SIP Early offer/early Media vs Delay Offer/Delay Media

Sip is a Request/Response protocol and it uses different mechanism to establish session.This article i am going to discuss about early/delay offer used by SIP for
capability negotiation

Early offer: SIP uses SDP (Session description protocol) in order to negotiate capability between two parties.In case of the early offer the SDP message is sent from the Calling party to calllee party in the initial invite message and the callee party respond with the negotiated capability  in the 200 OK it send to the calling
 party

Delay Offer : In this case the called party send the SDP message in the 200 OK message to the calling party and the calling party respond back with the negotiated parameter in the ACK message it send to callee party

Early Media : In case of early media the media between the callee and calling party establish before the session established between the two.In normal condition the SIP endpoint play ringing sound from its local source when it recieve a ringing message from the callee party.This is sometime confusing as no one knows what is happening at the callee party,Early media play a important role here what t does is the calle party send a message 183 instead of 180 and let the calling party establish a bearer channel between the two party so that they can share the message directly.

Delay Media : In case of delay media the media is established after the SIP session negotiation is complete