6.2.13 Explicit Congestion Notification support

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

6.2.13.1 General

An IMS-ALG may configure the IMS-AGW to transfer the ECN bits in the IP header transparent (see clause 6.2.13.2) or to act as an ECN endpoint (see clause 6.2.13.3). See clause 5.12.

6.2.13.2 ECN Active Indicated (ECN transparent)

Figure 6.2.13.2.1 shows the message sequence chart example for indicating ECN transparent.

Figure 6.2.13.2.1: Procedure to indicate ECN transparent negotiated

Upon receipt of the indication that ECN transparent has been negotiated, the IMS-AGW shall forward ECN bits within IP packets unmodified. Any RTCP feedback received shall be passed unchanged.

6.2.13.3 ECN support requested (ECN endpoint)

Figure 6.2.13.3.1 shows the message sequence chart example for requesting ECN endpoint.

Figure 6.2.13.3.1: Procedure to Request ECN endpoint

Upon receipt of a request to apply ECN the IMS-AGW shall set the ECN field of the IP header in accordance with 3GPP TS 26.114 [21] when sending any data packets.

Upon receipt of any IP headers indicating ECN Congestion Experienced (ECN-CE) the IMS-AGW shall trigger rate adaptation in accordance with 3GPP TS 26.114 [21].

NOTE: ECN endpoint requires the IMS-ALG to configure the IMS-AGW with all media attributes to allow rate adaptation even if no transcoding is required/supported in the IMS-AGW.

6.2.13.4 ECN Failure Indication (ECN endpoint)

Figure 6.2.13.4.1 shows the message sequence chart example for an ECN Failure Event.

Figure 6.2.13.4.1: Procedure to Report ECN Failure

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