WebRTC client (WIC-1) initiates a call, creating a Session Description Protocol (SDP) offer and sends it to the originating side eP-CSCF. The SDP offer may contain SRTP, ICE and Data Channel information. The WIC-1 IP address is IP1.
The eP-CSCF-1 receives the SDP offer from WIC-1. The eP-CSCF-1 allocates eIMS-AGW1 and configures it to terminate ICE procedures and provide interworking (e.g. transcoding, or transport interworking between a Data Channel and transport outside a Data Channel). eIMS-AGW1 allocates address IP2. It also requests the media gateway to provide a transport port suitable for a transparent forwarding of the media. IMS-AGW1 allocates port P2t for that purpose. Depending on configuration, it either configures the IMS-AGW1to terminate or to transparently forward the DTLS layer for transparent media. The eP-CSCF-1 shall not apply OMR procedures according to Annex Q
The eP-CSCF-1 describes the new media2 that result from the interworking and inserts the address IP2 at the eIMS-AGW1 in the SDP offer connection line it sends. Within the SDP offer, the eP-CSCF-1 also indicates that the media1 targeted for transparent forwarding (those media are called "transparent media" in what follows) can be selected instead and also encapsulates in SDP attribute(s) information about those transparent media (SDP attributes, SDP media line including transport protocol and port P2t, Data Channel related information, but no ICE related information. A DTLS fingerprint is included, and depending on configuration, it either contains the fingerprint received from the WIC-1 or a fingerprint allocated by eIMS-AGW1). To enable the check in step 6 wether intermediates which do not support switching to the encapsulated media are inserted into the media path, the eP-CSCF-1 also encapsulates the address IP2.
A possible intermediate (e.g. IBCF with attached TrGW or MTFC with attached MRFP) may also insert a media gateway into the user plane. Such an intermediate may optionally also support switching to the transparent media; this is required to allow the transparent media to be selected when its media gateway is present in the media path. Intermediates which do not support switching to the transparent media will be detected and will cause media 2 to be selected in step 6.
The possible intermediate can also apply OMR procedures according to Annex Q
to offer that its media gateway is bypassed by an optimised media path.
The intermediate replaces the transport address within the connection line in the SDP offer with the address IP3 allocated at its media gateway. If the intermediate supports switching to the transparent media, it also modifies the transparent media information encapsulated in SDP attribute(s) by replacing the encapsulated transport address with IP3 and the port information with P3t.
eP-CSCF-2 allocates eIMS-AGW2. It compares the transport address in the SDP connection line with the transport address in the transparent media information encapsulated in SDP attribute(s). Because both addresses match, the eP-CSCF-2 takes the transparent media information into consideration. Otherwise it shall ignore the transparent media information. The eP-CSCF-2 shall not apply OMR procedures according to Annex Q
and shall discard any received OMR related information. Because eP-CSCF2 knows that the served WIC-2 is a WIC, it decides that the transparent media information is appropriate and configures the eIMS-AGW2 to transparently forward those media; depending on configuration, eP-CSCF2 either configures the eIMS-AGW2 either transparently forward the DTLS layer or to terminate it, and it either forwards the fingerprint received as part of the transparent media information or a fingerprint allocated by eIMS-AGW2.
The eP-CSCF-2 forwards SDP offer with the transparent media1 and the transport address IP4 allocated at eIMS-AGW2 to WIC-2.
WIC2 selects media3 from the offered media1 and sends the selected media3 in the SDP answer to the eP-CSCF-2.
The eP-CSCF-2 forwards the SDP answer including connection information for eIMS-AGW2 and the unmodified media3, and includes an indication that the transparent media have been selected. Depending on configuration, it either forwards the fingerprint received from WIC2 or a fingerprint allocated by eIMS-AGW2.
The possible intermediate reconfigures its MGW to transparently pass media3.
The intermediate forwards the SDP answer with unmodified media3 and indication that the transparent media have been selected and includes address information IP7 of the controlled MGW.
According to received SDP answer, the eP-CSCF-1 knows that there is no bearer level protocol conversion. So eP-CSCF-1 deactivates media plane interworking in eIMS-AGW1.
The eP-CSCF-1 forwards the SDP answer to WIC-1. Depending on configuration, it either forwards the fingerprint received in step 12 or a fingerprint allocated by eIMS-AGW1.