Post on 24-Dec-2015
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 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 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
57
1
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