What is DDDS - Downlink Data Delivery Status?

DDDS stands for downlink data delivery status. The DDDS is basically an elementary procedure of NRUP protocol defined in 3gpp in 38.425 and 36.425 for 5G and 4G respectively.

So, in my words, when we need to give the feedback of downlink user data i.e DUD, then from the receiving node we need to send send DDDS message.

DUD goes from node hosting NR-PDCP to corresponding node and DDDS goes from corresponding node to the node hosting NR-PDCP to give the information about highest successfully delivered packet or highest successfully transmitted packets, lost packets etc.

 

Here are the main information carried by DDDS message:

1. Desired buffer size of corresponding RLC node.
2. Desired Data Rate.
3. Highest Transmitted NR PDCP SN
4. Highest delivered NR PDCP SN
5. Final frame indication
6. Lost packet report (start and end range)
7. Highest produced transmitted NR-PDCP SN.
8. Highest delivered to retransmitted NR-PDCP SN.

 

 Here are snippet taken from 3GPP spec 38.425 release 15.2

This is a DDDS packet example:

GPRS Tunneling Protocol

    Flags: 0x34

        001. .... = Version: GTP release 99 version (1)

        ...1 .... = Protocol type: GTP (1)

        .... 0... = Reserved: 0

        .... .1.. = Is Next Extension Header present?: Yes

        .... ..0. = Is Sequence Number present?: No

        .... ...0 = Is N-PDU number present?: No

    Message Type: T-PDU (0xff)

    Length: 16

    TEID: 0x00000001 (1)

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

    Extension header (NR RAN container)

        Extension Header Length: 3

        NR RAN Container

            NRUP

                0001 .... = PDU Type: DL Data Delivery Status (1)

                .... 0... = Highest Transmitted NR PDCP SN Ind: No

                .... .1.. = Highest Delivered NR PDCP SN Ind: Yes

                .... ..0. = Final Frame Indication: Frame is not final

                .... ...0 = Lost Packet Report: No

                000. .... = Spare: 0

                ...0 .... = Delivered NR PDCP SN Range Ind: No

                .... 0... = Data Rate Ind: No

                .... .0.. = Highest Retransmitted NR PDCP SN Ind: No

                .... ..0. = Highest Delivered Retransmitted NR PDCP SN Ind: No

                .... ...0 = Cause Report: No

                Desired buffer size for the data radio bearer: 12345

                Highest Successfully Delivered NR PDCP SN: 789

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

TEID : TEID of the receiving end

Next extension header type : 0x84 - NR RAN container 

Highest Transmitted NR PDCP SN Ind : Flag indicating that Highest Transmitted NR PDCP SN is present or not. Applicable only for UM mode.

Highest Delivered NR PDCP SN Ind : Flag indicating that Highest Delivered NR PDCP SN is present or not. Applicable only for AM mode.

Final Frame Indication : If this flag is set, it means that this is the last DDDS message. Do not expect any further DDDS for this TEID.

Lost Packet Report: This flag indicates that whether the lost packet report is present in this message or not.

Delivered NR PDCP SN Range Ind: this flag indicates the presence of Number of successfully delivered out of sequence PDCP Sequence Number range, Start of successfully delivered out of sequence PDCP Sequence Number range and End of successfully delivered out of sequence PDCP Sequence Number range.

Data Rate Ind : This flag indicates whether Desired Data Rate is present or not.

Highest Retransmitted NR PDCP SN Ind:  This flag indicates whether Highest Retransmitted NR PDCP SN is present or not.

Highest Delivered Retransmitted NR PDCP SN Ind:  Flag indicating whether Highest Delivered Retransmitted NR PDCP SN is present or not.

Cause Report: If Cause Value is present or not.

Next extension header type: Whether any further extension header is present or not.

Based on these flags the the corresponding field values would be present.