A WebRTC IMS Client (WIC) is an application using the WebRTC extensions specified in WebRTC 1.0 [85] except for those extensions specifically exempted by 3GPP specifications (e.g. TS 26.114) and providing access to IMS by interoperating with the WebRTC IMS access architecture defined in this Annex. Any IP access network with access to the internet may be used by a WIC; nevertheless WebRTC traffic is subject to the QoS and reachability limitations of this access network.

The WebRTC Web Server Function (WWSF) is the initial point of contact in the Web that controls access to the IMS communications services for the user. The WWSF has the following characteristics and functions:

• The WWSF is located either in the operator network or a third party network
• The WWSF provides the Web page presenting the user interface to the user for IMS access.
• The WWSF provides the JS WIC application for downloading to the browser on the UE.
• The WWSF manages the allocation of authorized IMS identities to WICs. The JS application downloaded from the WWSF controls which authentication method applies.

The P-CSCF enhanced for WebRTC (eP-CSCF) is a P-CSCF including the IMS-ALG functionality and with the following additional functions:

• The eP-CSCF shall support at least one WebRTC IMS client-to-network signalling protocol, e.g. SIP over WebSocket, REST/HTTP based interface, XMPP over WebSocket, etc.
• The eP-CSCF provides interworking between W2 and Mw.
• The eP-CSCF verifies that the UE is executing a WIC from an authorized WWSF.
• In the case of WIC registration of individual Public User Identity using IMS Authentication, the eP-CSCF shall relay the IMS authentication and registration information between W2 and Mw.
• Otherwise, i.e. for users authorized by the WWSF or WAF:
  • The eP-CSCF shall verify any UE authorization information received from the WIC;
  • The eP-CSCF shall verify that the WWSF is authorized to allocate IMS identities;
  • The eP-CSCF shall perform Trusted Node Authentication (TNA) in IMS, as defined in TS 33.203.
• The eP-CSCF shall control the media plane interworking functions provided by the eIMS-AGW, including those additional media plane functions specific to WebRTC.
• The eP-CSCF shall ensure via signalling that RTP streams are not multiplexed ("bundled") onto the same port.
• The eP-CSCF shall negotiate via signalling whether RTP and RTCP flows of an RTP stream are multiplexed onto the same port and shall configure the eIMS-AGW to (de)multiplex such flows if entities anchoring the session media path in the IMS domain do not support that capability.
• The eP-CSCF is located in the domain of the operator that provides the WWSF or with which the WWSF has a service level agreement.

The IMS-AGW enhanced for WebRTC (eIMS-AGW) is a standard IMS-AGW with the following additional mandatory characteristics and functions:

• The eIMS-AGW shall support the media plane interworking extensions as needed for WICs.
• The eIMS-AGW shall reside in the same network as the eP-CSCF.
• The eIMS-AGW shall support media security of type "e2ae" (as specified in TS 33.328) for media protocols specific to WebRTC, including media consent, and DTLS-SRTP as key exchange mechanism for media components using SRTP.
• The eIMS-AGW shall provide NAT traversal support including ICE
• The eIMS-AGW may be used to perform any transcoding needed for audio and video codecs supported by the browser.
• When GTT service is required, the eIMS-AGW shall perform transport level interworking between T.140 [87] over Data Channels and other T.140 transport options supported by IMS.
• When MSRP is transported over the data channel, the eIMS-AGW shall act as an MSRP B2BUA between MSRP over Data Channels and the other MSRP transport options supported by IMS.
• When BFCP service is required for conference floor control and BFCP is transported over Data Channels, the eIMS-AGW shall perform transport level interworking between BFCP over Data Channels and other BFCP transport options supported by IMS.
• The eIMS-AGW shall support the media plane optimization for WICs.
• The eIMS-AGW shall support that RTP and RTCP flows of an RTP stream between WIC and eIMS-AGW are multiplexed onto the same port, and shall support de-multiplexing such RTP and RTCP flows toward the core network.

The WebRTC Authorisation Function (WAF) has the following characteristics and functions:

• The WAF shall issue the authorisation token to WWSF.
• The WAF may either authenticate the user itself as part of the token issuance process, or it trusts the user identity provided by the WWSF.
• The WAF may either reside in the operator domain or the third party domain.

The WAF is not used in the case of IMS registration scenario using IMS Authentication, described in clause U.2.1.1.

The W1 reference point is between the UE and the WWSF. The HTTPS protocol is normally used to access the web page providing the User Interface for the WIC and to download the WIC JS application to the browser.

The W2 reference point is the signalling interface between the UE and the eP-CSCF. SIP over secure WebSocket is a non-mandatory option for W2 in Release 12, where the SIP/SDP procedures are based on Gm with enhancements to support extensions defined for WIC, and secure WebSocket is the supported transport protocol. Other protocols are allowed on W2 for WebRTC access but are not described in this document.

The Iq reference point is between the eP-CSCF and eIMS-AGW and is enhanced to control the additional bearer plane functions specific to WIC.

The W3 reference point is between the UE and eIMS-AGW. W3 carries the user plane between the UE and the network (see clause U.1.5).

The W4 reference point is the signalling interface between the WWSF and the WAF. The WWSF obtains an authorisation token from the WAF from which the user's identities, identities of WWSF and WAF, and a lifetime may be derived in the case of IMS registration scenarios based on web authentication and assignment from a pool of user identities.

The IMS AGW enhanced for WebRTC (eIMS-AGW) is the media plane interworking element with the functions described in clause U.1.3.4.

Figure U.1.5.1-1 shows the protocol architecture for support of MSRP, when transported over the data channel, from a WebRTC IMS client (WIC). When MSRP is transported over the data channel, the eIMS-AGW shall provide either a transport relay function from Data Channel to TCP or an MSRP B2BUA to allow interoperation with existing MSRP peer endpoints. UDP transport of DTLS shall be supported and TCP transport of DTLS may be supported to enable traversal of UDP-blocking NATs/firewalls.

Figure U.1.5.2-1 shows the protocol architecture for support of BFCP, when transported over the data channel for conference floor control, from a WebRTC IMS client (WIC). When BFCP service is required for conference floor control and BFCP is transported over Data Channels, the eIMS-AGW shall provide a transport relay function from Data Channel to TCP to allow interoperation with existing BFCP peer endpoints. UDP transport of DTLS shall be supported and TCP transport of DTLS may be supported to enable traversal of UDP-blocking NATs/firewalls.

Figure U.1.5.3-1 shows the protocol architecture for support of T.140 from a WebRTC IMS client (WIC). The eIMS-AGW shall provide a transport relay function from Data Channel to RTP to allow interoperation with existing T.140 peer endpoints. UDP transport of DTLS shall be supported and TCP transport of DTLS may be supported to enable traversal of UDP-blocking NATs/firewalls.

Figure U.1.5.4-1 shows the protocol architecture for support of Voice and Video from a WebRTC IMS client (WIC). Transcoding (i.e. allowing codec1 to be different from codec2) is optional. SRTP between the UE and the eIMS-AGW relies on keying material negotiated via DTLS.