4.22 Support of Explicit Congestion Notification (ECN)

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

4.22.1 General

The ECN profile used to trigger codec rate adaptation for Multimedia Telephony is defined in TS 26.114 [76] and affects the following IMS entities: UE, MGCF/IM-MGW, IBCF/TrGW, IMS-ALG/ IMS-AGW, MRFP/MRFC, and the MSC Server enhanced for ICS/MSC Server enhanced for SRVCC with SIP/CS-MGW.

As specified in TS 26.114 [76]:

– an MGCF/IM-MGW can be used for inter-working between an ECN-capable client in a 3GPP network that properly handles ECN-marked packets and a CS network;

– an IBCF/TrGW supporting Multimedia Telephony can be used for interworking between an ECN-capable entity in a 3GPP network that properly handles ECN-marked packets, and:

– a remote entity that does not use ECN;

– a remote entity that supports ECN in different way than what is specified for Multimedia Telephony clients;

– a network which does not handle ECN-marked packets properly.

A UE supporting Multimedia Telephony and ECN shall support the procedures described in TS 26.114 [76].

4.22.2 CS GERAN/UTRAN Interworking at MGCF/IM-MGW

If MGCF/IM-MGW supports Multimedia Telephony compliant ECN, it shall:

– support ECN Multimedia Telephony client procedures as described in TS 26.114 [76], except that the MGCF and IM-MGW do not determine whether ECN can be used based on the Radio Access Technology that is used towards the client; the MGCF/ IM-MGW act as an ECN endpoint towards the Multimedia Telephony terminal;

– support SDP ability to negotiate to ECN according to TS 26.114 [76];

– be capable of enabling end-to-end rate adaptation between Multimedia Telephony terminal and the CS terminal or IM-MGW by performing the following:

– negotiate the use of ECN with the Multimedia Telephony terminal, if it can be confirmed that the network used towards the Multimedia Telephony terminal properly handles ECN-marked packets;

NOTE 1: An operator can ensure that the network used towards the Multimedia Telephony terminal properly handles ECN-marked packets by setting corresponding requirements on network equipment, and verifying that those requirements are met. No active signalling ("probing") during normal operation is required to ensure this.

– trigger rate adaptation request towards the Multimedia Telephony terminal when receiving in the incoming IMS media flow IP packets marked with ECN-CE, regardless of whether the IM-MGW applies or does not apply transcoding;

– inter-work adaptation requests between the Multimedia Telephony terminal and the CS GERAN/UTRAN when the IM-MGW bridges compatible codec configurations between the interfaces without applying a transcoding function; if the IM-MGW prefers to receive a lower codec mode rate from the Multimedia Telephony terminal than what the CS network indicates, e.g. after having received IP packets with ECN-CE, the IM-MGW may replace the codec mode requested from the CS side with the codec mode that the IM-MGW prefers;

– perform media adaptation (e.g. reduce media bit-rate) towards the Multimedia Telephony terminal when receiving from the latter an adaptation request and the IM-MGW applies transcoding.

NOTE 2: For CS interworking the MGCF/MGW does not have to perform transcoding if the same codec is selected on the CS and PS network.

4.22.3 Interworking with non-ECN IP network and/or terminal at IBCF/TrGW

An IBCF/TrGW may support Multimedia Telephony using ECN and can be used to enable ECN within the local network when either the remote network cannot be confirmed to properly handle ECN-marked packets or the remote entity does not support or use ECN.

In order to support Multimedia Telephony using ECN when interworking with a remote network that cannot be confirmed to properly handle ECN-marked packets and/or with a remote terminal does not support or use ECN, the IBCF/TrGW shall:

– determine from local configuration if the remote network properly handles ECN-marked packets;

– determine with SDP offer/answer procedures if the remote entity supports ECN;

– support SDP ability to negotiate to ECN according to TS 26.114 [76];

– be capable of enabling end-to-end rate adaptation between the local Multimedia Telephony terminal and the remote entity or TrGW by performing the following towards the local Multimedia Telephony terminal:

– negotiate the use of ECN;

– support ECN Multimedia Telephony client procedures as described in TS 26.114 [76], except that the IBCF/TrGW does not determine whether ECN can be used based on the Radio Access Technology; the IBCF/TrGW acts as an ECN endpoint towards the ECN capable Multimedia Telephony terminal;

