I am trying to troubleshoot a latency issue for some of our networks,
and was wondering about this.Knowing that routing isn't always
symmetrical, is it possible for a traceroute to traverse a different
reverse path, than the path that it took to get there?
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).
.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
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
That's true but only if you have compiled traceroute
without "--enable-schroedinger" ;-))
-- Arnold
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.
Although I've only seen it as part of an April's Fool prank, it is
possible to do amazingly evil things to traceroutes, Whitehouse.Gov
going through Kremvax.Su Truth is such an elusive thing. Not
only do you need to worry about the network changing while you are
measuring it, you also need to worry about the network telling you the
truth about what happened to the packet.
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.)
People have come up with other ways of tracing routers through a
packet network, snmptrace, ip record route, beacon packets. But
they all have limitations compared to Van Jacobson's traceroute.
A testiment to the power of traceroute is its now considered necessary
functionality for any data network, not just TCP/UDP/IP networks.
OSItraceroute, MPLStraceroute, ATMtraceroute, DECNETtraceroute, etc.