Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!rpi!uupsi!sunic!nuug!sigyn.idt.unit.no!ugle.unit.no!spurv.runit.sintef.no!he
From: he@spurv.runit.sintef.no (Havard Eidnes)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Warning: Keep-Alive considered harmful
Summary: Death on ICMP unreachable also harmful
Message-ID: <1990Nov17.164522.9996@ugle.unit.no>
Date: 17 Nov 90 16:45:22 GMT
References: <1990Nov16.164448.9918@bwdls61.bnr.ca> <9011170344.AA20268@gaak.LCS.MIT.EDU>
Sender: news@ugle.unit.no
Organization: Computing Center at the University of Trondheim, Norway
Lines: 30

I agree that using TCP keepalives is a bad idea.  I just want to comment on
the specific example Michael A. Patton mentioned, making me able "with
Internet Hosts Requirements in hand" to point out one other area where
traditional implementations of TCP should be changed to improve robustness
in the case of temporary network failures.

In article <9011170344.AA20268@gaak.LCS.MIT.EDU> MAP@LCS.MIT.EDU (Michael A. Patton) writes:
>
>Just this morning I had a user complaint that they couldn't FTP
>a file between two distant hosts.  The problem resolved to a link that
>dropped out for several minutes every half hour or so, but the transfer
>time for the file they wanted was 45 minutes.  When the link dropped
>out, they would get punted because of keep-alives, then they had to
>start over.  If only they weren't running keep-alives on the FTP
>Server it would have worked, the person doing the transfer had enough
>patience, if only the computer had.

It is of course possible that this connection was blown away by the server
using TCP keepalives. However, one other possibility (perhaps more likely)
is that an intermediate gateway issued ICMP net unreachable messages while
the temporary network outage lasted.  Traditional implementations (including
the original BSD 4.3 version) of TCP blow away a live TCP connection when
they receive an ICMP net unreachable message. The Host Requirements state
that a host implementation of TCP MUST NOT do this (specifically: the ICMPs
net unreachable, host unreachable or source route failed should be
considered temporary failures and not permanent conditions). Some gateways
have the ability to turn off the sending of ICMP net unreachables, but this
is just a workaround for "broken" host implementations.

- Havard