SDU discard
For DRB, When the discardTimer expires for a PDCP SDU, or the successful delivery of a PDCP SDU is confirmed by PDCP status report, the transmitting PDCP entity shall discard the PDCP SDU along with the corresponding PDCP Data PDU.
For SRBs, when upper layers request a PDCP SDU discard, the PDCP entity shall discard all stored PDCP SDUs and PDCP PDUs.
Status reporting
Once a PDCP SDU has been sent and has been received by peer RLC entity, but the corresponding RLC confirmation has not been received. To avoid re-transmissions, UE can inform the network side of the packet reception status through PDCP status report.
Transmit operation
if RRC is configured with status reports, the PDCP status report needs to be triggered to receive PDCP:
– upper layer requests a PDCP entity re-establishment;
– upper layer requests a PDCP data recovery.
If a PDCP status report is triggered, the receiving PDCP entity shall:
– setting the FMC field to RX_DELIV;
– allocating a Bitmap field of length in bits equal to the number of COUNTs from and not including the first missing PDCP SDU up to and including the last out-of-sequence PDCP SDUs, rounded up to the next multiple of 8, or up to and including a PDCP SDU for which the resulting PDCP Control PDU size is equal to 9000 bytes, whichever comes first;
– setting in the bitmap field as ‘1’ for all PDCP SDUs that have been received;
Receive operation
– consider for each PDCP SDU, if any, with the bit in the bitmap set to ‘1’, or with the associated COUNT value less than the value of FMC field as successfully delivered, and discard the PDCP SDU
Source TS 38 323