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Date: Wed, 21 Jul 2004 10:20:24 -0700 (PDT)
From: Don Lewis <truckman@FreeBSD.org>
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cc: matt+list.sw.freebsd.current@publicly.neglect.us
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Subject: Re: fetch hangs, part 2
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On 21 Jul, Darren Pilgrim wrote:
>> From: Robert Watson [mailto:rwatson@freebsd.org] 
>> 
>> 
>> On Tue, 20 Jul 2004, Darren Pilgrim wrote:
>> 
>> > I've been having the same (or a similar problem).  Fetching the distfile
>> > for a port will hang.  I poked some more at this and found that all the
>> > sites fetch is hanging on successfully negotiate the TCP connection then
>> > go dead without closing the connection.  My workaround has been to edit
>> > the Makefile and remove the offending site from MASTER_SITES or manually
>> > fetch from ftp.freebsd.org. 
>> 
>> Could you try disabling TCP SACK
> 
> Set net.inet.tcp.delayed_ack=0?
> 
>> and see if things "get better"?  It's one
>> of the things that has changed in the TCP code recently.  This could well
>> be a user space fetch issue, but it would be worth trying it to see, if
>> only to rule it out.
> 
> It didn't change.  The connections still hung.

Smells somewhat like a broken path MTU discovery problem.  There might
be a hop in the path with a smaller than expected MTU and something
between there and the server that is filtering ICMP.

An interesting experiment would be to see what happens if a smaller than
normal MSS is requested.  This would probably be easiest for someone
with a PPP connection.