HTTP/2 协议相比较 HTTP1 有诸多优势，越来越多应用程序使用HTTP2协议。
HTTP2 与 HTTP1 区别
| HTTP1 | HTTP2 | 对比说明 | |
|---|---|---|---|
| 消息编码格式 | 文本 | 二进制 | HTTP2 的二进制编码格式效率更高，但是问题定位更麻烦，需借助解码工具查看头或body |
| 消息压缩 | gzip encoding 压缩 body |
gzip encoding 压缩 body; 默认使用 HPACK 压缩头 |
|
| HTTP请求/连接关系 | 单域多连接，每个连接同一时刻只允许一个请求 | 单域单连接，每个连接允许多请求并发(抽象为Stream) | HTTP2连接数量更少，特别是在TLS建连接耗时情况下效率提升明显；HTTP2 的多 Stream 并发并未解决 TCP 层面 HOL 问题，TCP顺序传包机制，如果前面包丢失重传，后面的包会被阻塞 |
| Server Push | 浏览器加载页面渲染后，再根据页面内定义的CSS/JS路径去请求资源 | 服务器端主动推送其猜测(??)的客户端需要缓存的内容 | |
| 消息优先级 | 协议本身没有 | 适合混杂页面，JS/CSS会被优先请求 |
体验HTTP/2接口
使用 curl 访问 HTTP/2 接口
curl 工具在 7.43.0 版本支持 http2 协议，在请求时候指定 --http2 选项，curl 客户端以 http2 协议发起请求，如果服务器支持 http2 ，则在消息头里返回的协议版本为 HTTP2。
curl -v --http2 -I https://1906714720.rsc.cdn77.org/http2/http2.html
* Trying 185.180.13.17...
* TCP_NODELAY set
* Connected to 1906714720.rsc.cdn77.org (185.180.13.17) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
* CAfile: /etc/ssl/certs/ca-certificates.crt
CApath: /etc/ssl/certs
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
* TLSv1.2 (IN), TLS handshake, Server hello (2):
* TLSv1.2 (IN), TLS handshake, Certificate (11):
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
* TLSv1.2 (IN), TLS handshake, Server finished (14):
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
* TLSv1.2 (OUT), TLS change cipher, Client hello (1):
* TLSv1.2 (OUT), TLS handshake, Finished (20):
* TLSv1.2 (IN), TLS handshake, Finished (20):
* SSL connection using TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256
* ALPN, server accepted to use h2
* Server certificate:
* subject: C=GB; L=London; O=DataCamp Limited; CN=rsc.cdn77.org
* start date: Sep 13 00:00:00 2019 GMT
* expire date: Jun 9 12:00:00 2020 GMT
* subjectAltName: host "1906714720.rsc.cdn77.org" matched cert's "*.rsc.cdn77.org"
* issuer: C=US; O=DigiCert Inc; CN=DigiCert SHA2 Secure Server CA
* SSL certificate verify ok.
* Using HTTP2, server supports multi-use
* Connection state changed (HTTP/2 confirmed)
* Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
* Using Stream ID: 1 (easy handle 0x7fffdb276b70)
> HEAD /http2/http2.html HTTP/2
> Host: 1906714720.rsc.cdn77.org
> User-Agent: curl/7.58.0
> Accept: */*
>
* Connection state changed (MAX_CONCURRENT_STREAMS updated)!
< HTTP/2 200
HTTP/2 200
< date: Fri, 11 Oct 2019 02:42:22 GMT
date: Fri, 11 Oct 2019 02:42:22 GMT
< content-type: text/html
content-type: text/html
< content-length: 17857
content-length: 17857
< etag: "570b88dc-45c1"
etag: "570b88dc-45c1"
< cache-control: no-cache
cache-control: no-cache
< access-control-allow-origin: *
access-control-allow-origin: *
< server: CDN77-Turbo
server: CDN77-Turbo
< x-cache: HIT
x-cache: HIT
< x-age: 252401
x-age: 252401
< accept-ranges: bytes
accept-ranges: bytes
<
* Connection #0 to host 1906714720.rsc.cdn77.org left intact
在 curl 命令返回消息中，有一行 * ALPN, server accepted to use h2 ，这个输出的意思是使用 ALPN: Application-Layer Protocal Negotiation 协商协议，协商到了 HTTP/2 协议。这个协商协议是为了客户端与服务端兼容而设计的，下面介绍HTTP协议协商。
协议协商与兼容性
HTTP/1.1 协议协商
为了协议更新时候方便向前兼容，HTTP/1.1引入了协议协商机制，这个机制在 RFC7230 的 Upgrade章节描述，流程为：
- 客户端发起协议选择请求，在请求 Header 中包含协商的协议列表
Connection: upgrade
Upgrade: protocol-name[/protocol-version]
如
GET /hello.txt HTTP/1.1
Host: www.example.com
Connection: upgrade
Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
- 服务端如果接受列表中的协议，则返回一个选择的协议，返回码为 101
HTTP/1.1 101 Switching Protocols
Connection: upgrade
Upgrade: protocol-name[/protocol-version]
[... data defined by new protocol ...]
如
HTTP/1.1 101 Switching Protocols
Connection: upgrade
Upgrade: HTTP/2.0
[... data stream switches to HTTP/2.0 with an appropriate response
(as defined by new protocol) to the "GET /hello.txt" request ...]
- 服务端还可以选择忽略协议协商，按照服务端协议返回数据
HTTP/1.0 200
[... data stream switches to HTTP/2.0 with an appropriate response
(as defined by new protocol) to the "GET /hello.txt" request ...]
HTTP/2 协议协商
HTTP/2 最初由谷歌的 SPDY 发展而来，SPDY 协议中包含了一个名为 NPN(Next Protocal Negotiation) 的SSL握手扩展。随着 SPDY 升级为 HTTP/2 ，NPN 也升级为 ALPN 。
ALPN 的协商流程为：
- 客户端在SSL握手的 ClientHello 请求中，通过ALPN扩展，发送要协商的协议列表
- 服务端在SSL握手的 ServerHello 请求中，通过ALPN扩展，返回选择的协议列表