Summary
-------

TCP_KEEPIDLE, TCP_KEEPCOUNT, and TCP_KEEPINTERVAL are non-standard socket options supported on several platforms to provide fine control over the TCP/IP keep alive mechanism. It should be possible to set these socket options via the setOption method defined by java.net.Socket and java.nio.channels.SocketChannel.

Problem
-------

When the SO_KEEPALIVE option is enabled, TCP probes a connection that has been idle for some amount of time.  The default value for this idle period is 2 hours which is too long for most applications. The TCP_KEEPIDLE, TCP_KEEPCOUNT, TCP_KEEPINTERVAL option can be used to affect this value for a given socket.

Solution
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Add a JDK-specific socket option that supports setting TCP_KEEPIDLE, TCP_KEEPCOUNT, TCP_KEEPINTERVAL, on platforms that support it. The option can be set/get through the existing set/getOption methods on Socket and NetworkChannel.

Specification
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To jdk.net.ExtendedSocketOptions, in the jdk.net module, add the following new socket options:


    /**
     * Keep-Alive idle time.
     *
     * <p>
     * The value of this socket option is an {@code Integer} that is the number
     * of seconds of idle time before keep-alive initiates a probe. The socket
     * option is specific to stream-oriented sockets using the TCP/IP protocol.
     * The exact semantics of this socket option are system dependent.
     *
     * <p>
     * When the {@link java.net.StandardSocketOptions#SO_KEEPALIVE
     * SO_KEEPALIVE} option is enabled, TCP probes a connection that has been
     * idle for some amount of time. The default value for this idle period is
     * system dependent, but is typically 2 hours. The {@code TCP_KEEPIDLE}
     * option can be used to affect this value for a given socket.
     *
     * @since 11
     */
    public static final SocketOption<Integer> TCP_KEEPIDLE

    /**
     * Keep-Alive retransmission interval time.
     *
     * <p>
     * The value of this socket option is an {@code Integer} that is the number
     * of seconds to wait before retransmitting a keep-alive probe. The socket
     * option is specific to stream-oriented sockets using the TCP/IP protocol.
     * The exact semantics of this socket option are system dependent.
     *
     * <p>
     * When the {@link java.net.StandardSocketOptions#SO_KEEPALIVE
     * SO_KEEPALIVE} option is enabled, TCP probes a connection that has been
     * idle for some amount of time. If the remote system does not respond to a
     * keep-alive probe, TCP retransmits the probe after some amount of time.
     * The default value for this retransmission interval is system dependent,
     * but is typically 75 seconds. The {@code TCP_KEEPINTERVAL} option can be
     * used to affect this value for a given socket.
     *
     * @since 11
     */
    public static final SocketOption<Integer> TCP_KEEPINTERVAL

    /**
     * Keep-Alive retransmission maximum limit.
     *
     * <p>
     * The value of this socket option is an {@code Integer} that is the maximum
     * number of keep-alive probes to be sent. The socket option is specific to
     * stream-oriented sockets using the TCP/IP protocol. The exact semantics of
     * this socket option are system dependent.
     *
     * <p>
     * When the {@link java.net.StandardSocketOptions#SO_KEEPALIVE
     * SO_KEEPALIVE} option is enabled, TCP probes a connection that has been
     * idle for some amount of time. If the remote system does not respond to a
     * keep-alive probe, TCP retransmits the probe a certain number of times
     * before a connection is considered to be broken. The default value for
     * this keep-alive probe retransmit limit is system dependent, but is
     * typically 8. The {@code TCP_KEEPCOUNT} option can be used to affect this
     * value for a given socket.
     *
     * @since 11
     */
    public static final SocketOption<Integer> TCP_KEEPCOUNT