A target is identified using SIP URI, TEL URI or IMEI. Not all SIP messages carry these identities. The LMISF by maintaining the IMS call state is able to determine the subsequent SIP messages that correspond to the same target. When a target is involved in multiple IMS sessions, the LMISF will have the logic to associate and correlate the SIP messages that are related to an IMS session. For example, the SIP messages that have the same Call Identity value can be treated as the SIP messages of a particular IMS session and hence, when reported to the LEMF (via DF2) can have the same Correlation Number.
LMISF will also examine the SIP messages that carry the SDP offer and SDP answer to determine the media information related to an IMS session.
When an IMS session is established, the media information is exchanged between the two end points of the media stream (e.g. target's UE and IMS-AGW in HPLMN) through the SDP offer and answer process. The combination of IP address of the end point (e.g. UE and IMS AGW) and UDP port numbers used to transport the RTP and RTCP are part of this SDP offer and answer along with other things like Codec information. The media packets (i.e. RTP streams) exchanged between the two end points of the media use those IP addresses and the port numbers (assigned for RTP).
One method that can be used to establish the correlation is to use the IP addresses and the UDP port numbers exchanged within the SDP offer and answer process and compare them with the IP addresses and UDP port numbers of the media packets to establish an association between the IMS session and the media.
In other words, the IP address and UDP port numbers associated with a media packet when compared with the IP address and UDP port numbers exchanged in the SDP offer and answer, one can determine to which IMS session a media packet corresponds to. Once that determination is made, these parameters may be used to establish a correlation.
When S-GW/BBIFF is asked to deliver the packets from the IMS Signalling Bearers to LMISF, it delivers everything above the GTP-U layer. S-GW/BBIFF does not look into the IMS packets above the GTP-U layer. Similarly, when the S-GW/BBIFF is asked to deliver the packets from the Media Bearer to the LMISF, it delivers everything above the GTP-U layer. It does not look into the Media packets above the GTP-U layer. However, the BBIFF knows that the Media Bearer and the IMS Signalling Bearer are related through the GTP protocol concepts defined in TS 29.274
The LMISF will generate a Correlation Number and include that Correlation Number while delivering the SIP messages to the DF2. When the media packets are received, LMISF will examine the Media packets to determine which IMS session, the Media packets are related to. Once determined, the LMISF will deliver the Media packets to the DF3 along with the Correlation Number previously stored against the IMS session.
Within the EPC, the MME sends the UE location to the S-GW within the Create Session Request and Create Bearer Response messages that it sends to the S-GW. The Create Session Request is sent from the MME to the S-GW when the default bearer is created. The Create Bearer Response is sent from the MME to the S-GW when a dedicated bearer used as Media Bearer is created.
In addition, the MME sends the UE location to the S-GW when a Bearer is modified (Modify Bearer Request and Update Bearer Response) or deleted (Delete Session Request and Delete Bearer Response).
The details of the above messages (i.e. Create Session Request etc.) are specified in TS 29.274
For the S8HR LI, the S-GW/BBIFF notifies the LMISF whenever the IMS signalling bearer (i.e. default bearer) or Media Bearer (i.e. dedicated bearer linked to the IMS Signalling Bearer) is created, modified, or deleted. The S-GW/BBIFF includes the UE Location that it receives from the MME when it notifies the IMS signalling bearer creation and Media Bearer creation events to the LMISF.
The LMISF should store the UE location as it stores the IMSI value (currently specified in TS 33.107), and include the same in the appropriate IRI events sent to the DF2 over the X2 reference point.
The DF2 delivers the UE Location to the LEMF (when required) as it is done for the non-roaming scenario or in a roaming with LBO scenario.