Friday, September 4, 2009

Implementing traceroute with System.Net: Part-I

In the last part, I started by giving links to my implementation of the Ping utility, that used System.Net.Sockets.

In this part, we will talk about traceroute. Traceroute is a general purpose utility that is used to discover a network path from the source to the destination. Traceroute is similar to Ping, in that it uses the ICMP protocol to send ICMP Echo request packets to the server. However, one key difference, is that it additionally uses the IP Time To Live (TTL) mechanism to specify the lifetime of the outgoing packets.

When the TTL expires on a packet, the receiving host must send an ICMP "Time Exceeded" message to the sender. The sender looks at this packet, and gets the IPAddress/HostName of the host that responded.

So, the basic algorithm for this goes as follows:

while (reply.address != dest.address)
int ttl = 1;
for i = 1 to 3
send a packet to the host at address dest.address with TTL=ttl
wait for reply
if (timeout) then
print "*"
print ipaddress of host thatresponded
// increase the TTL
ttl = ttl + 1;

This should be pretty easy to implement, given that we have already implemented a PING client in previous episodes, that does all the heavy lifting for us, in terms of marshalling the ICMP packet from managed code to network byte order, etc.

In the next part, we will modify the Ping utility to convert it into a Traceroute implementation.

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