Section: 5.19.1 NAS Message Redirection Procedure
Content: When DCNs are used, these steps are used to reroute a NAS message (and thereby a UE) from one CN node to another CN node during Attach, TAU or RAU procedure. These steps are also used by the MME/SGSN or HSS initiated Dedicated Core Network Reselection procedure in clause 5.19.3. Figure 5.19.1-1: NAS Message Redirection Procedure The procedure is started when a first new MME/SGSN decides to move the handling of an Attach Request, TAU Request or RAU Request to another CN node. 1. The first new MME/SGSN sends a Reroute NAS Message Request (original RAN message, reroute parameters, Additional GUTI/P-TMSI, UE Usage Type, and optionally the IMSI) to the RAN Node. The reroute parameter is a MMEGI (for E-UTRAN) or Null-NRI/SGSN Group ID (for UTRAN/GERAN) corresponding to the DCN that corresponds to the UE Usage Type. A UE provided Additional GUTI/P-TMSI (if available) from the NAS Request message is included. The MME/SGSN may determine the MMEGI or Null-NRI/SGSN Group ID corresponding to the DCN using DNS procedures. The original RAN message is the complete PDU received from the RAN which contains the original NAS Request message and all RAN IEs. The UE Usage Type shall be included, if available. For the condition to include the IMSI, see step 6 in clause 5.19.2. 2. The RAN node's NNSF selects a new MME/SGSN based on the MMEGI or Null-NRI/SGSN Group ID and possibly also based on an Additional GUTI/P-TMSI. If Additional GUTI/P-TMSI identifies an MME/SGSN within the set of valid nodes identified by MMEGI or Null-NRI/SGSN Group ID, it should be the selected node. Otherwise a valid CN node corresponding to the MMEGI or Null-NRI/SGSN Group ID will be selected. If no valid MME/SGSN is available within the set of valid nodes identified by MMEGI or Null-NRI/SGSN Group ID, the RAN node selects an MME/SGSN from the default DCN or selects the MME/SGSN that sent the Reroute Request, based on operator configuration. The MME/SGSN is selected from the network corresponding to the selected CN operator. 3. Dependent on RAT, the eNodeB/RNC sends the Initial UE message to the selected MME/SGSN or the BSC sends the UL-Unitdata message to the selected SGSN. The initial UE message/UL-Unitdata message includes the NAS Request message, the MMEGI or Null-NRI/SGSN Group ID, UE Usage Type and the IMSI if received from the first SGSN/MME in step 1. The MMEGI or Null-NRI/SGSN Group ID indicates that the message is a rerouted message and the second new MME/SGSN shall not reroute the NAS message. The UE Usage Type shall be included if received in the Reroute NAS Message Request to be used by the second new MME/SGSN to select SGW and PDN GW (see clauses 4.3.8.1 and 4.3.8.2).