L.2A Usage of SDP

24.2293GPPIP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP)Release 18Stage 3TS

L.2A.0 General

NOTE 1: When:

– establishing a session which is not an emergency session; or

– modifying a session which is not an emergency session;

and if the IMSVoPS indicator is received in the EPS network feature support information element (see 3GPP TS 24.301 [8J]) and no persistent EPS bearer context exists at the UE, the UE constructs SDP based on the restrictions indicated in the IMSVoPS indicator. Regardless whether the IMSVoPS indicator indicating voice is supported or not, m-lines can be set to "audio" and exclude voice codecs from the SDP answer or SDP offer. When a persistent EPS bearer context exists, m-lines can be set to "audio" and include voice codecs in the SDP answer or SDP offer.

NOTE 2: When the UE is accessing the IM CN subsystem via E-UTRAN, the appropriate specification for access domain selection is 3GPP TS 23.221 [6].

L.2A.1 Impact on SDP offer / answer of activation or modification of EPS bearer context for media by the network

If, due to the activation of EPS bearer context from the network the related SDP media description needs to be changed, the UE shall update the related SDP information by sending, within a SIP request, a new SDP offer for each of the existing SIP dialogs,

If the UE receives a modification request from the network for a EPS bearer context that is used for one or more media streams in an ongoing SIP session, the UE shall:

1) if, due to the modification of the EPS bearer context, the related SDP media description need to be changed, update the related SDP information by sending, within a SIP request, a new SDP offer for each of the existing SIP dialogs, and respond to the EPS bearer context modification request.

NOTE: The UE can decide to indicate additional media streams as well as additional or different codecs in the SDP offer than those used in the already ongoing session.

L.2A.2 Handling of SDP at the terminating UE when originating UE has resources available and IP-CAN performs network-initiated resource reservation for terminating UE

If the UE receives an SDP offer where the SDP offer includes all media streams for which the originating side indicated its local preconditions as met, if the precondition mechanism is supported by the terminating UE and the IP-CAN performs network-initiated resource reservation for the terminating UE and the available resources are not sufficient for the received offer, the terminating UE shall indicate its local preconditions and provide the SDP answer to the originating side without waiting for resource reservation.

NOTE: If the resource reservation is controlled by the EPS IP-CAN, the resource reservation request is initiated by the network after the P-CSCF has authorised the respective IP flows and provided the QoS requirements over the Rx interface to the PCRF as described in 3GPP TS 29.214 [13D].

L.2A.3 Emergency service

NOTE: When establishing an emergency session or when modifying an emergency session, the IMSVoPS indicator does not influence handling of SDP offer and SDP answer.