计算机网络第五版(英文版)ppt(5)解析

Routing Algorithm Basics(3)

Metrics used for optimization
Distance Number
of hops Time delay

Two major classes of routing algorithms
Nonadaptive

Routing Algorithms
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Shortest Path Routing Flooding Distance Vector Routing Link State Routing Hierarchical Routing
Routing Algorithm Basics(2)

All optimal routes from station A to other stations in the network, jointly constitute a sink tree Routers have to collaborate to build the sink tree for each source station or destination station.
or Static: the routing table is computed in advance, off-line, and downloaded to the routers when the network is booted. Adaptive or Dynamic: change their routing decisions to reflect changes in the topology, and usually the traffic as well.
计算机网络
The Network Layer
The Network Layer
Topics
Network Layer Design Issues
Store-and-Forward Packet Switching
Implementation of Connectionless Service
Shortest Path Routing Dijkstra Algorithm

Basic Idea: during each step, select a newly
reachable node at the lowest cost, and add the edge to that node, to the sink tree rooted at the source node built so far.
Flooding


Basic idea: Forward an incoming packet across every outgoing line, except the one it came through.������ Basic problem: how to avoid “drowning by packets”?������ Use a hop counter: after a packet has been forwarded across N routers, it is discarded.������ Be sure to forward a packet only once (i.e. avoid directed cycles). Requires sequence numbers per source router.������ Flood selectively: only in the direction that makes sense.������ Flooding makes sense only when robustness is needed, e.g. in military applications
Implementation of Connection-Oriented Service
Comparison of Virtual-Circuit and Datagram Subnets

Routing Algorithms
Correctness Simplicity Robustness Stability Fairness Optimality
Routing Algorithm Basics



The routing algorithm is that part of the network layer software responsible for deciding which output line an incoming packet should be transmitted on. If the subnet uses datagrams internally, this decision must be made anew for every arriving data packet since the best route may have changed since last time. If the subnet uses virtual circuits internally, routing decisions are made only when a new virtual circuit is being set up.
Shortest Path Routing



Idea: build a graph of the subnet, with each node of the graph representing a router and each arc of the graph representing a communication line. To choose a route between a given pair of routers, the algorithm just finds the shortest path between them on the graph. the labels (weights) on the arcs could be computed as a function of the distance, bandwidth, average traffic, communication cost, measured delay, and other factors. Shortest Path Criteria: sum of the weights all the way, or, the number of hops.