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From Adrian Bridgett <adr...@smop.co.uk>
Subject [users@httpd] SSL, flash, javascript pauses
Date Mon, 04 Dec 2006 15:48:18 GMT
Hi all, I've posted about this before on the mod-ssl list but was
redirected here. 

I'm seeing strange behaviour using SSL, it's been difficult to try and
dig out any useful information, but I didn't see a problem with
firefox 1.5, only 2.0 (and with MSIE).  However since we can't force
users to upgrade (shame!), I need to fix this at the server end in any
case.

What I see is that the client sends SYN, gets SYN/ACK, sends ACK, then
does nothign for several seconds (upto 15seconds) and then sends the
TLS Hello.

Another behaviour I've see is:
0.0s  > SYN
0.0s  < SYN/ACK (1)
0.0s  > ACK (2)
3.7s  < SYN/ACK - dup of 1  
3.7s  > ACK - dup of 2
10.0s < SYN/ACK - dup of 1
10.0s > ACK - dup of 2
14.9s > TLS Client Hello

This is a local network and there are no lost packets - I've traced
both ends and they both log this behaviour.  I'm suprised that Apache
is sending the dup SYN/ACK - it knows the client received it last time
since it has the ACK.  OTOH the really puzzling thing to me is why the
client doesn't send the TLS Client Hello in the first place.

I suspect that it's going horribly wrong _earlier_ and these delays
are just a symptom.

This is using sifr (which does some replacement of text with Flash
script using javascript).  If I create a page with say 30 embedded
flash objects (uniquely named varients of the flash file in question)
then it loads just fine. 

In the normal case, using sifr, the client fetches the page, css, js,
maybe a couple of copies of the flash file (yes, it refetches the same
file - doh!), then it start to exhibit this strange pausing behaviour.

My latest experiments involve running 2.2.3 (debian unstable) on my
laptop.  This works fine (I'm editing /etc/hosts so that the
certificate etc match).  However if I point to a debian stable Xen
host I get hangs.  I've set the MTU on lo to 1500 but localhost still
works.

I've tried tweaking nigh on every SSL setting (mutex, sessioncache,
random source, ssl-unclean-shutdown, keepalive,
ssh-accurate-shutdown), looked at the debug (can't see any errors, but
there is a lot there).  I've also tried symlinking /dev/random to
/dev/urandom on my client in case it was a lack of entrophy issue on
the client.  I've also tweaked keepalive, pipelining, max_requests etc
in the browser (and KA on the server) to no avail.  

Needless to say, this all works fine using http.  We aren't sending
any of the headers mentioned here and I've used mod-headers to remove
the range headers in case they were causing confusion:
http://www.adobe.com/cfusion/knowledgebase/index.cfm?id=fdc7b5c&pss=rss_flashplayer_fdc7b5c

However the slowdown also affects the browser even if flash isn't
installed.

I've been banging my head against the wall for a few days on this,
shortly I'll be putting spikes in the wall to end my misery :-)

Too much debug info to append here, but if there are specific bits
then I'll post them.

I'll be suggesting that we move to sifr v3 which does some preloading
hacks to avoid the multiple requests, but I'm a bit concerned that
there is an underlying problem here somewhere that will come back to
bite us.

Thanks in advance,

Adrian 
-- 
Adrian Bridgett - adrian@smop.co.uk
GPG key available on public key servers

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