I'm not positive on this, but I'm pretty sure it's mandatory in LTE. When you first register you send your IMSI (in the clear...) and they return a TMSI for you

I'm not positive on this, but I'm pretty sure it's mandatory in LTE. When you first register you send your IMSI (in the clear...) and they return a TMSI for you. My quick skim through the code looks like it'll track that too though: # Register IMSI as seen if a TMSI believed to # belong to the IMSI is seen.