Communication

Part IVMulticast 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 usageKey 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 multicastStanford

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 structureThe 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

100 Mb/s

R

1

2

R

1

2

R 1 2 R 12

The node addition algorithm

R

5

5 71

10

2

103

8

R

1 2

3

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

R1

1

2

3

4

5

6

R2 R3

Key:

Content query (multicast join)

Query redirect

Content delivery

Application level multicasting

• A survey on ALM