– trigger rate adaptation request towards the Multimedia Telephony terminal when receiving in the incoming IMS media flow IP packets marked with ECN-CE, regardless of whether the TrGW applies or does not apply transcoding; this requires that the IBCF provides the TrGW with the media configuration, even if transcoding is not supported or applied, when the IP termination is configured with ECN;

– forward adaptation requests between the Multimedia Telephony terminal and the remote entity when the TrGW bridges compatible codec configurations between the interfaces without applying a transcoding function; if the TrGW prefers to receive a lower codec mode rate from the Multimedia Telephony terminal than what the other SIP network indicates, e.g. after having received IP packets with ECN-CE, the TrGW may replace the codec mode requested from the other SIP network with the codec mode that the TrGW prefers;

– perform media adaptation (e.g. reduce media bit-rate) towards the Multimedia Telephony terminal when receiving from the latter an adaptation request and the TrGW applies transcoding.

NOTE: For this interworking the IBCF/TrGW does not have to perform transcoding if the same codec is selected on both sides of the TrGW.

4.22.4 Interworking with non-3GPP ECN IP terminal at IBCF/TrGW

An IBCF/TrGW supporting Multimedia Telephony compliant ECN can also be used to enable ECN end-to-end if the remote entity uses ECN in a different way than what is described in TS 26.114 [76], e.g. if the remote entity only supports probing for the ECN initiation phase or it needs the ECN feedback.

NOTE: For this interworking the IBCF/TrGW does not have to perform transcoding if the same codec is selected between both terminals.

4.22.5 ECN support at IMS-ALG/IMS-AGW

The P-CSCF shall be able to disallow the negotiation of ECN during SDP offer/answer exchanges if the IMS-AGW does not support transparent forwarding of the ECN bits or if the IMS core network or the access network used towards the Multimedia Telephony terminal do not properly handle ECN-marked packets.

An IMS-AGW supporting ECN shall be able to forward the ECN bits as instructed by the IMS-ALG.

The IMS-ALG/IMS-AGW may act as an ECN endpoint towards the access network and/or the IMS Core Network to enable ECN when the ECN connection cannot be established or maintained transparently (e.g. after PS-CS Access Transfer).

When acting as an ECN endpoint the IMS-AGW shall support end-to-end rate adaptation between the local terminal and the remote entity by performing the following:

– trigger rate adaptation request towards the ECN-capable peer when receiving in the incoming IMS media flow IP packets marked with ECN-CE, regardless of whether the IMS-AGW applies or does not apply transcoding;

– forward adaptation requests between the local and the remote peer when the IMS-AGW bridges compatible codec configurations between the interfaces without applying a transcoding function;

– perform media adaptation (e.g. reduce media bit-rate) towards the ECN-capable peer when receiving from the latter an adaptation request and the IMS-AGW applies transcoding.

4.22.6 ECN support at MRFC/MRFP

An MRFC/MRFP may support Multimedia Telephony using ECN and may act as an ECN endpoint to enable ECN with a local ECN-capable terminal within a local network that properly handles ECN-marked packets (see TS 23.333 [73]).

This requires that the MRFC/MRFP performs the following:

– support SDP ability to negotiate to ECN according to TS 26.114 [76];

– be capable of enabling end-to-end rate adaptation between the local Multimedia Telephony terminal and the MRFP by performing the following towards the local Multimedia Telephony terminal:

– negotiate the use of ECN;

– support ECN Multimedia Telephony client procedures as described in TS 26.114 [76], except that the MRFC/MRFP does not determine whether ECN can be used based on the Radio Access Technology; the MRFC/MRFP acts as an ECN endpoint towards the ECN capable Multimedia Telephony terminal;

– trigger rate adaptation request towards the Multimedia Telephony terminal when receiving in the incoming IMS media flow IP packets marked with ECN-CE;

– perform media adaptation (e.g. reduce media bit-rate) towards the Multimedia Telephony terminal when receiving from the latter an adaptation request.

4.22.7 CS GERAN/UTRAN Interworking at the MSC Server enhanced for ICS/MSC Server enhanced for SRVCC with SIP/CS-MGW

If the MSC Server enhanced for ICS/MSC Server enhanced for SRVCC with SIP/CS-MGW supports Multimedia Telephony compliant ECN, it shall support the procedures specified in clause 4.22.2 respectively for the MGCF and IM-MGW.