netdev - Re: wireless vs. alignment requirements

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Date:	Sun, 25 Nov 2007 18:04:17 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	dsd@...too.org, davem@...emloft.net, netdev@...r.kernel.org,
	linux-wireless@...r.kernel.org
Subject: Re: wireless vs. alignment requirements


> > Hmm. I don't think so. Take an AP for example. It gets a lot of packets
> > from stations. Now, if you're not QoS capable then all is well. But i
> > you are and some stations are as well then all those stations send QoS
> > packets (+2 bytes). Or take an AP connected via wireless (WPS), WPS has
> > +6 bytes so I get all incoming upstream traffic with such unaligned
> > headers.
> 
> The question is does this actually change all the time.  Let's
> say you took a random sample of a second worth of IP packets
> over wireless, what proportion of them are going to have the
> same hardware header length modulo 4?

I'd think that totally depends on the traffic. If you have a non-QoS AP
with WPS upstream connection, then the traffic to stations will be
four-byte aligned while the WPS upstream will be at a 2-byte-mod-4
boundary. And you'll have all packets from stations come in aligned and
all response packets from wherever come in as WPS.

johannes

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