What is DUD (Downlink User Data)?

DUD stands for Downlink User Data. The data sent from the node hosting NR-PDCP to the corresponding node.

So in my words, whenever we need to send split data towards corresponding node, we send DUD. Each DUD is associated with one NRUP sequence number. As per the 3gpp spec, even for re-transmitted DUD packets will contain a new NRUP sequence number.

 

DUD contains following info:

1. NR-U Sequence Number
2. DL Discard Blocks
3. Report Polling bit
4. Assistance info. Report polling flag
5. Retransmission flag

 

As I said that each DUD is associated with a new NRU sequence number even for the retransmission also, so the question is how do we identify a new transmission and retransmission?

So, the answer is that for a retransmission DUD, retransmission flag will be set as 1 with a new NRU sequence number and for a new transmission it will be set as zero.

If we need to indicate corresponding RLC node  for discarding few PCPD packets, then we can fill DL discard NR PDCP PDU SN with the number of blocks needs to be discarded.

To trigger the aperiodic DDDS, we need to set report polling bit as one. what's this bit is set as 1 corresponding node will send DDDS message. You can read about DDDS message here.

If the assistance information report polling flag is set as 1 then the corresponding note will send assistance information data.

 DUD is basically an elementary procedure of NRUP protocol defined in 3gpp in 38.425 and 36.425 for 5G and 4G respectively.

Example Packet of DUD:

GPRS Tunneling 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:

Please refer to see : What is difference between NRU Sequence number and PDCP Sequence number? Where to see it in wireshark DUD packets?