4. Protocol Operation

4. Protocol Operation

The offer/answer exchange assumes the existence of a higher layer protocol (such as SIP) which is capable of exchanging SDP for the purposes of session establishment between agents.

Protocol operation begins when one agent sends an initial offer to another agent. An offer is initial if it is outside of any context that may have already been established through the higher layer protocol. It is assumed that the higher layer protocol provides maintenance of some kind of context which allows the various SDP exchanges to be associated together.

The agent receiving the offer MAY generate an answer, or it MAY reject the offer. The means for rejecting an offer are dependent on the higher layer protocol. The offer/answer exchange is atomic; if the answer is rejected, the session reverts to the state prior to the offer (which may be absence of a session).

At any time, either agent MAY generate a new offer that updates the session. However, it MUST NOT generate a new offer if it has received an offer which it has not yet answered or rejected. Furthermore, it MUST NOT generate a new offer if it has generated a prior offer for which it has not yet received an answer or a rejection. If an agent receives an offer after having sent one, but before receiving an answer to it, this is considered a "glare" condition.

The term glare was originally used in circuit switched telecommunications networks to describe the condition where two switches both attempt to seize the same available circuit on the same trunk at the same time. Here, it means both agents have attempted to send an updated offer at the same time.

The higher layer protocol needs to provide a means for resolving such conditions. The higher layer protocol will need to provide a means for ordering of messages in each direction. SIP meets these requirements [7].