Broadcasting In Hypercube Networks

Sujith Nair12316EN005

The Flooding Approach

The Flooding Approach

Pros:

-reliable message delivery

Cons:

-multiple duplicate packets are received at nodes.

-high network bandwidth consumption

Single-Spanning-Tree Broadcast

LAN Bridges typically restrict all packet traffic toa single spanning tree.

This is done either by forbidding loops in thephysical topology or by running a distributedalgorithm among the bridges to compute aspanning tree.

Single-Spanning-Tree Broadcast

When a bridge receives a broadcast packet, itsimply forwards it onto every incident branch ofthe tree except the one on which it arrived.

Because the tree spans all segments and has noloops, the packet is delivered exactly once (in theabsence of errors) to every segment.

Single-Spanning-Tree Broadcast

Cons:

- Broadcast traffic is concentrated on certain links, which form a part of the spanning tree.

-This results in network congestion.

Single-Spanning-Tree Broadcast in a Hypercube

Computing a spanning-tree in a hypercube iscomputationally simple.

Moreover, edge-disjoint spanning trees exist in ahypercube, because of its inherent symmetry.

Hence, network congestion can be avoided by usingdifferent edge-disjoint spanning trees at differenttimes.

Reverse Path Forwarding

A node forwards a broadcast packet originating atsource S if and only if it arrives via the shortestpath from the node back to S (i.e., the “reversepath”).

The node forwards the packet out on all incidentlinks except the one on which the packet arrived.

Reverse Path Forwarding

To implement the basic reverse path forwardingalgorithm, a router must be able to identify theshortest path from the router back to any host.

In internetworks that use distance-vector routingfor unicast traffic, that information is preciselywhat is stored in the routing tables in everyrouter.

Reverse Path Forwarding Algorithm

1. run single-source shortest path.

i) for each node, the shortest path to every other node is computed.

ii)using the shortest path, an outgoing link is associated with each node.

iii)a routing table is made; for each node, an outgoing link is assigned.

Reverse Path Forwarding Algorithm

1. Consider a broadcast packet received at a node n, through link l. Let its source be s.

i) If in the routing table, the entry corresponding to s is l, then broadcast to all neighbors

ii) else discard.

Reverse Path Forwarding in Hypercube

In hypercube networks, the shortest unicast path toany host node can be found using the XORoperation.

Hence, distance-vector tables are not needed forperforming RPF in hypercube networks.

0011 xor 0100 = 0111

assume the unicast routing algorithm routes to anode which differs from the current node in the LSBbit.

Hence, when unicasting from 0011 to 0100, 0011sends the packet to 0010.

Thus, when a broadcast packet from 0100 isreceived at 0011, it is only further broadcasted if it isreceived via 0010.

The above operation can also be performed bymaintaining routing tables.

The trade-off between maintaining a routing tableat each node and performing an xor operation forevery broadcast , depends on the table look-uplatency and memory requirement at each node.

References

Deering, Stephen E. "Multicast routing in internetworksand extended LANs." ACM SIGCOMM ComputerCommunication Review 25.1 (1995): 88-101.

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