RFC 4944: Per-Hop Fragmentation and Reassembly Issues

Abstract

The first Internet Engineering Task Force (IETF) Working Group (WG) on Internet of Things (IoT), IPv6 over Low power WPAN (6LoWPAN), was established in 2005 to enable mechanisms to transmit Internet Protocol version 6 (IPv6) packets over IEEE 802.15.4. Since IPv6 supports packet sizes larger than the IEEE 802.15.4 maximum frame size, 6LoWPAN WG defined an adaptation layer by standardizing several documents such RFC 4944, RFC 6282 and RFC 6775. RFC 4944 describes the frame format for transmission of IPv6 packets and it defines mechanisms for header compression and fragmentation. RFC 6282 updates RFC 4944, however, it does not reconsider the 6LoWPAN fragment forwarding method. Indeed, considering that IEEE 802.15.4 comes with a Maximum Transmission Unit (MTU) size of 127 bytes, while a IPv6 datagrams with a 1280 byte MTU, an IPv6 packet could be fragmented into more than ten fragments at the 6LoWPAN adaptation layer. Then, using the 6LoWPAN route-over mesh network, the fragmented 6LoWPAN packet must be reassembled at every hop, which eventually causes latency, congestion, interference and packet losses. In this paper, we first provide a thorough background on RFC 4944. We then detail the potential fragment forwarding issues when employing the route over scheme. Finally, we present the ongoing efforts at the IETF to address the undesired issues.