Subject: [Sip] INVITE without SDP body.

From: Paraskevopoulos Pavlos <[email-masked]>
Date: Fri, 01 Nov 2002 15:23:13 +0200

Hello,

I 'm trying to build a SIP gateway, and I would appreciate a little help.

Think of that scenario

ISDN                     GW                         SIP-EP
----------------------------------------------------------------
-->SETUP                 --->INVITE(no SDP)       
       
             ALERTING<---                      180<-----
             CONNECT<---                       200<-----
                        ---->ACK(SDP)       

I want to send the INVITE request without SDP info. The SDP info will 
come later with the ACK.
Does the INVITE request have to contain a Required or Supported header 
with option tag 100rel?

Thanks in advance.

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---

From: Christer Holmberg <[email-masked]>
Date: Fri, 01 Nov 2002 15:46:30 +0200

Hi,

100rel is needed only if you want to use PRACK. It has nothing to do with the
fact if you have a SDP in the INVITE or not.

Note, however, that if you use PRACK you may have to send the SDP before you
send the ACK (in a PRACK), but that's another issue...

Regards,

Christer Holmberg
Ericsson Finland

---

From: Paraskevopoulos Pavlos <[email-masked]>
Date: Fri, 01 Nov 2002 16:30:32 +0200

Thanks for your response.
I agree but I 'm a bit confused because in the
draft-elwell-sippping-qsig2sip-03.txt (Interworking between SIP and QSIG),  
it is mentioned :

    If the SIP INVITE request does not contain SDP information and does 
not contain
    either a Required header or a Supported header with option tag 
100rel, the gateway
    SHALL NOT issue a QSIG SETUP message and SHALL send a SIP 488 (NOT 
ACCEPTABLE HERE) response.

What did I understand wrong?

Thanks again.

---

From: Christer Holmberg <[email-masked]>
Date: Sat, 02 Nov 2002 15:32:54 +0200

Hi,

>     If the SIP INVITE request does not contain SDP information and
> does not contain

I don't think you understood anything wrong :)

Specific SIP APPLICATIONS (in your case the QSIG-to-SIP interworking)
may require certain things (eg some SIP extensions, SDP in the INVITE
etc etc etc), but it is not a general SIP PROTOCOL requirement that you
must use PRACK if you don't have SDP in the INVITE.

Regards,

Christer Holmberg
Ericsson Finland