netdev - Re: [RFC] State resolution packet queue for IPsec

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Date:	Tue, 22 Jan 2013 12:10:29 +0100
From:	Steffen Klassert <steffen.klassert@...unet.com>
To:	David Miller <davem@...emloft.net>
Cc:	herbert@...dor.apana.org.au, netdev@...r.kernel.org
Subject: Re: [RFC] State resolution packet queue for IPsec

On Mon, Jan 21, 2013 at 04:17:16PM -0500, David Miller wrote:
> From: Steffen Klassert <steffen.klassert@...unet.com>
> Date: Mon, 21 Jan 2013 10:48:48 +0100
> 
> > I did a implementation of a state resolution packet queue for IPsec.
> > The original idea is described in git commit 14e50e57a
> > ([XFRM]: Allow packet drops during larval state resolution.)
> > 
> > Since we don't have any notifiers for xfrm state resolution, I used
> > a timer that does a relookup in given intervals. This is mainly to speed
> > up the start of tcp connections, so I tried to do the relookup in a higher
> > rate that tcp would resend the syn packet. However, the relookup intervals,
> > the queue size and the timeout value are choosen by gut instincts, so
> > could be suboptimal. If anyone has some insight on good choices of these
> > values, please let me know. All other comment are welcome too, of course.
> 
> This is probably the best way to solve this problem without adding
> xfrm_state resolution notifications.
> 
> But really why would notifications be so bad even if they would not
> be fine grained?
> 
> Any time a state is resolved, any state, you run what you have put
> currently into this new timer function.
> 

When we use state resolution notifiers, we have two problems.
First, we would probably not dequeue the packets on the same
cpu we enqueued them, this can introduce packet reorder. The
timer will at least try to run on the local cpu.

Second, as you already mentioned, even with state resolution
notifiers, we don't know if the inserted state is the one we
need and if our state bundle is complete with this state.
We would need to do a lookup whenever a state is inserted,
so we hog the cpu that tries to insert the states we need.

With the timer, we don't have these two problems for the price
that we have take a good choice on the relookup intervals.
Not sure what's better here.

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