In the NAS protocol, the attach request is a message which is sent by the UE to the network (the target is the MME) in order to perform an attach procedure.
The International Mobile Subscriber Number (IMSI) is a unique identification number which identifies precisely the user of the UE. The IMSI is a number with a maximum of 15 digits which is stored as a 64-bit number in the USIM; the IMSI number is also stored in the HSS of the core network. For privacy reasons (i.e. tracking the user of the UE over the radio interface) the IMSI is sent in clear as rarely as possible over the radio interface; in its place, another generated identifier is used, the Globally Unique Temporary Identity (GUTI).
The GUTI is associated to the UE by the MME and allows to hide the identity of the user (the IMSI). The GUTI is an 80-bit number.
For simplicity, let’s consider a particular case where:
- the UE has just been powered on
- the UE has already performed a search of available networks and has found a potentially suitable network
- no valid GUTI for the found network is stored on the USIM card
To attach to the network, the UE must use the NAS protocol to send an Attach Request to the MME.
Since there is no valid GUTI, the UE must transmit the IMSI in clear over the network (first through the LTE-Uu interface and then through the S1 interface) to the MME.
The Attach Request message is documented in the 3GPP TS 24.3031, section 8.2.4 .
The parameters are:
IMSI or old GUTI, last visited TAI (if available), UE Core Network Capability, UE Specific DRX parameters, Attach Type, ESM message container (Request Type, PDN Type, Protocol Configuration Options, Ciphered Options Transfer Flag), KSIASME, NAS sequence number, NAS-MAC, additional GUTI, P-TMSI signature, Voice domain preference and UE’s usage setting) message together with RRC parameters indicating the Selected Network and the old GUMMEI.