This time constant plays a major role in how long a UE has to wait
maximally before it might be scheduled. For instance, if a UE had a lot
of traffic, the average traffic will be high, pushing the PF coefficient
down. If another UE with no traffic in the past becomes active with
heavy traffic, the first UE might not be scheduled for an extended
period of time.
In the AW2S-Amarisoft UE CI test pipeline, we sometimes see that the
maximum number of RLC retransmissions was reached. While this might not
be the (only) cause, scheduling a UE more often might prevent such
situations, and we saw an improved stability.
After removing RA.crnti, there is no need to use a separate function
nr_generate_Msg3_dcch_dtch_response() to generate Msg.4 (yes, the
function mentions Msg3 but it does Msg3 response == Msg.4).
Also, using nr_generate_Msg4() has the advantage that it waits until we
have the reconfiguration available in RLC to be sent to the UE. Finally,
we can delete the old nr_generate_Msg3_dcch_dtch_response() as we don't
need it anymore.
When t_poll_retransmit expires we need to consider an SDU for
retransmission.
We used to take the first SDU (which could be a segment, not a full SDU)
of the wait list and put it in the retransmit list.
When testing with some UE (Amarisoft UE) a bad behavior was detected.
This is what happens for the SDU with SN 6 (for illustration purpose).
The gNB is sending an SDU in several pieces (it is segmented).
slot n: rlc sn 6 [1 .. 119[ p=0 RECEIVED
slot n+x: rlc sn 6 ]120 .. 180[ p=0 RECEIVED
slot n+x+y: rlc sn 6 ]181 .. 210] p=1 not RECEIVED
Then when t_poll_retransmit expires rlc retransmits only the PDU of
slot n (with p=1 this time) and the UE replies with ack 6. So nothing
happens on the gnb side.
(We would have expected ack 7 + nack 6 181..end.)
Then after t_poll_retransmit expires again rlc retransmits only PDU
of slot n and the UE still replies with ack 6.
This goes on forever (or some other timeout occurs.)
The logic is now changed.
When t_poll_retransmit expires we transfer all the SDUs in the wait list
having the same SN as the head of the wait list into the retransmit list.
Testing with Amarisoft UE, it seems to work properly.
The RA.crnti field is marked as being used by NSA mode, but it is not.
OTOH, it is used in the case of RA with Msg.3 with C-RNTI MAC CE. The
latter does not necessarily need it, though, as we can change the RNTI
of the RA process when we receive C-RNTI MAC CE.
This introduces a slight complication, though, as we cannot remove the
MAC UE Context (generated when receiving Msg.3) directly when parsing
the C-RNTI MAC CE. Previously, we had both rnti and crnti in the RA
struct, and when sending Msg.4 after Msg.3 with C-RNTI MAC CE, we would
remove the UE context (this is changed in the next commit). Therefore,
we employ nr_mac_trigger_release_timer() which releases the UE after
60ms, circumventing this complication.
This commit repairs the reestablishment after the introduction of the CU
UE ID in the CU, different from the DU UE ID. Now, the CU uses an
identifier different from the RNTI, used in the MAC, to refer to UEs.
To repair the reestablishment, a number of changes needed to be
introduced:
- Handle old_gNB_DU_UE_id in MAC
The MAC handles the old_gNB_DU_UE_id field in the DL RRC Message. The
commit adds encoding/decoding of the field. The MAC will drop the old
UE, but retain the CellGroup of the corresponding UE(!). Because a UE
releases the spCellGroup on reestablishment, the MAC does the same; it
needs to reapply the old configuration after a reconfiguration. For the
latter, introduce new variables to keep a "future" CellGroupConfig to be
applied (reconfigCellGroup) and a flag (expect_reconfiguration) that
applies the CellGroupConfig if a DL RRC Message transfer on DCCH, which
is assumed to be the reconfiguration.
- Add RNTI change in RLC
The RLC needs to reuse the old UE context. Hence, we simply change the
RNTI in the old context, and delete the new one.
- No PDCP UE ID change
The previous implementation of the PDCP used the RNTI; hence, the ID
needed to be switch (as above for the RLC). Since the PDCP now also uses
the CU UE ID, no identifier change is needed, and we remove all
corresponding code.
- No MAC modifications/update from CU
As foreseen by the spec, the CU does not modify or change the
cellGroupConfig in the RRC during reestablishment. This change will be
generalized in the future to all of the RRC. Also, the
nr_rrc_mac_remove_ue() function has been removed: the MAC handles any UE
changes autonomeously, without being triggered from the RRC explicitly.
- Remove reestablish_rnti_map
The RRC does not use RNTIs for UE identification. Hence, a
reestablish_rnti_map to link two UEs to each other is not necessary
anymore.
After a reestablishment, the PDCP sequence number counters have to be
reset. Implement an API to allow the RRC to reset the sequence numbers
for a particular UE in the case of reestablishment
- The CU/DU UE ID is used in DL RRC Message Transfer
- Note: The CU UE ID currently is still the RNTI
- on every DL RRC Message, the DU verifies that the UE exists
- Refactor RRC: common function to forward protected SRB PDUs
for reusing the old UE Context after reestablishment: we have to free a
UE context, but that also free's the RA process that we still need.
Remove here
Is this necessary?
Cedric Roux