Communication Part IV Multicast Communication* *Referred to slides by Manhyung Han at Kyung Hee University and Hitesh Ballani at Cornell University

Unicast, Broadcast versus Multicast Unicast –One-to-one –Destination – unique receiver host address Broadcast –One-to-all –Destination – address of network Multicast –One-to-many –Multicast group must be identified –Destination – address of group Key: Unicast transfer Broadcast transfer Multicast transfer

Multicast application examples Financial services – Delivery of news, stock quotes, financial indices, etc Remote conferencing/e-learning – Streaming audio and video to many participants (clients, students) – Interactive communication between participants Data distribution – e.g., distribute experimental data from Large Hadron Collider (LHC) at CERN lab to interested physicists around the world

IP multicast Highly efficient bandwidth usage Key Architectural Decision: Add support for multicast in IP layer Berkeley Gatech Stanford CMU Routers with multicast support

So what is the big issue … more than 20 years since proposal, but no wide area IP multicast deployment Scalability (with number of groups) -- Routers maintain per-group state IP Multicast: best-effort multi-point delivery service -- Providing higher level features such as reliability, congestion control, flow control, and security has shown to be more difficult than in the unicast case Can we achieve efficient multi-point delivery without IP-layer support?

Application layer multicast Stanford CMU Stan1 Stan2 Berk2 Overlay Tree Gatech Berk1 Berkeley Gatech Stan1 Stan2 Berk1 Berk2 CMU

Pros and Cons Scalability – Routers do not maintain per-group state – End systems do, but they participate in very few groups Potentially simplify support for higher level functionality – Leverage computation and storage of end systems – Leverage solutions for unicast congestion, error and flow control Efficiency concerns – redundant traffic on physical links – increase in latency due to end-systems

System structure The overlay comprises of : A central source (may be replicated for fault tolerance) A number of overcast nodes (standard PCs with lot’s of storage) - organized into a distribution tree rooted at the source - bandwidth efficient trees Final Consumers – members of the multicast group - allows unmodified HTTP clients to join

Bandwidth Efficient Overlay Trees “…three ways of organizing the root and the nodes into a distribution tree.” 10 Mb/s 100 Mb/s R 1 2 R 1 2 R 12 R 12

The node addition algorithm R R Physical network substrate Overcast distribution tree

The client side – how to join a multicast group Clients join a multicast group through a typical HTTP GET request Root determines where to connect the client to the multicast tree using – Status of overcast nodes – Location of client Root selects “best” server and redirects the client to that server

Client Joins R1R R2R2 R3R3 Key: Content query (multicast join) Query redirect Content delivery

Application level multicasting A survey on ALM