10.2.24 SDP Capability Negotiation (SDPCapNeg)

29.1623GPPInterworking between the IM CN subsystem and IP networksRelease 17TS

10.2.24.1 General

The SDP Capability Negotiation (SDPCapNeg) as specified in IETF RFC 5939 [57] is adopted as an optional functionality to negotiate capabilities and the associated configurations according to 3GPP TS 24.229 [1].

Upon receipt of an incoming SDP offer containing the attributes of SDP capability negotiation, e.g. offer AVPF and AVP together for the RTP profile negotiation using the "a=tcap", "a=pcfg" and "a=acfg" attributes, the IBCF shall:

– request the TrGW to reserve resources only for the default configuration without SDPCapNeg, and make the decision on support of the alternative configurations based on the IBCF/TrGW capability as provisioned before forwarding the SDP offer, i.e. handling SDPCapNeg at the controller level; or

– request the TrGW to reserve resources for all of these configurations by signalling SDPCapNeg to the TrGW, and update the SDP offer based on the response from the TrGW before forwarding.

NOTE: The additional benefit of signalling SDPCapNeg between the IBCF and the TrGW is to check the resource availability for the corresponding configurations and to avoid the further session failure in case of inadequate resources for the configuration changes in the final confirmation. However, due to the extra resources reserved only during the call establishment phase, there is increased risk of call establishment failures.

In case the IBCF decides to request the TrGW to reserve resources for all of those configurations, the IBCF shall:

– use legacy SDP attributes as specified in IETF RFC 4566 [53] to do the mapping of actual and potential configurations with the H.248 ReserveGroup concept; or

– use SDP extensions for SDP capability negotiation as specified in IETF RFC 5939 [57], if supported by the TrGW.

Before using SDP extensions for SDP capability negotiation as specified in IETF RFC 5939 [57] towards the TrGW, the IBCF shall perform the necessary checks (i.e. through auditing or via prior provisioning) to ensure that the TrGW supports the syntax and capabilities requested. For an auditing the procedure in subclause 6.1.8.1 is used with the "SDPCapNeg Supported Capabilities" as the object.

When receiving a request from the IBCF with information element "SDPCapNeg configuration" indicating the potential use of multiple configurations, the TrGW shall reserve resources for all of those configurations that it supports and shall send indicate the configurations for which it reserved resources in an "SDPCapNeg configuration" information element in the response. The IBCF shall update the SDP offer with SDPCapNeg configurations in the response from the TrGW and shall forward the SDP offer to the next hop.

The IBCF may also provide SDP configurations to the TrGW with no dependency on the incoming SDP offer, e.g. the IBCF may wildcard the supported configurations in order to construct or update an SDP offer with the addition of alternative configurations via SDPCapNeg attributes.

On receipt of an SDP answer with SDPCapNeg, the IBCF shall request the TrGW to configure the resources for the selected configuration. If the TrGW previously reserved any temporary resources for configurations that were not selected, the IBCF shall also request the TrGW to release those resources.

10.2.24.2 Message sequence chart

10.2.24.2.1 Audit SDPCapNeg Supported Capabilities

The IBCF may request the TrGW to report the current values assigned to distinct objects, here as the "SDPCapNeg Supported Capabilities" in the TrGW. This procedure may be used when the IBCF is unsure of the applicability for SDPCapNeg of the TrGW.

Figure 10.2.24.2.1.1 shows the message sequence chart example for the auditing.

Figure 10.2.24.2.1.1: Audit Value of the TrGW