From: John Beckett 

"Geo"  wrote in message
news::
> Seriously? I've seen programs for windows that can use either but I always
> assumed icmp was the default for all platforms.

I am no Unix guru, but I believe that Unix traceroute defaults to sending
UDP, and you need to use -I (uppercase i) to have it send ICMP.

My notes contain the info below; confirmed by man page:
http://www.linuxcommand.org/man_pages/traceroute8.html

traceroute host:
UDP packets with increasing TTL values are sent to the host, with three
packets sent for each TTL value.

Intermediate routers reply with 'ICMP Time Exceeded' when TTL = 0. The host
replies with 'ICMP Port Unreachable'.

The first packet is sent to port 33434. The port is incremented for subsequent hops.

The 'ICMP Time Exceeded' reply includes the header of the offending packet.
The port number in that header indicates the number of hops.

Windows TRACERT sends ICMP Echo Request packets. The host should reply with
'ICMP Echo Reply'. As UDP is commonly filtered, while ICMP is not, using
ICMP should work most of the time, when UDP might fail.

John

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