[lwip-users] TCP keep-alive not working on half-closed connections

[R. Diez]

Hi all:

I am using lwIP 2.0.3. I have a raw (embedded software) client that 
establishes a TCP connection to a server. When the server detects the 
incoming connection, it automatically sends data, so the client receives 
data without doing anything else.

Because the client never sends any data, I thought I could shutdown the 
Tx side of the connection on the client. The idea is that, if the server 
tries to read any data from the client, it will immediately fail. It is 
a way to detect programming errors early, because this kind of 
connections will never deliver any data to the server, so the software 
on the server should not try to read at all. The same applies to the 
client: any attempt to send data on this kind of connection would 
immediately be noticed.

In any case, I thought it was OK to keep using a half-closed TCP 
connection. In fact, it seems to be working fine (except for the 
keep-alive problem described below).

So the client calls tcp_connect( ... tcp_connected_fn connected ), and 
on that 'connected' callback, I am calling tcp_shutdown() like this:

  const int shutdownRx = 0;
  const int shutdownTx = 1;
  tcp_shutdown( m_pcb, shutdownRx, shutdownTx );

The trouble is, this changes the connection state (pcb->state) from 
ESTABLISHED to FIN_WAIT_1, which then turns quickly into FIN_WAIT_2. 
Afterwards, TCP keep-alive does not work anymore, because of this logic 
in tcp_slowtmr():

  /* Check if KEEPALIVE should be sent */
  if (ip_get_option(pcb, SOF_KEEPALIVE) &&
     ((pcb->state == ESTABLISHED) ||
      (pcb->state == CLOSE_WAIT))) {

If I then pull the server's Ethernet cable while the connection is 
transferring data, the connection never times out on the client.

I am no TCP/IP expert. Is there an error in my reasoning regarding 
half-closed connections? Can TCP keep-alive packets, whatever they are, 
still be sent and/or received on half-closed TCP connections?

Please copy me on any answers, as I am not (effectively) subscribed to 
this list.

Thanks in advance,
  rdiez