IP协议

	0                            15                              31
	-----------------------------------------------------------------
	|Version|HLength|   TOS         |       total length            |
	-----------------------------------------------------------------
	|       identification          |F|DF|MF| fragment offset       |
	-----------------------------------------------------------------
	|      TTL      | protocol      |       header checksum         |
	-----------------------------------------------------------------
	|                     source ip                                 |
	-----------------------------------------------------------------
	|                      destination ip                           |
	-----------------------------------------------------------------
	|                   options                    |  padding       |
	-----------------------------------------------------------------

1. DF 不能分片，
2. 分片的时候，前面的分组MF=1, 最后一个分组的MF=0
3. identification 表示源报文，
4. 分片时每个分组的相对便宜，一个单位8个字节

TCP协议

	0                            15                              31
	-----------------------------------------------------------------
	|          source port          |       destination port        |
	-----------------------------------------------------------------
	|                        sequence number                        |
	-----------------------------------------------------------------
	|                     acknowledgment number                     |
	-----------------------------------------------------------------
	|  HL   | rsvd  |C|E|U|A|P|R|S|F|        window size            |
	-----------------------------------------------------------------
	|         TCP checksum          |       urgent pointer          |
	-----------------------------------------------------------------

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