Traceroute (tracert) – Testing the Path

The Ping utility is only used to test connectivity between two hosts but traceroute provide information about the details of devices between the two hosts. Traceroute (tracert) is an also a utility that generates a list of hops that were successfully reached beside the path. The list provided by traceroute provide important information for verification and troubleshooting of the network.

If the data reaches the destination successfully, then the trace provides a complete list of the interfaces of every router in the path between the two hosts. If the data fails at some hop along the path, the address of the last router that responded to the trace can provide an indication of where the problem or security restrictions are found.

 

Round Trip Time (RTT)

The traceroute provide round trip time (RTT) for each hop alongside the path and indicate the hops which do not respond. The round trip time (RTT) is the time a packet takes to reach the remote host and for the acknowledge from the host to return. An asterisk (*) is used to indicate a loss or un-replied packet.

This information is important to locate a problematic router in the path to the remote host. If there are high response times or data losses from a particular hop; this is an indication that there is something wrong with the resources of the router or its connectivity.

IPv4 TTL and IPv6 Hop Limit

Traceroute used TTL field in IPv4 and the Hop Limit in the IPv6 in the layer 3 header. It also uses ICMP time exceeded message.

The figure shows the traceroute to http://fschub.com. The first series of messages sent from traceroute will have a TTL field value of 1. This causes the TTL to time out the IPv4 packet at the first router. This router then replies with an ICMPv4 messages. Traceroute now listed the address of the first hop.

Traceroute then increasingly increments the TTL field to 2, 3, 4... for each series of messages. This provides the trace with the address of each hop as the packets time out further down the path. The TTL field continues to be increased until the destination is reached; or it is incremented to a predefined maximum.

When trace reached to the final destination; the host responds with either an ICMP port unreachable message or an ICMP echo reply message instead of the ICMP time exceeded message.