> [TOC]

# 1、HTTP/1.1 发展中遇到的问题

## 1.1、发生的变化

![image-20220813224604630](assets/image-20220813224604630.png)

## 1.2、高延迟问题

![image-20220813224625917](assets/image-20220813224625917.png)

![image-20220813224836311](assets/image-20220813224836311.png)

## 1.3、巨大的头部

![image-20220813225004647](assets/image-20220813225004647.png)

## 1.4、HTTP1.1的优化

![image-20220813225037932](assets/image-20220813225037932.png)

## 1.5、HTTP/1.1 不支持服务器推送消息

![image-20220813225126902](assets/image-20220813225126902.png)

# 2、HTTP2特征概述

## 2.1、提出背景

![image-20220813225242683](assets/image-20220813225242683.png)

* http2协议可以在http协议下，也可以在TLS-SSL协议下使用。
* 浏览器强制要求HTTP2只能被HTTPS解析，也就是使用TLS-SSL协议。

## 2.2、特性

![image-20220813225444351](assets/image-20220813225444351.png)

![image-20220813225530773](assets/image-20220813225530773.png)

# 3、在TCP上从HTTP1升级到HTTP2

## 3.1、HTTP/2 是不是必须基于TLS/SSL 协议？

![image-20220816210600245](assets/image-20220816210600245.png)

## 3.2、h2与h2c

![image-20220816210746090](assets/image-20220816210746090.png)

## 3.3、H2C：不使用 TLS 协议进行协议升级

![image-20220816213310417](assets/image-20220816213310417.png)

![image-20220816213409707](assets/image-20220816213409707.png)

# 4、h2如何建立会话？

## 4.1、通讯过程

![image-20220816213628014](assets/image-20220816213628014.png)

![image-20220816213640440](assets/image-20220816213640440.png)

## 4.2、服务器端选择协议

![image-20220816213815311](assets/image-20220816213815311.png)

# 5、Stream、Message、Frame 间的关系

## 5.1、定义

![image-20220816214002799](assets/image-20220816214002799.png)

## 5.2、Stream、Message、Frame 间的关系

![image-20220816214249616](assets/image-20220816214249616.png)

![image-20220816214702279](assets/image-20220816214702279.png)

## 5.3、传输中无序

![image-20220816214729430](assets/image-20220816214729430.png)

* 同一个stream必须是有序的
* 跨越stream可以是无序的

![image-20220816214902643](assets/image-20220816214902643.png)

# 6、帧格式：Stream流ID的作用

## 6.1、对比websocket

![image-20220816215016793](assets/image-20220816215016793.png)

![image-20220816215110251](assets/image-20220816215110251.png)

## 6.2、作用

![image-20220816215232321](assets/image-20220816215232321.png)

![image-20220816215240030](assets/image-20220816215240030.png)

![image-20220816215249933](assets/image-20220816215249933.png)

![image-20220816215256014](assets/image-20220816215256014.png)

# 7、帧格式：帧类型及设置帧的子类型

## 7.1、定义

![image-20220816215731117](assets/image-20220816215731117.png)

* 帧长度
* 帧类型
* 帧标记
* 帧的id
* 帧数据

## 7.2、帧的类型

![image-20220816215758282](assets/image-20220816215758282.png)

![image-20220816215805092](assets/image-20220816215805092.png)

## 7.3、setting设置帧

* 可以设置多个key-value

![image-20220816215958065](assets/image-20220816215958065.png)

![image-20220816220048816](assets/image-20220816220048816.png)

# 8、HPACK 如何减少HTTP 头部的大小？

## 8.1、定义

![image-20220816220720751](assets/image-20220816220720751.png)

## 8.2、静态字典

![image-20220816220747802](assets/image-20220816220747802.png)

![image-20220816220927312](assets/image-20220816220927312.png)

![image-20220816220843281](assets/image-20220816220843281.png)

## 8.3、动态字典

动态表包含以**先进先出**的顺序维护的 header 字段列表。动态表中的第一个条目和最新条目在最低索引处，而动态表的最旧条目在最高索引处。

动态表最初是空的。当每个 header 块被解压缩时，将添加条目。动态表可以包含重复的条目（即，具有相同名称和相同值的条目）。因此，解码器不得将重复的条目视为错误。

编码器决定如何更新动态表，因此可以控制动态表使用多少内存。为了限制解码器的存储需求，动态表的 size 受到严格限制。

解码器在处理 header 字段表示列表时更新动态表。

# 9、HPACK 中如何使用Huffman树编码？

## 9.1、原理

![image-20220816221321050](assets/image-20220816221321050.png)

