ajlill at ajlc
Jan 20, 2010, 8:15 PM
Post #2 of 2
(336 views)
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Re: mythfilldatabase getting bad start date message from SD
[In reply to]
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f-myth-users [at] media writes:
> > Date: Thu, 07 Jan 2010 13:14:12 -0500
> > From: Tony Lill <ajlill [at] ajlc>
>
> > The interesting thing is that the tcp packet is split into two IP
> > datagrams, the second's payload starting right at 8T05:00:00Z
>
> I haven't seen anything more on this thread, and don't know if you're
> still having problems, but the fact that you're seeing this is a
> smoking gun for a bad load balancer at the SD end or some similar
> shenanigan---I've seen cheaply-made proxies/loadbalancers/whatever
> that assume that an entire GET is going to fit into the first
> datagram, even though TCP explicitly makes no such assurance since
> the MTU could well be tiny or the GET could be huge.
>
> (Of course, the -correct- behavior is to actually read until the end
> of the request, no matter how many datagrams and/or fragments that is,
> do reassembly, and -then- decide what to do with it, but that requires
> a possibly-large amount of buffer memory and the smarts to do the
> requisite reassembly of packets or fragments thereof, and many vendors
> just punt and assuem they'll never get in trouble for doing so---they
> just assume that -of course- MTU is always Ethernet's 1500 max and -of
> course- all GETs fit in that size, etc.)
>
> If I'm right, one way to test this would be to artificially decrease
> the MTU at the requesting side to something tiny (100 bytes? 50?) and
> then see if the SD end breaks.
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Once I figured out what was wrong, I turned on --dd-grab-all, so I
have no idea if it's still happening. Might try changing the MTU if I
get bored sometime.
--
Tony Lill, Tony.Lill [at] AJLC
President, A. J. Lill Consultants (519) 650 0660
539 Grand Valley Dr., Cambridge, Ont. N3H 2S2 (519) 241 2461
--------------- http://www.ajlc.waterloo.on.ca/ ----------------
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