作用：A Ping frame may serve either as a keepalive or as a means to verify that the remote endpoint is still responsive.
Ping和Pong的关系：
1. Upon receipt of a Ping frame, an endpoint MUST send a Pong frame in response…
2. A Pong frame sent in response to a Ping frame must have identical "Application data" as found in the message body of the Ping frame being replied to.