Etag
参考：http://baike.baidu.com/link?url=hhFAPq22BrAb89YBf8XjUSgbyL11FpfFCtmNyLgl_SaYcp68IBRmZhbXnHs5r5A-8Zk4IJ1i5CGFoAitmROKEa
1、HTTP/1.1引入了 Etag(Entity Tags).Etag