What content is sent in MSG3 during NR RACH in ENDC?

Hi Experts,
As you know, in LTE, UE send RRC connection request/re-establish req, etc as MSG3 in the UL grant received as part of MSG2.
What content is sent in MSG3 during NR RACH in ENDC?

If its NSA then we don’t have msg 3 it’s msg 1 and msg 2 then mac dci format given to UE as per requirement.
For SA we have msg 3.

Are you sure you are not talking about contention free RA?
In many NSA UE logs, I can see UE sending MSG3 and receiving msg4 too.

Ok, you mean HO scenario.
Having msg 3.

Nope. Even for initial NSA deployment.

I consider initial rach synchronisation.

Ok, msg 3 message name please.

Lte attach–>NSA deployment. Even in this case, I see msg3.

Yes, obvious at LTE end but not after SCG cell addition.
After scg cell addition we have SSB beam latch and RACH sync, i.e. msg 1 and msg 2 .
For msg 3 not seen.

Then, how can UE resolve contention?

It’s contention based resolution ok, but at L3 end message like RRC connection request not seen.

True, now my question is: what exactly are the contents of MSG3?
I see the MAC PDU as all zeros.
Is it possible that it’s just a MAC PDU full of padding?

Yes, all MAC PDU of msg 3 are zero in all 5G NR MAC, probably scrambled with T-RNTI!?
Oops there are cases when MAC PDU for rach are not zero… see this example:

But this is for SA log.

For SA, it is clear.
UE needs to send RRC req containing the NAS payload, so it will just that PDU as MSG3.

For NSA, in UE random access procedure for 5G, UE use contention-free as Huawei.
Does anyone see contention-based?

In the UE Random Acess Procedure:

UE → MSG1: Preamble → gNB
UE ← RAR ← gNB

In NSA mode UE adopts non-contention based random access to NR cell. The random access procedure should be completed with T304 timer. If UE fails random access, it will report SCG failure info to eNB, informing the failure cause.

Huawei implementation might be to implement contention free RACH always for SCG addition which I agree is a good implementation. But still, CB RACH is possible with NSA.

I have sgnb failure due random access failure.
Are you sure within the t304, in LTE t304 used for handover?

I wonder if Random Access Procedure in 5G is also contention-based, so UE will have RRC-connection in 4G + 5G together?

Contention based for 5G is not the same as 4G.

I think this is during UL sync issue.

Yes, agree.