PDU Number flag (PN) indicates the presence of a meaningful value of the N-PDU Number field. When present, this field is used in legacy (2G/3G) networks for support of RLC Acknowledged Mode (AM) mode handovers. Message Type-this field indicates the type of GTP-U message. 3GPP 29.060 defines the complete list of message types. Message types are included for path management (echo Open Radio Access Network (O-RAN) Systems Architecture and Design. DOI: https://doi.org/10.1016/B978-0-323-91923-4.00009-4 © 2022 Elsevier Inc. All rights reserved. 216 Open Radio Access Network (O-RAN) Systems Architecture and Design 8 bits Version Message Type Length Length Sequence Number ( Sequence Number N-PDU Number Next Extension Header Type FIGURE 6-1 GTP-U PDU Format. 8 bits PDU Type (DL=0, UL=1) Reserved Spare RQI (DL only) QoS Flow Identifier Padding to make PDU (n*4-2) octets (0..3 Bytes) Padding to make PDU (n*4-2) octets (0..3 Bytes) FIGURE 6-2 PDU session information format. request/response), tunnel creation, modification and deletion, and various error and other management uses. Length indicates the length of all payload (including extension headers) beyond the 8 bytes of the 8 mandatory header bytes. TEID identifies a tunnel endpoint in the receiving GTP-U protocol entity. The receiving end side of a GTP tunnel locally assigns the TEID value the transmitting side shall use, in a randomized manner. The "all zeroes" value shall not be used. The GTP protocol defines payload a T-PDU, typically an IP packet (datagram) to be trans- mitted between the client (UE) and the Internet (external packet network). A G-PDU is the definition of the combined T-PDU + GTP header (the complete datagram including GTP encapsulation). Note how GTP (and IPSec) headers add to the size of the IP packet being transported over the backhaul interface, which may (when Maximum Transfer Unit (MTU) sizes are badly configured) lead to high percentages of fragmentation of IP traffic over the mid/backhaul interface. Fragmentation is both costly to implement 