Record-Route header field
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... the parameter comp=sigcomp to its URI in the Record-Route header
field.
If a user agent client ...
... proxy performing Record-
Route inspects the Record-Route header field in the response and the
Contact header field in the request that triggered this response (see
...
... proxy SHOULD add
comp=sigcomp to its entry in the Record-Route header field. If this
URI does not contain the parameter comp=sigcomp ...
... interface.
This way, the proxy does not need to rewrite the Record-Route header
field on the response.
Proxies ...
... requests (Record-Route of the 200 OK). Therefore, P2 modifies its
entry in the Record-Route header field of the 200 OK (5). In the
INVITE (3), P2 did not used the comp=sigcomp ...