Sajeer,
Traceroute shows you the path of the packet.
Each line of output represents a hop (router to router) from the source address to the destination address.
Each packet contains a field called Time To Live (TTL), which is the maximum number of hops a packet can travel before it is discarded. TTL ensures a "TTL time exceeded" message is sent back to the source at each hop and an "ICMP Destination/PORT Unreachable" message (or a ICMP Echo reply) is sent back to the source when the final destination is reached. The TTL is decremented at each hop and when it reaches 1 the router replies with the timeout or reply message.
Example:
- TTL = 1
- Packet sent to gateway router and times out
- “TTL time exceeded” message sent back from the gateway router
- TTL = 2
- Packet sent to gateway router and decrements by 1
- Packet sent to next hop router and times out
- “TTL time exceeded” message sent back from the next hop router
- TTL = 3
- Packet sent to gateway router and decrements by 1
- Packet sent to next hop router and decrements by 1
- Packet sent to next hop router #2 and times out
- “TTL time exceeded” message sent back from the next hop router #2
This process continues until the destination router is reached; at that point a “ICMP Destination/PORT Unreachable” is sent back (in the case of UDP tracerotue).
There are three types of traceroutes:
- UDP traceroute (default for Unix/Linux)
- ICMP traceroute (default for Windows)
- TCP traceroute
Note: A hop that outputs * * * does not mean that router is down; instead it means the router at that hop does not respond to the type of packet you were using for traceroute. For example, a router may be configured to not respond to ICMP echo requests (pings) for an ICMP traceroute.
For more information:
http://www.cisco.com/c/en/us/support/docs/ip/ip-routed-protocols/22826-traceroute.html
-Alex
