question concerning traceroute?

Thu Oct 17 15:43:01 UTC 2002

On Thu, Oct 17, 2002 at 07:45:39AM -0700, Stephen Stuart wrote:

  Traceroute sends UDP datagrams and receives ICMP datagrams in order to
  show you what it shows you. It is possible for the ICMP datagrams to
  return via a different path than the UDP datagrams took outbound (it
  is also possible that they will not return).

remark it is also possible for the (forward or reverse)
path to change in the middle of the measurement,
such that traceroute output would lead you
to believe a path that never existed anywhere
on the Internet (i.e., one that is not manifested
in the current physical Internet) 
and you would not be able to confirm for sure
without asking the contacts for the IP links
in question how they're connected.

traceroute is a disconcertingly blunt hammer;
that we continue to use it to essentially
nail moving jello to a wall says more about us 
than about anything on the Internet
(and is quite the testimony to van who thought it up
and implemented it in a few hours 20 years ago 
and noone has come up with anything better since.)

(caida has a few hundred gigabytes of traceroute-like
output on disk, so it's at least auspicious for the
mass storage industry if not the jello nailing mission)

k

  > .or will it provide a trace of the path the packet took to reach the
  > destination?

  This is not the "or" case of the question you asked previously.

  Traceroute will display the path that the UDP datagrams took to get to
  the destination you specified. No information will be presented about
  the return path that the ICMP datagrams took.

  > According to definition, is should take the same path

  This is not a correct assertion.

  > but are there any other cases that I should be aware of?

  The traceroute man page lists a few.

  Stephen