前言
http2可以承载在TCP上或者TLS上，分别简称h2和h2c
当访问一个服务的时候，到底用http1.1还是http2，client和server是可以协商的。
[https://www.cnblogs.com/hugetong/p/13410276.html]
TLS承载HTTP2
RFC规定，TLS承载HTTP2时，TLS必须实现ALPN功能。
implementations that support HTTP/2 over TLS MUST use protocol negotiation in TLS
nginx里可以这样配置：这样配置之后，5000端口可以同时为http2与http1.1提供服务。
server { listen 0.0.0.0:5000 http2 ssl; ssl_certificate /data/sni/sni_test3.cer; ssl_certificate_key /data/sni/sni_test3.key; location / { proxy_pass http://httpt7/; } }
使用curl测试如下：
http2
╰─>$ curl -v -k https://t9:5000/ * Trying 192.168.7.9:5000... * Connected to t9 (192.168.7.9) port 5000 (#0) * ALPN, offering h2 * ALPN, offering http/1.1 * successfully set certificate verify locations: * CAfile: /etc/ssl/certs/ca-certificates.crt CApath: none * TLSv1.3 (OUT), TLS handshake, Client hello (1): * TLSv1.3 (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, Change cipher spec (1): * TLSv1.2 (OUT), TLS handshake, Finished (20): * TLSv1.2 (IN), TLS handshake, Finished (20): * SSL connection using TLSv1.2 / ECDHE-RSA-AES256-GCM-SHA384 * ALPN, server accepted to use h2 * Server certificate: * subject: C=CN; ST=BeiJing; L=BeiJing; O=tong.com; OU=tong; CN=caotong_test3; emailAddress=tong@local * start date: Sep 24 10:00:10 2019 GMT * expire date: Sep 21 10:00:10 2029 GMT * issuer: C=CN; ST=BeiJing; L=BeiJing; O=Tartaglia; CN=TTTrust; emailAddress=ca@tartaglia.org * SSL certificate verify result: unable to get local issuer certificate (20), continuing anyway. * 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 0x5635fa55b8b0) > GET / HTTP/2 > Host: t9:5000 > user-agent: curl/7.70.0 > accept: */* > * Connection state changed (MAX_CONCURRENT_STREAMS == 128)! < HTTP/2 200 < server: tong.localhost < date: Fri, 31 Jul 2020 07:41:11 GMT < content-type: text/html < content-length: 660 < last-modified: Thu, 02 Apr 2020 08:45:48 GMT < etag: "5e85a63c-294" < accept-ranges: bytes < <!DOCTYPE html> <html> xxx </html> * Connection #0 to host t9 left intact
http1
╰─>$ curl -v -k --http1.1 https://t9:5000/ * Trying 192.168.7.9:5000... * Connected to t9 (192.168.7.9) port 5000 (#0) * ALPN, offering http/1.1 * successfully set certificate verify locations: * CAfile: /etc/ssl/certs/ca-certificates.crt CApath: none * TLSv1.3 (OUT), TLS handshake, Client hello (1): * TLSv1.3 (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, Change cipher spec (1): * TLSv1.2 (OUT), TLS handshake, Finished (20): * TLSv1.2 (IN), TLS handshake, Finished (20): * SSL connection using TLSv1.2 / ECDHE-RSA-AES256-GCM-SHA384 * ALPN, server accepted to use http/1.1 * Server certificate: * subject: C=CN; ST=BeiJing; L=BeiJing; O=tong.com; OU=tong; CN=caotong_test3; emailAddress=tong@local * start date: Sep 24 10:00:10 2019 GMT * expire date: Sep 21 10:00:10 2029 GMT * issuer: C=CN; ST=BeiJing; L=BeiJing; O=Tartaglia; CN=TTTrust; emailAddress=ca@tartaglia.org * SSL certificate verify result: