8 Protocol using SDP for session-mode messaging and session-mode messaging conferences
24.2473GPPMessaging service using the IP Multimedia (IM) Core Network (CN) subsystemRelease 17Stage 3TS
8.1 Introduction
8.2 Functional entities
8.2.1 User Equipment (UE)
For the purpose of session-mode messaging and session-mode messaging conferences, the UE shall implement the role of:
– an SDP offerer as described in clause 8.3.1; and
– an SDP answerer as described in clause 8.3.2.
8.2.2 Media Resource Function Controller (MRFC)
The MRFC shall implement the role of an intermediate node as described in clause 8.3.3.
8.2.3 Application Server (AS)
The AS shall implement the role of an SDP offerer, as described in clause 8.3.1, and an SDP answerer, as described in clause 8.3.2 when engaged in a session mode session between a SDP offerer and SDP answerer.
NOTE: An AS, that is on the signalling path for the related SIP signalling, is not mandated to terminate the related MSRP.
8.3 Role
8.3.1 SDP offerer
When an SDP offerer wants to create a session mode messaging session, the SDP offerer shall populate the SDP as specified in clause 6.1 in 3GPP TS 24.229 [5]. In addition the SDP offerer shall also include an SDP media description for an MSRP session. The SDP media description shall include:
a) an SDP msrp-cema attribute, in accordance with RFC 6714 [19];
b) the supported MIME types in the SDP accept-types or accept-wrapped-types attributes, in accordance with RFC 4975 [9];
c) an SDP path attribute, with a value for the authority component that does not resolve to a valid IP address, and with a valid session-id component, in the MSRP URI component of the attribute ; and
d) an SDP setup attribute, in accordance with RFC 6135 [18].
The SDP may also include an SDP max-size attribute, in accordance with RFC 4975 [9].
The SDP offerer may want to indicate to the other user(s), that the SDP offerer is prepared to receive isComposing information, then it shall add the MIME type "application/ im-iscomposing+xml" to the SDP accept-type or access-wrapped-types attributes.
NOTE: When the SDP offerer receives an SDP answer, if the SDP media description for the MSRP session does not contain an SDP msrp-cema attribute, the SDP offerer might fall back to the RFC 4975 [9] procedures, in accordance with RFC 6714 [19]. In such case, the SDP offerer will send a new SDP offer that does not include an SDP msrp-cema attribute, and in which the authority component in the MSRP URI component of the SDP path attribute is set in accordance with RFC 4975 [9].
Otherwise, if the SDP answer contains an SDP setup attribute with a "passive" value, the SDP offerer shall set up a TCP connection (if not already available) when an IP-CAN bearer with sufficient QoS is available.
In accordance with RFC 6135 [18], the SDP offerer shall not include an SDP connection attribute in the initial SDP offer. For file transfer, the SDP shall also include SDP media level attributes in accordance with RFC 5547 [15], with the exception that it shall include the SDP file-selector attribute with at least a size parameter.
For file transfer, once the MSRP 200 (OK) response for the last MSRP SEND request is received, the SDP offerer shall terminate the MSRP session associated with the file transfer by sending an SDP offer where the port value of the SDP media description for the MSRP session is zero. In case there are no other SDP media descriptions associated with the SIP session, the SDP offerer shall send a SIP BYE request in order to terminate the SIP session.
8.3.2 SDP answerer
SDP answerer shall populate the SDP answer as specified in clause 6.1 in 3GPP TS 24.229 [5]. In addition the SDP answerer shall include an SDP media description for an MSRP session. The SDP media description shall include:
a) an SDP msrp-cema attribute, if the precondition described in RFC 6714 [19] for including the attribute is fulfilled. Otherwise the SDP answerer will fall back to the RFC 4975 [9] procedures;
b) the supported MIME types in the SDP accept-types or accept-wrapped-types attributes, in accordance with RFC 4975 [9];
c) if the precondition described in RFC 6714 [19] for including the SDP msrp-cema attribute is fulfilled, an SDP path attribute, with a value for the authority component that does not resolve to a valid IP address, and with a valid session-id component, in the MSRP URI component of the attribute;
d) if the precondition described in RFC 6714 [19] for including the SDP msrp-cema attribute is not fulfilled, an SDP path attribute in accordance with RFC 4975 [9]; and
e) an SDP setup attribute, in accordance with RFC 6135 [18].
The SDP answerer may also include an SDP max-size attribute, in accordance with RFC 4975 [9].
If the SDP answerer receives the MIME type "application/im-iscomposing+xml" in the SDP accept-types or accept-wrapped-types attribute and the SDP answerer accepts the exchange of isComposing information the SDP answerer shall add the MIME type "application/im-iscomposing+xml" to the SDP accept-types or access-wrapped types attributes.
If the SDP answer contains an SDP setup attribute with an "active" value, the SDP answerer shall set up a TCP connection (if not already available) when an IP-CAN bearer with sufficient QoS is available.
For file transfer, the SDP answerer shall behave in accordance with RFC 5547 [15].