Date:   Thu, 10 Aug 2017 12:13:14 -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 Thu, Aug 10, 2017 at 11:30:51AM -0700, Tom Herbert wrote:
> On Thu, Aug 10, 2017 at 9:30 AM, Shaohua Li <shli@...nel.org> wrote:
> > 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.
> >
> That is assuming one particular use case. Generally, if you want to
> ensure all packets for a flow take the same path you'll need tx_hash
> and make it persistent (disable flow bender). For instance, if you
> were doing UDP encapsulation like in VXLAN the UDP source port
> selection is unaffected by saved flow label for the lifetime of the
> flow. So we would still hit #2 in that case and the stateful device
> doesn't see whole flow. It might be just as easy to move tx_hash in
> skc_common so that it's available in TW state for this purpose. Then
> when moving to TW state just copy the tx_hash.

Hi Tom,

My original implementation is to add a tx_hash in tw sock, we then copy sock's
tx_hash to the tw tx_hash. This does makes things simplier. One concern from
Eric is this will increase the size of tw sock. If we move tx_hash to
skc_common, all sock size will increase, is this acceptable?

Thanks,
Shaohua

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