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定义编码方案与实际传输最高有效位不一样。详细基本语法
8.4.2.1 Basic rules的Short-integer
Short-integer = OCTET
; Integers in range 0-127 shall be encoded as a one octet value with the most significant bit set
; to one (1xxx xxxx) and with the value in the remaining least significant bits.
解释部分，最高有效位要置为1.而xxxxxxx才是编码方案的值