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.
How It Works
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
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.
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.
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
The MME does not
perform GUTI Reallocation if the subscriber is marked for offload or if the
subscriber is executing an outbound handover procedure.
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.
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
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
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.
diagram illustrates the messages exchanged during network-initiated GUTI
Figure 1. GUTI
The MME sends GUTI
REALLOCATION COMMAND message to the UE. The time duration for the T3450 timer
starts. This timer starts when the MME initiates a Globally Unique Temporary
Identifier (GUTI) reallocation procedure by sending a GUTI REALLOCATION COMMAND
message to the UE and stops upon receipt of the GUTI REALLOCATION COMPLETE
The UE sends a GUTI REALLOCATION
COMPLETE message to the MME on completion of the GUTI Re-allocation procedure.
The T3450 timer stops once the MME receives the GUTI REALLOCATION COMPLETE
configuration command is used to configure the periodicity (time interval) /
frequency of GUTI Reallocation for a UE: