Re: [Sip-implementors] Building route set from provisional responses

[Paul Kyzivat]

On 10/12/16 12:40 PM, Brett Tate wrote:

Hi,


Since these are B2BUAs, there are *three* different
early dialogs here:
- UAC:B2BUA1
- B2BUA1:B2BUA2
- B2BUA2:UAS

All the rules regarding route sets, etc. apply
separately to each.


I agree.  However, I just wanted to mention that the 3gpp specs appear to
be requiring B2BUA Application Servers to add Record-Route instead of
replacing a gruu Contact.  I'm not sure how well the 3gpp requirement will
work since nothing has been specified for building (record routing) routes
for use when sending a request to a gruu location.


It always seemed to me that you should't be a UA without putting in your 
own Contact URI. But I have scoured 3261 for support for that opinion 
and have not found it. So I have (reluctantly) accepted that B2BUAs 
using Record-Route to stay in the path while leaving the Contact URI 
unchanged are valid.


But Record-Route does cause problems with GRUU. If you aren't careful 
you can end up in sub-optimal routing spirals.


A long time ago I worked on some proposals for additions to IMS that 
made things work with GRUU. It was complex. And I don't know if that 
stuff was ever adopted.


Thanks,
Paul

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Re: [Sip-implementors] Building route set from provisional responses

[Brett Tate]

Hi,

> Since these are B2BUAs, there are *three* different
> early dialogs here:
> - UAC:B2BUA1
> - B2BUA1:B2BUA2
> - B2BUA2:UAS
>
> All the rules regarding route sets, etc. apply
> separately to each.

I agree.  However, I just wanted to mention that the 3gpp specs appear to
be requiring B2BUA Application Servers to add Record-Route instead of
replacing a gruu Contact.  I'm not sure how well the 3gpp requirement will
work since nothing has been specified for building (record routing) routes
for use when sending a request to a gruu location.
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Re: [Sip-implementors] Building route set from provisional responses

[Paul Kyzivat]

On 10/12/16 8:16 AM, Gagandeep Singh wrote:

Considering below scenario. (Please view in fixed-width font)

UAC  B2BUA1 B2BUA2 UAS
|--INVITE->|  |  |
|  |--INVITE->|  |
|  |  |--INVITE->|
|  |  |  |
|  |  |<---180---|
|  |<---180---|  |
|<---180---|  |  |
|  |  |  |
|<---183(100rel)---|  |  |
|  |  |  |
|--PRACK-->|  |  |
|  |(-PRACK?>)|  |
|  |  |  |

(1) Route set due to 180: , 
(2) Route set due to 183: 


Since these are B2BUAs, there are *three* different early dialogs here:
- UAC:B2BUA1
- B2BUA1:B2BUA2
- B2BUA2:UAS

All the rules regarding route sets, etc. apply separately to each.

B2BUA1 has not received a reliable provisional from B2BUA2, so it cannot 
send a PRACK.


People like to think that there is a single dialog between UAC and UAS, 
but that can't be with B2BUAs. They can try to maintain the illusion by 
carefully correlating the messages on each side. But as soon as you 
depart from proxy behavior you are going to encounter cases where that 
strategy breaks down. As a B2BUA developer it is your responsibility to 
follow all the rules on each side of your B2BUA.


Thanks,
Paul



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Re: [Sip-implementors] Building route set from provisional responses

[Dale R. Worley]

Gagandeep Singh  writes:
> Considering below scenario. (Please view in fixed-width font)
>
> UAC  B2BUA1 B2BUA2 UAS
> |--INVITE->|  |  |
> |  |--INVITE->|  |
> |  |  |--INVITE->|
> |  |  |  |
> |  |  |<---180---|
> |  |<---180---|  |
> |<---180---|  |  |
> |  |  |  |
> |<---183(100rel)---|  |  |
> |  |  |  |
> |--PRACK-->|  |  |
> |  |(-PRACK?>)|  |
> |  |  |  |
>
> (1) Route set due to 180: , 
> (2) Route set due to 183: 
>
> PRACK should be routed as per (2), which will not be possible if route set
> (1) is used.

As all the responders have indicated, within a dialog, the UAS MUST
provide the same Record-Routes for all the provisional responses, which is
indeed the set of Record-Routes that was attached to the INVITE when it
received it.

When B2BUAs are operating, there are two methods of operation:

(1) operate the incoming and outgoing dialogs as specified in RFC 3261,
that is, be a UAC connected to a UAS, or

(2) act as a "quasi-proxy", which does not fully terminate either the
incoming or outgoing dialog, but also modifies the messages within the
dialog in ways that are not permitted by RFC 3261.

In the latter case, the B2BUA has to ensure that the message flow
appears to the upstream and downstream devices to be operating according
to RFC 3261.  (That is not easy to do.)

In the example you diagram, if B2BUA1 is operating in mode (1), then the
PRACK terminates at B2BUA1; B2BUA1 sends a 200 response for it.  But
also, the 180 and the 183 going to UAC do not contain Record-Route
headers for B2BUA2 or UAS; B2BUA1 is the UAS for the dialog with UAC.
B2BUA1 MUST NOT send a PRACK to B2BUA2 because B2BUA1 has not received a
100rel provisional response from B2BUA2.

