3.5. HTTP/2 Connection Preface
In HTTP/2, each endpoint is required to send a connection preface as a final confirmation of the protocol in use and to establish the initial settings for the HTTP/2 connection. The client and server each send a different connection preface.
The client connection preface starts with a sequence of 24 octets, which in hex notation is:
0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a
That is, the connection preface starts with the string PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n). This sequence MUST be followed by a SETTINGS frame (Section 6.5), which MAY be empty. 
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The server connection preface consists of a potentially empty SETTINGS frame (Section 6.5) that MUST be the first frame the server sends in the HTTP/2 connection.

HTTP/2 Connection Preface / HTTP/2连接前奏
在HTTP/2中，要求两端都要发送一个连接前奏，作为对所使用协议的最终确认，并确定HTTP/2连接的初始设置。客户端和服务端各自发送不同的连接前奏。
客户端连接前奏以一个24字节的序列开始，用十六进制表示为：
0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a
即，连接前奏以字符串"PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n"开始。这个序列后面必须跟一个可以为空的 SETTINGS 帧( 6.5节 )。
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服务端连接前奏包含一个可能为空的 SETTINGS 帧( 6.5节 )，它必须由服务端在HTTP/2连接中首先发送。