netdev - Re: [PATCH V4 net 0/2] ipv6: fix flowlabel issue for reset packet

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Date:   Thu, 10 Aug 2017 09:30:33 -0700
From:   Shaohua Li <shli@...nel.org>
To:     Tom Herbert <tom@...bertland.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>, Shaohua Li <shli@...com>
Subject: Re: [PATCH V4 net 0/2] ipv6: fix flowlabel issue for reset packet

On Wed, Aug 09, 2017 at 09:40:08AM -0700, Tom Herbert wrote:
> On Mon, Jul 31, 2017 at 3:19 PM, Shaohua Li <shli@...nel.org> wrote:
> > From: Shaohua Li <shli@...com>
> >
> > Please see below tcpdump output:
> > 21:00:48.109122 IP6 (flowlabel 0x43304, hlim 64, next-header TCP (6) payload length: 40) fec0::5054:ff:fe12:3456.55804 > fec0::5054:ff:fe12:3456.5555: Flags [S], cksum 0x0529 (incorrect -> 0xf56c), seq 3282214508, win 43690, options [mss 65476,sackOK,TS val 2500903437 ecr 0,nop,wscale 7], length 0
> > 21:00:48.109381 IP6 (flowlabel 0xd827f, hlim 64, next-header TCP (6) payload length: 40) fec0::5054:ff:fe12:3456.5555 > fec0::5054:ff:fe12:3456.55804: Flags [S.], cksum 0x0529 (incorrect -> 0x49ad), seq 1923801573, ack 3282214509, win 43690, options [mss 65476,sackOK,TS val 2500903437 ecr 2500903437,nop,wscale 7], length 0
> > 21:00:48.109548 IP6 (flowlabel 0x43304, hlim 64, next-header TCP (6) payload length: 32) fec0::5054:ff:fe12:3456.55804 > fec0::5054:ff:fe12:3456.5555: Flags [.], cksum 0x0521 (incorrect -> 0x1bdf), seq 1, ack 1, win 342, options [nop,nop,TS val 2500903437 ecr 2500903437], length 0
> > 21:00:48.109823 IP6 (flowlabel 0x43304, hlim 64, next-header TCP (6) payload length: 62) fec0::5054:ff:fe12:3456.55804 > fec0::5054:ff:fe12:3456.5555: Flags [P.], cksum 0x053f (incorrect -> 0xb8b1), seq 1:31, ack 1, win 342, options [nop,nop,TS val 2500903437 ecr 2500903437], length 30
> > 21:00:48.109910 IP6 (flowlabel 0xd827f, hlim 64, next-header TCP (6) payload length: 32) fec0::5054:ff:fe12:3456.5555 > fec0::5054:ff:fe12:3456.55804: Flags [.], cksum 0x0521 (incorrect -> 0x1bc1), seq 1, ack 31, win 342, options [nop,nop,TS val 2500903437 ecr 2500903437], length 0
> > 21:00:48.110043 IP6 (flowlabel 0xd827f, hlim 64, next-header TCP (6) payload length: 56) fec0::5054:ff:fe12:3456.5555 > fec0::5054:ff:fe12:3456.55804: Flags [P.], cksum 0x0539 (incorrect -> 0xb726), seq 1:25, ack 31, win 342, options [nop,nop,TS val 2500903438 ecr 2500903437], length 24
> > 21:00:48.110173 IP6 (flowlabel 0x43304, hlim 64, next-header TCP (6) payload length: 32) fec0::5054:ff:fe12:3456.55804 > fec0::5054:ff:fe12:3456.5555: Flags [.], cksum 0x0521 (incorrect -> 0x1ba7), seq 31, ack 25, win 342, options [nop,nop,TS val 2500903438 ecr 2500903438], length 0
> > 21:00:48.110211 IP6 (flowlabel 0xd827f, hlim 64, next-header TCP (6) payload length: 32) fec0::5054:ff:fe12:3456.5555 > fec0::5054:ff:fe12:3456.55804: Flags [F.], cksum 0x0521 (incorrect -> 0x1ba7), seq 25, ack 31, win 342, options [nop,nop,TS val 2500903438 ecr 2500903437], length 0
> > 21:00:48.151099 IP6 (flowlabel 0x43304, hlim 64, next-header TCP (6) payload length: 32) fec0::5054:ff:fe12:3456.55804 > fec0::5054:ff:fe12:3456.5555: Flags [.], cksum 0x0521 (incorrect -> 0x1ba6), seq 31, ack 26, win 342, options [nop,nop,TS val 2500903438 ecr 2500903438], length 0
> > 21:00:49.110524 IP6 (flowlabel 0x43304, hlim 64, next-header TCP (6) payload length: 56) fec0::5054:ff:fe12:3456.55804 > fec0::5054:ff:fe12:3456.5555: Flags [P.], cksum 0x0539 (incorrect -> 0xb324), seq 31:55, ack 26, win 342, options [nop,nop,TS val 2500904438 ecr 2500903438], length 24
> > 21:00:49.110637 IP6 (flowlabel 0xb34d5, hlim 64, next-header TCP (6) payload length: 20) fec0::5054:ff:fe12:3456.5555 > fec0::5054:ff:fe12:3456.55804: Flags [R], cksum 0x0515 (incorrect -> 0x668c), seq 1923801599, win 0, length 0
> >
> > The flowlabel of reset packet (0xb34d5) and flowlabel of normal packet
> > (0xd827f) are different. This causes our router doesn't correctly close tcp
> > connection. The patches try to fix the issue.
> >
> Shaohua,
> 
> Can you give some more detail about what the router doesn't close the
> TCP connection means? I'm guessing the problem is either: 1) the
> router is maintaining connection state that includes the flow label in
> a connection tuple. 2) some router in the path is maintaining
> connection state, but when the flow label changes the flow's packet
> are routed through a different router that doesn't have a state for
> the flow it drops the packet. #1 should be easily fix in the router,
> flow labels cannot be used as state. #2 is the known problem that
> stateful firewalls have killed our ability to use multihoming.

The #2 is exactly the problem we saw.
 
> Another consideration is that sk_txhash is also used in routing
> decisions by the local host (flow label is normally derived from
> txhash). If you want to ensure that connections are routed
> consistently for timewait state you might need sk_txhash saved also.

As far as I understood, we don't use sk_txhash for routing selection. The code
does routing selection with flowlabel user configured, at that time we don't
derive fl6.flowlabel from skb->hash (which is from sk_txhash). The code always
does routing selection first and then uses ip6_make_flowlabel to build packet
data where we derive flowlabel from skb->hash.

Thanks,
Shaohua