Subject: [Sip] Questions on draft-polk-sip-rph-in-responses-00

From: Dean Willis <[email-masked]>
Date: Tue, 14 Aug 2007 11:43:01 -0500

The draft draft-polk-sip-rph-in-responses-00 allows UAS(es) to  
include resource priority headers in responses.

I'm trying to figure out exactly what these headers really mean in  
responses.

RFC 4412 explains what the RPH is used for in requests:

    1.  The request can be given elevated priority for access to PSTN
        gateway resources, such as trunk circuits.

    2.  The request can interrupt lower-priority requests at a user
        terminal, such as an IP phone.

    3.  The request can carry information from one multi-level priority
        domain in the telephone network (e.g., using the facilities of
        Q.735.3 [Q.735.3]) to another, without the SIP proxies  
themselves
        inspecting or modifying the header field.

    4.  In SIP proxies and back-to-back user agents, requests of higher
        priorities may displace existing signaling requests or bypass
        PSTN gateway capacity limits in effect for lower priorities.

This all has to do with prioritizing the media channel that is being  
established by the request in-question.

So what does it mean to have an RPH in a response? It doesn't seem  
that we're using the response to affect the priority of the media  
channel. Are we perhaps reporting the priority that was granted to  
the media channel on the assumption that the priority granted might  
differ from the priority requested?

--
Dean

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