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tags: [计算机网络, TCP, 状态机, 三次握手, 四次挥手]
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create time: 2026-05-18 01:30
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---
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# TCP 段结构与状态机
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## 概述
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TCP（Transmission Control Protocol）是互联网最核心的可靠传输协议。理解其段结构和完整状态机，是排查线上网络问题的基本功。
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## TCP 首部格式（32-bit fields）
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```
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┌─────────────┬─────────────┬───────────────────┐
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│ Source Port │ Destination Port│ Sequence No │
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│ 2 bytes │ 2 bytes │ 4 bytes │
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├─────────────┼───────────────┤ │
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│ Ack No │ Data Offset | Reserved│ECN│ CWR│ ACK │ PSH │ RST │ SYN │ FIN │
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│ 4 bytes │ 4bits │ 3 bits │ │ │ │ │ │
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├─────────────┴─────────────┴───────────────────┤
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│ Window Size │
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│ 2 bytes │
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├───────────────────┬───────────────────────────┤
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│ Checksum │ Urgent Pointer │
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│ 2 bytes │ 2 bytes │
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├───────────────────┴───────────────────────────┤
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│ Options (variable) │
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│ Max: 40 bytes │
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└───────────────────────────────────────────────┘
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Minimum header: 20 bytes
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Maximum header: 60 bytes (with options)
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```
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### 关键字段详解
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| 字段 | 大小 | 说明 |
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|------|------|------|
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| **Source/Dest Port** | 各 2B | 源端口和目的端口 |
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| **Sequence Number** | 4B | 本报文第一个字节的序号（32 bit，约 4GB） |
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| **Acknowledgment Number** | 4B | 期望收到的下一个字节序号（ACK=1 时有效） |
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| **Data Offset** | 4 bit | 首部长度，单位 4 bytes（最小 5 = 20B，最大 F = 60B） |
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| **Reserved** | 3 bit | 保留字段，必须为 0 |
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| **ECN** | 2 bit | ECN-Echo + CWR：显式拥塞通知 |
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| **URG** | 1 bit | 紧急指针有效 |
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| **ACK** | 1 bit | 确认号有效（除首次 SYN 外几乎所有包都设 ACK=1） |
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| **PSH** | 1 bit | 推送标志，告诉接收端立即提交应用层 |
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| **RST** | 1 bit | 复位连接（异常关闭） |
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| **SYN** | 1 bit | 同步序列号（建立连接） |
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| **FIN** | 1 bit | 结束标志（优雅关闭） |
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| **Window Size** | 2B | 接收窗口大小，流量控制基础 |
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| **Checksum** | 2B | 覆盖首部 + 数据的 CRC |
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| **Urgent Pointer** | 2B | URG=1 时偏移量，指示紧急数据位置 |
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### 常用 TCP Options
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| Option | 大小 | 说明 |
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|--------|------|------|
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| NOP | 1B | No Operation，用于填充对齐 |
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| MSS | 4B | Maximum Segment Size，通常 1460（1500-MT-20IP） |
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| Window Scale | 4B | 扩大窗口因子（2^x倍，最大 14 = 16384 倍） |
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| SACK Perm / SACK | 可变 | Selective Acknowledgment，选择性确认 |
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| Timestamps | 10B | TSval + TSecr，RTT 测量和防回绕 |
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## TCP 状态机——完整图
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```mermaid
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stateDiagram-v2
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[*] --> CLOSED
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note right of CLOSED
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TCP 不活跃状态
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只有监听套接字在此
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end note
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CLOSED --> LISTEN: socket() + listen()
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LISTEN --> SYN_RCVD: 收到 SYN (server only)
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LISTEN --> ESTABLISHED: connect() + remote SYN-ACK (client side rare)
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note right of SYN_SENT
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client sent SYN
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waiting for SYN-ACK
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end note
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SYN_RCVD --> ESTABLISHED: 3-Way Handshake Complete
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SYN_SENT --> ESTABLISHED: Got SYN-ACK
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ESTABLISHED --> CLOSE_WAIT: Application calls close()
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ESTABLISHED --> FIN_WAIT_1: Remote sends FIN
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FIN_WAIT_1 --> FIN_WAIT_2: Local ACK received
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FIN_WAIT_1 --> CLOSING: Simultaneous close
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FIN_WAIT_2 --> TIME_WAIT: Remote FIN received
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CLOSING --> TIME_WAIT: Remote ACK received
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CLOSE_WAIT --> LAST_ACK: Local application close
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LAST_ACK --> CLOSED: Final ACK received
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TIME_WAIT --> CLOSED: 2 * MSL timeout
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note right of TIME_WAIT
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Must wait 2*MSL
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Ensures final ACK arrives
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Prevents old duplicate packets
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end note
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note right of CLOSE_WAIT
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Server side closing
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Application must call close()
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end note
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```
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### 所有 11 种状态一览
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| # | 状态 | 触发条件 | 谁处于？ |
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|---|------|---------|---------|
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| 1 | CLOSED | 初始/终止 | 双方初始状态 |
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| 2 | LISTEN | `listen()` 系统调用 | 服务端 |
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| 3 | SYN_SENT | `connect()` 发出 SYN | 客户端 |
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| 4 | SYN_RCVD | 收到 SYN | 服务端 |
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| 5 | ESTABLISHED | 三次握手完成 | 双方 |
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| 6 | FIN_WAIT_1 | 本地调用 close() | 主动关闭方 |
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| 7 | FIN_WAIT_2 | FIN_WAIT_1 的 ACK 到达 | 主动关闭方 |
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| 8 | CLOSE_WAIT | 收到远程 FIN | **被动关闭方**（服务器！）|
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| 9 | CLOSING | 同时关闭 | 极少见 |
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| 10 | LAST_ACK | 本地发送最后一个 FIN | 被动关闭方 |
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| 11 | TIME_WAIT | 发送完最后一个 ACK | 主动关闭方 |
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> [!tip] 哪些状态是你最常看到的？
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> - **ESTABLISHED**: 正常工作
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> - **TIME_WAIT**: 高频短连接的服务端（如 nginx、API 网关）
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> - **CLOSE_WAIT**: ⚠️ **应用 Bug！** 说明服务器收到了客户端的关闭请求但没调用 close()
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### CLOSE_WAIT 堆积排查
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```bash
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# 查看 CLOSE_WAIT 数量
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$ ss -tan state close-wait | wc -l
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4521
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# 是谁引起的？查进程
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$ ss -tan state close-wait src :80
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State Recv-Q Send-Q Local Address:Port Peer Address:Port Process
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CLOSE-WAIT 0 0 10.0.0.1:80 203.0.113.5:45678 app=nginx(1234)
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# 解决方案: 修复代码中未正确关闭的文件描述符/HTTP连接
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```
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## 关联笔记
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- [[hhs/NETWORK/TCP三次握手与四次挥手]] — 详细解析握手/挥手过程
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- [[hhs/NETWORK/TCP可靠传输机制]] — 序列号/确认号的深入讲解
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- [[hhs/NETWORK/TCP拥塞控制]] — TCP 窗口的动态管理
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