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Unformatted text preview: ertisements ensure that over time the nodes all have all
the information necessary to compute correct routes. Second, in both vector and
link-state protocols, periodic advertisements are the fundamental mechanism used
to overcome the effects of link and node failures (as well as packet losses).
3. Integrating advertisements: In this step, a node processes all the advertisements it
has recently heard and uses that information to produce its version of the routing
Because the network topology can change and because new information can become
available, these three steps must run continuously, discovering the current set of neighbors, disseminating advertisements to neighbors, and adjusting the routing tables. This
continual operation implies that the state maintained by the network switches is soft: that
is, it refreshes periodically as updates arrive, and adapts to changes that are represented
in these updates. This soft state means that the path used to reach some destination could
change at any time, potentially causing a stream of packets from a source to destination to
arrive reordered; on the positive side, however, the ability to refresh the route means that
the system can adapt by “routing around” link and node failures. We will study how the
routing protocol adapts to failures in the next lecture.
A variety of routing protocols have been developed in the literature and several different ones are used in practice. Broadly speaking, protocols fall into one of two categories
depending on what they send in the advertisements and how they integrate advertisements to compute the routing table. Protocols in the ﬁrst category are called vector protocols because each node, n, advertises to its neighbors a vector, with one component per
destination, of information that tells the neighbors about n’s route to the corresponding CHAPTER 18. NETWORK ROUTING - I 6 WITHOUT ANY FAILURES destination. For example, in the simplest form of a vector protocol, n advertises its cost to
reach each destination as a vector of destination:cost tuples. In the integration step, each
recipient of the advertisement can use the advertised cost from each neighbor, together
with some other information (the...
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This document was uploaded on 02/26/2014 for the course CS 6.02 at MIT.
- Fall '13