[SIPForum-discussion] Offer-answer

Garron, James jgarron at sonusnet.com
Wed Jun 29 17:01:43 UTC 2011

Yes, 18x after OA is complete UAS can send additional 18x messages,
however they should not contain SDP and if they do they should be



3.1.2.  INVITE request without SDP


   When a UAC does not include a SDP body in the INVITE request, it

   expects an offer to be received with the first reliable non-failure

   response.  And a UAS MUST include an offer in the first reliable non-

   failure response and should not include any SDP in the other

   responses to the INVITE request.  The UAC will send the answer in the

   request to acknowledge the response, i.e.  PRACK or ACK request of

   the reliable response.  Other than that, no offer/answer exchanges

   can occur in the messages within the INVITE transaction.


      NOTE: In Figure 2, the UAS should not include SDP in the responses

      F6 and F9.  However, the UAC should prepare to receive SDP bodies

      in F6 and/or F9, and just ignore them to handle a peer that does

      not conform to the recommended implementation.


      UAC                   UAS

       | F1  INVITE (no SDP) |


       | F2     1xx          |


       |                     |

       | F3 1xx-rel (SDP)    | <- The first 1xx-rel must contain SDP

       |<--------------------|    as the offer.

       | F4   PRACK (SDP)    | <- A PRACK request to the first 1xx-rel

       |-------------------->|    must contain SDP as the answer.

       | F5 2xx PRA (no SDP) | -

       |<--------------------| |

       |                     | |

       | F6 1xx-rel (no SDP) | <- The subsequent 1xx-rel should not

       |<--------------------| |  contain SDP.

       | F7   PRACK          | |

       |-------------------->| | UAC can send a new offer in an UPDATE

       | F8 2xx PRA          | | request after F4.

       |<--------------------| v

       |                     |

       | F9 2xx INV (no SDP) | <- The final response should not

       |<--------------------|    contain SDP.

       | F10    ACK          |



        Figure 2: Example of Offer/Answer with 100rel Extension (2)


   Note that in the case that the UAC needs to prompt the user to accept

   or reject the offer, the reliable provisional response with SDP as an

   offer (Pattern 4) can result in the retransmission until the PRACK

   request can be sent.  The UAC should take care to avoid this

   situation when it sends the INVITE request without SDP.




From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of mrinal kuila
Sent: Wednesday, June 29, 2011 1:45 AM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Offer-answer


Hi  Forum,


Coiuld you please answer my below question.

Once offer answer is complete Is it possible to UAS to send more 18x

         UAC               UAS
        |                 |
         |<----18x (o)-----| - provisional responses with REQUIRE 100rel
         |                 |
         |-----PRACK(a)--->| - send PRACK with answer
         |<---- 2xx--------|                 |
          |                 |
         |                 |
         |<----18x --------| - >         |                 |




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