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

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Date:   Mon, 17 Jul 2017 21:02:57 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Shaohua Li <shli@...nel.org>
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, 2017-07-17 at 14:53 -0700, Shaohua Li wrote:
> 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.

What happens then when flowlabel changes as I described ?

See commit 3acf3ec3f4b0 ("tcp: Change txhash on every SYN and RTO
retransmit")

> > 
> > >   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.

Sure, if you can find a way to keep storage as small as possible.

Current size is dangerously approaching 256 bytes, so we might soon use
one additional cache line (64 bytes)