6.2.5 Traffic Policing

23.3343GPPIP Multimedia Subsystem (IMS) Application Level Gateway (IMS-ALG) - IMS Access Gateway (IMS-AGW) interface: Procedures descriptionsRelease 17TS

This procedure is identical to that of clause 6.2.1 apart from the IMS-ALG optionally requesting the IMS-AGW to police the media stream flow according to one or more of the following media policing(s) through the IMS-AGW, in accordance with IETF RFC 2216 [10].

NOTE 1: In order to support services that generate media with variable bitrate (e.g. video), the policing function needs to provide sufficient buffer space. The derivation of the correspondent token bucket size can be based on the estimated traffic volume over the expected maximum burst length (supposed time interval of the burst). For example, for MTSI media, 3GPP TS 26.114 [21] clause 7.5.5 recommends a default period of 2 seconds for the derivation of average bitrates and provides further considerations regarding suitable time periods for speech and video.

NOTE 2: The decision to apply or not traffic policing is general for all sessions with the same media characteristics (i.e. not user specific). The conditions which media policing to apply are beyond the scope of this specification. This can be based on the media characteristics of the session (e.g. media type).

The following media policing shall be supported at the IMS-AGW:

– Sustainable Data Rate (SDR) Policing:
To request policing of the sustainable data rate of a media stream, the IMS-ALG shall request media policing for that media stream and shall provide the sustainable data rate, and shall provide a maximum burst size (MBS) indicating the expected maximum size of packet bursts for that media stream. The IMS-AGW shall then measure the data rate for the received packets within that media stream as per IETF RFC 2216 [10] for "Token Bucket", where r = SDR and b = MBS. If the permissible sustainable data rate is exceeded, the IMS-AGW shall discard packets to reduce the data rate to the permissible sustainable data rate.

NOTE 3: The IMS-ALG can derive the sustainable data rate from bandwidth parameters if it receives them within an SDP media description. The IMS-ALG can:
– use the "b=AS" bandwidth modifier if received within an SDP body; or
– if the IMS-ALG supports the enhanced bandwidth negotiation mechanism, use the "maximum supported bandwidth for the receiving direction" property value from the "a=bw-info" SDP attribute received within an SDP answer.
If the value of the "maximum supported bandwidth for the receiving direction" for the selected RTP payload type differs from the value of the "b=AS" bandwidth modifier then the IMS-ALG can use "maximum supported bandwidth for the receiving direction" property value from the "a=bw-info" SDP attribute to determine the sustainable data rate for the selected RTP payload.

The following media policing may be supported in addition at the IMS-AGW ; if supported then the following applies:

– Peak Data Rate Policing:
To request policing of the peak data rate of a media stream, the IMS-ALG shall request media policing for that media stream and shall provide the peak data rate, and may provide a Delay Variation Tolerance indicating the expected maximum delay variation due to jitter for that media stream. The IMS-AGW shall then measure the data rate for the received packets within that media stream. If the permissible peak data rate is exceeded, the IMS-AGW shall discard packets to reduce the data rate to the permissible peak data rate. If both peak data rate and sustainable data rate have been provided for the same media stream, the IMS-AGW shall discard packets to reduce the data rate to the permissible peak data rate and should discard packets to reduce the data rate to the permissible sustainable data rate.