Symptom:
At apparently random rate, some mobiles which attach to 4G coming from 3G, during a TAU MME encounters an error and prematurely detaches these subscribers.

After a successful radio attach of UE to eNB  4G radio coming from 3G RAN:

*) MME retransmits "Modify Bearer Request" even thought it received the response already   <--- THIS might be a DIFFERENT issue, or the Cause of the MME error. (?)

*)  MME logs the following error:  "Completing NAS triggered MME IM Exit Procedure  procedure due to Error"

Please note that the mobile is _Connected_ and that the Procedure cause by error is  _IM (idle mode) Exit Procedure_

*) Finally, MME does not wait for a "detach accept", instead it shuts down the S1.


/!\ The Impact is a total loss of service.

Conditions:
The conditions which would systematically (at every attach) produce this error - are not found.

However, during on-field tests, at every 15th call, the error is triggered during a TAU from 3G RAN to 4G radio access.

Update: Issue is due to the SGW and/or PGW not sending a Linked EPS Bearer ID IE in response to the Create Session Request from the MME.  The GTPv2 stack on the MME is enforcing this requirement on the subsequent Modify Bearer Response, rather than the Create Session Response.