Does a TCP socket connection have a "keep alive"?

In JAVA Socket – TCP connections are managed on the OS level, java.net.Socket does not provide any in-built function to set timeouts for keepalive packet on a per-socket level. But we can enable keepalive option for java socket but it takes 2 hours 11 minutes (7200 sec) by default to process after a stale tcp connections. This cause connection will be availabe for very long time before purge. So we found some solution to use Java Native Interface (JNI) that call native code(c++) to configure these options.

Windows OS

In windows operating system keepalive_time & keepalive_intvl can be configurable but tcp_keepalive_probes cannot be change.By default, when a TCP socket is initialized sets the keep-alive timeout to 2 hours and the keep-alive interval to 1 second. The default system-wide value of the keep-alive timeout is controllable through the KeepAliveTime registry setting which takes a value in milliseconds.

On Windows Vista and later, the number of keep-alive probes (data retransmissions) is set to 10 and cannot be changed.

On Windows Server 2003, Windows XP, and Windows 2000, the default setting for number of keep-alive probes is 5. The number of keep-alive probes is controllable. For windows Winsock IOCTLs library is used to configure the tcp-keepalive parameters.

int WSAIoctl(
    SocketFD, // descriptor identifying a socket
    SIO_KEEPALIVE_VALS, // dwIoControlCode
    (LPVOID) lpvInBuffer, // pointer to tcp_keepalive struct (DWORD)
    cbInBuffer, // length of input buffer
    NULL, // output buffer
    0, // size of output buffer
    (LPDWORD) lpcbBytesReturned, // number of bytes returned
    NULL, // OVERLAPPED structure
    NULL // completion routine
);

Linux OS

Linux has built-in support for keepalive which is need to be enabling TCP/IP networking in order to use it. Programs must request keepalive control for their sockets using the setsockopt interface.

int setsockopt(int socket, int level, int optname, const void *optval, socklen_t optlen)

Each client socket will be created using java.net.Socket. File descriptor ID for each socket will retrieve using java reflection.

For Windows according to Microsoft docs

• KeepAliveTime (REG_DWORD, milliseconds, by default is not set which means 7,200,000,000 = 2 hours) - analogue to tcp_keepalive_time
• KeepAliveInterval (REG_DWORD, milliseconds, by default is not set which means 1,000 = 1 second) - analogue to tcp_keepalive_intvl
• Since Windows Vista there is no analogue to tcp_keepalive_probes, value is fixed to 10 and cannot be changed