Subject: Re: ICMP specification
To: Andrew Brown <>
From: Marc Slemko <>
List: tech-net
Date: 10/04/1998 14:46:48
On Sun, 4 Oct 1998, Andrew Brown wrote:

> i've noticed two things rather recently.
> 1) microsoft's tracert program uses icmp echo requests (with varying
> ttls) instead of udp packets to high numbered ports to accomplish the
> same task.
> 2) the rfc (792) for icmp states that no icmp messages should be sent
> about icmp messages.
> is there an addendum to rfc792 that alters this restriction?  or is
> microsoft merely lucky that it works and everyone is being a little
> more permissive with what they send?

Something updates it; perhaps 1122.

The requirement that ICMP isn't sent about ICMP is too restrictive; it is
now (more properly) ICMP errors shouldn't be sent about ICMP errors.  The
way MS does it is actually better in many ways, but there will be systems
where it doesn't work but UDP will, there will be cases where UDP doesn't
work but it does, etc.

At the time Unix traceroute was implemented, the world was a different
place and many routers wouldn't send ICMP in response to ICMP.