(CPU cycles) and prone to packet loss (the whole IP packet is discarded when a single fragment is lost) and as such should be avoided. The GTP proto- col implementation should as such support Path MTU Discovery (MTUD) in its implementation. Chapter 6 User-plane application components 217 GTPvl-U tunnel endpoints do not need to change the hop count/or Time To Live (TTL) values in the IP header or to perform any IP routing. PDCP protocol As we explained before, the main role of the Packet Data Convergence Protocol (PDCP) layer is conversion of end-to-end (typically, IP, but could be Ethernet or other protocols such as in URLLC) to a packet that can be transmitted over-the-air, including header compression, authentication/ciphering and re-ordering and duplicate detection through sequence number addition. A PDCP header is added that contains the following fields: Sequence Number (SN)-this allows the receiver to support in-sequence delivery of the packet to the upper layer including duplicate detection. PDCP sequence numbers 12 bits wide [UM DRBs, AM Data Radio Bearers (DRBs), and Signaling Radio Bearers (SRBs)] or 18 bits wide (UM DRBs and AM DRBs). Data/Control (D/C) bit to differentiate Data and Control PDUs. Control means PDCP control packets and not SRBs. The MAC-I field which carries a message authentication code calculated for integrity protection using the AES, SNOW, and ZUC algorithms as specified in 3GPP 33.501. For SRBs, the MAC-I field is always present and set to 0 if integrity protection is not configured. For DRBs, the MAC-I field is only present when the bearer is configured for integrity protection. Additional bits associated with PDCP CONTROL PDUs that are used to carry either PDCP status reports of interspersed RoHC feedback PDU Type-used to separate PDCP status report from interspersed ROHC feedback Frame Missing Count (FMC) indicates the COUNT value of the first missing PDCP SDU within the reordering window Bitmap to define missing SDUs 8 bits PDCP SN PDCP SN (cont.) MAC-I (optional) MAC-I (cont.)