Designing A Routing Protocol
Design Issues
The designer of a routing protocol needs to make a number of decisions
concerning the features of the future network that best fit the
particular specifications and available mechanisms. Often, the
decision involves making a choice between two opposing features.
Examples of such choices include:
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Centralized vs. distributed routing: Under a
centralized routing protocol, a central processor collects information
about the status of each link (up or down, its utilization and
capacity) and processes this information to compute a routing table
for each node.
In distributed routing, the routers cooperate using a distributed
routing protocol to create mutually-consistent routing tables.
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Source-based vs. hop-by-hop routing: In
source-based routing, the header of the packet carries the entire
route. In hop-by-hop routing, the header carries only the destination
address; each router along the path chooses the next hop.
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These alternatives represent two extremes. Intermediate strategies, in
which the header contains more than just the minimum (the destination
address), but also specifies some of the whole path, are also used.
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Often, the question is: What is the best trade-off between
routing specificity and the overhead incurred in control messages and
the increased size of the packet's header?
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Deterministic vs. stochastic routing: A
deterministic router forwards packets toward a destination node along
exactly one path, which depends only on the address of the
destination.
A stochastic router maintains more than one next hop for each possible
destination; it randomly picks one of the available hops when
forwarding the packets.
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Dynamic vs. static routing: With
dynamic routing, the choice of routes depends on parameters
describing the current state of the network (for example,
the workload of some of the links). Static (or
state-independent) routing ignores the state of
the network. Routes are selected on the basis of the
network topology (for example, by selecting a
shortest-length path). Telephone networks once were static,
but state-dependent routing with multiple paths is now the
norm. The Internet uses both static and dynamic routing.
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