IP头格式口诀：
版头类全分组三，
存活协议checksum，
源目选项占的宽。
IP头最少占20bytes
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| ver(4bit) | Hlen(4bit) | TOS(Type of service) 8bit | Total Length |
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| Idendification(16 bit) | Flag(3bit)| Offset(13bit) |
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| TTL(8bit) | Protocol(8bit) | checksum(16bit) |
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| source address (32bit) |
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| destination address(32bit) |
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| option(if has) |
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