

• IGMP manages group membership in multicast groups within local networks (MLD does the same on IPv6 networks)
• PIM (or MOSPF) are the equivalent of routing protocols for multicast, providing multicast routing when the multicast router is not local

IGMP

• Internet Group Management Protocol version 3
• used to communicate between a multicast router and local multicast hosts
• the router needs to know which hosts require which multicast stream(s), so as to only forward streams that are needed
• a host requesting a stream from its router results in the router recording this information
• this request expires if not refreshed often enough: soft state in the router
• messages sent over IP (protocol number 2) with TTL 1
• IGMP routers send Membership Queries, IGMP hosts send Membership Reports
• RFC 3376

PIM

• Protocol Independent Multicast is protocol-independent in the sense that it relies on a not-otherwise defined routing protocol to find routes
• PIM dense mode (PIM-DM), RFC 3973
• in dense mode, multicast data is sent to all routers except those that send prune messages
• PIM sparse mode (PIM-SM), RFC 4601
• in sparse mode, multicast data is broadcast over a tree rooted at a designated router called the Rendezvouz Point (RP)
• also PIM Source-Specific Multicast (PIM-SSM) and Bidirectional PIM (BIDIR-PIM), a variant of PIM-SM

MBONE

• for a while, there was a generic multicasting infrastructure called the MBONE (multicast backbone)
• the 6-bone was a similar infrastructure for IPv6 traffic
• the MBONE was a collection of multicast routers willing to carry multicast traffic and to run multicast routing protocols
• a host that was not directly connected to a multicast router could register with a remote MBONE router and exchange packets using unicast IP