Multicast Routing Protocols

The Need for Multicast Routing n Routing based on member information –Whenever a multicast router receives a multicast packet it checks the group ID of the message and forwards the packet only if there is a member of that group in networks connected to it n Member information exchange –For delivering a multicast packet from the source to the destination nodes on other networks, multicast routers need to exchange the information they have gathered from the group membership of the hosts directly connected to them

Basic Router Model n Since hosts can send any time to any group, routers must be prepared to receive on all link-layer group addresses –And know when to forward or drop packets n What does a router keep track of? –interfaces leading to receivers –sources when utilizing source distribution trees –prune state depending on the multicast routing protocol (e.g. Dense Mode)

Data Distribution Concepts n Routers maintain state to deliver data down a distribution tree n Source trees –Router keeps (S,G) state so packets can flow from the source to all receivers –Trades off low delay from source against router state

Data Distribution Concepts n Shared trees –Router keeps (*,G) state so packets flow from the root of the tree to all receivers –Trades off higher delay from source against less router state

Data Distribution Concepts n How is the tree built? –On demand, in response to data arrival »Dense-mode protocols (PIM-DM and DVMRP) »MOSPF –Explicit control – Sparse-mode protocols (PIM-SM and CBT)

Data Distribution Concepts n Building distribution trees requires knowledge of where members are –flood data to find out where members are not (Dense-mode protocols) –flood group membership information (MOSPF), and build tree as data arrives –send explicit joins and keep join state (Sparse-mode protocols)

Data Distribution Concepts n Construction of source trees requires knowledge of source locations –In dense-mode protocols you learn them when data arrives (at each depth of the tree) –Same with MOSPF –In sparse-mode protocols you learn them when data arrives on the shared tree (in leaf routers only) »Ignore since routing based on direction from RP »Pay attention if moving to source tree

Data Distribution Concepts n To build a shared tree you need to know where the core (RP) is –Can be learned dynamically in the routing protocol (Auto-RP, PIMv2) –Can be configured in the routers

Data Distribution Concepts n Shared trees make sense for –Many low-rate sources –Applications that don’t require low delay –Consistent policy and access control across most participants in a group –When most of the source trees overlap topologically with the shared tree

Multicast Routing Protocols—Characteristics n Types of multicast protocols –Dense-mode »Flood and prune behavior –Sparse-mode »Explicit join behavior

Multicast Routing Protocols—Characteristics n Dense-mode protocols –Assumes dense group membership »Branches that are pruned don’t get data »Pruned branches can later be grafted to reduce join latency –DVMRP—Distance Vector Multicast Routing Protocol –Dense-mode PIM—Protocol Independent Multicast

Multicast Routing Protocols—Characteristics n Sparse-mode protocols »Assumes group membership is sparsely populated across a large region »Uses either source or shared distribution trees »Explicit join behavior—assumes no one wants the packet unless asked »Joins propagate from receiver to source or Rendezvous Point (Sparse mode PIM) or Core (Core Based Tree)

Intra-Domain Multicast Routing Protocols n Similar to unicast routing protocols - such as Routing Information Protocol (RIP) and Open Shortest Path First (OSPF) protocol -, there should be multicast routing protocols such that multicast routers can determine where to forward multicast messages n Existing multicast protocols: –Distance Vector Multicast Routing Protocol (DVMRP) based on the RIP unicast routing protocol –Multicast Extensions to OSPF (MOSPF) protocol based on the OSPF unicast routing protcol –Protocol Independent Multicast Sparse Mode (PIM-SM) protocol: »performs better when group members are sparsely distributed

Protocol Independent Multicast (PIM) n The major proposed (and used) multicast protocols perform well if group members are densely packed and bandwidth is not a problem n However, the fact that DVMRP periodically floods the network and the fact that MOSPF sends group membership information over the links, make these protocols not efficient in cases where group members are sparsely distributed among regions and the bandwidth is not plentiful n To address these issues, PIM contains two protocols: PIM-DM and PIM-SM n Although these two algorithms belong to PIM and they share similar control messages, they are essentially two different protocols

