1. Reliable Multicast Group
Neelofer T.
CMSC 621
Oct 25, 2007

2. What is Reliable Multicasting?
Reliability could mean messages are delivered:
In order
In timely manner
No duplicates
Processes may enter/leave the group
Processes may fail while communicating

3. Reliable Multicasting Issues
Protocol
TCP is point-to-point
Multicast is not connection oriented
Protocols based on IP Multicast
Applications over UDP
Reliable multicasting can cause network congestion:
Large amounts of data transmission since multiple recipients
Control messages for reliability

4. Small Scale Group For small scale group: Large scale groups cause:
Reliable, connection oriented, point-to-point connections are feasible
Messages have sequence numbers
Receivers acknowledge messages
Request retransmission of missing messages
Large scale groups cause:
Feedback implosion
Congestion due to repair messages

5. Hierarchical Feedback Control
RMTP (reliable multicast transport protocol)
Designated receivers act as coordinators for sub-groups
Receivers unicast feedback to DR
DRs retransmit data to sub-group
Hierarchy of DRs reduces load on original sender

7. SRM (Scalable Reliable Multicast)
Receivers help each other
Receiver multicasts retransmission request after a specific time period
Closest neighbor with requested data will multicast the message
Distant neighbors timeout later than the closest ones
If data is not retransmitted by anyone when a node timesout, then it multicasts the data

8. PGM (Pragmatic General Multicast)
Ensures that all receivers can get reliable, ordered, duplicate data or can detect message loss
NACK based protocol
Suited for applications where messages can join and leave group at any time
Uses routers for group control

9. References
Tanenbaum and Steen, “Distributed Systems Principles and Paradigms”
Peled and Vardi, “Formal Techniques for Networked and Distributed Systems”
C. Kenneth Miller, “Reliable Multicast Protocols and Applications”
Gemmel et al, “The PGM Reliable Multicast Protocol”
RFC 3940, “NACK-Oriented Reliable Multicast Protocol (NORM)”

10. Wu, “Verification analysis of Reliable Multicast protocol”

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