> 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?

See RFC 1812 for example.  ICMP messages come in two flavours, ICMP error
messages and ICMP query messages.  ICMP echo requests are in the latter
group.  RFC 1812 says

    4.3.2.7 When Not to Send ICMP Errors

       An ICMP error message MUST NOT be sent as the result of receiving:

       o An ICMP error message, or

This is actually not a change, it is in fact identical to how most
implementers parsed the original text in RFC 792, i.e.

   The ICMP messages typically report errors in the processing of
   datagrams.  To avoid the infinite regress of messages about messages
   etc., no ICMP messages are sent about ICMP messages.  Also ICMP

where the first sentence suggests this is talking about sending errors
in response to errors rather than a general restriction.

Dennis Ferguson