PIM-Sparse Mode (PIM-SM) n PIM-SM has a key difference with existing dense-mode protocols (DVMRP, MOSPF) n In PIM-SM protocol routers need to explicitly announce their will for receiving multicast messages of multicast groups, while dense- mode protocols assumes that all routers need to receive multicast messages unless they explicitly send a prune message

Sparse Mode Protocol Independent Multicast (PIM-SM) I. Phase

Sparse Mode Protocol Independent Multicast (PIM-SM) II. Phase

n Explicit join model –Receivers join to the Rendezvous Point (RP) –Senders register with the RP –Data flows down the shared tree and goes only to places that need the data from the sources –Last hop routers can join source tree if the data rate warrants by sending joins to the source n RPF check for the shared tree uses the RP n RPF check for the source tree uses the source Sparse Mode PIM

n Only one RP is chosen for a particular group n RP statically configured or dynamically learned (Auto-RP, PIM v2 candidate RP advertisements) n Data forwarded based on the source state (S, G) if it exists, otherwise use the shared state (*, G) n Draft: draft-ietf-idmr-pim-sm-specv2-00.txt n Draft: draft-ietf-idmr-pim-arch-04.txt Sparse Mode PIM

Sparse Mode PIM Example Receiver 1 B E AD Source C Receiver 2 RP Link Data Control

Sparse Mode PIM Example Receiver 1 B E AD Source Receiver 1 Joins Group G C Creates (*, G) State, Sends (*, G) Join to the RP C Receiver 2 RP Join

Sparse Mode PIM Example Receiver 1 B E A RP D Source RP Creates (*, G) State C Receiver 2

Sparse Mode PIM Example Receiver 1 B E A RP D Source Source Sends Data A Sends Registers to the RP C Receiver 2 Register

Sparse Mode PIM Example Receiver 1 B E A RP D Source RP de-encapsulates Registers Forwards Data Down the Shared Tree Sends (S,G) Join Towards the Source C Receiver 2 Join

Sparse Mode PIM Example Receiver 1 B E A RP D Source RP Sends Register-Stop Once Data Arrives Natively C Receiver 2 Register-Stop

Sparse Mode PIM Example Receiver 1 B E A RP D Source C Sends (S, G) Joins to Join the Shortest Path (SPT) Tree C Receiver 2 (S, G) Join

Sparse Mode PIM Example Receiver 1 B E A RP D Source When C Receives Data Natively, It Sends Prunes Up the RP tree for the Source. RP Deletes (S, G) OIF and Sends Prune Towards the Source C Receiver 2 (S, G) RP Bit Prune (S, G) Prune

Sparse Mode PIM Example Receiver 1 B E A RP D Source New Receiver 2 Joins E Creates State and Sends (*, G) Join C Receiver 2 (*, G) PIM Join (*, G) IGMP Join

Sparse Mode PIM Example Receiver 1 B E A RP D Source C Adds Link Towards E to the OIF List of Both (*, G) and (S, G) Data from Source Arrives at E C Receiver 2

Sparse Mode PIM Example Receiver 1 B E A RP D Source New Source Starts Sending D Sends Registers, RP Sends Joins RP Forwards Data to Receivers through Shared Tree C Receiver 2 Source 2 (*, G) PIM Register

Multicast Routing Protocols Comparison Multicast Routing Protocol Unicast Protocol Dependence DVMRPRIP MOSPFOSPF PIM-SM -

Source Specific Protocol Independent Multicast (PIM-SS) n Only one S source can send data to an (S,G) channel, where G is a multicast address n In such a way the problem of global assignment of the multicast address is eliminated, since the used addresses are local considering to the actual sender n Each hosts are responsible to use different multicast addresses n The distribution tree according to an (S,G) SSM channel is always rooted in the S source, in such a way RP-based shared trees are not necessary

ASM and SSM routing

Broadcast Membership MOSPF Problem of learning group membership Flood and Prune DVMRP Rendezvous Mechanism PIM-SM

Intra-domain Multicast

MSDP Peering

Inter-domain Multicast

BBC, 2005

Multicast, state-of-the-art