Section: 12.3.8 The interaction with APN congestion control using the PGW Back-Off Time
Content: When detecting that a given APN is congested, the PGW shall either use the PGW Back-Off Time mechanism (see clause 4.3.7.5 of 3GPP TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [3]) or the APN level overload control mechanism (i.e. providing an Overload Control Information IE with an APN-List included) for that APN, but not both together for the same APN, e.g. if the PGW provides an Overload Control Information IE with an APN-List set to "APN1", it shall not reject Create Session Request messages for "APN1" with a PGW Back-Off Time until the Period-Of-Validity of the overload information previously sent has expired. The PGW may however use both mechanisms concurrently for different APNs, e.g. the PGW may reject Create Session Request messages for the"APN2" with a PGW Back-Off Time IE, if the APN2 is also congested and if there is no on-going APN-level overload control mechanism for that APN. When rejecting a Create Session Request due to APN congestion, the PGW shall set the "APN Congestion" cause, regardless of the aforementioned mechanisms. If the MME/S4-SGSN or ePDG/TWAN has one mechanism active for a given APN and PGW, (e.g. an MME has received a PGW Back-Off Time) and if subsequently it receives information for the same APN and PGW for another mechanism, (e.g. the MME receives an Overload Control Info IE with APN-List included for the same APN), then it shall deactivate/stop the earlier mechanism and consider only the information received for the latter mechanism. Different PGWs may use concurrently different mechanisms for the same APN.