Implementers of intermediate caches (proxies) have found it useful to convert the media type of certain entity bodies. A proxy might, for example, convert between image formats in order to save cache space or to reduce the amount of traffic on a slow link. HTTP has to date been silent on these transformations.
Serious operational problems have already occurred, however, when these transformations have been applied to entity bodies intended for certain kinds of applications. For example, applications for medical imaging, scientific data analysis and those using end-to-end authentication, all depend on receiving an entity body that is bit for bit identical to the original entity-body.
Therefore, if a response includes the no-transform directive, an intermediate cache or proxy MUST NOT change those headers that are listed in section 13.5.2 as being subject to the no-transform directive. This implies that the cache or proxy must not change any aspect of the entity-body that is specified by these headers.