 ## 9.2、构造过程

![image-20220816221558274](assets/image-20220816221558274.png)

# 10、HPACK 中整型数字的编码

这里编码的是索引位置，即使下文11章节所说的索引表下标

## 10.1、小于31

![image-20220816230238907](assets/image-20220816230238907.png)

## 10.2、大于31

![image-20220816230356798](assets/image-20220816230356798.png)

![image-20220816230421388](assets/image-20220816230421388.png)

![image-20220816230523265](assets/image-20220816230523265.png)

# 11、HPACK 中HEADER 的编码格式

## 11.1、HEADER帧的格式

![image-20220816230631593](assets/image-20220816230631593.png)

![image-20220816230701522](assets/image-20220816230701522.png)

## 11.2、动态表

![image-20220816230731894](assets/image-20220816230731894.png)

## 11.3、字面编码

![image-20220816230813032](assets/image-20220816230813032.png)

## 11.4、格式

![image-20220816230857683](assets/image-20220816230857683.png)

![image-20220816231045912](assets/image-20220816231045912.png)

![image-20220816231317892](assets/image-20220816231317892.png)

![image-20220816231327159](assets/image-20220816231327159.png)

![image-20220816231628647](assets/image-20220816231628647.png)

![image-20220816231647378](assets/image-20220816231647378.png)

![image-20220816231657774](assets/image-20220816231657774.png)

## 11.5、动态表控制

![image-20220816231728498](assets/image-20220816231728498.png)

# 12、服务器端的主动消息推送

## 12.1、价值

![image-20220816232717019](assets/image-20220816232717019.png)

![image-20220816232729367](assets/image-20220816232729367.png)

![image-20220816232913454](assets/image-20220816232913454.png)

## 12.2、帧格式

![image-20220816233032609](assets/image-20220816233032609.png)

## 12.3 可以禁用

![image-20220816233326240](assets/image-20220816233326240.png)

# 13、stream的状态变迁

## 13.1、stream特性

![image-20220816233654866](assets/image-20220816233654866.png)

## 13.2、message特性

![image-20220816233732465](assets/image-20220816233732465.png)

## 13.3、发送例子

![image-20220816233807759](assets/image-20220816233807759.png)

![image-20220816233818110](assets/image-20220816233818110.png)

## 13.4、stream状态的变化

![image-20220816233905535](assets/image-20220816233905535.png)

# 14、RST_STREAM帧及常见错误码

## 14.1、定义

![image-20220816234215070](assets/image-20220816234215070.png)

## 14.2、常见的错误码

![image-20220816234234494](assets/image-20220816234234494.png)

![image-20220816234241382](assets/image-20220816234241382.png)

![image-20220816234247670](assets/image-20220816234247670.png)

# 15、不同请求的优先级

## 15.1、Priority 优先级设置帧

![image-20220816234332719](assets/image-20220816234332719.png)

## 15.2、优先级

![image-20220816234537477](assets/image-20220816234537477.png)

![image-20220816234913183](assets/image-20220816234913183.png)

* exclusive：是否独占

# 16、不同于 TCP 的流量控制

## 16.1、为什么需要？

![image-20220817205611078](assets/image-20220817205611078.png)

![image-20220817210023672](assets/image-20220817210023672.png)

## 16.2、如何进行？

![image-20220817210119059](assets/image-20220817210119059.png)

## 16.3、流量控制帧

![image-20220817210326885](assets/image-20220817210326885.png)

## 16.4、流控制窗口

![image-20220817210516483](assets/image-20220817210516483.png)

## 16.5、并发流

![image-20220817210535722](assets/image-20220817210535722.png)

# 17、HTTP/2 的问题

## 17.1、TCP 以及 TCP+TLS 建链握手过多的问题

![image-20220817210751019](assets/image-20220817210751019.png)

## 17.2、多路复用与 TCP 的队头阻塞问题

![image-20220817210827738](assets/image-20220817210827738.png)

## 17.3、TCP的问题

![image-20220817210845887](assets/image-20220817210845887.png)

# 18、七层负载均衡做了些什么？

## 18.1、四层负载均衡

![image-20220817211116912](assets/image-20220817211116912.png)

## 18.2、七层负载均衡协议转换举例

![image-20220817211157133](assets/image-20220817211157133.png)

## 18.3、协议转换

![image-20220817211448937](assets/image-20220817211448937.png)

## 18.4、WAF 防火墙

![image-20220817211544225](assets/image-20220817211544225.png)

## 18.5、负载均衡算法

![image-20220817211644003](assets/image-20220817211644003.png)

## 18.6、缓存功能

![image-20220817211702271](assets/image-20220817211702271.png)