What is NRUP Protocol ?

NRUP is one of the widely talked protocol in 5G-NR. It is even applicable if you are working with Split bearer or NSA mode in 5G.

So, Let's understand What is NRUP protocol? What is the use of NRUP protocol in LTE and 5G?

NRUP stands for “New Radio User Plane” Protocol. Before going to understand what exactly NRUP is, I hope you know about 5G architecture. I strongly recommend to read this post before proceeding: How does DL data travel in 5G-NSA mode with Split Bearer?

The below diagram is a truncated diagram from the 5G-NSA mode with Split Bearer from the above link.

So, what do you think, Where is the NRUP protocol located? Between CU - DU or between NR-PDCP & RLC ?  

NRUP is between the node hosting NR-PDCP and the corresponding node. The corresponding node can be DU, neighbour eNB, neighbour gNB i.e over F1U, X2U or XnU interface respectively for downlink data flow. As per 3GPP's word taken from 38.425:

The NR user plane protocol layer is using services of the transport network layer in order to allow flow control of user data packets transferred from the node hosting NR PDCP to the corresponding node.

The next question will come to the mind is that, Why do we need NRUP? Isn't GTP protocol enough for sending data from one node to another?

The Answer is, No. GTP doesn't have any mechanism to allow flow control of user data packets. We cannot track whether the data is successfully delivered or not, what are missing packets or the retransmission of a packet, how much do we need to send etc. Hence we need a NRUP protocol. We indicate the presence of NRUP header as a part of GTP-U extension header by means of "NR RAN Container".

Does it work with RLC AM or RLC UM?

NRUP works for both the type of flow RLC AM as well as RLC UM too.

Is it UE level or Radio Bearer (RB) level? When do we configure/setup the NRUP protocol?

NRUP protocol is per RB level, means, each NRUP  instance would be associated with only one radio bearer. At the time of creating tunnels (eg. tunnels between CU & DU or between CU & eNB or forwarding tunnel in case of Xn HO), we need to create an NRUP instance associated with it.

NRUP works for UL data also?

3GPP spec says NRUP is for the data packets transferred from the node hosting NR PDCP to corresponding node as highlighted above, which means it is only applicable for DL data not for UL data.

NRUP uses two elementary procedures to do its job:

        a. Downlink User Data

        b. Downlink Data Delivery Status

        c. Transfer of Assistance Information  

You can on each of the procedure to read in details about them.

To active the flow control of the user data from the sender node (node hosting NR PDCP) to the receiver node (corresponding node), each Downlink User Data (DUD) packets are marked with NR Sequence Number. It is different from PDCP Sequence number. You can read about "What is difference between NRU Sequence number and PDCP Sequence number?" in details.

What are the services provided by NRUP protocol?

 These are few services listed as per 3GPP 38.425 v16.1:

a. Addition of NR-SN in DUD packets.

b. Information of successfully delivered (in RLC AM mode) or transmitted (in RLC UM mode) of transmitted packets.

c. Information regarding lost (DUD) packets.

d. Information regarding the packets needs to be discarded at RLC level.

e. Desired buffer size indication to NR-PDCP

f. Desired data rate indication to NR-PDCP

g. Information of successfully delivered (in RLC AM mode) or transmitted (in RLC UM mode) of retransmitted packets.

h. Radio link related information & QoS related information.

This is the generic structure of frame of NRUP protocol taken from 3GPP 38.425 v16.1

Spare fields should be filled with 0.

Payload part is octet aligned with the help of padding.

Total size of the frame shall not exceed 1018 octets.