Message-ID: <CALx6S34C5aT+UX-Rz39iu5FX-U-EAw23ZiBEcGP90HZRj4xf+g@mail.gmail.com>
Date:   Tue, 15 Aug 2017 17:15:46 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     Shaohua Li <shli@...nel.org>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH V4 net 0/2] ipv6: fix flowlabel issue for reset packet

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.

> 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.

> 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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