215 lines
7.4 KiB
Markdown
215 lines
7.4 KiB
Markdown
---
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tags: [计算机网络, HTTP/2, HPACK, Multiplexing, Server Push]
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create time: 2026-05-18 02:40
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---
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# HTTP/2 多路复用
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## 概述
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HTTP/2 (RFC 7540) 是 HTTP 协议自 1999 年 HTTP/1.1 以来的第一次重大修订。核心改进：**二进制分帧层**，让一个 TCP 连接上能并发多个请求，彻底消除 Head-of-Line blocking（应用层层面）。
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```mermaid
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flowchart TD
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A["HTTP/1.1<br/>问题重重"] -->|"串行请求"| B["每个请求等响应"]
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B --> C["慢 ❌"]
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D["HTTP/2<br/>二进制分帧"] -->|"多路复用"| E["并发请求/响应"]
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E --> F["快 ✅"]
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style A fill:#FF6B6B,color:#fff
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style D fill:#98FB98,color:#000
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```
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## HTTP/2 的三层层级
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```
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┌─────────────────────────────────────┐
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│ Application Layer │
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│ HTTP Frames / Streams │ ← 你操作的对象
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├─────────────────────────────────────┤
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│ Frame Layer │ ← HPACK + Flow Control
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│ Binary Framing + Headers │
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├─────────────────────────────────────┤
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│ Transport Layer │ ← TCP (or QUIC for H3)
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│ Connection Management │
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└─────────────────────────────────────┘
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```
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## 二进制帧类型（Frame Types）
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| Frame | 说明 |
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|-------|------|
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| **HEADERS** | 请求/响应头部 |
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| **DATA** | 实际数据载荷 |
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| RST_STREAM | 异常关闭流 |
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| SETTINGS | 通信参数协商 |
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| PUSH_PROMISE | 服务器推送 |
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| PING | 延迟测量 |
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| GOAWAY | 优雅关闭连接 |
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| WINDOW_UPDATE | 流量控制 |
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| CONTINUATION | HEADERS 太长时的延续帧 |
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### HEADERS 帧结构
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```
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+---------------+
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|Pad Length? |(0-7 bits)
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|Padding (8-bit)|
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|E |
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|+---------------+----------------------------------------------+
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| |Header Block Fragment (*) |
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|+-----------------------------------------------------+
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|... |
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+-----------------------------------------------------+
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```
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## Stream — 多路复用的核心
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HTTP/2 将每个请求/响应绑定到一个唯一的 **Stream ID**：
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```
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TCP Connection: client:45678 ↔ server:443
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Stream 1: GET /index.html → 200 OK
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Stream 3: GET /style.css → 200 OK
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Stream 5: POST /api/login → 200 OK
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Stream 7: GET /app.js → 200 OK
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Client Server
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│ │
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│ ───HEADERS(stream=1)──→ │ GET /index.html
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│ ───DATA(stream=1, 5KB)──→ │
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│ │
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│ ←──HEADERS(stream=1, 200 OK)── │
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│ ←──DATA(stream=1, 15KB)── │
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│ │
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│ ───HEADERS(stream=3)──→ │ GET /style.css
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│ ←──HEADERS(stream=3, 200 OK)── │
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│ ←──DATA(stream=3, 3KB)── │
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│ │
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│ ───HEADERS(stream=5)──→ │ POST /api/login
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│ ───DATA(stream=5, 50B)──→ │ body: {"user":"alice"}
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│ ←──HEADERS(stream=5, 200 OK)── │
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│ ←──DATA(stream=5, 80B)── │ body: {"token":"abc"}
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```
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### Stream 状态机
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```
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idle → reserved(local) ———→ half closed(local) —→ closed
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↑ ↓ ↑
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│ half closed(remote) │
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│ ↓ │
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└──── open ←───────── half closed(remote) ◄─────┘
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↑ ↑
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│ half closed(local)
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│ ↓
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send HEADERS closed
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recv HEADERS
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```
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### 优先级分组（Stream Dependency）
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```
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Stream 1 (weight=16) [root]
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├── Stream 3 (weight=8): /index.html
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│ ├── Stream 7 (weight=4): style.css
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│ └── Stream 9 (weight=2): logo.png
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├── Stream 5 (weight=8): API data
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└── Stream 11 (weight=1): tracking pixel
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```
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## HPACK — Header 压缩
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HTTP/1.1 每次请求都重复发送 `User-Agent`、`Cookie`、`Accept` 等头部，浪费带宽。HPACK 用两种技术解决：
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| 技术 | 说明 |
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|------|------|
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| **Header Table** (动态表) | 服务端缓存常用头，后续请求用索引代替 |
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| **Huffman Coding** | 对字符串值做霍夫曼编码压缩 |
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### HPACK 编码示例
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原始 HTTP/1.1 请求头部（重复发送）：
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```
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GET /page HTTP/1.1
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Host: example.com
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User-Agent: Mozilla/5.0 ...
