LTE: Why User plane PDCP Data PDU has D/C bit in the packet header ?

Question

I found 1st bit which is used to indicate that packet carrying Data/Control information. My question is that why first bit is used even packet itself known as "PDCP Data PDU".

Comments

First of all I would like mention the difference between PDCP PDU formats.
PDCP Data PDUs and PDCP Control PDUs.
PDCP Control PDU - Carries controlling information such as ROHC Feedback and PDCP Status report.
These information goes on U-Plane, and there is no ciphereing , integrity protection, assigning sequence numbers to these.

PDCP Data PDU - Can be Control plane data(SRBs) and User plane  data(DRBs).

Dont get confused between control plane data and control PDU.
Control plane data means (For signalling purposes)- SRB1, SRB2

And the PDCP Control PDUs:-
PDCP Control PDU - Carries controlling information such as ROHC Feedback and PDCP Status report.
These information goes on U-Plane.

PDCP Data PDU - Can be Control plane data(SRBs) and data pdu(DRBs).

D/C - This is used to distinguish between "PDCP data PDU s and PDCP Control PDUs.
 
For SRBs there is MAC-I (32 bits) and PDCP SN -5 in the header.

Answer 1

D/C bit is used to know whether the PDU is for user data or pdcp control data. .

This control data does not mean C-Plane data. It is control data being used in PDCP layer.
(Source: Share Technote )

As control messages go through the PDCP so for this we will take it as control PDU
and other data which is data PDU for PDCP.

we apply integrity protection only to the control pdu and Herader compression to only data PDU,But ciphering/deciphering is applied to both control and data PDU.

PDCP layer view u can see from this picture

Comments

First of all I would like mention the difference between PDCP PDU formats.
PDCP Data PDUs and PDCP Control PDUs.
PDCP Control PDU - Carries controlling information such as ROHC Feedback and PDCP Status report.
These information goes on U-Plane, and there is no ciphereing , integrity protection, assigning sequence numbers to these.

PDCP Data PDU - Can be Control plane data(SRBs) and User plane  data(DRBs).

Dont get confused between control plane data and control PDU.
Control plane data means (For signalling purposes)- SRB1, SRB2

And the PDCP Control PDUs:-
PDCP Control PDU - Carries controlling information such as ROHC Feedback and PDCP Status report.
These information goes on U-Plane.

PDCP Data PDU - Can be Control plane data(SRBs) and data pdu(DRBs).

D/C - This is used to distinguish between "PDCP data PDU s and PDCP Control PDUs.
 
For SRBs there is MAC-I (32 bits) and PDCP SN -5 in the header.