Re: [Sip] RE: Delivering request-URI and parameters to UAS via proxy

[Paul Kyzivat <[e-mail]>]

I am still confused by the distinction between Target and P-CPI.

I agree that you have described rules for updating Target that are 
different from those for updating P-CPI. But it seems to me that is a 
distinction without a significant difference. For the recipient of the 
request, the two are used the same way, toward the same end. If anything 
I would view the rules for Target to be just a bug fix on P-CPI.

	Paul

Christer Holmberg wrote:
> Hi John,
> 
>> I guess what still confuses me is, when both Target and P-CPI 
>> are used, which comes first, i.e., which represents the 
>> earlier target? When I read the draft, I thought Target was 
>> earlier. From various clarifying emails I now get the 
>> impression that Target is later. Can you confirm?
> 
> I am not sure what you mean by "earlier" target. 
> 
> Target, if present, always represents the current target. A receiving
> entity that receives a request without a Target header has to assume
> that the Request-URI represents the current target. When no Target
> header is available on the request then the Target is inserted when a
> reroute is performed.
> 
> P-CPI is ONLY used to maintain the R-URI which is rewritten by the
> contact of the UAS. I.e. by the home proxy which is usually at the end
> of a chain of proxies.
> 
> When both Target and P-CPI are used: Target always represents the
> current target. P-CPI always represents the AOR received by the home
> proxy which may in some cases be the same as the current target but it
> may also be the last route (not equal to the current target) taken to
> deliver the request. 
> 
>> Picking up on Francois' point about History-Info, with the 
>> introduction of Target and P-CPI we do indeed have a lot of 
>> URIs, and of course History-Info can already convey all these 
>> URIs and any others. The difference is that History-Info does 
>> not give particular semantics to each of the URIs it conveys 
>> - they are simply a succession of targets.
> 
> Yes, we agree with your analysis regarding History-Info. The
> UA-loose-draft discusses further issues with using History-Info.
> 
>> With Target and P-CPI we are aiming to define specific 
>> semantics. I am concerned whether we will be able to define 
>> these semantics tightly enough to ensure consistent 
>> implementations.
> 
> Target always represents the current target. See above.
> 
> P-CPI does not represent the current target in all cases. See above.
> 
>> The more URIs we try to define, the harder it will be to assign each
> one a clearly distinguishable meaning. I hope the next draft will help.
> 
> In the next version of the draft I will remove as much as possible about
> P-CPI, in order not to cause confusion. I will only keep some text where
> I describe the semantical difference between Target and P-CPI.
> 
> I also do agree with one of your previous comments, that we should
> clarify the meaning of all the URIs we're using - and that should be
> done no matter if we adopt Target or not.
> 
> Regards.
> 
> Christer
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
>>> -----Original Message-----
>>> From: Christer Holmberg [mailto:[e-mail]]
>>> Sent: 15 January 2008 08:48
>>> To: Francois Audet
>>> Cc: [e-mail]; DRAGE, Keith (Keith); Paul Kyzivat; Elwell, John
>>> Subject: RE: [Sip] RE: Delivering request-URI and parameters to UAS 
>>> via proxy
>>>
>>>
>>> Hi,
>>>
>>> P-CPI could probably be useful in some cases, in addition to Loose 
>>> Route/Target. And, as the draft says, P-CPI will still have 
>> to be used 
>>> in IMS, because there are procedures defined for it.
>>>
>>> However, again, the purpose of the draft was to provide an 
>> alternative 
>>> to the Loose Route alternative, and that alternative is Target only.
>>>
>>> I am working on an updated version of the draft to make that more 
>>> clear.
>>>
>>> Regards,
>>>
>>> Christer
>>>
>>>
>>>
>>>
>>>
>>>> Hum. I guess then P-Called-ID would then be useful with 
>> Loose-route 
>>>> as well (although now I'm thinking that History-Info covers it).
>>>>
>>>> I think explaining all that in great and precise details, with a 
>>>> concrete example would be very useful.
>>>>
>>>> And then we could compare P-Target with Loose-route. 
>>>
>>>
>>>
>>>
>>>>> -----Original Message-----
>>>>> From: Christer Holmberg [mailto:[e-mail]]
>>>>> Sent: Monday, January 14, 2008 03:56
>>>>> To: Audet, Francois (SC100:3055)
>>>>> Cc: [e-mail]; DRAGE, Keith (Keith); Paul Kyzivat; 
>> Elwell, John
>>>>> Subject: RE: [Sip] RE: Delivering request-URI and
>>> parameters to UAS
>>>>> via proxy
>>>>>
>>>>>
>>>>> Hi Francois,
>>>>>
>>>>>> I think what you meant by Target was more the "Current" 
>>>>>> target as opposed to the Initiatl Target.
>>>>> Yes.
>>>>>  
>>>>>> And if that's the case, then I don't see why it is
>>> different from
>>>>>> P-Called-ID (although I might be missing something 
>> with what the 
>>>>>> P-Called_ID is supposed to be).
>>>>> In the draft we try to explain the difference. But, we are
>>>> working on
>>>>> the text to make it more clear.
>>>>>
>>>>> The P-CPI is inserted when the R-URI is rewritten by 
>> the Contact 
>>>>> address of the UAS. RFC3455 calls that operation
>>>> "retargeting", but we
>>>>> don't think that is the definition for retarget used in the 
>>>>> ua-loose-route draft, which says:
>>>>>
>>>>> "When a home proxy receives a request and accesses a
>>>> location service,
>>>>> the resulting contact(s) obtained from the location service are 
>>>>> considered the last hop in the route towards the entity
>>>> addressed by
>>>>> the Request-URI.  Since that target, almost by definition,
>>>> can claim
>>>>> the identity of the URI prior to translation, the operation
>>>> is one of
>>>>> routing and not retargeting."
>>>>>
>>>>> So, if we follow the definitions in the ua-loose-route
>>> draft, P-CPI
>>>>> would be inserted due to a reroute - not retarget.
>>>>>
>>>>> But, no matter whether we call it retarget or reroute,
>>> the point is
>>>>> that the P-CPI is inserted when the R-URI is rewritten with the 
>>>>> Contact address of the UAS. The scope of Target is wider
>>> than that,
>>>>> and can be used in any retargeting situation.
>>>>>
>>>>> Regards,
>>>>>
>>>>> Christer
>>>>>
>>>>>
> 


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