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 Headers
   For the purpose of defining the behavior of caches and non-caching
   proxies, we divide HTTP headers into two categories:
      - End-to-end headers, which are  transmitted to the ultimate
        recipient of a request or response. End-to-end headers in
        responses MUST be stored as part of a cache entry and MUST be
        transmitted in any response formed from a cache entry.

      - Hop-by-hop headers, which are meaningful only for a single
        transport-level connection, and are not stored by caches or
        forwarded by proxies.
   The following HTTP/1.1 headers are hop-by-hop headers:
      - Connection
      - Keep-Alive
      - Proxy-Authenticate
      - Proxy-Authorization
      - TE
      - Trailers
      - Transfer-Encoding
      - Upgrade
   All 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