The media sessions between Tobias and Theresa are negotiated via SIP and SDP signalling. As in our example both UEs make use of a dedicated signalling PDP context for the transport of SIP signalling, they will have to establish one or more media PDP contexts for the transport of the media streams.
The procedure for establishing media PDP contexts is called "resource reservation". Both UEs perform these procedures completely independently of each other.
The establishment of media PDP contexts may consume some time and may even fail: when, say, insufficient resources are available over the wireless link. This means that, until the resources have been reserved, it cannot be guaranteed that the agreed media sessions can be established at all.
Therefore, Theresa's UE should not inform her about the incoming session request (INVITE): that is, it should not start to ring until it has confirmation that resource reservation has succeeded locally as well as at the calling user's end.
In order to achieve this, both UEs exchange preconditions during the SDP offer/answer negotiation, which basically instruct:
Tobias's UE to send a SIP UPDATE request to Theresa's UE, when resource reservation has succeeded at Tobias's UE; and
Theresa's UE not to ring until it receives the SIP UPDATE request from the remote end and has also successfully reserved its own/local resources.
Furthermore, the preconditions indicate what should happen with a session if a ...