netdev - Re: [RFC net 1/2] net: set skb hash for IP6 TCP reset packet

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Date:   Mon, 17 Jul 2017 14:53:22 -0700
From:   Shaohua Li <shli@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, Kernel-team@...com,
        Florent Fourcot <flo@...rcot.fr>
Subject: Re: [RFC net 1/2] net: set skb hash for IP6 TCP reset packet

On Mon, Jul 17, 2017 at 01:51:51AM -0700, Eric Dumazet wrote:
> On Thu, 2017-07-13 at 10:56 -0700, Shaohua Li wrote:
> > From: Shaohua Li <shli@...com>
> > 
> > Please see below tcpdump output:
> 
> > The tcp reset packet has a different flowlabel, which causes our router
> > doesn't correctly close tcp connection.
> 
> This looks a bug in your router, because (IPv6 only) flowlabel is not
> part of the tuple identifying a TCP flow.

Actually it's for load balance between several routers.
> 
> >   The reason is the normal packet
> > gets the skb->hash from sk->sk_txhash, which is generated randomly.
> > ip6_make_flowlabel then uses the hash to create a flowlabel. The reset
> > packet doesn't get assigned a hash, so the flowlabel is calculated with
> > flowi6.
> > 
> > The solution is to save the hash value for timeout sock and use it for
> > reset packet.
> 
> I am a bit unsure why we need to add yet another field in TCP timewait
> structure, since :
> 
> 1) flowlabel can vary during a TCP flow lifetime.
> 2) flowlabel is different unde synflood (each syncookie gets a random
> flowlabel), and if 3rd packet comes back from the client to finish 3WHS,
> the flowlabel will again be different from the one that SYNACK used.

Is it acceptable we reuse tw_flowlabel as Florent Fourcot suggested? It makes
no sense to change flowlabel for no reason.

Thanks,
Shaohua