C.3 PCRF procedures

29.2133GPPPolicy and charging control signalling flows and Quality of Service (QoS) parameter mappingRelease 17TS

C.3.1 General

The procedures in clause C.3 are only invoked when the following two conditions are met:

1. Both the local and remote UE have utilized the ICE protocol for media traversal of NATs (see clause C.2.1 for details on how this is determined); and

2. The IP-CAN which is servicing the IMS session does not support the concept of a default bearer.

C.3.2 Deriving additional flow descriptions

The PCRF may need to develop additional flow descriptions (beyond those provided by the P-CSCF) for a media component based on additional candidate addresses present in the SDP offer/answer exchange. The PCRF shall follow the procedures defined in IETF RFC 8445 [67] and IETF RFC 8839 [68] for forming candidate pairs based on the data contained within the received Codec-Data AVP. For each candidate pair created based on the ICE procedures and not already present in the received flow descriptions, the PCRF shall add an uplink and downlink flow description for each media component.

NOTE 1: The uplink SDP represents the local candidates while the downlink SDP represents the remote candidates.

Following the ICE procedures for forming candidate pairs will result in some flow descriptions which would never be exercised. In particular, while the UE will send connectivity checks (and ultimately its media stream) from its host candidate, from the PCEF perspective, this will appear as being from the server reflexive address. Given this, the PCRF should not form flow descrptions using host candidate addresses and should only form additional flows based on server reflexive addresses and relay addresses.

As candidates are removed from the SDP via subsequent offer/answer exchanges, the PCRF shall update its candidate pair list and shall remove any flow descriptors no longer being used.

NOTE 2: If the default candidate (the candidate used to populate the "c=" and "m=" lines of both the uplink and downlink SDP) is chosen, then an updated SDP offer/answer will not be done, and any extra flow descriptions not being used by the session will not be removed.

C.3.3 Gating control

For each additional flow description the PCRF adds to a media component (per clause C.3.2), the PCRF shall enable the flow in order to allow connectivity checks to pass.

C.3.4 Bandwidth impacts

Per clause C.2.5 ICE is designed to have minimal impact on bandwidth policy control. However, it is possible that media will begin flowing while the ICE connectivity checks are still in progress. Given the possibility that no session update will be made (the default candidates will be chosen by the ICE protocol), it is not recommended that the PCRF adjust the bandwidth parameters provided by the P-CSCF.