netdev - Re: [PATCH v1 net] tcp: Don't drop SYN+ACK for simultaneous connect().

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Date: Thu, 04 Jul 2024 10:01:48 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>, "David S. Miller"
	 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	 <kuba@...nel.org>, David Ahern <dsahern@...nel.org>
Cc: Lawrence Brakmo <brakmo@...com>, Kuniyuki Iwashima <kuni1840@...il.com>,
  netdev@...r.kernel.org
Subject: Re: [PATCH v1 net] tcp: Don't drop SYN+ACK for simultaneous
 connect().

On Wed, 2024-07-03 at 20:57 -0700, Kuniyuki Iwashima wrote:
> RFC 9293 states that in the case of simultaneous connect(), the connection
> gets established when SYN+ACK is received. [0]
> 
>       TCP Peer A                                       TCP Peer B
> 
>   1.  CLOSED                                           CLOSED
>   2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...
>   3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT
>   4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED
>   5.  SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
>   6.  ESTABLISHED  <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
>   7.               ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED
> 
> However, since commit 0c24604b68fc ("tcp: implement RFC 5961 4.2"), such a
> SYN+ACK is dropped in tcp_validate_incoming() and responded with Challenge
> ACK.
> 
> For example, the write() syscall in the following packetdrill script fails
> with -EAGAIN, and wrong SNMP stats get incremented.
> 
>    0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3
>   +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
> 
>   +0 > S  0:0(0) <mss 1460,sackOK,TS val 1000 ecr 0,nop,wscale 8>
>   +0 < S  0:0(0) win 1000 <mss 1000>
>   +0 > S. 0:0(0) ack 1 <mss 1460,sackOK,TS val 3308134035 ecr 0,nop,wscale 8>
>   +0 < S. 0:0(0) ack 1 win 1000
> 
>   +0 write(3, ..., 100) = 100
>   +0 > P. 1:101(100) ack 1
> 
>   --
> 
>   # packetdrill cross-synack.pkt
>   cross-synack.pkt:13: runtime error in write call: Expected result 100 but got -1 with errno 11 (Resource temporarily unavailable)
>   # nstat
>   ...
>   TcpExtTCPChallengeACK           1                  0.0
>   TcpExtTCPSYNChallenge           1                  0.0
> 
> That said, this is no big deal because the Challenge ACK finally let the
> connection state transition to TCP_ESTABLISHED in both directions.  If the
> peer is not using Linux, there might be a small latency before ACK though.

I'm curious to learn in which scenarios the peer is not running Linux:
out of sheer ignorance on my side I thought simult-connect was only
possible - or at least made any sense - only on loopback. 

Thanks,

Paolo