 datagrams with IPv6/UDP/RTP headers compressor Radio link decompressor Reference header Reference header Figure 2.15 Example ROHC compression/decompression of RTP/UDP/IP headers for communication over a radio link 16]. reaches the maximum number of retransmissions for a PDCP PDU, the UE informs the gNB, but does not trigger RLF³ 3 [8]. 2.2.3.2 Header Compression Function There are multiple header compression algorithms referred to as profiles in 3GPP speci- fications [11]. Each profile is specific to a particular network layer, transport layer, or upper layer protocol combination, for example, TCP/IP or RTP/UDP/IP. The PDCP enti- ties associated with DRBs carrying user-plane data can be configured by upper layers to use header compression. Each PDCP entity uses at most one ROHC compressor instance and at most one ROHC decompressor instance. As we mentioned earlier, the ROHC algorithm reduces the size of transmitted RTP/UDP/IP header by removing the redundancies. This mechanism starts by classifying header fields into different classes according to their variation pattern. The fields that are classified as inferred are not sent. The static fields are sent initially and then are not sent anymore and the fields with varying information are always sent. The ROHC mechanism is based on a context, 4 which is maintained, by both ends, that is, the compressor and the decompressor (see Fig. 2.15). The context encompasses the entire header and ROHC information. Each context has a context ID, which identifies the flows. The ROHC scheme operates in one of the following three operation modes [16]: The UE declares a RLF when one of the following criteria is met: (1) Expiry of a timer started after indication of radio problems from the physical layer (if radio problems are recovered before the timer expires, the UE stops the timer), random-access procedure failure, or RLC failure. After an RLF is declared, the UE stays in RRC_CONNECTED and selects a suitable cell and then initiates RRC connection reestablishment. The UE further