Definitely don't use it as a cache key. I see two things happening:
1) The cache sends an If-Modified-Since request, *with* the Request-ID
from the new request. Server returns a 304, and keeps the request-ID for
more info.
2) The cache opts to not send an If-Modified-Since request, instead
serving up files locally. This is due to a cache config setting to
keep possibly stale documents around for a short time (not recommended
but impossible to control) or an Expires: header on the content with a
later date than the current one. In this case, it's true that the server
does not get the Request-ID, but then the server would never see that
access anyways.
Both cases are just like the current situation when it comes to collecting
information, so I don't see it as a problem. I believe Simon Spero suggested
that there were mechanisms in HTTP-NG to relay this type of information at a
later point in time back to the server.
Brian
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