12 Extensions within the present document

24.3803GPPMission Critical Push To Talk (MCPTT) media plane controlProtocol specificationRelease 18TS

12.1 Session description types defined within the present document

12.1.1 General

This clause contains definitions for SDP parameters that are specific to SDP usage with MCPTT and therefore are not described in an RFC.

12.1.2 SDP "fmtp" attribute for MCPTT

12.1.2.1 General

This clause defines the structure and syntax of the SDP "fmtp" attribute, when used to negotiate an MCPTT media plane control channel. The MCPTT media plane control channel, and the protocols used on the control channel, is described in the present specification.

12.1.2.2 Semantics

In an SDP offer and answer, the "mc_queueing" fmtp attribute is used to indicate support of the Floor Request message queueing mechanism, as defined in the present specification.

In an SDP offer, the "mc_priority" fmtp attribute indicates (using an integer value between ‘1’ and ‘255’) the maximum floor priority that the offerer requests to be used with Floor Request messages sent by the offerer. In an SDP answer, the attribute parameter indicates the maximum priority level that the answerer has granted to the offerer. The value must be equal or less than the value provided in the associated SDP offer.

NOTE 1: If the "mc_priority" fmtp attribute is not used within an SDP offer or answer, a default priority value is assumed.

In an SDP offer, the "mc_granted" fmtp attribute parameter indicates that the offerer supports the procedure where the answerer indicates, using the fmtp attribute in the associated SDP answer, that the floor has been granted to the offerer.

NOTE 2: When the "mc_granted" fmtp attribute is used in an SDP offer, it does not indicate an actual request for the floor. The SDP "mc_implicit_request" fmtp attribute can be used to request the floor. In an SDP answer, the attribute indicates that the floor has been granted to the offerer.

NOTE 3: Once the offerer has been granted the floor, the offerer has the floor until it receives a Floor Revoke message, or until the offerer itself releases the floor by sending a Floor Release message, as described in the present specification.

In an SDP offer, the "mc_implicit_request" fmtp attribute indicates that the offerer implicitly requests the floor (without the need to send a Floor Request message). In an SDP answer, the attribute parameter indicates that the answerer has accepted the implicit floor request. Once the answerer grants the floor to the offerer, the answerer will send a Floor Granted message.

NOTE 4: The usage of the "mc_implicit_request" fmtp attribute in an SDP answer does not mean that the answerer has granted the floor to the offerer, only that the answerer has accepted the implicit floor request.

In an SDP answer, the "mc_ssrc" fmtp attribute indicates ssrc value of the offerer. The value may be equal to the value provided in the associated SDP offer or different if a collision is detected. If the associated SDP offer does not indicate the ssrc value then server can determine an appropriate value. This value is returned in an SDP answer only if the answerer accepts the implicit floor request offered in SDP.

In an SDP offer, the "mc_no_floor_ctrl" fmtp attribute indicates that the offerer wants to use pre-established session call control during the session without floor control over the offered media plane control channel. In an SDP answer, the attribute parameter indicates that the answerer has accepted the request to use the offered media plane control channel for pre-established session call control during the session without floor control.

12.1.2.3 Syntax

Table 12.1.2.3-1: SDP "fmtp" attribute for the MCPTT media plane control channel

fmtp-attr-mpcp = "a=fmtp:" "MCPTT" SP attr-param-list

attr-param-list = attr-param *(COLON attr-param)

attr-param = mc_queueing / mc_priority / mc_granted / mc_implicit_request / mc_ssrc / mc_no_floor_ctrl

mc_queueing = "mc_queueing"

mc_priority = "mc_priority=" 1*2(DIGIT)

mc_granted = "mc_granted"

mc_implicit_request = "mc_implicit_request"

mc_ssrc = "mc_ssrc=" 1*(DIGIT)

mc_no_floor_ctrl = "mc_no_floor_ctrl"