upstream
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... header field in the request that triggered this response (see
example in Section 9). It looks for the URI of the next upstream
(closer to the user agent client) hop in the route ...
... Contact header field of the response if the URI of the next upstream
hop in the route set contained the parameter comp=sigcomp ...
... Proxies that receive compressed messages from one side of the dialog
(e.g., upstream) and uncompressed messages from the other side (e.g.,
downstream) MAY use the mechanism described above.
...