请访问原文链接：ALPN (Application-Layer Protocol Negotiation) 查看最新版。原创作品，转载请保留出处。
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ALPN (Application Layer Protocol Negotiation)是 TLS 的扩展，允许在安全连接的基础上进行应用层协议的协商。ALPN 支持任意应用层协议的协商，目前应用最多是 HTTP/2 的协商。当前主流浏览器，都只支持基于 HTTPS 部署的 HTTP/2，因为浏览器是基于 ALPN 协议来判断服务器是否支持 HTTP/2 协议。
About
See MDN Web Docs Glossary: Definitions of Web-related termsALPN
Application-Layer Protocol Negotiation (ALPN) is a TLS extension which indicates what application layer protocol is negotiating the encryped connection without requiring additional round trips (sysin).
| Protocol | Identification sequence |
|---|---|
| HTTP/1.1 | 0x68 0x74 0x74 0x70 0x2F 0x31 0x2E 0x31 (“http/1.1”) |
| HTTP/2 | 0x68 0x32 (“h2”) |
| HTTP/2 over cleartext TCP | 0x68 0x32 0x63 (“h2c”) |
Specifications
| Specification | Status | Notes |
|---|---|---|
| RFC 7301 | IETF RFC | Initial definition. |
Support
浏览器和服务端都支持ALPN 协商，是用上 HTTP/2 的大前提。
- 关于浏览器支持
In May 2016, Google released Chrome build 51, eliminating support for SPDY and NPN in favor of HTTP/2 and ALPN. Although concern was expressed publicly and privately about the negative consequences before the change was made, Google went ahead (sysin), providing a brief explanation of its reasons for dropping NPN.
At that time Chrome was the only major browser that didn’t support NPN, so users could regain HTTP/2 access by switching to another browser. However, by mid‑2017 all of the most popular browser vendors except Safari (version 10) had dropped support for NPN, starting with the following versions (see the Protocol Details section on the page for each browser at Qualys SSL Labs):
- Chrome 51
- Edge 12
- Firefox 53
- Internet Explorer 11
Opera 38
大部分 Web Server 都依赖 OpenSSL 库提供 https服务，是否支持 ALPN 完全取决于使用的 OpenSSL 版本，OpenSSL 1.0.2 版本才开始支持 ALPN。
The table summarizes Linux operating system support for ALPN and NPN as of September 2017.
| Operating System | OpenSSL Version | ALPN/NPN Support |
|---|---|---|
| CentOS/Oracle Linux/RHEL 6.5+, 7.0-7.3 | 1.0.1e | NPN |
| CentOS/Oracle Linux/RHEL 7.4+ | 1.0.2k | ALPN and NPN |
| Debian 7.0 | 1.0.1e | NPN |
| Debian 8.0 | 1.0.1k | NPN |
| Debian 9.0 | 1.1.0f | ALPN and NPN |
| Ubuntu 12.04 LTS | 1.0.1 | NPN |
| Ubuntu 14.04 LTS | 1.0.1f | NPN |
| Ubuntu 16.04 LTS | 1.0.2g | ALPN and NPN |
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