请求头分类
End-to-end（端对端）
必须全部带给目标服务器，不会被中途变化或去掉
Hop-by-hop（逐跳头）
比如客户端发请求，要路过代理(例如Nginx)，头可以被自动删掉，来到真正服务器上是没有的
进入HTTP文档RFC 2616: Hypertext Transfer Protocol -- HTTP/1.1 (rfc-editor.org)
原文：
13.5.1 End-to-end and Hop-by-hop HeadersFor the purpose of defining the behavior of caches and non-cachingproxies, we divide HTTP headers into two categories:- End-to-end headers, which are transmitted to the ultimaterecipient of a request or response. End-to-end headers inresponses MUST be stored as part of a cache entry and MUST betransmitted in any response formed from a cache entry.- Hop-by-hop headers, which are meaningful only for a singletransport-level connection, and are not stored by caches orforwarded by proxies.The following HTTP/1.1 headers are hop-by-hop headers:- Connection- Keep-Alive- Proxy-Authenticate- Proxy-Authorization- TE- Trailers- Transfer-Encoding- UpgradeAll other headers defined by HTTP/1.1 are end-to-end headers.Other hop-by-hop headers MUST be listed in a Connection header,(section 14.10) to be introduced into HTTP/1.1 (or later).上文说以下都是Hop-by-hop头，除了这些都是End-to-end：
- Connection
- Keep-Alive
- Proxy-Authenticate
- Proxy-Authorization
- TE
- Trailers
- Transfer-Encoding
- Upgrade
总结：
端到端(End-to-end)
一定会被最终目标接收，必须保存在由缓存生成的响应中，头必须被转发
逐跳头(Hop-by-hop)
只对单次转发有效，会因通过缓存或代理而不再转发
HTTP/1.1和以后版本中如果要使用Hop-to-hop头，需提供Connection头字段
Connection:keep-alive头的作用
这种情况，客户端(浏览器)与Nginx服务器(代理服务器)建立keep-alive连接，不会与Tomcat建立keep-alive
如果是直接访问的Tomcat服务器，则会与tomcat建立keep-alive关系
客户端与服务端的连接保活技术，可以减少TCP的连接创建断开额外开销，减轻服务器负载，减少响应时间