[Serusers] Initiate a call from SER

Samuel Osorio Calvo samuel.osorio at nl.thalesgroup.com
Thu Jul 8 10:49:02 CEST 2004


Hi,

SIP's main entity are User Agents (UAs), which can be devided into UAC
(User Agent Client) and UAS (User Agent Server), depending on the
*transaction* they are involved into.
UAC are transaction-level entities initiating requests and accepting
responses. On the other side, UAS are transaction-level entities
accepting requests and issuing responses.

Thus, a normal SIP softphone instantiates (depending on the
implementation may differ the exact way) a UAC whenever sending an
INVITE and the *same* UAC waits for the response (usually an OK).
If acting as receiving point, it will create a UAS instantiation as
soon as it receives an incoming INVITE and will issue the appropriate
answer.

The point is that a server is mainly a UAS waiting for incoming
requests BUT it will need to instantiate a UAC to send requests (just as
forwarding INVITES, ACK, CANCEL,etc...) only after receiving incoming
requests.  By itself, a server MUST NOT send a request except to forward
incoming ones.
Next diagram (I expect it arrives with the right alignment) tries to
explain the transaction-level entities involved in a generic SIP
transaction. (INVITE+OK). I hope it will be useful to clarify how SIP
works.

               softphone                                      proxy
server                                        softphone
                   UAC----------------------------------------->UAS +
UAC----------------------------------------->UAS

Thus a SIP application starting a request (initiating a transaction)
will create a UAC transaction and will expect a UAS to receive and
process it.

I expect not to confuse you with the words UAS and UAC with the usual
meaning of UA (=softphone). 

Summarizing, only softphones and end-point applications will initiate
requests, that will be forwarded by servers within the "backbone" to the
appropriate end-point.

I hope it is more clear now,

Samuel.


P.S. This is the normal SIP behaviour........I'm afraid some situations
might be different (NOTIFY requests).



Unclassified

>>> GR S <gr_sh2003 at yahoo.com> 07/08/04 09:00AM >>>
Hi,

--- Klaus Darilion <klaus.mailinglists at pernau.at> wrote:
> So maybe you should start with your intention - what feature are you

> trying to setup?
> ser can create requests ( transaction = request+response(s) ) and you

> can use this feature via the FIFO.

Thanks for the reply.

Sorry, I misnderstood the meaning of transaction. I was only thinking
about SIP requests, not
responses. But i am still confused about call initiating. Please
correct me if i am wrong on this,
the original poster was asking about initiating calls from SER and
AFAIK proxies dont initiate
calls. Can they initiate calls? If yes, on what situations? When you
say SER can create requests,
do you mean methods like CANCEL?

> klaus

TIA,

=====
Girish Gopinath  <gr_sh2003 at yahoo.com>


		
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