What is difference between NRU Sequence number and PDCP Sequence number?

First thing first, If you don't know what is DUD (Downlink User Data), then please first read it.

PDCP SN is a sequence number associated with each PDCP PDU by PDCP. Whereas, NRU sequence number is associated with each DUD (Downlink User Data). 

DDDS (Downlink Data Delivery Status)  reports the lost packets, highest transmitted or highest delivered packets with PDCP SN instead of NRU-SN.

Let's see where to find PDCP-SN & NRU-SN in DUD packet. You may have seen the below packet example in my earlier post of DUD. Let's start from where we left -

Example Packet of DUD:

GPRS Tunnelling Protocol

    Flags: 0x34

    Message Type: T-PDU (0xff)

    Length: 1392

    TEID: 0xpppppp (xxxxxx)

    Next extension header type: NR RAN container (0x84)

    Extension header (NR RAN container)

        Extension Header Length: 2

        NR RAN Container

            NRUP

                0000 .... = PDU Type: DL User Data (0)

                .... 0... = Spare: False

                .... .0.. = DL Discard Blocks: Not Present

                .... ..0. = DL Flush: Not Present

                .... ...0 = Report Polling: Not Requested

                000. .... = Spare: 0

                ...0 .... = Request Out Of Seq Report: No

                .... 0... = Report Delivered: No

                .... .0.. = User Data Existence Flag: No

                .... ..0. = Assistance Info. Report Polling Flag: No

                .... ...0 = Retransmission Flag: No

                NR-U Sequence Number: 187387

        Next extension header type: No more extension headers (0x00)

Wireshark Snippet of Downlink User Data:

DUD with NRUP header

Where to find PDCP SN in the DUD packet ?

Let's directly jump to the hex dump and have a look at a typical DUD packet. (I have truncated some part of the message and kept only relevant info)

So, to find out the PDCP SN you need to look at the 4^th word into the Wireshark Hexdump and need to pick the 3 nibbles as shown in the below diagram. This is for PDCP SN size as 12 bit.

Hex dump of a DUD

Here, 813 (in hex) i.e. 2067 (in decimal) is our PDCP SN. But my NR-U sequence is 02 db fb (in Hex) i.e 187387 as shown in "DUD with NRUP header" figure and can be seen into the 3rd word of the "Hex dump of a DUD" figure also.

Similarly, for the PDCP SN size as 18 bit we need to read 5 nibble.

Always remember, NRU-SN is a part of NR Header whereas PDCP SN is a part of payload in DUD message.