netdev - Re: TCP_DEFER_ACCEPT brokenness?

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Message-ID: <Pine.LNX.4.64.0701300104130.17450@twinlark.arctic.org>
Date:	Tue, 30 Jan 2007 01:05:12 -0800 (PST)
From:	dean gaudet <dean@...tic.org>
To:	netdev@...r.kernel.org
cc:	mtk-manpages@....net
Subject: Re: TCP_DEFER_ACCEPT brokenness?

ping.  i received no response on this one..

thanks
-dean

On Sat, 30 Dec 2006, dean gaudet wrote:

> hi... i'm having troubles matching up the tcp(7) man page description of 
> TCP_DEFER_ACCEPT versus some comments in the kernel (2.6.20-rc2) versus 
> how the kernel actually acts.
> 
> the man page says this:
> 
>    TCP_DEFER_ACCEPT
>         Allows a listener to be awakened only when data arrives on
>         the socket.  Takes an integer value (seconds), this can bound
>         the maximum number of attempts TCP will make to complete the
>         connection.  This option should not be used in code intended to
>         be portable.
> 
> which is a bit confusing because it talks both about seconds and
> "attempts".  (and doesn't mention what happens when the timeout finishes
> -- i could see dropping the socket or passing it to userland anyhow as
> possibilities... but in fact the socket is dropped).
> 
> the setsockopt code in tcp.c does this:
> 
>         case TCP_DEFER_ACCEPT:
>                 icsk->icsk_accept_queue.rskq_defer_accept = 0;
>                 if (val > 0) {
>                         /* Translate value in seconds to number of
>                          * retransmits */
>                         while (icsk->icsk_accept_queue.rskq_defer_accept < 32 &&
>                                val > ((TCP_TIMEOUT_INIT / HZ) <<
>                                        icsk->icsk_accept_queue.rskq_defer_accept))
>                                 icsk->icsk_accept_queue.rskq_defer_accept++;
>                         icsk->icsk_accept_queue.rskq_defer_accept++;
>                 }
>                 break;
> 
> so at least the comment agrees with the man page -- however the code
> doesn't... the code finds the least n such that val < (3<<n)...  but these
> are timeouts and they're cumulative -- it would be more appropriate to
> search for least n such that
> 
>         val < (3<<0) + (3<<1) + (3<<2) + ... + (3<<n)
> 
> but that's not all that's wrong... i'm not sure why, for val == 1 it
> computes n=0 correctly (verified with getsockopt) but then it defers
> way more timeouts than 2.  here's a tcpdump example where the timeout
> was set to 1:
> 
> 1167532741.446027 IP 127.0.0.1.56733 > 127.0.0.1.53846: S 1792609127:1792609127(0) win 32792 <mss 16396,sackOK,timestamp 249615 0,nop,wscale 5>
> 1167532741.446899 IP 127.0.0.1.53846 > 127.0.0.1.56733: S 1785169552:1785169552(0) ack 1792609128 win 32768 <mss 16396,sackOK,timestamp 249616 249615,nop,wscale 5>
> 1167532741.446122 IP 127.0.0.1.56733 > 127.0.0.1.53846: . ack 1 win 1025 <nop,nop,timestamp 249616 249616>
> 1167532745.249902 IP 127.0.0.1.53846 > 127.0.0.1.56733: S 1785169552:1785169552(0) ack 1792609128 win 32768 <mss 16396,sackOK,timestamp 250566 249616,nop,wscale 5>
> 1167532745.249912 IP 127.0.0.1.56733 > 127.0.0.1.53846: . ack 1 win 1025 <nop,nop,timestamp 250566 250566,nop,nop,sack 1 {0:1}>
> 1167532751.648046 IP 127.0.0.1.53846 > 127.0.0.1.56733: S 1785169552:1785169552(0) ack 1792609128 win 32768 <mss 16396,sackOK,timestamp 252166 250566,nop,wscale 5>
> 1167532751.648058 IP 127.0.0.1.56733 > 127.0.0.1.53846: . ack 1 win 1025 <nop,nop,timestamp 252166 252166,nop,nop,sack 1 {0:1}>
> 1167532764.448456 IP 127.0.0.1.53846 > 127.0.0.1.56733: S 1785169552:1785169552(0) ack 1792609128 win 32768 <mss 16396,sackOK,timestamp 255366 252166,nop,wscale 5>
> 1167532764.448473 IP 127.0.0.1.56733 > 127.0.0.1.53846: . ack 1 win 1025 <nop,nop,timestamp 255366 255366,nop,nop,sack 1 {0:1}>
> 1167532788.452409 IP 127.0.0.1.53846 > 127.0.0.1.56733: S 1785169552:1785169552(0) ack 1792609128 win 32768 <mss 16396,sackOK,timestamp 261366 255366,nop,wscale 5>
> 1167532788.452430 IP 127.0.0.1.56733 > 127.0.0.1.53846: . ack 1 win 1025 <nop,nop,timestamp 261366 261366,nop,nop,sack 1 {0:1}>
> 1167532836.453520 IP 127.0.0.1.53846 > 127.0.0.1.56733: S 1785169552:1785169552(0) ack 1792609128 win 32768 <mss 16396,sackOK,timestamp 273366 261366,nop,wscale 5>
> 1167532836.453539 IP 127.0.0.1.56733 > 127.0.0.1.53846: . ack 1 win 1025 <nop,nop,timestamp 273366 273366,nop,nop,sack 1 {0:1}>
> 
> 
> now honestly i don't mind if 1s works correctly (because
> apache 2.2.x is broken and sets TCP_DEFER_ACCEPT to 1 ... see
> <http://issues.apache.org/bugzilla/show_bug.cgi?id=41270>).
> 
> but even if i use more reasonable timeouts like 30s it doesn't
> behave as expected based on the docs.
> 
> not sure which way this should be resolved -- or how long the code has 
> been like this...  perhaps the current behaviour should just become the 
> documented behaviour (whatever the current behaviour is :).
> 
> -dean
> -
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