In traceroute, when a * is recieved, it could mean the router didn't reply due to various reasons- either the router is busy processing other packets or ICMP is blocked. I think few ISPs configure some routers on the Internet to filter ICMP as ICMP traffic is the least priority of all the Internet traffic.

But eventually after a while, I'm able to get to the next hop and the destination- how will that be possible when an intermediate router is busy? Does it mean when a * is received at the source, the source increments the TTL by 1 and sends a trace probe packet and the intermediate router that was busy just decrements the TTL of that packet and forwards the packet to the next hop router?

  • The answer to your last sentence is "yes". – Todd Wilcox Jul 20 '15 at 18:29
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    Routers generally route packets. If they have spare time, they might send you TTL exceeded response packets. The two are not related. – Craig Constantine Jul 20 '15 at 18:36

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