1. Wireless Ad Hoc Multicast and ODMRP CS 218 Fall 2017

2. General Multicasting Approach
Build a tree rooted at the multicast source
That includes short paths to all multicast destinations
Update tree as nodes join and leave
Rarely perform major surgery on tree when Internet routing changes
Won’t work well in mobile situations
Source moves
Destination moves
Any intermediate routing nodes also move
How to do multicast in ad hoc wireless networks?

3. Per-Source Tree Multicast
Each source supports own separate tree
“Probing and Pruning” tree maintenance
Does not scale well with many sources
Requires frequent updates if sources move rapidly S1 S2 R1 R2

4. Core-based Shared Tree Multicast
Core or RP (Rendezvous Point) is the root
Tree maintenance:
soft state
“Off-center” RP
longer paths than shortest path tree RP S1

5. Wireless Tree Multicast Limitations in High MobilityRP
In a mobile situation, tree is fragile: connectivity loss, multipath fading
Need to refresh paths very frequently
High control traffic overhead

6. Mesh Solution: Forwarding Group MulticastFG
Forwarding Group
All the nodes inside the “bubble” forward the M-cast packets via “restricted” flooding
Multicast Tree replaced by Multicast “Mesh” Topology
Flooding redundancy helps overcome displacements and fading
FG nodes selected by tracing shortest paths between M-cast members

7. Forwarding Group Concept
A set of nodes in charge of forwarding multicast packets
Supports shortest paths between any member pairs
Flooding helps overcome displacements and channel fading

8. Mesh vs Tree Forwarding
Richer connectivity among multicast members
Unlike trees, frequent reconfigurations are not needed

9. ODMRP (On Demand Multicast Routing Protocol)
Uses forwarding group multicast concept
Tree replaced by mesh
On-demand approach
Soft state

10. FG Maintenance (On-Demand Approach)
A sender periodically floods JOIN messages when it has data to send
All intermediate nodes set up route to sender (backward pointer)
Receivers update Member Tables
Keeps track of which multicast groups the receiver belongs to
Also periodically broadcast Join Tables
Tells forwarders and multicast sender that he’s a member
Nodes on path to sources set FG_Flag; FG nodes forward Join Tables

11. Soft State Approach
No explicit messages required to join/leave multicast group (or FG)
An entry of a receiver’s Member Table expires if no Join Request is received from that sender entry during MEM_TIMEOUT
Nodes in the forwarding group are demoted to non-forwarding nodes if not refreshed (no Join Tables received) within FG_TIMEOUT

12. Fast Movers Benefit From Dense Networks

13. High Overhead for Few Nodes, Lower for More Nodes