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Accept: text/html
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Cookie: session=abc123
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Accept-Encoding: gzip
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```
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重复 10 次后，HTTP/2 只发一次完整头部，后续使用动态表索引：
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```
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索引 1: Host: example.com (查表! 不重传)
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索引 4: User-Agent: ... (查表!)
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索引 5: Accept: text/html (查表!)
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索引 6: Cookie: session=abc123 (查表!)
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索引 7: Accept-Encoding: gzip (查表!)
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```
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**节省效果：** header 体积减少 70%~90%。
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## Server Push
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服务器可以主动向客户端推送资源，无需客户端请求：
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```
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Client: GET /index.html
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Server: HEADERS(stream=2, 200 OK) — HTML content
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Server: PUSH_PROMISE(stream=2, promised_stream=4)
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→ style.css
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Server: HEADERS(stream=4, 200 OK)
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Server: DATA(stream=4, CSS content)
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← 客户端在还没解析到 <link> 之前就已经收到了 CSS!
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```
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```bash
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# Go 中启用 HTTP/2 Server Push
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pusher, ok := w.(http.Pusher)
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if ok {
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pusher.Push("/static/style.css", nil)
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}
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```
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> [!warning] Push 的实际争议
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> Push 被广泛认为弊大于利：浏览器可能已经缓存了资源（导致冗余传输），且无法像 Client-initiated 那样利用浏览器已有的缓存决策。现代实践中更多使用 Preload `<link rel="preload">` 替代 Push。
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## HTTP/2 vs HTTP/1.1 对比
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| 特性 | HTTP/1.1 | HTTP/2 |
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|------|---------|--------|
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| 格式 | 文本 | **二进制** |
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| 并发 | 需要 N 个连接 | **单连接多路复用** |
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| Header | 明文全量发送 | **HPACK 压缩** |
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| 服务端推送 | ❌ | ✅ (可选) |
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| 头部压缩 | ❌ (需 HPACK extension) | ✅ 原生支持 |
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| 头部顺序 | 按发送顺序 | 可重新排序 |
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| HoL Blocking | 严重 (TCP 层) | **消除** (Stream 独立) |
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| TLS 要求 | 不需要 | **推荐** (nginx/apache 默认 h2c) |
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## h2c — HTTP/2 Clear Text (未加密)
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HTTP/2 可以通过 ALPN 升级（TLS 下的 h2）或 HTTP Upgrade 机制（纯 TCP 的 h2c）：
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```bash
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# curl 测试 h2c
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curl --http2 -H "Upgrade-H2C: true" http://localhost:8080/api
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# Go net/http 中 h2c 支持需 import golang.org/x/net/http2
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import "golang.org/x/net/http2"
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import "golang.org/x/net/http2/h2c"
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```
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> [!tip] 生产环境始终用 HTTPS + h2
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> 浏览器仅支持 TLS 下的 HTTP/2 (ALPN = "h2")。纯 TCP 的 h2c 主要用于微服务间通信。
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## 关联笔记
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- [[hhs/NETWORK/HTTP/1.1完全指南]] — HTTP/2 的基础
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- [[hhs/NETWORK/HTTP/3与QUIC]] — HTTP/2 的 UDP 替代品
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- [[hhs/NETWORK/HTTPS与TLS握手]] — HTTP/2 通常配合 HTTPS
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