On Mon, Dec 24, 2007 at 03:46:02PM -0500, Joey Hess wrote:
> Robert Woodcock wrote:
> > On Mon, Dec 24, 2007 at 10:07:47AM -0500, Joey Hess wrote:
> > > ICMP (-I) works, TCP SYN (-T) doesn't.
First,
This router is completely broken. It foges packets from the
destination. As everybody else does it differently I'm pretty sure you
can find an RFC that tells you this is wrong.
We can't program against all possible router faults.
How traceroute decides that it reaches the destination I don't
remember. Traceroute however takes ages: It sends out one packet,
waits for the result or times out, and then tries the next hop.
(after trying the same hop 3 times).
Mtr tries to do things a bit more parallel. This means that it sends
out request for multiple hops at once. It determins it's reached the
destination when at a certain hop, it gets back a reply from the
destination. This is exactly what you've seen.
In practise, we COULD probably detect this situation: mtr sends out
requests for 5 hops further than it knows it is reasonable. So the
first round, it sends out a request for hops 1 through 5. If it then
gets a reply from hop 4, it will in the next round probe for 1 through
9. On the other hand, it concludes it reached the destination on the
reply to the first probe. This probably happens before the probe for
hop 2 goes out (at 200ms).
Although you might want different results, mtr is behaving "as
designed", and the fact that you get odd results is the fault of
your odd network, and not mtr's.
Roger.
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