sockets - How to set the keepalive timeout in Android?

Question

I'd like to lower the TCP keepalive time on a Socket I'm opening from 2 hours to something on the order of ten minutes. I can make it use keepalive with socket.setKeepAlive(true), but how can I control the time before a keepalive packet is sent?

It looks like I could do this if I was using the NDK, but I want to distribute this code as a jar, so that's not ideal for me.

Answer 1

I think it might be quite important to be able to set the keepalive timeouts on an per app level, especially on a mobile device, because it might be under bad network conditions (wifi/mobile). If the app does not send (m)any data but uses a persistent connection, the socket will not detect whether the connection is lost, unless it sends tcp keepalive probes. Setting this option is usually possible via the setsockopt(2) call, but the android sdk provides only the setKeepAlive(boolean) option. Deeper in the stack, that functions calls libcore.io.ForwardingOs.setsockoptInt(...), which is not available directly, nor the required file descriptor. By using java reflection, setting the keepalive timeouts is possible anyway, e.g like this:

private final static int SOL_TCP = 6;

private final static int TCP_KEEPIDLE = 4;
private final static int TCP_KEEPINTVL = 5;
private final static int TCP_KEEPCNT = 6;

protected void setKeepaliveSocketOptions(Socket socket, int idleTimeout, int interval, int count) {
  try {
    socket.setKeepAlive(true);
    try {
      Field socketImplField = Class.forName("java.net.Socket").getDeclaredField("impl");
      if(socketImplField != null) {
        socketImplField.setAccessible(true);
        Object plainSocketImpl = socketImplField.get(socket);
        Field fileDescriptorField = Class.forName("java.net.SocketImpl").getDeclaredField("fd");
        if(fileDescriptorField != null) {
          fileDescriptorField.setAccessible(true);
          FileDescriptor fileDescriptor = (FileDescriptor)fileDescriptorField.get(plainSocketImpl);
          Class libCoreClass = Class.forName("libcore.io.Libcore");
          Field osField = libCoreClass.getDeclaredField("os");
          osField.setAccessible(true);
          Object libcoreOs = osField.get(libCoreClass);
          Method setSocketOptsMethod = Class.forName("libcore.io.ForwardingOs").getDeclaredMethod("setsockoptInt", FileDescriptor.class, int.class, int.class, int.class);
          if(setSocketOptsMethod != null) {
            setSocketOptsMethod.invoke(libcoreOs, fileDescriptor, SOL_TCP, TCP_KEEPIDLE, idleTimeout);
            setSocketOptsMethod.invoke(libcoreOs, fileDescriptor, SOL_TCP, TCP_KEEPINTVL, interval);
            setSocketOptsMethod.invoke(libcoreOs, fileDescriptor, SOL_TCP, TCP_KEEPCNT, count);
          }
        }
      }
    }
    catch (Exception reflectionException) {}
  } catch (SocketException e) {}
}

This works at least until the following requirements are met:

• libcore.io.ForwardingOs.setsockoptInt/4 exists at current sdk version
• java.net.Socket has an impl member at the current sdk version
• java.net.Socket->impl is instance of java.net.SocketImpl at the current sdk version
• java.net.SocketImpl has a fd member at the current sdk version
• TCP_KEEPIDLE, TCP_KEEPINTVL and TCP_KEEPCNT have the same values (4, 5 and 6) at the current sdk version and all android devices / architectures.

That seems to be true at least for android versions from 4.0.1 / November 2011 up to recent version 5.1.1 r9.

See luni/src/main/java/libcore/io/Os.java, luni/src/main/java/java/net/Socket.java and luni/src/main/java/java/net/SocketImpl.java from the platform/libcore repository. TCP_KEEPIDLE, TCP_KEEPINTVL and TCP_KEEPCNT seem to have the same values for android versions since 2.2.3 r2 and all architectures. This can be validated e.g. by executing find . -name tcp.h | xargs grep -ho "TCP_KEEPw+s+d+" | sort | uniq -c in the android platform/ndk repository.