This clause describes the state variables used in PDCP entities in order to specify the PDCP protocol. The state variables defined in this clause are normative. All state variables are non-negative integers, and take values from 0 to [2³² - 1]. PDCP Data PDUs are numbered integer sequence numbers (SN) cycling through the field: 0 to [2^{[pdcp-SN-SizeUL]} - 1] or 0 to [2^{[pdcp-SN-SizeDL]} - 1] or 0 to [2^{[sl-PDCP-SN-Size]} - 1]. The transmitting PDCP entity shall maintain the following state variables:

1. TX_NEXT
   This state variable indicates the COUNT value of the next PDCP SDU to be transmitted. The initial value is 0, except for SRBs configured with state variables continuation. For target SRB configured with state variables continuation, the initial value is the value stored in PDCP entity for the corresponding source SRB. For source SRB configured with state variables continuation, the initial value is the value stored in PDCP entity for the corresponding target SRB.

The receiving PDCP entity shall maintain the following state variables:

1. RX_NEXT
   This state variable indicates the COUNT value of the next PDCP SDU expected to be received. The initial value is 0, except for sidelink broadcast and groupcast, for SRBs configured with state variables continuation, for multicast MRBs whose PDCP COUNT is not synchronized as indicated by upper layer, and for broadcast MRBs. For NR sidelink communication for broadcast and groupcast or sidelink SRB4 for NR sidelink discovery, the initial value of the SN part of RX_NEXT is (x +1) modulo (2^{[sl-PDCP-SN-Size]}), where x is the SN of the first received PDCP Data PDU. For multicast MRBs whose PDCP COUNT is not synchronized as indicated by upper layer, and for broadcast MRBs, the initial value of the SN part of RX_NEXT is (x +1) modulo (2^{[PDCP-SN-SizeDL]}), where x is the SN of the first received PDCP Data PDU. For target SRB configured with state variables continuation, the initial value is the value stored in PDCP entity for the corresponding source SRB. For source SRB configured with state variables continuation, the initial value is the value stored in PDCP entity for the corresponding target SRB.
2. RX_DELIV
   This state variable indicates the COUNT value of the first PDCP SDU not delivered to the upper layers, but still waited for. The initial value is 0, except for sidelink broadcast and groupcast, for SRBs configured with state variables continuation, and for MRBs. For NR sidelink communication for broadcast and groupcast or sidelink SRB4 for NR sidelink discovery, the initial value of the SN part of RX_DELIV is (x - 0.5 × 2^{[sl-PDCP-SN-Size-1]}) modulo (2^{[sl-PDCP-SN-Size]}), where x is the SN of the first received PDCP Data PDU. For multicast MRBs whose PDCP COUNT is not synchronized as indicated by upper layer, and for broadcast MRBs, the initial value of the SN part of RX_DELIV is set to (x - 0.5 × 2^{[PDCP-SN-SizeDL-1]}) modulo (2[PDCP-SN-SizeDL]), where x is the SN of the first received PDCP Data PDU. For multicast MRBs, the initial value of RX_DELIV is set, if provided, by initialRX-DELIV in TS 38.331. For target SRB configured with state variables continuation, the initial value is the value stored in PDCP entity for the corresponding source SRB. For source SRB configured with state variables continuation, the initial value is the value stored in PDCP entity for the corresponding target SRB.
3. RX_REORD
   This state variable indicates the COUNT value following the COUNT value associated with the PDCP Data PDU which triggered t-Reordering. For target SRB configured with state variables continuation, the initial value is the value stored in PDCP entity for the corresponding source SRB. For source SRB configured with state variables continuation, the initial value is the value stored in PDCP entity for the corresponding target SRB.

1. Window_Size This constant indicates the size of the reordering window. The value equals to 2^{[pdcp-SN-SizeDL] - 1} for SRB/DRB/MRB and 2^{[sl-PDCP-SN-Size] - 1} for SLRB.

The transmitting PDCP entity shall maintain the following timers:

1. discardTimer
   This timer is configured only for DRBs. The duration of the timer is configured by upper layers TS 38.331. In the transmitter, a new timer is started upon reception of an SDU from upper layer as specified in clause 5.2.1.
2. discardTimerForLowImportance
   This timer is configured only for DRBs. The duration of the timer is configured by upper layers TS 38.331. In the transmitter, a new timer is started upon reception of an SDU belonging to a low importance PDU Set from upper layer as specified in clause 5.2.1.
   The receiving PDCP entity shall maintain the following timers:
3. t-Reordering
   The duration of the timer is configured by upper layers TS 38.331, except for the case of NR sidelink communication or sidelink SRB4. For NR sidelink communication or sidelink SRB4, the t-Reordering timer is determined by the UE implementation. This timer is used to detect loss of PDCP Data PDUs as specified in clause 5.2.2. If t-Reordering is running, t-Reordering shall not be started additionally, i.e. only one t-Reordering per receiving PDCP entity is running at a given time.