ICMP isnt the problem. Some of the load balancers and proxy setups didnt
allow ECN frames through. ICMP blocking just breaks path mtu discovery and
accessing the site via IPsec, via mobile ip and a few other things.
And you can tunnel data over ack sequence spaces, IP over http is trivial.
There are reasons proper proxy setups have passwords outgoing and do not let
any control data/header info across untouched
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