lte-pdcp-header: PDCP packet split and error in decoding PDCP packet header

[Franci Gjeci]

Hi,

I have implemented the handover in nr, which works as it should and the traffic gets rerouted to the new gnb. The handover is triggered manually at 0.17 sec after the simulation starts.

However, I do get the following error in the pdcp layer, which occurs in the UE side. After inspecting the error, below I am trying to explain what happens.

What do you think could solve this error?

Thank you, Franci

// GNB sending pdcp packet of size 590 bytes

LtePdcp:DoTransmitPdcpSdu(0x5589a0937800, 2, 3, 588)

LtePdcp:DoTransmitPdcpSdu(): PDCP header: D/C=1 SN=1196

// UE first round: Decodes only 142 bytes from 590

+0.179390175s [ Rnti 2] LtePdcp:DoReceivePdu(0x5589a08bc480, 2, 3, 142)

+0.179390175s [ Rnti 2] LtePdcp:DoReceivePdu(): PDCP header: D/C=1 SN=1196

// UE second round: Tries to decode the remaining 448 bytes, which does have any more a PDCP header (it is present in the first 142 bytes), thus obviously leading to an error, when PDCP tries to remove the header from the packet

+0.179390175s [ Rnti 2] LtePdcp:DoReceivePdu(0x5589a08bc480, 2, 3, 448)

**** Error ****

assert failed. cond="m_dcBit == DATA_PDU", +0.179390175s 3 file=../src/lte/model/lte-pdcp-header.cc, line=114 terminate called without an active exception

[Bill Tao]

Hi, Franci.

I am quite interested in an HO implementation in NR, and it would be helpful for my project too, so I would like to follow your progress.

For your error, why does the UE PDCP attempt to extract the header twice? Does it only happen at the handover?

Cheers,

Bill

[Biljana B.]

Hi Franci, 

that is great! Regarding BIlls' answer, first of all, thanks a lot for helping other 5G-LENA users. What I don't understand is why RLC forwards to PDCP in two pieces the PDCP SDU??? Something happened wrong in RLC, and I suspect that it is some modified version of RLC. I wrote also on some other threads, that error could be if you did some modifications in the RLC layer. If that is not the case can you specify which ns-3-dev branch are you using?

Kind regards,

Biljana

[Bill Tao]

You are right, Biljana. It appears RLC forwards the PDCP header and payload separately to PDCP, judging by the packet size to  LtePdcp:DoTransmitPdcpSdu().

Bill

[Biljana B.]

I have just opened an issue: https://gitlab.com/cttc-lena/nr/-/issues/123.

If you have any further information regarding this, please write in the Gitlab issue.

Thanks!