webscoket 中的 mask 细节

掩码用于给客户端到服务端的帧数据加密（异或的方式，非常简单），对此RFC6455中给了一些细节如下：

The masking key is contained completely within the frame, as defined in Section 5.2 as frame-masking-key. It is used to mask the "Payload data" defined in the same section as frame-payload-data, which includes "Extension data" and "Application data".

我们看到，这个写的很清楚，掩码是给 payload  进行加密用的，这个位置会产生一个误解，就是，如果我没有 payload 的时候，是不是可以不需要掩码？比如一个 ping 帧，不带任何数据是否不需要mask？

答案是否定的！

协议中有这么一段话：

In the WebSocket Protocol, data is transmitted using a sequence of frames. To avoid confusing network intermediaries (such as intercepting proxies) and for security reasons that are further discussed in Section 10.3, a client MUST mask all frames that it sends to the server (see Section 5.3 for further details). (Note that masking is done whether or not the WebSocket Protocol is running over TLS.) The server MUST close the connection upon receiving a frame that is not masked