[SIPForum-discussion] IMS call: Answer just when 180 is sent out

sreekant nair sreekant_nair at yahoo.com
Tue Aug 21 16:26:39 UTC 2007

Hi Gopalarathnam, 

I think RFC 3262 (section 3 in particular) should help you out. See a small excerpt below. 
   The UAS MAY send a final response to the initial request before
   having received PRACKs for all unacknowledged reliable provisional
   responses, unless the final response is 2xx and any of the
   unacknowledged reliable provisional responses contained a session
   description.  In that case, it MUST NOT send a final response until
   those provisional responses are acknowledged.  If the UAS does send a
   final response when reliable responses are still unacknowledged, it
   SHOULD NOT continue to retransmit the unacknowledged reliable
   provisional responses, but it MUST be prepared to process PRACK
   requests for those outstanding responses.  A UAS MUST NOT send new
   reliable provisional responses (as opposed to retransmissions of
   unacknowledged ones) after sending a final response to a request.To depicting your scenario 
Invite (SDP)------>

<------- 100 Trying

<------- 180 (SDP)

<------- 200 Ok for Invite (No SDP)

From the above excerpt it is clear that the above scenario is invalid(assuming that the 
REQUIRE header was included saying 100Rel). The only final that can be sent without waiting 
for PRACK are NON-200OK final responses. If the final response is a 200OK we have to wait 
for PRACK for earlier 18Xs(see earlier paragraphs). 
However had the 18X not contained an SDP, the 200OK must contain the answer SDP and so can be sent I believe.  
Sreekant Nair

----- Original Message ----
From: Gopalarathnam Sambasivan <s.gopalarathnam at gmail.com>
To: discussion at sipforum.org; imsclub at googlegroups.com
Sent: Tuesday, August 21, 2007 9:33:57 AM
Subject: [SIPForum-discussion] IMS call: Answer just when 180 is sent out

Hello All,

Just after the moment 180 ringing is sent in a scenario with require and supported 100rel, state to be waiting for PRACK

If terminating side answers the call (200 OK), what will happen? what is the expected behaviour? 

What about SDP negotiation with PRACK in this case?


Please tell me the sections of RFC/TS which describes this scenario.


Thanks in Advance.


Best Regards,



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