HDATA — Intermediaries Consideration

The HTTP protocol may be routed through an HTTP proxy such as squid.
Such proxies are transparent to the applications, though implementors
should be alert to the effects of rogue caching.

Interface engines may also be placed between the consumer and the
provider; these differ from proxies because they actively alter the
content and/or destination of the HTTP exchange and are not bound the
rules that apply to HTTP proxies. Such agents are allowed, but must
mark the http header to assist with troubleshooting.

Any agent that modifies an HTTP request or Response content other than
under the rules for HTTP proxie must add a stamp to the HTTP headers
like this:

request-modified-[identity]: [purpose]
response-modified-[identity]: [purpose]

The identity must a single token defined by the administrator of the
agent that will sufficiently identify the agent in the context of use.
The header must specify the agents purpose in modifying the content.
End point systems must not use this header for any purpose; it's aim
is to assist with system troubleshooting.

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This concern is added as another General Consideration to section 6.1.2.