Computer Networks. Routing

Transcription

1 Computer Networks Routing

2 Topics Link State Routing (Continued) Hierarchical Routing Broadcast Routing Sending distinct packets Flooding Multi-destination routing Using spanning tree Reverse path forwarding Multicast Routing Pruning spanning tree

3 Link State Routing Each router must do the following: 1. Discover its neighbors, learn their network address. 2. Measure the delay or cost to each of its neighbors. 3. Construct a packet telling all it has just learned. 4. Send this packet to all other routers. 5. Compute the shortest path to every other router.

4 Measuring Line Cost Line cost is measured by sending an ECHO message and measuring the return time. Waiting time in router queue can be added to include the line traffic load

5 Building Link State Packets (a) A subnet. (b) The link state packets for this subnet.

6 Distributing the Link State Packets The packet buffer for router B.

7 Updating the Routing Tables After receiving the link state packets, each router creates a labeled graph of the network. The shortest path to each destination is found using Dijkstra s shortest path algorithm

8 Hierarchical Routing As networks grow in size, the router routing tables grow proportionally. In hierarchical routing, the routers are divided into regions, with each router knowing all the details about how to route packets to destinations within its own region, but knowing nothing about the internal structure of other regions.

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10 Broadcast Routing In some applications, hosts need to send messages to many or all other hosts. Sending a packet to all destinations simultaneously is called broadcasting

11 Sending distinct packets A broadcasting method that requires no special features from the subnet. It simply sends a distinct packet to each destination. Not only is the method wasteful of bandwidth, but it also requires the source to have a complete list of all destinations.

12 Flooding Although flooding is ill-suited for ordinary point-to-point communication, for broadcasting it might be a good solution, especially if other methods are not applicable. The problem with flooding is it generates too many packets and consumes too much bandwidth.

13 Multi-destination Routing In this method, each packet contains either a list of destinations or a bit map indicating the desired destinations. When a packet arrives at a router, the router checks all the destinations to determine the set of output lines that will be needed. The router generates a new copy of the packet for each output line to be used and includes in each packet only those destinations that are to use the line.

14 Spanning Tree A spanning tree is a subset of the subnet that includes all the routers but contains no loops. If each router knows which of its lines belong to the spanning tree, it can copy an incoming broadcast packet onto all the spanning tree lines except the one it arrived on. This method makes excellent use of bandwidth, generating the absolute minimum number of packets necessary to do the job. Each router must have knowledge of the spanning tree for the method to be applicable.

15 Reverse Path Forwarding Reverse Path Forwarding is an attempt to approximate the spanning tree broadcast, even when the routers do not know anything at all about spanning trees. When a broadcast packet arrives at a router, the router checks to see if the packet arrived on the line that is normally used for sending packets to the source of the broadcast. If so, assume it is the first copy to arrive at the router, so Otherwise, discarded it.

16 Reverse Path Forwarding(Example)

17 Reverse Path Forwarding(Example)

18 Multicast Routing Some applications require that widely-separated processes work together in groups, for example, a group of processes implementing a distributed database system. Sending a message to a group is called multicasting, and its routing algorithm is called multicast routing.

19 Host Groups Multicasting requires group management. When a process joins a group, it informs its host of this fact. Either hosts must inform their routers about changes in group membership, or routers must query their hosts periodically. Routers tell their neighbors, so the information propagates through the subnet.

20 To do multicast routing, each router computes a spanning tree covering all other routers. When a process sends a multicast packet to a group, the first router examines its spanning tree and prunes it, removing all lines that do not lead to hosts that are members of the group.

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