了解nginx’uscuping’指令的不同值

而

listen [::]：443 ssl：让nginx监听服务器上的所有ipv6地址,端口443(::: 443)

[::]：443默认情况下不会对ipv4进行nginx响应,除非你指定参数ipv6only = off：

listen [::]：443 ipv6only = off;

按照文档：http://nginx.org/en/docs/http/ngx_http_core_module.html#listen

ssl：

The ssl parameter (0.7.14) allows specifying that all connections
accepted on this port should work in SSL mode.

http2：

The http2 parameter (1.9.5) configures the port to accept 07001 connections.

这并不意味着它只接受HTTP / 2连接.

按照RFC7540

A client that makes a request for an “http” URI without prior
knowledge about support for HTTP/2 on the next hop uses the HTTP
Upgrade mechanism. The client does so by making an HTTP/1.1 request
that includes an Upgrade header field with the “h2c” token.

A server
that does not support HTTP/2 can respond to the request as though the
Upgrade header field were absent.

HTTP/1.1 200 OK
Content-Length: 243
Content-Type: text/html

A server that supports HTTP/2
accepts the upgrade with a 101 (Switching Protocols) response. After
the empty line that terminates the 101 response,the server can begin
sending HTTP/2 frames.

总结一下：

不支持HTTP / 2的客户端永远不会向服务器请求
HTTP / 2通信升级：它们之间的通信将是完全的
HTTP1 / 1.

支持HTTP / 2的客户端将要求服务器(使用HTTP1 / 1)进行HTTP / 2升级：

>如果服务器已准备好HTTP / 2,则服务器将注意到客户端
因此：它们之间的通信将切换到HTTP / 2.
>如果服务器未准备好HTTP / 2,则服务器将忽略该服务器
使用HTTP1 / 1进行升级请求应答：之间的通信
他们应该保持足够的HTTP1 / 1.

也许更多地总结一下：http://qnimate.com/http2-compatibility-with-old-browsers-and-servers/

但是,nginx doc通过TLS声明了以下有关HTTP / 2的内容：

Note that accepting HTTP/2 connections over TLS requires the
“Application-Layer Protocol Negotiation” (ALPN) TLS extension support,
which is available only since OpenSSL version 1.0.2.

确保旧客户符合此要求.