Which router port address should respond with time to live exceeded in transit message?

Question

Given the following situation:

PC --- |aa  RTR1  bb| --- |aa  RTR2  bb| --- |aa  RTR3  bb| etc  

Each of the |aa rtr bb| is meant to be a router with two ports aa and bb.

When I do a trace route from my PC, which router port address should respond with time to live exceeded in transit message?

I seem to remember being taught to think of the router as being in as many parts as ports, so that in my scenario when aa is forwarding the packet to bb and decrements the TTL to 0, it will be the address of the aa port in the failure message. I am trying to find the definitive answer.

Answer 1

Since each router sends you the ICMP packet using aa and not using bb, the packet it sends will have aa's address and not bb's.

The router generates an ICMP packet and sends it to you. The source address the packet will have is the source address that this packet is sent from, aa's address.