TCP Hand Shakes


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。

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