Rule 13: Configure ETags13(Chapter 13 of High performance Web Sites)

　　Entity tags (ETags) are a mechanism that web servers and browsers use to validate
cached components.Before jumping into the details of ETags, let’s review how components
are cached and validated.

　　An ETag is a string that uniquely identifies a specific version of
a component.The only format constraint is that the string must be quoted.The origin
server specifies the component’s ETag using the ETag response header.

　　Reconfigure or remove ETags.