If B2BUA1 is operating in mode (2), it still MUST NOT send a PRACK to
B2BUA2.  Since it decided to send the 183 itself, it knows that it MUST
provide the response to the PRACK it receives for the 183 -- despite
that it MUST have provided the 183 with the same Record-Route headers
that were on the 180 going to UAC.

Dale
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Re: [Sip-implementors] Building route set from provisional responses

[Brett Tate]

> Suppose UAC receives multiple provisional responses
> before confirmation of the dialog (2xx), and each
> provisional response has a different set of Record-Route
> headers.  (Each provisional response is within the same
> early dialog. i.e. same From-tag and To-tag.)
>
> In this scenario:
> 1. Should UAC update its route set according to latest
> Record-Route headers received?
> 2. Or should UAC retain the route set formed from first
> provisional response?
>
> It is noted that "final" route set will be built from
> Record-Routes in 2xx response.

In my opinion, the answer to the question should be irrelevant since the
UAS should be providing the same Record-Route within the 101-299 responses
to INVITE (excluding 199 situation mentioned within RFC 6228).  However,
the answer is relevant since there are vendors not including the
Record-Route because they ignore the fact that the sent 1xx might create a
dialog because of dropped/reordered responses or UAC forgot dialog (e.g.
because of race condition or unwillingness to maintain an infinite number
of early dialogs).

More specifically for interoperability reasons, I think that the UAS
should interpret RFC 3261 section 12.1.1's MUST statement to include
Record-Route as applying to every 101-198 response to INVITE during call
setup since they don't truly know that it won't create an early dialog.

With that said, RFC 6141 updated some of the rules mentioned within RFC
3261 (although I think just associated within Contact instead of also
Record-Route).  I don't recall either RFC having any normative statements
indicating/precluding UAC to allow reliable 1xx's present/missing
Record-Route to update the route set during call setup; however from a
compliance perspective, it is easier to defend not updating the route set.
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Re: [Sip-implementors] Building route set from provisional responses

[Gagandeep Singh]

Considering below scenario. (Please view in fixed-width font)

UAC  B2BUA1 B2BUA2 UAS
|--INVITE->|  |  |
|  |--INVITE->|  |
|  |  |--INVITE->|
|  |  |  |
|  |  |<---180---|
|  |<---180---|  |
|<---180---|  |  |
|  |  |  |
|<---183(100rel)---|  |  |
|  |  |  |
|--PRACK-->|  |  |
|  |(-PRACK?>)|  |
|  |  |  |

(1) Route set due to 180: , 
(2) Route set due to 183: 

PRACK should be routed as per (2), which will not be possible if route set
(1) is used.

Kindly guide where am I missing.

Warm Regards
Gagandeep Singh

On Wed, Oct 12, 2016 at 4:27 PM, ankur bansal  wrote:

> Hi Gagan,
>
> UAC constructs dialog-set when early dialog is created(on first prov rsp)
> and this is maintained for rest of dialog .
> Which means on getting first prov response ,route-set is filled.And not
> updated  on getting further prov responses.
>
> Later when 200ok comes,route-set can be updated again in dialog-set.
>
> Can you please explain scenario where provisional responses in same dialog
> coming via different route ?
>
> Regards
> Ankur Bansal
>
> On Wed, Oct 12, 2016 at 1:38 PM, Gagandeep Singh 
> wrote:
>
>> Hello
>>
>> Suppose UAC receives multiple provisional responses before confirmation of
>> the dialog (2xx), and each provisional response has a different set of
>> Record-Route headers.
>> (Each provisional response is within the same early dialog. i.e. same
>> From-tag and To-tag.)
>>
>> In this scenario:
>> 1. Should UAC update its route set according to latest Record-Route
>> headers
>> received?
>> 2. Or should UAC retain the route set formed from first provisional
>> response?
>>
>> It is noted that "final" route set will be built from Record-Routes in 2xx
>> response.
>>
>> Warm Regards
>> Gagandeep Singh
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>>
>
>
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Re: [Sip-implementors] Building route set from provisional responses

[ankur bansal]

Hi Gagan,

UAC constructs dialog-set when early dialog is created(on first prov rsp)
and this is maintained for rest of dialog .
Which means on getting first prov response ,route-set is filled.And not
updated  on getting further prov responses.

Later when 200ok comes,route-set can be updated again in dialog-set.

Can you please explain scenario where provisional responses in same dialog
coming via different route ?

Regards
Ankur Bansal

On Wed, Oct 12, 2016 at 1:38 PM, Gagandeep Singh 
wrote:

> Hello
>
> Suppose UAC receives multiple provisional responses before confirmation of
> the dialog (2xx), and each provisional response has a different set of
> Record-Route headers.
> (Each provisional response is within the same early dialog. i.e. same
> From-tag and To-tag.)
>
> In this scenario:
> 1. Should UAC update its route set according to latest Record-Route headers
> received?
> 2. Or should UAC retain the route set formed from first provisional
> response?
>
> It is noted that "final" route set will be built from Record-Routes in 2xx
> response.
>
> Warm Regards
> Gagandeep Singh
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>
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