[SIPForum-discussion] How to make Re-Invite without SDP

[Norbert.Moura at swisscom.com]

Hi all,

This is a comment posted on the sipforum. My question is : do you expect the SDP version in the 200OK to be increased, compared to the initial SDP offer in the first INVITE from A ?

I assume that is one way of doing session timer, but re-INVITE
without SDP was not really intended for that.  Sending a re-INVITE
with the SAME sdp as the previous offer is a more elegant way of
implementing session timer and the way I have normally seen it done.


Here is a classic use of re-INVITE with no SDP. Consider the situation
when you need to do mid call media re-negotiation that needs to be
started by an intermediary ( for example an SBC ). In that case, the
SBC, acting as a B2BUA that stands in the middle of the signaling
between two dialogs that it is managing, solicits an offer from one of
the dialogs  ( lets call it side A) by sending it an INVITE without
SDP. Then plays the offer received in the OK from side A as a
re-INVITE to the other side ( lets call it side B) and plays the
answer back to the side A in an ACK with SDP when the answer is
received from  side B. This call flow is pretty useful when dealing
with call transfer through the SBC.

Thanks,
Norbert

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