9.2.11 Explicit Congestion Notification Support

29.1633GPPInterworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networksTS

9.2.11.1 General

A MGCF and IM-MGW may support Multimedia Telephony using Explicit Congestion Notification (see IETF RFC 3168 [129], IETF RFC 6679 [130], and 3GPP TS 26.114 [104]), and may then act as an ECN endpoint to enable ECN with a local ECN-capable terminal within a local network that properly handles ECN-marked packets.

9.2.11.1a Incoming Call Interworking from SIP to ISUP/BICC at I-MGCF

If the MGCF receives a SDP Offer containing ECN negotiation, see IETF RFC 6679 [130], and if all of the following statements are true:

– the MGCF supports ECN according to 3GPP TS 26.114 [104];

– the IM-MGW supports ECN according to 3GPP TS 26.114 [104];

– the IM-MGW knows (via configuration) that the preceding (IMS) network supports ECN according to 3GPP TS 26.114 [104]; and

– the IM-MGW supports at least some of the initialisation methods offered within the "a=ecn-capable-rtp",

NOTE 1: Only the "leap" initialisation method is supported over the Mn interface in this release.

The MGCF shall:

– act as an end point for ECN;

– select an initialisation method supported by the IM-MGW;

– determine if application specific feedback or ECN feedback messages shall be used, taking into account whether the IM-MGW supports ECN feedback messages, and the negotiation procedures in 3GPP TS 26.114 [104];

– determine if ECN XR summary reports can be used, taking into account whether they are supported at the IM-MGW and the negotiation procedures in 3GPP TS 26.114 [104];

– return an SDP Answer according to 3GPP TS 26.114 [104] and the capabilities of the IM-MGW, containing the "a=ecn-capable-rtp" attribute; and

NOTE 2: ECN XR summary reports and RTCP AVPF ECN feedback message are not supported in this release.

– indicate to the IM-MGW that it shall apply the ECN procedures (according to 3GPP TS 26.114 [104]) and act as an ECT endpoint.

The IM-MGW should not send RTCP XR ECN summary reports.

9.2.11.1b Outgoing Call Interworking from ISUP/BICC to SIP at O-MGCF

When creating the SDP Offer the MGCF may initiate ECN negotiation (in accordance with 3GPP TS 26.114 [104]) , indicating the capabilities of the IM-MGW (e.g. supported initialisation methods, support of ECN feedback message, and support of ECN XR summary reports).

NOTE: ECN XR summary reports and RTCP AVPF ECN feedback message are not supported in this release.

If the MGCF receives the SDP Answer also containing ECN attributes (indicating successful ECN negotiation) then it shall indicate to the IM-MGW that it shall apply the ECN procedures (according to 3GPP TS 26.114 [104]) and act as an ECT endpoint.

The IM-MGW should not send RTCP XR ECN summary reports.

9.2.11.1c Detection of ECN failures by IM-MGW

If the IM-MGW detects an ECN-related error case, for example non-ECT in the received packets when ECT(0) was expected or detecting a very high packet loss rate when ECN is used, the IM-MGW shall notify the MGCF. The MGCF should then initiate a session re-negotiation to disable ECN.

9.2.11.2 Message sequence chart

Figure 9.2.11.2.1 shows the message sequence chart example for requesting Explicit Congestion Notification.

Figure 9.2.11.2.1: Procedure to Request ECN

Upon receipt of a request to apply Explicit Congestion Notification the IM-MGW shall set the ECN field of the IP header in accordance with 3GPP TS 26.114 [104] when sending any data packets.

Upon receipt of any IP headers indicating ECN Congestion Experienced (ECN-CE) the IM-MGW shall trigger rate adaptation in accordance with 3GPP TS 26.114 [104].

Figure 9.2.11.2.2 shows the message sequence chart example for an ECN Failure Event.

Figure 9.2.11.2.2: Procedure to Report ECN Failure

When the MGCF receives a Notification indicating that a failure has occurred, the MGCF may trigger a new SDP offer to disable ECN.