"Since a non-trivial number of network elements discard well-known
ICMP packets the results of our tests do not offer hope for protocol
designers proposing to use new ICMP messages to signal various
network path properties to end systems (e.g., for explicit corruption
notification, handoff or outage notification, etc.)."
Yes, this is a very serious issue. Very often, a server that is
configured for a service will open exactly the port necessary for that
service, and drop all other traffic. For example, a web server will
accept TCP packets sent to ports 80 and 443, but it will either drop
everything else, or be located behind a firewall that drops everything
else.
This restriction affects the way we design protocol extensions. I see
that as an argument for "in-band" signaling, e.g. parameters in TCP
packets or in IP headers of TCP packets, by opposition to "out of band",
e.g. ICMP messages.
-- Christian Huitema
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