A.18 Putting a MTSI speech call to hold or to resume the call from the SS / 5GS

34.229-53GPPInternet Protocol (IP) multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP)Part 5: Protocol conformance specification using 5G System (5GS)Release 16TSUser Equipment (UE) conformance specification

Expected sequence

Step	Direction		Message	Comment
	UE	SS
1	<-		INVITE	SS sends INVITE with a SDP offer to hold or resume the call
2	->		100 Trying	Optional: The UE responds with a 100 Trying provisional response
3	->		200 OK	The UE responds to INVITE with 200 OK to indicate that the UE is no more sending any media (call hold) or resumes sending media (call resume)
4	<-		ACK	The SS acknowledges the receipt of 200 OK for INVITE

Specific Message Contents

INVITE (Step 1)

Use the default message “INVITE for MT call setup” in Annex A.2.9 of TS 34.229-1 [2], applying conditions A1 and A5, with the below exceptions. The SS uses the same URI in the request line as the UE has sent in the Contact header of the original INVITE request creating this dialog.

The SS shall include support for precondition in the Supported header field.

In case of Call Hold, the SS shall include the same lines in the SDP body as finally accepted for the MTSI call, i.e., the last SDP sent by the SS, with the following exceptions:

– version number of the SDP shall be incremented; and

– each media line shall carry direction attribute “a=sendonly”.

In case of Call Resume, the SS shall include the same lines in the SDP body as sent in the message for Call Hold with the following exceptions:

– version number of the SDP shall be incremented; and

– each media line shall carry direction attribute “a=sendrecv”.

100 Trying for INVITE (Step 2)

Use the default message “100 Trying for INVITE” in Annex A.2.2 of TS 34.229-1 [2], applying condition A2.

200 OK for INVITE (Step 3)

Use the default message “200 OK for other requests than REGISTER or SUBSCRIBE” in Annex A.3.1 of TS 34.229-1 [2], applying conditions A2, A5, A11, A20 and A22 with the following exceptions:

Header/param	Value/remark
Require
option-tag	precondition
Content-Type
media-type	application/sdp
Content-Length	header shall be present if UE uses TCP to send this message and if there is a message body
value	length of message-body
Message-body	SDP answer to the SDP offer contained in the INVITE including: – All mandatory SDP lines as specified in RFC 4566 [38]. – The same number of media lines (“m=”) as in the INVITE. – All the media lines having directionality as "recvonly" In case of Call Hold: All the media lines having direction attribute “a=recvonly”. In case of Call Resume: All the media lines having direction attribute “a=sendrecv”.

ACK (Step 4)

Use the default message “ACK” in Annex A.2.7 of TS 34.229-1 [2], applying conditions A2, A3 and A5.