首先，客户端向服务端发出一个 HTTP 请求，请求中携带了服务端规定的信息，并在信息中表明希望将协议升级为 WebSocket。这个请求被称为升级请求，双端升级协议的整个过程叫做握手。然后服务端验证客户端发送的信息，如果符合规范则将协议替换成 WebSocket，并将升级成功的信息响应给客户端。最后，双方就可以基于 WebSocket 协议互相推送信息了。现在，我们需要学习的第一个知识点就是握手。
双端握手
我们先来看看 RFC6455 对客户端握手的规定，原文锚点链接为 Opening Handshak。此段原文如下：
The opening handshake is intended to be compatible with HTTP-based server-side software and intermediaries, so that a single port can be used by both HTTP clients talking to that server and WebSocket clients talking to that server. To this end, the WebSocket client's handshake is an HTTP Upgrade request: GET /chat HTTP/1.1 Host: server.example.com Upgrade: websocket Connection: Upgrade Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ== Origin: http://example.com Sec-WebSocket-Protocol: chat, superchat Sec-WebSocket-Version: 13 In compliance with [RFC2616], header fields in the handshake may be sent by the client in any order, so the order in which different header fields are received is not significant.