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From: barmar@think.com (Barry Margolin)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Warning: Keep-Alive considered harmful
Message-ID: <1990Nov19.063111.21768@Think.COM>
Date: 19 Nov 90 06:31:11 GMT
References: <1990Nov16.164448.9918@bwdls61.bnr.ca> <9011170344.AA20268@gaak.LCS.MIT.EDU>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
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Unfortunately, keep-alives are sometimes needed to work around deficiencies
in application protocols.  For instance, there's no way for a server telnet
to detect when the client host has crashed (it could send an IAC
Are-You-There, but there's no standard for the response, so it would
confuse the process receiving the input).

However, I think the common design of keep-alives is incorrect.  The
connection shouldn't be killed as a result of keep-alive timeouts.
Instead, the purpose of keep-alives should be to elicit RSTs from the other
host.  Timeouts can be due to any number of reasons, but a RST indicates
unambiguously that the connection is unusable, because the other end
rebooted or closed the connection itself (perhaps network problems
prevented the FIN from getting through).  If a host crashes, the keepalive
won't actually notice this until it comes back up, which is probably good
enough.

Yes, this will not catch all half-open connections.  If the host dies for
good, or crashes and is given a new address, the other hosts won't
automatically kill their connections to it.  But it's better to leave some
useless connections open than to close some useful connections.  Pinging
for RSTs is fail-safe.

--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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