Re: [PATCH v2 net-next] tcp: Refine SYN handling for PAWS.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From:   Kuniyuki Iwashima <kuniyu@amazon.com>
Date:   Tue, 28 Mar 2023 11:42:57 -0700
> 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. [1]
> 
> 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 62727, 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 2819965600, win 62643, options [mss 8961,sackOK,TS val 1224784076 ecr 1029816180,nop,wscale 7], length 0
> IP 10.0.0.215.60000 > 10.0.3.249.10000: Flags [.], ack 1, win 491, options [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 62727, 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, options [nop,nop,TS val 1224794914 ecr 1029816181,nop,nop,sack 1 {4156821004:4156821005}], 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 dup ACK in tcp_send_challenge_ack()
> to increase the correct counter and rate-limit it properly.
> 
> Let's check the SYN flag after the PAWS tests to avoid adding unnecessary
> overhead for most packets.
> 
> Link: https://docs.aws.amazon.com/elasticloadbalancing/latest/network/introduction.html [0]
> Link: https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html#client-ip-preservation [1]
> Fixes: 0c24604b68fc ("tcp: implement RFC 5961 4.2")

Sorry, I forgot to remove Fixes tag.
I'll post v3.