# Evaluation of 6 LoWPAN Generic Header Compression in the Context of a RPL Network
## 2 Related Work  
Modified and Improved Header Compression (MIHC) [7] relies on the fact that many IPv6 packets sent by a given node are usually transmitted to the same destination, meaning that many packets contain the same information in their headers. The core concept is to remove redundant fields in IPv6 headers of correlated packets. To do so, the first packet of a sender node contains an uncompressed IPv6 header. A receiver node will generate a Unique Pair Identifier (UPI), a random and unique 8-bit number, and will store the IPv6 header along with the UPI in a table that links them. Afterwards, the receiver node will transmit the UPI to the sender node. Subsequent packets transmitted by that sender node to the same receiver node will use the UPI instead of the full IPv6 header. This approach increases the throughput significantly, mainly for bigger packets that would otherwise need fragmentation. In recent years, the IETF worked on Static Context Header Compression (SCHC), a replacement for 6 LoWPAN that was discussed by [PERSON] et al. [8]. It relies on a static context shared between the sender and receiver, simplifying the compression process. Compression rules known by each node in the network define the operation. When transmitting a packet, the compressor selects the appropriate rule to compress the header, replacing it with an identifier. The standard also defines a fragmentation mechanism, to split larger packets into fragments that fit within the maximum frame size. The SCHC fragmentation is specifically designed for Low-Power Wide Area Networks (LPWANs)' star topology and very small payload sizes. Moreover, SCHC offers reliability by including the No-ACK, ACK-always and ACK-on-Error modes. The benefits of using SCHC have also been studied by [PERSON] et al. [9]. They compared the overhead of the SCHC compression and fragmentation mechanism with 6 LoWPAN compression, showing that it is the only protocol that allows the use of end-to-end IPv6 connectivity on devices using SigFox, LoRa and DASH-7. One limitation of the SCHC protocol is that the context is static and must be distributed before deployment. [PERSON] et al. [10] introduce two context registration mechanisms for SCHC networks. The first mechanism extends the SCHC protocol with an adaptation layer, which is responsible for context registration. The selection of the appropriate action is determined based on specific rules and analysis of incoming packets. Another approach for managing SCHC context configuration involves using the Internet Control Message Protocol for IPv6 (ICMPv6) control messages. An SCHC Control Message is defined to facilitate the configuration of rules for both communication ends. The authors show that the proposed solutions for context registration can lead to a reduced energy consumption and enable resource-constrained devices to self-register and establish their context with the corresponding network gateway. Robust Header Compression (ROHC) is another compression technique designed to reduce the overhead of network and transport layer headers. It was developed by [PERSON] et al. [11] to address the challenges in scenarios with a limited bandwidth and lossy links. ROHC achieves compression by identifying and encoding redundancies in packet headers. Initially, packets are sent uncompressed, but after the redundancy identification and encoding, only the difference between their headers is sent. A context is kept by both the compressor and decompressor. ROHC can handle packet loss and packet corruption by including a mechanism that ensures that the decompression remains reliable. Although ROHC is a powerful header compression technique, it can be challenging to implement it on resource-constrained devices, because ROHC requires a significant computational effort. Table 1 summarises the compression formats discussed in this section. There are other compression techniques for protocols in the context of 6 LoWPAN. [PERSON] [12] wrote a draft proposal for a compression format for ICMPv6 headers. However, it was never finished.