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

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Message-ID: <1471537092.29842.62.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Thu, 18 Aug 2016 09:18:12 -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, 2016-08-18 at 14:48 +0200, Florian Westphal wrote:
> commit ceaa1fef65a7c2e ("tcp: adding a per-socket timestamp offset")
> added the main infrastructure that is needed for per-connection
> randomization, in particular writing/reading the on-wire tcp header
> format takes the offset into account so rest of stack can use normal
> tcp_time_stamp (jiffies).
> 
> So only two items are left:
>  - add a tsoffset for request sockets
>  - extend the tcp isn generator to also return another 32bit number
>  in addition to the ISN.
> 
> Re-use of ISN generator also means timestamps are still monotonically
> increasing for same connection quadruple.

I like the idea, but the implementation looks a bit complex.

Instead of initializing tsoffset to 0, we could simply use

jhash(src_addr, dst_addr, boot_time_rnd)

This way, even syncookies would be handled, and we do not need to
increase tcp_request_sock size.