netdev - Re: [RFC 1/3] tcp: randomize tcp timestamp offsets for each connection

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Message-ID: <1472134541.14381.126.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Thu, 25 Aug 2016 07:15:41 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Florian Westphal <fw@...len.de>
Cc:     netdev@...r.kernel.org
Subject: Re: [RFC 1/3] tcp: randomize tcp timestamp offsets for each
 connection

On Thu, 2016-08-25 at 11:06 +0200, Florian Westphal wrote:
> So I gave this a try and it does avoid this tcp_request_sock increase,
> but I feel that getting boot_time_rnd is too easy.

Fair enough, I didn't think very hard about it.

> 
> I tried a few other ideas but nothing satisfying/simpler came out of it
> (e.g. i tried to also hash the isn but that gets scaled w. current
>  clock so it doesn't work).
> 
> Are you more concerned wrt. complexity or the reqsk increase?
> 

No, a reqsk increase is really fine.

I guess I was simply worrying your work would make my future submission
more tricky.

Here at Google we have been using usec resolution in TCP timestamps for
a while for all our DC traffic, and we have to upstream this at some
point.

It would be nice if the randomization was optional, because having usec
timestamps with a common base (ie no per flow random base) helps a lot
to understand the host delays at transmit time, and receive time.

I will review your patch more in depth today, thanks.