1. Distance-vector Multicast Routing Protocol (DVMRP)

2. Distance-vector Multicast Routing Protocol
DVMRP consists of two major components:
a conventional distance-vector routing protocol (like RIP)
a protocol for determining how to forward multicast packets, based on the routing table

3. Multicast Forwarding A DVMRP router forwards a packet if
the packet arrived from the link used to reach the source of the packet (Reverse path forwarding - RPF)
similar (but not quite the same) to flooding each packet once
if downstream links have not pruned the tree

4. Example Topology g g s g

5. Phase 1: Flood Using Truncated Broadcast

6. Phase 2: Prune g g
prune (s,g)
prune (s,g) s g

7. Phase 3: Graft g g g
report (g)
graft (s,g)
graft (s,g) s g

8. Phase 4: Steady State g g g s g

9. Multicast Routing: MOSPF

10. Multicast OSPF (MOSPF)
Add-on to OSPF (Open Shortest-Path First, a link-state, intra-domain routing protocol)
Multicast-capable routers flag link state routing advertisements
Each router indicates groups for which there are directly-connected members

11. MOSPF (Cont.)
Link-state advertisements augmented with multicast group addresses to which local members have joined
Link-state routing algorithm augmented to compute shortest-path distribution tree from any source to any set of destinations

12. Link state: Each router floods link state advertisement
Multicast: add membership information to “link state”
Each router computes multicast tree for each active source,
builds forwarding entry with outgoing interface list. S1 R1 R2 X Y Z

13. Z has network map, including membership at X and Y
Z computes shortest path tree from S1 to X and Y
Z builds multicast entry with one outgoing interface
W, Q, R, each build multicast entries S1 Z W Q X R R1 Y R2

14. Link state advertisement with new topology may require
re-computation of tree and forwarding entry S1 Z W Q X R R1 Y R2

15. R3 T Link state advertisement (T) with new membership (R3) may
require incremental computation and addition of interface
to outgoing interface list (Z) S1 Z R3 W Q T X R R1 Y R2

16. Impact on Route Computation
Can’t pre-compute all source multicast trees
Compute on demand when first packet from a source S to a group G arrives
Forward packet onto outgoing interfaces that correspond to local portion of the tree