java.net.Socket TCP keep-alive用法

Question

How to use java.net.Socket setKeepAlive(boolean b) API?

I am hosting a simple server using Socket . The clients can connect and send data. I will not close the connection unless the client sends end of stream. The client can continue to hold a connection for any length of time. Data transfer will be going on, but suddenly I see that there is a connection loss without any logs whatsoever. Once the client is restarted (not server), the transmission begins again. I am not setting any timeout as by default it is infinity. I want to understand what setKeepAlive actually does so that I will be able to decide whether I need to set that to true . Can someone help me understand the same? thanks.

Answer 1

As the documentation for setKeepAlive() says, it will enable (or disable) the SO_KEEPALIVE option on the socket.

When the keepalive option is set for a TCP socket and no data has been exchanged across the socket in either direction for 2 hours (NOTE: the actual value is implementation dependent), TCP automatically sends a keepalive probe to the peer. This probe is a TCP segment to which the peer must respond. One of three responses is expected: 1. The peer responds with the expected ACK. The application is not notified (since everything is OK). TCP will send another probe following another 2 hours of inactivity. 2. The peer responds with an RST, which tells the local TCP that the peer host has crashed and rebooted. The socket is closed. 3. There is no response from the peer. The socket is closed. The purpose of this option is to detect if the peer host crashes. Valid only for TCP socket: SocketImpl

Here is another reference explaining the SO_KEEPALIVE option.

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Note that in networking, connections can be lost at any time for a myriad of reasons. If the connection traverses a NAT router, the entry in the NAT table could expire (when the connection is idle) and the connection is lost due to that. The client could cease to function, or be suspended (especially laptops and mobile devices), or a cable could be disconnected, or WiFi (or cellular) signal could be interfered with, or ... the list can go on. Your server needs to be written to cope gracefully with loss of connection.

Answer 2

I face the similar problem in my case

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.