观察TCP的握手机制
快速看一下TCP connection的具体过程，简单写了一个TCP server 配合wireshark来做一些分析：
建立三步握手
- SYN
C(client)向S（server）发送请求，SYN标志位为1，产生一个随机的32bit Seq Num:0x0b97d19c，TCP header如下：
Source port: 54133 (54133)
Destination port: mit-dov (91)
[Stream index: 0]
Sequence number: 0x0b97d19c
Header length: 44 bytes
Flags: 0x002 (SYN)
Window size value: 65535
[Calculated window size: 65535]
Checksum: 0xfc9c [validation disabled]
- ACK+SYN
S收到后，将SYN和ACK标志位至为1，同时产生一个新的Seq Num：0x0f8baa6e和一个Ack Num：0x0b97d19d（C的Seq Num+1）
Source port: mit-dov (91)
Destination port: 54133 (54133)
[Stream index: 0]
Sequence number: 0x0f8baa6e
Acknowledgment number: 0x0b97d19d
Header length: 44 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 65535
[Calculated window size: 65535]
Checksum: 0xb0cc [validation disabled]
- ACK
C收到后，先校验Seq Num对不对，如果正确，将ACK标志位至为1，同时产生新的Ack Num为0x0f8baa6f(S的Seq Num+1)。
Source port: 54133 (54133)
Destination port: mit-dov (91)
[Stream index: 0]
Sequence number: 0x0b97d19d
Acknowledgment: 0x0f8baa6f
Header length: 32 bytes
Flags: 0x010 (ACK)
Window size value: 8235
[Calculated window size: 131760]
[Window size scaling factor: 16]
Checksum: 0xcf36 [validation disabled]
至此，三步握手完成,链接建立，但是我们发现，在第三步的时候，C向S通知了自己window窗的大小：8235和 scaling factor：16。关于流控和滑动窗口不在这里讨论，但是当S收到后，需要调整自己的window size然后给C一个应答：
Source port: mit-dov (91)
Destination port: 54133 (54133)
[Stream index: 0]
Sequence number: 0x0f8baa6f
Acknowledgment number: 0x0b97d19d
Header length: 32 bytes
Flags: 0x010 (ACK)
Window size value: 8235
[Calculated window size: 131760]
[Window size scaling factor: 16]
Checksum: 0xcf34 [validation disabled]
Options: (12 bytes), No-Operation (NOP), No-Operation (NOP), Timestamps
数据传输
4个包交换完毕后，链接已经建立，然后C向S发数据：”iam:a”。这个时候，PSH和ACK两个flag被置为1，PSH通知S有数据到来，需要写入。同时将数据装入TCP的data部分。由于有了data，下一条seq num的值要加上data的size（5）
Ack: 1, Len: 5
Source port: 54133 (54133)
Destination port: mit-dov (91)
[Stream index: 0]
Sequence number: 0x0b97d19d
[Next sequence number: 0x0b97d19d+0x5]
Acknowledgment number:0x0f8baa6f
Header length: 32 bytes
Flags: 0x018 (PSH, ACK)
Window size value: 8235
[Calculated window size: 131760]
[Window size scaling factor: 16]
Checksum: 0x6552 [validation disabled]
Options: (12 bytes), No-Operation (NOP), No-Operation (NOP), Timestamps
[SEQ/ACK analysis]
Data (5 bytes)0000 69 61 6d 3a 61 iam:a
Data: 69616d3a61
[Length: 5]
S收到后，要给出ACK应答，重点关注Ack Num：
Source port: mit-dov (91)
Destination port: 54133 (54133)
[Stream index: 0]
Sequence number:0x0f8baa6f
Acknowledgment number: 0x0b97d1a2
Header length: 32 bytes
Flags: 0x010 (ACK)
Window size value: 8235
[Calculated window size: 131760]
[Window size scaling factor: 16]
Checksum: 0x69bf [validation disabled]
Options: (12 bytes), No-Operation (NOP), No-Operation (NOP), Timestamps
随后的一个包是S向C发的[PSH,ACK]，然后C应答[ACK]。过程同上。
四次挥手
TCP结束时需要四个包，假设C向S请求关闭连接。S会向C发送[FIN,ACK]:
Source port: 54133 (54133)
Destination port: mit-dov (91)
[Stream index: 0]
Sequence number: 12 (relative sequence number)
Acknowledgment number: 14 (relative ack number)
Header length: 32 bytes
Flags: 0x011 (FIN, ACK)
Window size value: 8234
[Calculated window size: 131744]
[Window size scaling factor: 16]
Checksum: 0x3485 [validation disabled]
C收到后，会先[ACK]，然后同样发送一个[FIN,ACK]到S：
Source port: 54133 (54133)
Destination port: mit-dov (91)
[Stream index: 0]
Sequence number: 12 (relative sequence number)
Acknowledgment number: 14 (relative ack number)
Header length: 32 bytes
Flags: 0x011 (FIN, ACK)
Window size value: 8234
[Calculated window size: 131744]
[Window size scaling factor: 16]
Checksum: 0x3485 [validation disabled]
S收到后，会给C一个[ACK]，然后连接终止。