From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <c2d313a519d57ee0aa369809aabc8aae@akira.nop.cx>
To: 9fans@9fans.net
From: Tim Wiess <tim@nop.cx>
Date: Thu, 17 Apr 2008 13:59:31 -0700
In-Reply-To: <41b9f1bf163c2d3c41e182407477eb28@quanstro.net>
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Subject: Re: [9fans] telnet vs. godaddy whois
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>> On Thu, 17 Apr 2008 09:18:31 BST Charles Forsyth <forsyth@terzarima.net>  wrote:
>>> > having said that, i now suspect that sending one byte into a zero-window is
>>>  not the problem.
>>>
>>> because the one-byte probe can only be done if there is data to send, and i
>>> already knew that a plain connection (dial only) to that port also failed:
>>
>> Not setting the PSH bit on a pure ACK (== no data is being
>> sent) seems to fix this (see ip/tcp.c around line 2530).  May
>> be it tickles a bug on the receiver (0 byte read?).
>
> this does work for me.  is there some subtile reason *to* set the psh bit
> on a pure ack?  in certain circumstances?
>
> good call. from rfc793, p 42:
>
>       [...]  If the the user signals a push function then the
>       data must be sent even if it is a small segment.
>
> minooka; 9diff ip/tcp.c
> /n/sources/plan9//sys/src/9/ip/tcp.c:2529,2535 - ip/tcp.c:2529,2535
>   			}
>   		}
>
> - 		if(sent+dsize == sndcnt)
> + 		if(sent+dsize == sndcnt && dsize)
>   			seg.flags |= PSH;
>
>   		/* keep track of balance of resent data */
>
> - erik

    I noticed this some time ago when I was doing some work in the
    stack and thought it was very questionable.  But I never got a
    chance to go back and do further research.  Nevertheless I think
    it's the wrong behavior.