TCP 三次握手

sequenceDiagram
    participant C as Client
    participant S as Server
    C->>S: relative_seq=0, raw_seq=n1
    S->>C: relative_seq=0, raw_seq=n2, relative_ack=1, raw_ack=n1+1
    C->>S: relative_seq=1, raw_seq=raw_ack, relative_ack=1, raw_ack=n2+1

在浏览器内发起网址请求，不考虑网络层通信细节，只对于 TCP 进行三次握手建立链接的过程，抓包具体分析如下

seq与ack均分为relative和raw，即相对值与实际值。

第一次握手

Transmission Control Protocol, Src Port: 55879, Dst Port: 80, Seq: 0, Len: 0
    Source Port: 55879
    Destination Port: 80
    [Stream index: 49]
    [TCP Segment Len: 0]
    Sequence number: 0    (relative sequence number)
    Sequence number (raw): 3271402248
    [Next sequence number: 1    (relative sequence number)]
    Acknowledgment number: 0
    Acknowledgment number (raw): 0
    1011 .... = Header Length: 44 bytes (11)
    Flags: 0x002 (SYN)
        000. .... .... = Reserved: Not set
        ...0 .... .... = Nonce: Not set
        .... 0... .... = Congestion Window Reduced (CWR): Not set
        .... .0.. .... = ECN-Echo: Not set
        .... ..0. .... = Urgent: Not set
        .... ...0 .... = Acknowledgment: Not set
        .... .... 0... = Push: Not set
        .... .... .0.. = Reset: Not set
        .... .... ..1. = Syn: Set
            [Expert Info (Chat/Sequence): Connection establish request (SYN): server port 80]
        .... .... ...0 = Fin: Not set
        [TCP Flags: ··········S·]
   
   ......

客户端请求，相对seq值为 0，实际为 3271402248。ack 值为 0

Flags中设置 Syn。

第二次握手

Transmission Control Protocol, Src Port: 80, Dst Port: 55879, Seq: 0, Ack: 1, Len: 0
    Source Port: 80
    Destination Port: 55879
    [Stream index: 49]
    [TCP Segment Len: 0]
    Sequence number: 0    (relative sequence number)
    Sequence number (raw): 2899824743
    [Next sequence number: 1    (relative sequence number)]
    Acknowledgment number: 1    (relative ack number)
    Acknowledgment number (raw): 3271402249
    1000 .... = Header Length: 32 bytes (8)
    Flags: 0x012 (SYN, ACK)
        000. .... .... = Reserved: Not set
        ...0 .... .... = Nonce: Not set
        .... 0... .... = Congestion Window Reduced (CWR): Not set
        .... .0.. .... = ECN-Echo: Not set
        .... ..0. .... = Urgent: Not set
        .... ...1 .... = Acknowledgment: Set
        .... .... 0... = Push: Not set
        .... .... .0.. = Reset: Not set
        .... .... ..1. = Syn: Set
            [Expert Info (Chat/Sequence): Connection establish acknowledge (SYN+ACK): server port 80]
        .... .... ...0 = Fin: Not set
        [TCP Flags: ·······A··S·]
    
    ......

服务端应答，ack值为上一步的seq值+1，所以相对ack值为 0+1=1，ack值为3271402248+1=3271402249。相对seq值为 0，实际seq值为 2899824743。

Flags中设置 Syn和Acknowledgment。

第三次握手

Transmission Control Protocol, Src Port: 55879, Dst Port: 80, Seq: 1, Ack: 1, Len: 0
    Source Port: 55879
    Destination Port: 80
    [Stream index: 49]
    [TCP Segment Len: 0]
    Sequence number: 1    (relative sequence number)
    Sequence number (raw): 3271402249
    [Next sequence number: 1    (relative sequence number)]
    Acknowledgment number: 1    (relative ack number)
    Acknowledgment number (raw): 2899824744
    0101 .... = Header Length: 20 bytes (5)
    Flags: 0x010 (ACK)
        000. .... .... = Reserved: Not set
        ...0 .... .... = Nonce: Not set
        .... 0... .... = Congestion Window Reduced (CWR): Not set
        .... .0.. .... = ECN-Echo: Not set
        .... ..0. .... = Urgent: Not set
        .... ...1 .... = Acknowledgment: Set
        .... .... 0... = Push: Not set
        .... .... .0.. = Reset: Not set
        .... .... ..0. = Syn: Not set
        .... .... ...0 = Fin: Not set
        [TCP Flags: ·······A····]
    
    ......

客户端应答，相对seq值为 1，实际seq值为上一步服务端响应的实际ack值，相对ack值为 1，实际ack值为上一步服务端响应的实际seq值+1。

Flags中设置 Syn和Acknowledgment。

至此，TCP链接建立完毕。