Some features of the HTTP/1.1 protocol, such as Digest Authentication, depend on the value of certain end-to-end headers. A cache or non-caching proxy SHOULD NOT modify an end-to-end header unless the definition of that header requires or specifically allows that.
A cache or non-caching proxy MUST NOT modify any of the following fields in a request or response, nor may it add any of these fields if not already present:
A cache or non-caching proxy MUST NOT modify or add any of the following fields in a response that contains the no-transform Cache- Control directive, or in any request:
A cache or non-caching proxy MAY modify or add these fields in a response that does not include no-transform, but if it does so, it MUST add a Warning 14 (Transformation applied) if one does not already appear in the response.