Exercise: Sending ICMP Messages With Ping & Traceroute

Traceroute

Another very useful debugging tool is traceroute. The traceroute man page describes this tool as “print the route packets take to network host."

How It Works

Traceroute uses the TTL exceeded ICMP messages to discover the intermediate routers on the path towards a destination. The principle behind traceroute is very simple.

• When a router receives an IP packet whose TTL is set to 1, it decrements the TTL and is forced to return a TTL exceeded ICMP message to the sending host.

• To discover all routers on a network path, a simple solution is to first send a packet whose TTL is set to 1, then a packet whose TTL is set to 2, and so on. When the TTL is set to 1, the first router on the path returns a TTL expired packet, which is how its IP address can be discovered. When TTL is set to 2, the second router on the path returns a TTL expired packet, and so on. In this way, we are able to discover IP addresses of all routers on the path to the destination from the sending host. traceroute actually sends three packets with each TTL value.

Run the following call to traceroute to get a traceroute output of a path to ietf.org from one of Educative’s servers.

Usage

Sample Output #1

Here’s what some simple traceroute output may look like. Notice that the output is organized in rows and columns where each hop is represented by one row. Here’s what each column means:

The traceroute output above shows a 3-hop path (in the instance of writing this course - the number of hops and their IP addresses may be different now) between a host at Educative and one IETF’s servers. For each hop, traceroute provides the IPv4 address of the router that sent the ICMP message and exactly three measured round-trip-times between the source and this router.

Sample Output #2

You may also get something slightly more complicated like the above. Here, there is more than one next-hop each packet can take. For example, the first hop shows 2 different IP addresses:

 1  216.239.63.174 (216.239.63.174)  27.718 ms 72.14.232.108 (72.14.232.108)  11.264 ms 216.239.63.174 (216.239.63.174)  27.598 ms

So there are multiple routes towards the destination and probes are sent to each possible next hop.

Note Some routers are configured by their administrators not to respond to ICMP messages. In such cases, traceroute shows * * * when it times out waiting for the response. Also, by default, the traceroute utility on our platform goes to a maximum of 30 hops.

In the next lesson, we’ll study IPv4 Data Link Layer Address Resolution