问浏览器与CDN缓存协商的几点问题？

Access-Control-Max-Age:1800 Cache-Control:max-age=7200 Content-Type:text/html; charset=utf-8 Date:Fri, 10 Nov 2017 09:34:06 GMT Expires:Fri, 10 Nov 2017 11:34:06 GMT Last-Modified:Fri, 10 Nov 2017 09:20:00 GMT Proxy-Connection:Keep-Alive Server:NWS_SP Transfer-Encoding:chunked via:proxy A X-Cache-Lookup:Hit From MemCache Gz X-NWS-LOG-UUID:51f99ad4-51a2-4644-9eb5-7b724174b9df 97f4d63ee025a7dce855a20d775b5b1d 浏览器跟CDN缓存协商时，为啥有时候响应Transfer-Encoding:chunked，有时候又不带。这个是基于什么样策略响应的？