Question

1) a) If there are 9 routers between the client and the server they are trying to reach what is the minimum size of the TTL field in the UDP segment sent out by Traceroute that will actually reach the server?

b) If there are 19 routers between a client and a server how many UDP packets will traceroute send out to determine the complete path between the two hosts? Assume the network is static (i.e. no change in the path traversed by the packets) and there is no disruption or packets losses on the network and that all routers and the server are configured to send ICMP packets.

c)

Answer 1

TTL stands for time to live . This field is present in IPV4 header. This field is used for various purposes.like consider we don't want any packet to get into infinite loop on internet. So we set TTL field some value now each time this packet go from one hop to another this TTL field get decremented. Now if TTL=0 and packet is not yet reached to destination then that packet will get discarded.

[1] a) If there are 9 routers between server and client then server will set TTL = 10 to get packet at client. Because if TTl=10 at server and server is connected like this

Server-------R1------R2--------R3-------R4------R5------R6------R7------R8-------R9---------client

Now when packet go from S to R1 then TTL get decremented to 9. Then when packet arrive from R1 to R2 TTL becomes 8 and so on. When packet is at R9 TTL=1 now packet go to client with TTL=0 since it is destination IP of packet hence it will be recieved.

[b]

If there are 19 router between client and server then number of UDP packets sent will be = 20.

Because at first server will send packet with TTL=1. Now this packet will be discarded at R1. then again server will send another UDP packet with TTL=2 then that packet will get discarded at R2. and similarly 19 different UDP packets will be discarded.

After that server will send UDP packet with TTL=20 then that packet will go till R19 where its TTL field will be 1 then after that packet will go from R19 to client where TTL will become 0 and packet is recieved.