RFC 3486:Compressing the Session Initiation Protoc...
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next-hop


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... URI. This URI, referred to as the next-hop URI, is the Request-URI of the request or an entry in ...
... header field. If the next-hop URI contains the parameter comp=sigcomp, the client ...
... 2]. If the next-hop URI is a SIPS URI, the request SHOULD be compressed ...
... For requests within a dialog, a next-hop URI with the comp=sigcomp ...


... If a proxy detects that the next-hop proxy for a request is the proxy ...



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