Section: 4.7.4 Support for Application / Service Layer Rate Adaptation
Content: The E-UTRAN/UTRAN and the UE support the RFC 3168 [55] Explicit Congestion Notification (ECN), as described in TS 36.300[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [5], TS 25.401[ None ]  [16] and TS 26.114[ IP Multimedia Subsystem (IMS); Multimedia telephony; Media handling and interaction ]  [56]. The IP level ECN scheme enables the E-UTRAN/UTRAN to trigger a rate adaptation scheme at the application / service / transport layer. To make sufficient time available for end-to-end codec rate adaptation the E-UTRAN/UTRAN should attempt to not drop any packets on a bearer for a default grace period of at least 500 ms after it has indicated congestion with ECN on the bearer for packets within the packet delay budget. During this ECN grace period the E-UTRAN/UTRAN should also attempt to meet the QCI characteristics / QoS class associated with the bearer. NOTE 1: Note that the receiving end-point should interpret all ECN-CE signals received within one end-to-end round-trip time as one "congestion event" (see IETF RFC 3168 [55] and TS 26.114[ IP Multimedia Subsystem (IMS); Multimedia telephony; Media handling and interaction ]  [56]). The MBR of a particular GBR bearer may be set larger than the GBR. NOTE 2: Enforcement of APN-AMBR / UE-AMBR is independent of whether the MBR of a particular GBR bearer has been set larger than the GBR (see clause 4.7.3). The EPC does not support E-UTRAN/UTRAN-initiated "QoS re-negotiation". That is, the EPC does not support an eNodeB/RNC initiated bearer modification procedure. If an eNodeB/RNC can no longer sustain the GBR of an active GBR bearer then the eNodeB/RNC should simply trigger a deactivation of that bearer.