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Re: PRACK: Does an early-session SDP offer fullfills the rule to insert SDP in first reliable response




Hi,

 >
> > I've changed the subject, because I don't think this is related to the
> > re-transmission of 18x. It is a little related to the discussion about
> > relaxing the must-insert-SDP-in-first-reliable-18x, but let's still have
> > a separate thread for it.
> >
> >>>  > >  > >>>Is the flows below valid according to recent arguments?
> >>>  > >  > >>>
> >>>  > >  > >>>   UAC                  UAS
> >>>  > >  > >>>    |----invite(SDP)--->|
> >>>  > >  > >>>    |<--- 183(SDP)------|
> >>>  > >  > >>>    |-----prack(SDP)--->|
> >>>  > >  > >>>    |<--- 200(SDP)------|
> >>>  > >  > >>>
> >>>  > >  > >>>       flow 1
> >>>  > > Does it means,the first flow is allowed?
> >>>  >
> >>>  > Yes.
> >>>  > > I think, the restriction the first reliable response
> >> must contain
> >>> SDP if  > > the INVITE without SDP  > > should restrict the called
> >>> user.
> >>>  >
> >>>  > That's the way it is now.
> >>>
> >>>
> >>> Sure, I means, the first reliable response from the called
> >>> user must contain NORMAL SESSION SDP in order to communication,but
> >>> AS(application server) shouldn't be restricted by this rule
> >>> as AS may only concern about early-session for pronunciation. Or, the
> >>> AS is also restricted if early-session plays the same role as normal
> > session.
> >> The AS is an artifact of IMS, it has no role in any of the
> >> ietf standards. It has been expected that components playing
> >> various roles such as this are bound by normal distinctions
> >> between UAC, UAS, Proxy, etc.
> >>
> >> IMO it isn't a good idea to introduce a new kind of UA (or
> >> proxy) that is bound by different rules.
> >
> > I don't think a new role is proposed. The AS was just an example, but
> > from a SIP perspective it is of course behaving as a UAS in this case :)
> >
> >> You might consider the "early-session" mechanism specified by RFC 3959.
> >> But I have never heard of it being implemented, so it may not be useful
> > to you.
> >
> > I have been assuming that we ARE talking about the 3959 mechanism, and
> > the question has been whether an "early-session" SDP offer fullfills the
> > MUST rule to include SDP offer in the first reliable response.
> >
> > IF we decide to relax the rule I guess it doesn't really matter, but if
> > we maintain the rule I guess it is a valid question.
>
> Frankly I don't expect to see 3959 ever used. But if it is...
>
> IMO the "early-session" and the "session" must each independently comply
> with the o/a rules. So if you take any call flow using early-session,
> and transform it in either of the following ways, the transformed flow
> should still be valid according to the normal rules for o/a using "session":
>
> 1) remove all the body parts with content-disposition of "early-session"
> and content-type "application/sdp". Then if that results in
> multipart/mixed body parts containing only an application/sdp body part
> with c-d of session, remove the multipart, leaving just the sdp in its
> place. Then remove any Supported/Required references to early-session.t
>
> 2) remove all the body parts with content-disposition of "session" and
> content-type "application/sdp". Then if that results in multipart/mixed
> body parts containing only an application/sdp body part with c-d of
> early-session, remove the multipart, leaving just the sdp in its place.
> Then replace occurrences of C-D:early-session with C-D:session. Then
> remove any Supported/Required references to early-session.
>

If I catch you, you mean, if early-session and session exist in the same
signal,how to place the content-type/content-disposition.You can place
content-type/content-disposition with just the SDP,and place
"Content-Type: multipart/mixed" at the front.

e.g:

Content-Type: multipart/mixed;boundary=---boundary-boundary---

---boundary-boundary---
Content-Type: application/SDP
Content-Disposition: session
v=0
o=
s=session SDP
c=

---boundary-boundary---
Content-Type: application/SDP
Content-Disposition: early-session
v=0
c=

---boundary-boundary---


> In theory, the above would allow the UAC to initiate both, but in
> practice that would make no sense.
>

In the initial INVITE,the UAC needn't initiate the early-session.
But if UAC want to alert the CALLED-UA (e.g UAC want to play some
special music for CALLED-UA), UAC can initiate the early-sesion.



> If what I say is correct, then it relaxes no restrictions on anybody.
>
> I don't understand why anybody would bother with early-session. For the
> cases where it might be useful I think the same effect can be
> accomplished easier by simply establishing an early dialog with one
> to-tag, and a different early/final dialog with a different to-tag.
>

Yes.
Early-session plays the same role as the fork early dialog, but if 199
is not implemented, and the call is forwarded, there may exist several
fork early dialogs, which one the UAC shold choose is problem.

IMO, the two methods both have advantages!
 

Regards,
Eric

>    Paul

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