mboned: UDP port conflict mtrace/traceroute

I think it would be great if we can capture in an mtrace errata for
later memory the reason why practically the method of per-packet port
number scheme in traceroute is highly useful and also widely
deployed.

The reason is that you want to parallelize per-hop packets so they
can overlap in time. To do this, you need a unique identifier in each
packet that can be recognized in the ICMP unreachable reply going
back to the sender. As ICMP APIs are not standardized, those
APIs may not always deliver all fields of the actual ICMP unreach
packet, so the destination port number is the safest identifier to
rely on.

Ultimately, we should ask for IANA to put for the 254 port numbers
following the traceroute assigned port number something like
"reserved for tracroute", and maybe someone who wants an easy RFC
number could write a one-pager outlining this scheme informationally ;-)

Cheers
    Toerless

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