Another team member asked me today if Lync was “early offer” or “delayed offer” and why it would matter. He had been asked this by a Cisco partner configuring the CUCM side and wasn’t sure what he was being asked.
Simply put, the difference between the two is when the SDP (Session Description Protocol) information is sent. The information in the SDP headers will establish accepted audio codecs such as G.711. Early-offer refers to the SDP information being sent within the SIP INVITE message, this allows the calling party to specify it’s codec right at the start. With delayed-offer, we withhold the SDP information and have the called party reply with it’s SDP first in the 200 OK response.
So, what does Lync use? Early-offer, and this the standard for most VoIP (even non-Lync) deployments. Can delayed-offer be used? In short, kind of but not without some issues. I’ve seen reports of audio delays and at least with Lync 2010, media bypass was not supported with delayed-offer.
So what should you say when asked? Early-offer.