Section: 7.10 Fallback to GTPv1 mechanism
Content: An EPC entity shall assume that each GTP processing node that it is about to communicate with is GTPv2 capable. Before the first GTP tunnel is setup for a given UE/node, the EPC node shall always send a version 2 (GTPv2) message to a peer node. As an exception, during an inter-SGSN handover, even if the target SGSN is GTPv2 capable, the source SGSN shall send a GTPv1 message "Forward Relocation Request" to the target SGSN if the PDP Context(s) for this UE were established to GGSN(s), or if there is no active PDP context and the source or target SGSN does not support SRNS relocation w/o PDN connection over GTPv2 (see clause 7.3.1). A GTPv2 entity shall fallback to GTPv1 only if either a "Version Not Supported" message in GTPv1 format as specified in 3GPP TS 29.060[ General Packet Radio Service (GPRS); GPRS Tunnelling Protocol (GTP) across the Gn and Gp interface ]  [4] is received from the peer node (this indicates that the peer GTP entity does not support GTPv2), or if a GTPv2 message is received with Cause value "Fallback to GTPv1". If a GTPv1 "Version Not Supported" message in received, a GTPv2 entity may fallback to GTPv1. 3GPP TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [3] (see annex D) and 3GPP TS 23.060[ General Packet Radio Service (GPRS); Service description; Stage 2 ]  [35] specify GTP version usage during the mobility between a UTRAN/GERAN and an E-UTRAN. A GTPv2 entity may receive a GTPv2 message with a Cause value "Fallback to GTPv1" in the following cases: - an S4 SGSN receives the Cause code "Fallback to GTPv1" in a GTPv2 Context Response message over S16 interface. When an UE has activated a PDP context via S4 SGSN to GGSN and inter-SGSN RAU is underway, the old S4 SGSN shall include the Cause value "Fallback to GTPv1" in a GTPv2 Context Response message over S16 interface. In this case, the new S4 SGSN shall abort the ongoing GTPv2 procedure and send a GTPv1 "SGSN Context Request" message to the old S4 SGSN. The fallback to GTPv1 is performed only for this UE in the current procedure. - an MME receives the Cause code "Fallback to GTPv1" in a GTPv2 Context Response message over the S3 interface. When an UE has active PDP context(s) via an S4 SGSN and a TAU is underway, the old S4 SGSN may include the Cause value "Fallback to GTPv1" in a GTPv2 Context Response message over the S3 interface. In this case, the MME shall abort the ongoing GTPv2 procedure and should send a GTPv1 "SGSN Context Request" message to the old S4 SGSN. The fallback to GTPv1 is performed only for this UE. Fallback to GTPv1 shall not occur on already established GTP tunnels without change of the peer nodes of the communication bearer.