Dear all Communications Experts, I am wondering whether we can know the information of both incoming and outgoing route of a packet from/to a particular server or not? In other words, is there any way for me to know on which route (through which IP addresses, which Autonomous Systems did a packet traverse to reach to my PC) and vice-versa? Knowing that in order to know via which IP addresses a packet is sent from my PC to a server, I can run "traceroute". However, what I also want to know is via which IP addresses did the server send the ACK to my PC.(As you also might know that the incoming and outgoing route not always symmetric, especially for those long-enough route).

1 Answer 1


It is impossible to know the route for sure. You can try to guess the path with traceroute, but as you already noted, the return path may be asymmetric. The internet routing infrastructure is extremely dynamic. It cannot even be guaranteed that the same path will be taken from one packet to the next. Furthermore, with traceroute, devices can be configured not to respond to these probes so you also will frequently find blind spots with this tool.

The only way you can be sure is if you control or have configuration information for all the devices in the equation and are able to determine the paths based on configuration.

  • Thanks James for your prompt response to my question. So can I properly guess the forth/back route from/to my PC to/from a particular IP, if I have the whole AS-relationship bidirected graph which is monthly updated like this one: irl.cs.ucla.edu/topology ?
    – wgq23ggee
    Mar 23, 2015 at 9:12

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