Solved–Assignment 4– Solution

Description

1 Objective

The purpose of this assignment is to implement a distributed asynchronous distance vector routing algorithm. By completing this assignment, you will learn about distributed computing in a realistic network environment.

2 Overview

You will develop a program called Router to emulate the behavior of a router executing a mod-ified version of the distance vector routing algorithm discussed in class. Several instances of your router program will be started on possibly different hosts that communicate with each other using TCP.

To have a controlled environment for this assignment, rather than directly communicating with each other, the routers will communicate through a relay server as depicted in Figure 1. The relay server forwards packets passing through it to the appropriate receiving routers, very much like a switch. Unfortunately, the relay server is lazy and sometimes drops packets that go through it!

ROUTER 10 ROUTER
0 2
3 1
ROUTER ROUTER
1 1 3

(a) Network topology with 4 routers.

Router Router
0 2
Relay
Server
Router Router
1 3

(b) Relay server forwards packets.

Figure 1: Routers and relay server.

3 Relay Server

The relay server is implemented as a TCP server that listens on a specific port to communicate with the routers. From the perspective of routers, the relay server forwards routing packets to appropriate destination routers.

Network Topology: Once the relay server starts, it reads the network topology informa-tion from a text file. The topology information is presented as the adjacency matrix of the network formed by the routers. An example topology file is provided to you. It is possible to specify several topologies in a single file. All topologies in a file should have the same number of routers and be separated from each other by a blank line. If there are n routers

2

Assignment 4 CPSC 441

in the network, each is uniquely identified with an ID in f0, 1, . . . , n 1g. During the ex-
ecution, the server can be instructed to cycle through the specified topologies to emulate link cost change in the network.

Routing Packets: The packets exchanged between routers and the relay server are of type DvrPacket, which is implemented as a Java class. The source code for DvrPacket is provided to you along with tester code showing how to use it. The class DvrPacket is serializable so that its instances can be conveniently read from or written to the socket using object IO streams. Each DvrPacket contains the unique IDs of its sender and receiver. The relay server’s ID is denoted by SERVER, which is defined in class DvrPacket.

4 Router Design

Every router has a unique ID that is assigned to it when the router object is created. In your implementation, in addition to the cost of the minimum cost path to each destination (i.e., the mincost vector), you should also keep track of the neighboring router that is on the minimum cost path to the destination (i.e., the next hop router).

In the following, we describe the main tasks performed by a router.

Initialization: When started, each router must send a HELLO packet to the relay server and then wait to receive a HELLO response from the server. A HELLO packet is an instance of DvrPacket whose type is set to HELLO. The HELLO response contains the link cost vector associated with the router. The link cost vector is sent via the instance variable DvrPacket.mincost. For the routers that are not directly connected to this router, the corresponding link cost values are set to INFINITY. The number of routers in the network is equal to the length of the link cost vector received in the HELLO message from the re-lay server. Upon receiving the initial distance vector from the server, the router should initialize its internal distance vector and any other data structures needed by your imple-mentation.

Operation: The router has to update its distance vector when it receives a routing packet from its directly connected neighbors. The values received in a routing packet (carried in DvrPacket.mincost) from some other router i, contain i’s current mincost vector to all other routers in the network. Regular DvrPackets sent by routers are of type ROUTE.

Termination: When receiving a routing packet of type QUIT from the relay server, the router has to stop its operation, perform clean up and then return its forwarding table to the calling process. The forwarding table consists of both the mincost and nexthop vectors.

3
Assignment 4 CPSC 441

5 Routing Update

Updating Neighbors: This part is slightly different from the algorithm described in class. Rather than updating neighboring routers only when there is a change in the local mincost vector, your router should also update its neighbors periodically regardless of any change in its mincost vector. This is to deal with lost routing packets. The periodic update mecha-nism is controlled by a timer. Whenever the timers expires, the local mincost vector is sent to all neighbors. The exact update mechanism is as follows:

1: if local mincost vector changes then

2: send the local mincost vector to neighbors

3: re-start the timer

4: else

5: do nothing

6: end if

7: if timer expires then

8: send the local mincost vector to neighbors

9: re-start the timer

10: end if

Notice that the timer is not modified if the local mincost vector does not change. The timer is started after the router initialization is completed (after HELLO is processed), and is canceled during the router termination (before processing QUIT). The time-out interval for the timer is specified as an input parameter. Note that mincost vectors are sent in routing packets of type ROUTE.

Link Cost Change: If a router receives a ROUTE packet with DvrPacket.sourceid set to SERVER, it means that the network topology has changed. The new link cost vector for the router is included in the received DvrPacket.mincost array.

6 Software Interfaces

For detailed information about the following methods and classes, refer to the Javadoc documentation provided in the source code.

Define a Java class named Router, which includes the following public methods:

Router(int routerId, String serverName,

4
Assignment 4 CPSC 441

int serverPort, int updateInterval)

This is the constructor to initialize the router program.

RtnTable start()

Starts the router and runs the routing algorithm in a loop until terminated by a QUIT packet. The forwarding table of the router is returned once this method terminates. To return the forwarding table, simply create an object of type RtnTable and return it, as follows:

return new RtnTable(mincost, nexthop);

where mincost and nexthop are the mincost and nexthop vectors at the router.

Your implementation should include appropriate exception handling code to deal with various exceptions that could be thrown in the program. A skeleton class, named Router, is provided on D2L. Notice the import statement
import cpsc441.a4.shared.*

at the top of the file. The package cpsc441.a4.shared contains classes DvrPacket and RtnTable to be used in your program. A jar file named a4.jar containing the package cpsc441.a4.shared is provided to you on D2L. Make sure to add this jar file to your class path when compiling and running your program. For example, if you are using Java command line tools, use the option -cp to include a4.jar in your class path.

The source code for classes DvrPacket and RtnTable is also provided to you as a source of documentation for using these classes:

DvrPacket: This class defines the structure of the messages exchanged between routers (via the relay server). Each DvrPacket has a mincost array and specifies the packet’s sender and receiver IDs, and the type of the packet. As mentioned earlier, there are three types of packets, namely ROUTE, HELLO and QUIT packets.

RtnTable: This is a wrapper class used by the Router.start() method to return the forwarding table of the router.

The relay server is provided to you in a Java jar file called rserver.jar. The server comes with a readme file that explains how to run it and set its various parameters.

Restrictions

You are not allowed to change the signature of the methods provided to you. You can however define other variables, methods or classes in order to complete the assignment.

5
Assignment 4 CPSC 441

Your program should use the library file a4.jar. Do not use DvrPacket.java or RtnTable.java in your code. The source code is provided to you only for documenta-tion purposes.

You have to write your own code for sending and receiving DvrPackets over TCP. Ask the instructor if you are in doubt about any specific Java classes that you want to use in your program.

6