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The Globally Unique Temporary Identity (GUTI) is assigned to the UE by the MME the GUTI is used to support subscriber identity confidentiality. The GUTI has two parts, the Globally Unique Mobility Management Entity Identifier (GUMMEI), which identifies the network, and the M-TMSI, which identifies the device. This feature enables GUTI Re-allocation for an UE based on time and frequency of access attempts per UE.
The MME currently performs GUTI allocation during UE attaches. The GUTI once allocated is retained until the DB associated with the UE is purged. This feature introduces MME support to perform GUTI Reallocation for securing the TMSI allocated to UE. GUTI Reallocation is triggered based on configured frequency of access attempts or periodicity.
A configured frequency of "n" requests triggers GUTI Reallocation for every "nth" ATTACH / TAU / SERVICE REQUEST received from the UE. Here 'n' is the sum of the received ATTACH / TAU/ SERVICE Request. A configured periodicity of "t" minutes triggers GUTI Reallocation at every "t" minutes for a UE.
The frequency-based GUTI reallocation is independent of the configured periodicity. However, periodicity-based GUTI reallocation attempts are relative to the last attempted UE GUTI Reallocation time. The last attempted GUTI Reallocation time for a UE is updated whenever a GUTI Reallocation for a UE is attempted irrespective of the trigger (frequency/periodicity).
The MME initiates GUTI Reallocation only if the NAS signaling connection with the UE is present. If the NAS signaling connection is not present the UE shall not be paged. If the NAS signaling connection with the UE is absent, GUTI reallocation is performed whenever the NAS signaling connection with the UE is established.
 Note |
GUTI Reallocation is not triggered when UE is always in connected mode as, GUTI Reallocation based on periodicity is performed only when the either Attach, periodic TAU, Service request is received by MME and the configured periodicity time has been reached. For a UE that is always in connected mode neither of these events occur. The session are not disturbed during GUTI Reallocation, idle-active transitions are a frequent occurrence in the network, therefore GUTI Reallocation should happen for most UE's at the configured periodicity/frequency during service request procedure. |
The Reallocated GUTI is sent in the NAS Attach Accept, NAS TAU Accept and NAS GUTI Relocation Command messages.
The MME does not perform GUTI Reallocation if the subscriber is marked for offload or if the subscriber is executing an outbound handover procedure.
The GUTI reallocation retries for UE's which do not adhere to specifications is limited by the MME. MME detaches such UEs after "10" consecutive failure attempts of GUTI Reallocation. This behavior and number of consecutive failures to trigger detach is not configurable.
The frequency and periodicity configured to trigger authentication/GUTI reallocation requires the new session setup message (NAS Attach/TAU) to be processed by the Session Manager instance which has the corresponding MME DB for the subscriber. If the MME DB is not available the frequency and periodicity triggers will not work. For example, if the mobile identifier in the NAS Attach/TAU message is a foreign GUTI and additional GUTI is not present, the MME does not trigger authentication/GUTI reallocation for the subscriber based on frequency/periodicity.
Reallocated GUTI is not sent in TAU accept for TAU with type TA Update. In this scenario, once the frequency criteria for TAU is met, GUTI reallocation is performed on receiving the next periodic TAU or Service request. This to prevent the case where, TAU complete for an TAU accept with Reallocated GUTI is not received by MME. Wherein upon receiving a paging trigger, MME needs to page the UE in both the TAI lists (before and after TAU) with both the GUTI (previous and reallocated). In the case of SGSN , paging message is sent to the RNC with acknowledged PTMSI and unacknowledged (reallocated) PTMSI. However paging is sent only for the current RAI. Similarly in the case of MME, MME has to send paging message to the eNodeB's with acknowledged GUTI and unacknowledged GUTI (reallocated). But paging needs to be sent in both current TAI list and previous TAI list.
The following diagram illustrates the messages exchanged during network-initiated GUTI re-allocation:
The following configuration command is used to configure the periodicity (time interval) / frequency of GUTI Reallocation for a UE:
config
call-control-profile <profile_name>
[ remove ] guti reallocation [ frequency <frequency> | periodicity <duration> ]
end Notes:
This section provides information regarding show commands and/or their outputs in support of the GUTI reallocation feature in MME.
The following new fields are added to the show output to display the configured GUTI Reallocation parameters:
The following new disconnect reason is added for GUTI Reallocation:
The following new fields are added to the show output to display the configured GUTI Reallocation parameters:
Below is an example displaying the EMM Statistics listed above:
EMM Statistics:
. . .
GUTI Reallocation:
Attempted : 176807
Success : 176691
Failures : 116 Below is an example displaying the Total EMM Control Messages listed above:
Total EMM Control Messages::
. . .
GUTI Reallocation:
Attach Accept: 180094 Retransmissions: 0
TAU Accept: 892098 Retransmissions: 0
GUTI Reallocation Cmd: 389986 Retransmissions: 0 The following new field is added to the show output to display the configured GUTI Reallocation parameters:
The following new fields are added to the show output to display the configured GUTI Reallocation parameters:
Below is an example displaying the statistics listed above:
show mme-service db record imsi 123456710100158
Friday September 18 09:25:19 EDT 2015
DB RECORD
=========
Sessmgr Instance : 1
Imsimgr Instance : 1
MME Service : mmesvc
Lookup Keys
-----------
IMSI : 123456710100158
Service-id : 7
GUTI
PLMN : 123456
MME Group ID : 32777
MME Code : 2
M-TMSI : 3221491713
REALLOCATED GUTI
PLMN : 123456
MME Group ID : 32777
MME Code : 2
M-TMSI : 3221491713
Call-ID : 00004e62
GUTI Allocated time : Fri Sep 18 08:29:16 2015 The following bulk statistics are included in the MME Schema in support of this feature:
The following bulk statistics are included in the MME Schema in support of this feature:
For descriptions of these variables, see "MME Schema Statistics" in the Statistics and Counters Reference.
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