4.7.6 P-TMSI reallocation procedure

24.0083GPPCore network protocolsMobile radio interface Layer 3 specificationRelease 18Stage 3TS

4.7.6.0 General

A temporary mobile station identity for GPRS services, the Packet-TMSI (P-TMSI), is used for identification within the radio interface signalling procedures. The structure of the P-TMSI is specified in 3GPP TS 23.003 [10]. The P-TMSI has significance only within a routing area. Outside the routing area the P-TMSI has to be combined with the routing area identification (RAI) to provide for an unambiguous identity.

The purpose of the P-TMSI reallocation procedure is to provide identity confidentiality, i.e. to protect a user against being identified and located by an intruder (see 3GPP TS 43.020 [13], 3GPP TS 23.060 [74] and 3GPP TS 33.102 [5a]).

Usually, P-TMSI reallocation is performed at least at each change of a routing area (Such choices are left to the network operator).

The reallocation of a P-TMSI is performed by the unique procedure defined in subclause 4.7.6. This procedure can only be initiated by the network in state GMM-REGISTERED.

P-TMSI can also be implicitly reallocated in the attach or routing area updating procedures (see subclauses 4.7.3.1 and 4.7.5). The implicit reallocation of a P-TMSI is described in the corresponding subclauses.

NOTE: Normally, the P-TMSI reallocation will take place in conjunction with another GMM procedure, e.g. at routing area updating (see 3GPP TS 29.002 [37]).

4.7.6.1 P-TMSI reallocation initiation by the network

The network shall initiate the P-TMSI reallocation procedure by sending a P-TMSI REALLOCATION COMMAND message to the MS and shall start the timer T3350.

The P-TMSI REALLOCATION COMMAND message shall contain a new combination of P-TMSI, RAI and optionally a P-TMSI signature or a DCN-ID or both allocated by the network.

The network may suspend the transmission of user data during the P-TMSI reallocation procedure.

4.7.6.2 P-TMSI reallocation completion by the MS

Upon receipt of the P-TMSI REALLOCATION COMMAND message, the MS shall store the routing area identifier (RAI) and the P-TMSI. Furthermore, the MS shall send a P-TMSI REALLOCATION COMPLETE message to the network.

If a P-TMSI signature is present in the P-TMSI REALLOCATION COMMAND message, the MS shall store the new P-TMSI signature and shall if available delete the old P-TMSI signature. If no P-TMSI signature is present in the P-TMSI REALLOCATION COMMAND message, the old P-TMSI signature, if available, shall be kept.

If the P-TMSI REALLOCATION COMMAND message contains the DCN-ID IE, then the MS shall store the included DCN-ID value together with the PLMN code of the registered PLMN in a DCN-ID list in a non-volatile memory in the ME as described in 3GPP TS 24.301 [120], annex C.

4.7.6.3 P-TMSI reallocation completion by the network

Upon receipt of the P-TMSI REALLOCATION COMPLETE message, the network shall stop the timer T3350 and shall consider both the old and the new P-TMSI and the corresponding P-TMSI signatures as valid until the old P-TMSI can be considered as invalid by the network (see subclause 4.7.1.5).

In A/Gb mode, the GMM layer shall notify the LLC layer that the P-TMSI has been changed (see 3GPP TS 44.064 [78a]).

4.7.6.3A Abnormal cases in the MS

The following abnormal cases can be identified:

If different new P-TMSIs are included in subsequent P-TMSI REALLOCATION COMMAND messages due to an aborted or repeated P-TMSI reallocation procedure, the MS shall behave as described in subclause 4.7.6.2. Furthermore, the MS shall take action on the P-TMSI as described in subclause 4.7.1.5.

4.7.6.4 Abnormal cases on the network side

The following abnormal cases can be identified:

a) Lower layer failure

If a lower layer failure is detected before the P-TMSI REALLOCATION COMPLETE message is received, the network shall consider both the old and the new P-TMSI as occupied until the old P-TMSI can be considered as invalid (see subclause 4.7.1.5).

During this period the network:

– may first use the old P-TMSI for paging for an implementation dependent number of paging attempts in the case of network-originated transactions. Upon response from the MS, the network may re-initiate the P-TMSI reallocation procedure. If no response is received to the paging attempts, the network may use the new P-TMSI for paging for an implementation dependent number of paging attempts. Upon response from the MS, the network shall consider the new P-TMSI as valid and the old P-TMSI as invalid. If no response is received to the paging attempts, the network may use the IMSI for paging for an implementation dependent number of paging attempts;

NOTE: Paging with IMSI causes the MS to re-attach as described in subclause 4.7.9.1.

– shall consider the new P-TMSI as valid if it is used by the MS (see subclause 4.7.1.5); or

– may use the identification procedure followed by a new P-TMSI reallocation, if the MS uses the old P-TMSI.

b) Expiry of timer T3350

The P-TMSI reallocation procedure is supervised by the timer T3350 (see example in figure 4.7.6/1). On the first expiry of timer T3350, the network shall reset and restart timer T3350 and shall retransmit the P-TMSI REALLOCATION COMMAND message. This retransmission is repeated four times, i.e. on the fifth expiry of timer T3350, the network shall abort the P-TMSI reallocation procedure and shall follow the rules for case a as described above.

c) P-TMSI reallocation and GPRS attach procedure collision

If the network receives an ATTACH REQUEST message before the ongoing P-TMSI reallocation procedure has been completed, the network shall proceed with the GPRS attach procedure after deletion of the GMM context.

d) P-TMSI reallocation and an MS initiated GPRS detach procedure collision

If the network receives a DETACH REQUEST message before the ongoing P-TMSI reallocation procedure has been completed, the network shall abort the P-TMSI reallocation procedure and shall progress the GPRS detach procedure.

e) P-TMSI reallocation and a routing area updating procedure collision

If the network receives a ROUTING AREA UPDATE REQUEST message before the ongoing P-TMSI reallocation procedure has been completed, the network shall abort the P-TMSI reallocation procedure and shall progress the routing area updating procedure. The network may then perform a new P-TMSI reallocation.

f) P-TMSI reallocation and a service request procedure collision

If the network receives a SERVICE REQUEST message before the ongoing P-TMSI reallocation procedure procedure has been completed, the network shall progress both procedures.

Figure 4.7.6/1 3GPP TS 24.008: P-TMSI reallocation procedure