🤝 传输控制协议：可靠送达的握手🤝 TCP: The Reliable Handshake

上一片里，DNS 帮小电脑查到了门牌号。可包裹送到张三手里，丢了算谁的？小电脑寄生日祝福，得保证一件不少。 Last leaf, DNS found the house number for the little computer. But who pays if a parcel goes missing? Sending birthday wishes to Sam, it must deliver every piece.

第 1 步：开寄先握三下手Step 1: Shake hands three times first

小电脑抱着箱子就寄吗？先敲门。Does the little computer just ship the box? It knocks first.

小电脑和张三三次握手确认都在家再开寄 — 小电脑喊：在家吗（SYN）？张三回：在家，寄吧（SYN-ACK）！小电脑说：好，开寄（ACK）！三句确认都在家，路才通。这就是三次握手。 The little computer calls: home (SYN)? Sam answers: home, send it (SYN-ACK)! It replies: OK, sending (ACK)! Three lines confirm both sides are ready. That is the three-way handshake.

路上小包走散了，有的还丢了，怎么办？Packets scatter on the road, and some get lost — now what?

第 2 步：拆包编号，缺了补寄Step 2: Number the pieces, resend what's lost

大箱拆成编号小包乱序到达按序拼回缺号补寄 — 大箱拆成 1 到 5 号小包，到时顺序全乱。张三按编号拼回：1、2、3 上架了，4 号空着。小电脑喊：缺 4 号，请补寄！丢包重传，一件不少。 The big box is split into packets 1 to 5, arriving all jumbled. Sam shelves them by number: 1, 2, 3 are up, slot 4 is empty. The little computer calls: #4 is missing, please resend! Lost packets are resent, so nothing is lost.

件件都补寄，会不会太慢？Resending every piece — isn't that slow?

第 3 步：稳车还是纸飞机Step 3: Steady van or paper plane

TCP稳车保网页文件邮件一件不少UDP纸飞机保视频游戏够快 — TCP 开稳车：网页、文件、邮件一件不能少。路堵了它会减速让行，这就是流量控制。UDP 坐纸飞机：视频通话、游戏只求快。下一片，我们看谁来指路。 TCP drives a steady van: web pages, files and mail must all arrive. When roads jam it slows down — that is flow control. UDP rides a paper plane: video calls and games just want speed. Next leaf, we meet the guide at the crossing.

🎮 当一回收件人（60 秒）🎮 Be the receiver (60 seconds)

你是张三：先握手，再按 1、2、3 点小包上架。缺了哪号，就发回执请补寄，共拼好 4 条消息。You are Sam: shake hands first, then shelve packets 1, 2, 3 in order. If a number is missing, send a receipt to ask for it. Rebuild 4 messages.

一句话记住它：TCP 是可靠快递：先握手，拆包编号送，缺了补寄，不多不少按序到。 Remember it in one line: TCP is reliable delivery — shake hands, send numbered pieces, resend gaps, all in order.

先三次握手确认都在家，再开寄Shake hands three times to confirm, then send 按编号组装，缺号发回执要求补寄Reassemble by number; ask for resends with receipts 网页文件邮件用 TCP，视频游戏追快用 UDPFiles and mail use TCP; video and games chase speed with UDP

内容参考 OpenStax 等公开教材，多来源核对 · AI 生成、人工审核 · 发现错误欢迎指正，帮这片叶子长得更好。 Based on OpenStax and other open textbooks, cross-checked across sources · AI-generated, human-reviewed · Spotted a mistake? Tell us — help this leaf grow.