generic-message  =  start-line
                             *message-header
                             CRLF
                             [ message-body ]
         start-line       =  Request-Line / Status-Line

其中在rfc2543中规定

CR        =  %d13 ; US-ASCII CR, carriage return character 
LF        =  %d10 ; US-ASCII LF, line feed character
项目十进制字符串表示
CR13\r
LF10\n

也就是说在一个SIP消息中

headline\r\n
key:v\r\n
\r\n
some_body\r\n

所以CRLF就是 \r\n 

参考