netdev - Re: [PATCH v1 net] tcp: Refine SYN handling for PAWS.

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Message-ID: <20230328164119.44065-1-kuniyu@amazon.com>
Date:   Tue, 28 Mar 2023 09:41:19 -0700
From:   Kuniyuki Iwashima <kuniyu@...zon.com>
To:     <edumazet@...gle.com>
CC:     <davem@...emloft.net>, <kuba@...nel.org>, <kuni1840@...il.com>,
        <kuniyu@...zon.com>, <netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH v1 net] tcp: Refine SYN handling for PAWS.

From:   Eric Dumazet <edumazet@...gle.com>
Date:   Tue, 28 Mar 2023 01:13:09 -0700
> On Mon, Mar 27, 2023 at 4:06=E2=80=AFPM Kuniyuki Iwashima <kuniyu@...zon.co=
> m> wrote:
> >
> > Our Network Load Balancer (NLB) [0] has multiple nodes with different
> > IP addresses, and each node forwards TCP flows from clients to backend
> > targets.  NLB has an option to preserve the client's source IP address
> > and port when routing packets to backend targets.
> >
> > When a client connects to two different NLB nodes, they may select the
> > same backend target.  Then, if the client has used the same source IP
> > and port, the two flows at the backend side will have the same 4-tuple.
> >
> > While testing around such cases, I saw these sequences on the backend
> > target.
> >
> > IP 10.0.0.215.60000 > 10.0.3.249.10000: Flags [S], seq 2819965599, win 62=
> 727, options [mss 8365,sackOK,TS val 1029816180 ecr 0,nop,wscale 7], length=
>  0
> > IP 10.0.3.249.10000 > 10.0.0.215.60000: Flags [S.], seq 3040695044, ack 2=
> 819965600, win 62643, options [mss 8961,sackOK,TS val 1224784076 ecr 102981=
> 6180,nop,wscale 7], length 0
> > IP 10.0.0.215.60000 > 10.0.3.249.10000: Flags [.], ack 1, win 491, option=
> s [nop,nop,TS val 1029816181 ecr 1224784076], length 0
> > IP 10.0.0.215.60000 > 10.0.3.249.10000: Flags [S], seq 2681819307, win 62=
> 727, options [mss 8365,sackOK,TS val 572088282 ecr 0,nop,wscale 7], length =
> 0
> > IP 10.0.3.249.10000 > 10.0.0.215.60000: Flags [.], ack 1, win 490, option=
> s [nop,nop,TS val 1224794914 ecr 1029816181,nop,nop,sack 1 {4156821004:4156=
> 821005}], length 0
> >
> > It seems to be working correctly, but the last ACK was generated by
> > tcp_send_dupack() and PAWSEstab was increased.  This is because the
> > second connection has a smaller timestamp than the first one.
> >
> > In this case, we should send a challenge ACK instead of a dup ACK and
> > increase the correct counter to rate-limit it properly.
> 
> OK, but this seems about the same thing to me. A challenge ACK is a dup ACK=
>  ?

Yes, I thought you distinguished them with the counter, and I just
followed it :)

e37158991701 ("tcp: refine SYN handling in tcp_validate_incoming")

> 
> It is not clear why it matters, because most probably both ACK make no
> sense for the sender ?

Yes, but it's tricky that which counter is increased depends on the
two timestamp.


> 
> >
> > Let's check the SYN bit after the PAWS tests to avoid adding unnecessary
> > overhead for most packets.
> >
> > Link: https://docs.aws.amazon.com/elasticloadbalancing/latest/network/int=
> roduction.html [0]
> > Link: https://docs.aws.amazon.com/elasticloadbalancing/latest/network/loa=
> d-balancer-target-groups.html#client-ip-preservation [1]
> > Fixes: 0c24604b68fc ("tcp: implement RFC 5961 4.2")
> 
> The core of the change was to not send an RST anymore.
> I did not change part of the code which was not sending an RST :)

I see.
Should I replace the tag with add 'CC: stable # backport ver', or
respin for net-next without the tag ?

Thanks,
Kuniyuki


> 
> 
> 
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > ---
> >  net/ipv4/tcp_input.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index cc072d2cfcd8..89fca4c18530 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -5714,6 +5714,8 @@ static bool tcp_validate_incoming(struct sock *sk, =
> struct sk_buff *skb,
> >             tp->rx_opt.saw_tstamp &&
> >             tcp_paws_discard(sk, skb)) {
> >                 if (!th->rst) {
> > +                       if (unlikely(th->syn))
> > +                               goto syn_challenge;
> >                         NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABRE=
> JECTED);
> >                         if (!tcp_oow_rate_limited(sock_net(sk), skb,
> >                                                   LINUX_MIB_TCPACKSKIPPED=
> PAWS,
> > --
> > 2.30.2
> >