-If the gNB crashes when the X2 connection is established with the eNB, an sctp shutdown event is received from the eNB. However the x2 data related with
the connection were not removed from the RB tree. Then if the gNB tried to reconnect, it made the eNB exit because the eNB had kept the previous data.
Now we remove the x2 data as soon as the sctp shutdown event is received so that the eNB does not exit anymore at the next connection attempts.
Exit gNB if connection to eNB failed - to be modified if needed.
We may want to try to connect over and over again until we succeed
but the modifications to the code to get this behavior are complex.
Exit on error is a simple solution.
Before this commit, when the gNB crashes, the eNB keeps running.
Same, if the eNB crashes, the gNB keeps running.
In a far past when this happened, the other program (the one not
crashing) was forced to exit.
Then this behavior was changed, for some reason.
But the code was not finished, so now we have a system in an inconsistent
state.
So either we accept that the connection between eNB and gNB can break,
that is one of the programs crashes, and we clean the state of the
program that keeps running. But this is a complex work and it will
surely not survive very long, because someone will change something
in the code later that will break this complex behavior.
Or, simpler, we go back to initial behavior, which is: the program that
did not crash does actually exit when the other crashes.
This commit provides the second solution.
It can easily be reverted whenever someone wants to implement the complex
solution.
This commit introduces several enhancements in X2AP and RRC to deal
with ENDC. The main goal is to deal with UE's on/off and with various
errors.
The following work is included:
- DC prep and DC overall X2 timers
- X2 messages for UE release and DC timers' timeout
Open problems:
- what to do when the UE sends an scg Failure? This commit removes
the UE. And this removal may not be correctly done
- UE removal in the gNB's PHY layer (a next commit is coming,
but it is hackish and will need a rework)
An NR-capable UE will transmit some NR measurements.
Once received, the whole process on nr-cell addition is started.
It will fail quickly.
Many things are still missing.
What is not handled properly with this commit:
- in x2ap_eNB_handle_sgNB_add_req we hardcode the gNB ID
- nr triggering is done if we receive a measurement but this measurement is
not analyzed at all
- no timers (T_dc_overall, T_dc_prep) and as is the code fails and then the
next triggering after UE reconnection will not work (x2 ID are not well
handled here and there in the code)
Note also that the gNB part has not been looked at all for this commit.
This is work in progress...
Laurent THOMAS