Dale Worley wrote:
On Tue, 2008-12-02 at 16:53 -0500, Paul Kyzivat wrote:
As I read 3261, a proxy can't do this. The section on adding headers
from the URI to the message is in section 8, for UACs. In section 16,
for proxies, there is no comparable language. Instead, the proxy must
remove any parameters that aren't allowed in the R-URI, which would
include the headers. And it may indeed add additional headers, but I
believe it is restricted in doing so by Table 2. Table 2 doesn't permit
the proxy to add, modify, or delete an Expires header.
Now this does seem a bit inconsistent. If a URI in a 3xx includes
headers, then they would apply if the 3xx is returned to the UAC, but
not apply if the 3xx is recursed by a proxy.
Interesting... People have discussed proxies for a long time as if they
handled target URIs similarly to UASs generating initial requests, that
is, header-parameter parts are extracted from the request-URI and turned
into headers.
Its amazing isn't it, what we think the specs say vs. what they actually
say?
I have to admit that before replying to you I wasn't certain where
headers in URIs were applied. I had to review 3261 before I decided that
it was only UACs, not Proxies.
Looking at RFC 3261 section 16.6 step 2, none of this is discussed.
Step 5 says "The proxy MAY add any other appropriate header fields to
the copy at this point." which seems to allow the proxy great latitude.
Proxies *do* have a lot of latitude. But they are constrained by Table
2. I know a lot of people don't like table 2 - there are a lot of
limitations in what it can describe, and it isn't maintained as well as
it might be. Yet there are things, like this, that are not stated
anywhere else.
Table 2 does forbid the proxy from modifying the Expires header. But
perhaps that's an oversight?
We need to input from a real old-timer to answer that. Its before my time.
One could also argue that the proxy rules
apply to UACs when they're recursing on 3xx responses, as they're
"acting like proxies" then.
That would have been one way to do it. But that isn't how it was written
up. And they can't be exactly the same. A UAC can act on an http: URI in
a 3xx, where a proxy isn't permitted to.
What this will really do is revive all the chatter about banning forking
and recursing on 3xx.
Thanks,
Paul
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