Section 7.7. Summary

7.7. Summary

This chapter focused on layer 3 of the protocol stack reference model. Two least-cost- path algorithms are Dijkstra's algorithma centralized least-cost algorithm designed to find the best path from a source to a destination through the optimization of path costsand the Bellman-Ford algorithma distributed least-cost approach by which each node retrieves information from reachable nodes to each of its neighbors. Non-least-cost algorithms deflection , or flood algorithmsare not optimal but they have their own useful applications.

A number of practical routing protocols exist. One category is intradomain routing protocols, such as OSPF and RIP. A router equipped with OSPF is aware of its local link-cost status and periodically sends updates to all surrounding routers. OSPF is based on link-state routing , by which routers exchange packets carrying status information of their adjacent links. A router collects all the packets and determines the network topology, thereby executing its own shortest-route algorithm. RIP is one of the most widely used routing protocols in the Internet infrastructure; routers exchange information abour reachable networks and the number of hops. RIP uses the Bellman-Ford algorithm.

By contrast, interdomain routing protocols , such as BGP, let two contributing routers exchange routing information even if they are located in two different autonomous systems. Each router sends information to all internal neighbors and decides whether the new route is possible.

Congestion control at the network layer is a major issue. In computer networks, congestion represents some form of overload, generally resulting from resources shortage at links and devices. Unidirectional congestion control can take several forms: back-pressure signaling , transmission of choke packets , and traffic policing . Bidirectional congestion control is a host-based resource-allocation scheme. In this category, random early detection is one congestion- avoidance technique. An approximation method to calculate link blocking follows two simple rules: two links in series and two links in parallel. These two rules can be used to estimate the blocking in any complex combination of links.

The next chapter presents fundamentals of the transport layer and end-to-end protocols. The transport layer is responsible for signaling and file transfer.