SO_KEEPALIVE：
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
从上面一段英文，what is “the peer”？
希望给一个比较专业的解释：
