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TCP keep alives

Dave Perry

I have a problem with the TCP connection being dropped. The far end drops the connection after a few minutes.

I believe I need to send TCP keepalives periodically to stop the connection from dropping, but I don't see a way to do this. I tried sending a zero length buffer to tcp_send() (with the 3rd param set to 0). That didn't seem to do anything.

Then I tried sending one byte of garbage data periodically, and this keeps the connection up.

I don't want to send garbage. How can I send a proper keepalive (an empty packet with the ACK flag set)?

Dave

  • 0

    It is currently not possible to send 0-length packets with TCP. Also TcpNet does not support keep-alive mechanism. When a keep-alive timer has expired, TCP closes the connection, if not already done by far end.

    A solution is (as you have already figured out) to send a dummy TCP data packet with at least 1 byte or to close a TCP connection and open a new one when data needs to be send out. This is how our HTTP server, Telnet server and other TCP based applications work. We do not keep TCP connections alive for long time. We simply open a TCP connection, transfer some data and then close the connection.

  • 0 in reply to Franc Urbanc

    I am writing an FTP client. The control socket times out before the data socket has finished transferring data. I will have to find some sequence to send which is harmless to the FTP server. I don't have control over which FTP server will be used. This doesn't seem good to me.

    Do you know if Keil is planning to support tcp keepalive at some point?

    Dave

  • 0 in reply to Dave Perry

    You can send STAT command during a file transfer. It will be used for a harmless keep-alive data transfer in your case.

    RFC 959 says:

    This command shall cause a status response to be sent over the control connection
in the form of a reply. The command may be sent during a file transfer (along
with the Telnet IP and Synch signals--see the Section on FTP Commands) in which
case the server will respond with the status of the operation in progress,
or it may be sent between file transfers.

  • 0 in reply to Franc Urbanc

    Thanks. That would work, or perhaps a telnet negotiation sequence like IAC NOP. There are ways around it.

    Dave