when UE send GUTI, IMSI, S-TMSI?

Question

When UE sends GUTI/STMSI to MME and what all massages are involved?

Answer 1

UE sends IMSI during Attach Request and NW allocates a different identity to UE , that is GUTI in attach accept.
Now UE can use GUTI if it attaches again with Attach Request and Tracking Area Update Request.
S-TMSI is generally used as UE identity in the RRC Connection Request or in Paging.

Answer 2

UE sends IMSI an Attach Request message in case of initial cell selection. Network allocates a unique identity to UE, that is GUTI in attach accept. UE can use GUTI if it attaches again (stored information cell selection) with Attach Request or Tracking Area Update Request. S-TMSI is used as UE identity in the RRC Connection Request and in dedicated paging messages.