Traceroute probes

Question

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?

Answer 1

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?

The intermediate router is busy forwarding packets, and a packet from a different router is just a packet to it, so it gets forwarded. The primary job of a router is to forward packets as fast as possible. Low on its priority list is generating ICMP messages, and it will do that when it gets time, but if it is busy forwarding packets, it may put that off until the traceroute times out for that router.

When the router is an intermediate router, it is not generating the ICMP error message, it is simply forwarding a packet that contains an ICMP error message generated by a different router. The intermediate router only knows that it is forwarding a packet (its primary job), not what the packet contains.

Answer 2

Modern routers handle packet forwarding in hardware (for maximum speed and efficiency). While handling control traffic, that is traffic destined for the router itself, in CPU (software). There are few reasons that come to mind (maybe there are more) why a router wouldn't respond to ICMP messages. ICMP handling is disabled (for sequrity porpuses) - it makes it harder for someone to initiate a DDOS attack on that router. ICMP has low handling priority - The router would rather handle OSPF/BGP first to maintain connectivity, rather than this low priority task.