1UCLA CSD Mario Gerla

Adaptive Shared Tree Multicast in Mobile Wireless Networks

GLOBECOM 98

Ching-Chuan Chiang

Mario Gerla

Lixia ZhangComputer Science Department

University of California, Los Angeles

2UCLA CSD Mario Gerla

Per-Source Tree Multicast

Like DVMRP and PIM-DM in wired net

Each source supports own separate tree

“Probing and Pruning” tree maintenance

Reverse Path Forwarding “Fast Source” problem

S1S2

R1

R2

3UCLA CSD Mario Gerla

RP-based Shared Tree Multicast

RP (Rendezvous Point)-based “Shared” tree

Similar to wired network PIM-SM, CBT

Tree maintenance: hard state soft state

Traffic concentration “off-center” RP

RP

S1

4UCLA CSD Mario Gerla

Shared Tree vs. Per-source Tree

Shared Tree:scalabilityless sensitive to fast

source longer path traffic concentration off center RP

Per-Source Tree:shortest pathtraffic distributionno central node scalability problem fast source problem

RP

S1

R2

R1

R3

R4

S2

5UCLA CSD Mario Gerla

Conclusions

Shared Tree scheme: lower throughput in heavy load lower control O/H

Adaptive scheme: like a Per Source tree scheme in heavy load like a Shared tree scheme in high mobility

Forwarding Group Multicast scheme: more robust & lower O/H than shared tree

6UCLA CSD Mario Gerla

Wireless Tree Multicasting (eg. Shared Tree)

• In a mobile situation, tree is fragile: connectivity loss, multipath fading

• Need to refresh paths very frequently

• High control traffic overhead

RP

7UCLA CSD Mario Gerla

Proposed solution: Forwarding Group Multicast

• All the nodes inside the “bubble” forward the M-cast packets via “restricted” flooding

• Flooding redundancy helps overcome displacements and fading• FG nodes selected by tracing shortest paths between M-cast members

FG

FG

FG

FG

FG

Forwarding Group

8UCLA CSD Mario Gerla

Key Concepts of ODMRP[5]

• Mesh topology

• Forwarding group concept

• On-demand approach

• Soft state

9UCLA CSD Mario Gerla

Why a Mesh?

• Richer connectivity among multicast members

• Unlike trees, frequent reconfigurations are not needed

10UCLA CSD Mario Gerla

Forwarding Group Concept• A set of nodes in charge

of forwarding multicast packets

• Supports shortest paths between any member pairs

• Flooding redundancy helps overcoming displacements and channel fading

11UCLA CSD Mario Gerla

FG Maintenance (On-Demand Approach)

• A sender periodically floods control messages when it has data to send

• All intermediate nodes set up route to sender

• Receivers update their Member Tables ; periodically broadcast Join Tables

• Nodes on path to sources set FG_Flag; FG nodes create and forward Join Tables to neighbors

12UCLA CSD Mario Gerla

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