Date:   Thu, 17 Aug 2017 15:55:21 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Tom Herbert <tom@...bertland.com>
CC:     Shaohua Li <shli@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH V4 net 0/2] ipv6: fix flowlabel issue for reset packet

On Tue, Aug 15, 2017 at 05:15:46PM -0700, Tom Herbert wrote:
> On Tue, Aug 15, 2017 at 3:42 PM, Shaohua Li <shli@...nel.org> wrote:
> > On Tue, Aug 15, 2017 at 07:08:31AM -0700, Tom Herbert wrote:
> >> On Mon, Aug 14, 2017 at 7:52 PM, Shaohua Li <shli@...nel.org> wrote:
> >> > On Fri, Aug 11, 2017 at 06:00:20PM -0700, Tom Herbert wrote:
> >> >> On Thu, Aug 10, 2017 at 12:13 PM, Shaohua Li <shli@...nel.org> wrote:
> >> >> > 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?
> >> >>
> >> >> I think that can only be measured by how critical it is to
> >> >> persistently route all packets the same exact way for every
> >> >> connection. Page one of the IP book clearly states that IP packets can
> >> >> be dropped, duplicated, or received out of order. Received OOO implies
> >> >> that packet for the same flow are allowed to take different paths. The
> >> >> requirement that packets for the same flow must always take the same
> >> >> path through the network was created by stateful middleboxes-- it's
> >> >> not inherent in the architecture of IP networking. Unfortunately,
> >> >> we're seeing this become more and more of a problem as more devices
> >> >> are multi-homed (like smart phones) and these network requirement
> >> >> cripple our ability to take advantage of features like that.
> >> >>
> >> >> Personally, I wish the middleboxes fix the problem they created, but I
> >> >> suppose we need to be pragmatic at least in the short term.
> >> >
> >> > Hmm, I still hesitate to add a new field in skc_common. Fixing current problem
> >> > looks propriate in current stage. I'd defer fixing the generic issue till it's
> >> > necessary.
> >> >
> >> Shaohua,
> >>
> >> An alternative would be to not initialize sk_txhash, but instead defer
> >> hash computation to use flow dissector in the TX path when the hash is
> >> needed (to get flow label, src port for UDP encap, route for
> >> multipath, etc.). At the first hash computation in TX path  the result
> >> in sk_txhash. TW state there is no socket so flow dissector is
> >> always used but that should yield the same hash. No extra fields would
> >> be needed and additional cost is negligible.
> >
> > Hi Tom,
> >
> > Did you mean revert 877d1f6291f8(net: Set sk_txhash from a random number)? This
> > could fix the issue. So in normal case we calculate the sk_txhash using flow
> > dissector but in negative routing case we use the random hash, is this what you
> > want?
> >
> No. What you'd want is something like a sysctl that sets an alternate
> mode for sk_txhash processing. sk_txhash is derived from flow
> dissector for the first TX packet and then it's never allowed to
> change. Maybe this should be called persistent-hash mode.
Hi Tom,

We had been using the auto_flowlabels=1 (i.e. essentially enable flowlabel)
mainly because we want to take the benefit of dst_negative_advice() when
tcp_write_timeout() happens.

During our test, our system handles quite well with changing flowlabel.
The only exception we have hit is the TCP_RST sent from an inet_timewait_sock.

If we keep the flowlabel consistent (or persistent sk_txhash), there
is no practical usage for us to turn on flowlabel and the problem also goes
away.  We have it off for now.

>
> > There seems to have other bug in this side. From my understanding, commit
> > 265f94ff54d6(net: Recompute sk_txhash on negative routing advice) tries to
> > select a different route. But the multipath selection code
> > (rt6_multipath_select) doesn't use sk_txhash or skb->hash, it does use
> > fl6.flowlabel, but that is the flowlabel user sets. So looks like the commit
> > doesn't change anything.
> >
> The routing functions typically don't use sock of skbuff, but use flow
> structs instead. It may be reasonable to add a hash to those.
The localhost's mutlipath selection is another existing issue.  AFAICT,
it does not take the sk_txhash (or skb->hash) into account and the following
dst_negative_advice() will also have no effect in the route selction.
It is another issue to be fixed and to be figured out how to pass the
sk_txhash down. (1)

Shaohua is proposing to record the 20 bits of the sk_txhash in the
tw_flowlabel of the 'struct inet_timewait_sock'.  The tw_flowlabel could
potentially be used to do the multipath selection once we figured out
how to tackle (1).

Thanks,
Martin


>
> > What's the 'src port for UDP encap'? I can't find the code setting skb->hash
> > to sk_txhash in UDP side.
> >
> udp_flow_src_port is function call by UDP encaps to set source port.
> This is call skb_get_hash. sk_set_txhash is function to set txhash
> right now to random value. skb_set_hash_from_sk set skb->hash when
> skbuff is owned by socket (skb_set_owner_w).
>
> Thanks,
> Tom
>
>
> > Thanks,
> > Shaohua

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