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Date:	Fri, 24 Aug 2007 00:40:14 -0400
From:	John Heffner <>
To:	TJ <>
Subject: Re: Problem with implementation of TCP_DEFER_ACCEPT?

TJ wrote:
> client SYN > server LISTENING
> client < SYN ACK server SYN_RECEIVED (time-out 3s)
>                  server: inet_rsk(req)->acked = 1
> client ACK > server (discarded)
> client < SYN ACK (DUP) server (time-out 6s)
> client ACK (DUP) > server (discarded)
> client < SYN ACK (DUP) server (time-out 12s)
> client ACK (DUP) > server (discarded)
> client < SYN ACK (DUP) server (time-out 24s)
> client ACK (DUP) > server (discarded)
> client < SYN ACK (DUP) server (time-out 48s)
> client ACK (DUP) > server (discarded)
> client < SYN ACK (DUP) server (time-out 96s)
> client ACK (DUP) > server (discarded)
> server: half-open socket closed.
> With each client ACK being dropped by the kernel's TCP_DEFER_ACCEPT
> mechanism eventually the handshake fails after the 'SYN ACK' retries and
> time-outs expire.
> There is a case for arguing the kernel should be operating in an
> enhanced handshaking mode when TCP_DEFER_ACCEPT is enabled, not an
> alternative mode, and therefore should accept *both* RFC 793 and
> TCP_DEFER_ACCEPT. I've been unable to find a specification or RFC for
> implementing TCP_DEFER_ACCEPT aka BSD's SO_ACCEPTFILTER to give me firm
> guidance.
> It seems incorrect to penalise a client that is trying to complete the
> handshake according to the RFC 793 specification, especially as the
> client has no way of knowing ahead of time whether or not the server is
> operating deferred accept.

Interesting problem.  TCP_DEFER_ACCEPT does not conform to any standard 
I'm aware of.  (In fact, I'd say it's in violation of RFC 793.)  The 
implementation does exactly what it claims, though -- it "allows a 
listener to be awakened only  when  data  arrives  on  the  socket."

I think a more useful spec might have been "allows a listener to be 
awakened only when data arrives on the socket, unless the specified 
timeout has expired."  Once the timeout expires, it should process the 
embryonic connection as if TCP_DEFER_ACCEPT is not set.  Unfortunately, 
I don't think we can retroactively change this definition, as an 
application might depend on data being available and do a non-blocking 
read() after the accept(), expecting data to be there.  Is this worth 
trying to fix?

Also, a listen socket with a backlog and TCP_DEFER_ACCEPT will have reqs 
sit in the backlog for the full defer timeout, even if they've received 
data, which is not really the right thing to do.

I've attached a patch implementing this suggestion (compile tested only 
-- I think I got the logic right but it's late ;).  Kind of ugly, and 
uses up a bit in struct inet_request_sock.  Maybe can be done better...


View attachment "tcp_defer_accept.patch" of type "text/plain" (2738 bytes)

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