[asterisk-users] 180 Ringing with SDP

[Raj Jain]

180 w/ SDP is valid, although not ideal. 183 w/ SDP is a better choice for
early-media. The SIP specifications do not dictate what a UAC should do when
it receives 180 w/ SDP. It depends on the policy implemented in the UAC. 

As far as Asterisk is concerned, it could treat 180 w/ SDP same as 183 w/
SDP. However, there should be a global flag in sip.conf that controls
whether SDP in 180 should be interpreted (early media played) or ignored
(local ring back generated).

Here is a sample policy from RFC 3960 (this doesn't exactly talk about 180
w/ SDP but suggests what a UAC can do when it receives early-media packets
in conjunction w/ a 180 response):

   With this in mind, a UAC should develop its local policy regarding
   local ringing generation.  For example, a POTS ("Plain Old Telephone
   Service")-like SIP User Agent (UA) could implement the following
   local policy:

      1. Unless a 180 (Ringing) response is received, never generate
         local ringing.

      2. If a 180 (Ringing) has been received but there are no incoming
         media packets, generate local ringing.

      3. If a 180 (Ringing) has been received and there are incoming
         media packets, play them and do not generate local ringing.

         Note that a 180 (Ringing) response means that the callee is
         being alerted, and a UAS should send such a response if the
         callee is being alerted, regardless of the status of the early
         media session.

Thanks,

Raj
 

> -----Original Message-----
> From: asterisk-users-bounces at lists.digium.com 
> [mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of 
> Alex Balashov
> Sent: Monday, June 18, 2007 8:27 PM
> To: Asterisk Users Mailing List - Non-Commercial Discussion
> Subject: Re: [asterisk-users] 180 Ringing with SDP
> 
> On Mon, 18 Jun 2007, Jared Smith wrote:
> 
> > I could be totally off base here, but it's my understanding 
> that a 180 
> > is telling Asterisk to generate ringing on it's side, and 
> that a 183 
> > (with SDP) would tell Asterisk that the call is progressing 
> and that 
> > it should play the early media specified in the SDP.  I'm 
> sure someone 
> > there's probably someone on the list who is more intimate with the 
> > details of SIP that can enlighten us further on the subtle 
> differences 
> > between the 180 and 183 provisional responses.
> 
>    A cursory interpretation of the RFC suggests that 180 
> Ringing is a message designed solely to convey ringback, and 
> that it is the payload of the 183 response that may be used 
> to convey additional details about the nature of the call's 
> progress.  Therefore, a 180 would be an inappropriate vehicle 
> for early media SDP information.
> 
>    21.1.5 183 Session Progress
> 
>     The 183 (Session Progress) response is used to convey information
>     about the progress of the call that is not otherwise 
> classified.  The
>     Reason-Phrase, header fields, or message body MAY be used 
> to convey
>     more details about the call progress.
> 
> -- Alex
> 
> 
> --
> Alex Balashov
> Evariste Systems
> Web    : http://www.evaristesys.com/
> Tel    : +1-678-954-0670
> Direct : +1-678-954-0671
> 
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