unable to get local issuer certificate (20), continuing anyway. > GET / HTTP/1.1 > Host: t9:5000 > User-Agent: curl/7.70.0 > Accept: */* > * Mark bundle as not supporting multiuse < HTTP/1.1 200 OK < Server: TONG < Date: Fri, 31 Jul 2020 07:45:34 GMT < Content-Type: text/html < Content-Length: 660 < Connection: keep-alive < Last-Modified: Thu, 02 Apr 2020 08:45:48 GMT < ETag: "5e85a63c-294" < Accept-Ranges: bytes < <!DOCTYPE html> <html> xxx </html> * Connection #0 to host t9 left intact
TCP承载HTTP2
nginx配置：
server { listen 0.0.0.0:5000 http2; location / { proxy_pass http://httpt7/; } }
curl访问，必须采用显示约定的方式进行访问。显示约定是指client事先知道了对方是http2，curl的话，要参数 http2-prior-knowledge指定
╰─>$ curl -v -k --http2-prior-knowledge http://t9:5000/ * Trying 192.168.7.9:5000... * Connected to t9 (192.168.7.9) port 5000 (#0) * 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 0x55f343ece8b0) > GET / HTTP/2 > Host: t9:5000 > user-agent: curl/7.70.0 > accept: */* > * Connection state changed (MAX_CONCURRENT_STREAMS == 128)! < HTTP/2 200 < server: TONG < date: Fri, 31 Jul 2020 07:48:36 GMT < content-type: text/html < content-length: 660 < last-modified: Thu, 02 Apr 2020 08:45:48 GMT < etag: "5e85a63c-294" < accept-ranges: bytes < <!DOCTYPE html> <html> xxx </html> * Connection #0 to host t9 left intact
一般来说，server应该有能力与client协商版本。但是很遗憾目前nginx还不支持。这个feature已经被提出，还没有被开发。见：https://trac.nginx.org/nginx/ticket/816
h2c的时候怎么协商？
首先client发一个http1.1的请求过去，并携带着header 希望upgrade到http2，如果server愿意，将回一个101的response
如下例子： （这个例子是nignx不支持，拒绝的情况）
╰─>$ curl -v -k --http2 http://t9:5000/ * Trying 192.168.7.9:5000... * Connected to t9 (192.168.7.9) port 5000 (#0) > GET / HTTP/1.1 > Host: t9:5000 > User-Agent: curl/7.70.0 > Accept: */* > Connection: Upgrade, HTTP2-Settings > Upgrade: h2c > HTTP2-Settings: AAMAAABkAAQCAAAAAAIAAAAA > * Received HTTP/0.9 when not allowed * Closing connection 0 curl: (1) Received HTTP/0.9 when not allowed
（假如不拒绝，将回复如下：）
HTTP/1.1 101 Switching Protocols Connection: Upgrade Upgrade: h2c [ HTTP/2 connection ...
当server只支持http1时，试图升到http2的client会回落至http1正常工作，如下：
╰─>$ curl -v -k --http2 http://t9:5000/ * Trying 192.168.7.9:5000... * Connected to t9 (192.168.7.9) port 5000 (#0) > GET / HTTP/1.1 > Host: t9:5000 > User-Agent: curl/7.70.0 > Accept: */* > Connection: Upgrade, HTTP2-Settings > Upgrade: h2c > HTTP2-Settings: AAMAAABkAAQCAAAAAAIAAAAA > * Mark bundle as not supporting multiuse < HTTP/1.1 200 OK < Server: TONG < Date: Fri, 31 Jul 2020 08:01:12 GMT < Content-Type: text/html < Content-Length: 660 < Connection: keep-alive < Last-Modified: Thu, 02 Apr 2020 08:45:48 GMT < ETag: "5e85a63c-294" < Accept-Ranges: bytes < <!DOCTYPE html> <html> xxx </html> * Connection #0 to host t9 left intact
总结一下
client访问http2 server的时候，分两种情况，一种是自适应版本的协商方式。一种是显式指定版本为http2的方式。
自适应的方式分两种情况tls承载与tcp承载。
tls承载时，使用ALPN。tcp承载时，client使用Upgrade header尝试http2，serveri接受便回复101开始http2，server不接受就正常用http1答复。