[SIPForum-discussion] Offer-Answer Model

[gaurang.rathod at wipro.com]

I have a scenario as follows and looking forward for your suggestion:

 

  SIPGW1                         SIPGW2

    |        INVITE with SDP        |

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

    |                               |

    |          100 Trying           |

    |------------------------------>|

    |                               |

    | 183 Session Progress with SDP |

    |------------------------------>|

    |                               |

    |             PRACK             |

    |<----------------------------->|

    |                               |

    |       200 OK for PRACK        |

    |<----------------------------->|

    |                               |

    |        UPDATE with SDP        |

    |<----------------------------->|

    |                               |

    |  200 OK for UPDATE with SDP   |

    |<----------------------------->|

    |                               |

    |       200 OK for INVITE       |

    |<----------------------------->|

 

 

As a part of SIPGw implementation,

 

1) If we have a 183 with SDP (or multiple 183s with SDP), without any
UPDATE, 200 OK would not contain any SDP.

When PRACK is used, we really shouldn't be "copying" the answer in
further 183s and the 200. 

 

2) If we have a 183 with SDP followed by UPDATE with SDP and 200 OK
response for UPDATE with SDP, then 200 OK for INVITE SHOULD NOT contain
an SDP. 

 

3)      If we have a 183 with SDP followed by UPDATE with SDP and
non-200 OK response for UPDATE, then we SHOULD copy the latest 183's SDP
in the 200 OK for INVITE (Treat the scenario as no UPDATE occurred)

 

What I'm concerned is about the scenario mentioned below (I couldn't
find any information in RFC 3311 for SDP-less UPDATE).

 

Scenario *) We have a 183 with SDP. CAN this be followed by an UPDATE
WITHOUT SDP (session refresh)?

 

My Understanding

================

 

Since a 200 OK for INVITE has not been received, the session has not
been established and UPDATE without SDP is out of question. 

 

My conclusion

=============

 

If we have a 183 with SDP, followed by SDP-less UPDATE, the 200 OK for
UPDATE won't contain an SDP. Thus we SHOULD copy 183's SDP in 200 OK for
INVITE.

 

Regards,

Gaurang Rathod

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