Page 145 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes dynamic. The static table does not alter frequently while dynamic table is updated frequently
whenever some change in the Internet is advertised like failure of some route or addition of a
better route. Routing protocols are used for dynamic routing tables. They are based on
combination of rules and procedures that enable routers to inform each other about the changes
in the Internet and share the information about the Internet or their neighborhood.

Unicast Routing

The majority of IP addresses are unicast addresses that are meant for a single recipient. Unicast
connections are one-to-one connections. A connectionless and connection oriented protocol can
use unicast addresses irrespective of whether connection exists between a specific pair of hosts.
In unicast routing the router forwards the incoming packet through one of its port as defined in
the routing table. A router that is attached to several networks has to determine the optimal path
for a packet so that a router chooses the route with the shortest metric. Metric is defined as the
cost assigned for passing through a network. The total metric for a particular route is the sum of
the metrics of the network that build up the route. The metric assigned to each network depends
upon the protocol used. The unit of metric cost is hop count and protocol like routing information
protocol assigns equal metric to each networks. If it assigns a metric of 1 hop to each network
then a packet traversing 15 networks will have metric value as 15 hop count. The assignment of
metric varies from protocols to protocols based on the services required from the network.

Interior and Exterior Routing

An Internet is divided into autonomous systems so that routing protocols may handle Internet
effectively and efficiently. An autonomous system (AS) is a group of networks under the
administration of a single authority and therefore routing inside an AS is called as interior routing
while routing among autonomous systems is called exterior routing. There exist many standard
and proprietary interior gateway protocols. Some of them are Routing Information Protocol (RIP)
and Open Shortest Path Fist (OSPF). The exterior protocol is Border Gateway Protocol (BGP).

Hierarchical Routing

Because of the global nature of Internet system and ever growing networks in size, it becomes
more difficult to centralize the system management and operation. For this reason, the system
must be hierarchical such that it is organized into multiple levels, with several group loops
connected with one another at each level. The routers are divided into regions with each router
knowing all the details about how to route packets within its own region but knowing nothing
about the internal structure of other regions. Therefore, hierarchical routing is commonly used
for such a system as shown in the Figure 9.2.
A set of networks interconnected by routers within a specific area using the same routing
protocol is called domain.

Two or more domains may be further combined to form a higher-order domain.
A router within a specific domain is called intra-domain router. A router connecting
domains is called inter-domain router.

A network composed of inter-domain routers is called backbone.
Each domain, which is also called operation domain, is a point where the system operation is
divided into plural organizations in charge of operation. Domains are determined according to
the territory occupied by each organization.

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