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From: narten@PURDUE.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: ICMP in ISO
Message-ID: <8704210222.AA05136@gwen.cs.purdue.edu>
Date: Mon, 20-Apr-87 21:22:28 EST
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Posted: Mon Apr 20 21:22:28 1987
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>It is interesting to note that a TCP/UPD Echo service is trivial to
>implement, since all it takes is a swap of addresses (and ports in some
>implementations), which makes it no more intrusive than ICMP Echo.

While you might get away with this at the TCP layer, it leads to
interesting behavior when done with UDP or at lower layers.  A certain
dedicated IP gateway we have does this very thing when generating ICMP
errors, and answering ICMP echo requests.  Imagine further, a very
widely distributed version of ping that allowed one to send ICMP echo
requests to IP broadcast addresses.  While pings are harmless enough,
I have seen port unreachables fly across our Ethernet that "originate"
from an IP broadcast address.

Thomas