Network Coding and Wireless Physical-layer ... - Jacobs University

14 Chapter 2: Introduction to Digital Communication Systems and Networks
twice by the same router. The last one is called selective flooding, meaning that routers
only send the packets only along the lines going approximately to the right direction [7].
2. Multi-destination Routing
In this method, each packet contains either a list of destinations or a bit map indicating
the desired destinations. When a packet arrives at a router, the router calculates the
best route for each destination and determines the set of all output lines needed by all
calculated best routes. The router generates a copy of the packet for each output line and
includes, in the packet, a list of those destinations using the line. This process repeats
for every router along the best routes until every destination receives the packet [7].
3. Spanning Tree
A spanning tree is a subset of the subnet. It is a graph whose set of nodes comprises all
routers in the network. Every pair of nodes are connected by a sequence of edges but
there is no loop in the graph. Figure 2.5(b) gives an example of the spanning tree. In
this method, the router simply copies an incoming packet to all output lines belonging to
the spanning tree except the one on which the packet arrives [7].
4. Reverse Path Forwarding
This algorithm is an attempt to approximate the previous one, even when the routers
know nothing about the spanning tree. In this algorithm, when a packet arrives at a
router, the router checks to see if it arrives on the line normally used for sending packets
to the broadcasting source. If that is the case, there is a good chance that packet has
followed the best route from the source and is the first copy arrived at the router. The
router therefore copies the packet onto all lines except that on which the packet arrives.
However, if the packet arrives on a line other than the preferred one used for reaching the
source, the packet is discarded as a duplicate [7].
Multicast Routing in Wireline Networks
Multicasting requires group management so that packets are sent only to those interested
in it and authorized to see it. In multicast routing, each router computes a spanning tree