1. Introduction

1. Introduction

The Generic Framework for Static Context Header Compression (SCHC) and Fragmentation specification [RFC8724] describes two mechanisms: i) a protocol header compression scheme and ii) a frame fragmentation and loss recovery functionality. Either can be used on top of radio technologies, such as the four Low-Power Wide Area Networks (LPWANs) listed in [RFC8376], which are Sigfox, LoRaWAN, NB-IoT, and IEEE 802.15.4w. These LPWANs have similar characteristics, such as star-oriented topologies, network architecture, and connected devices with built-in applications.

SCHC offers a great level of flexibility to accommodate all these LPWAN technologies. Even though there are a number of similarities between them, some differences exist with respect to the transmission characteristics, payload sizes, etc. Hence, there are optimal parameters and modes of operation that can be used when SCHC is used on top of a specific LPWAN technology.

In ACK-on-Error mode in [RFC8724], the SCHC Packet is fragmented into pieces called tiles, where all tiles are the same size except for the last one, which can be smaller. Successive tiles are grouped in windows of fixed size. A SCHC Fragment carries one or several contiguous tiles, which may span multiple windows. When sending all tiles from all windows, the last tile is sent in an All-1 SCHC Fragment. The SCHC receiver will send a SCHC ACK reporting on the reception of exactly one window of tiles after receiving the All-1 SCHC Fragment. In case of SCHC Fragment losses, a bitmap is added to the failure SCHC ACK, where each bit in the bitmap corresponds to a tile in the window. If SCHC Fragment losses span multiple windows, the SCHC receiver will send one failure SCHC ACK per window with losses.

This document updates the SCHC protocol for frame fragmentation and loss recovery. It defines a SCHC Compound ACK format and procedure, which are intended to reduce the number of response transmissions (i.e., SCHC ACKs) in the ACK-on-Error mode of SCHC. The SCHC Compound ACK extends the failure SCHC ACK message format so that it can contain several bitmaps, with each bitmap being identified by its corresponding window number. The SCHC Compound ACK is backwards compatible with the SCHC ACK as defined in [RFC8724], and introduces flexibility, as the receiver has the capability to respond to the All-0 SCHC Fragment, providing more Downlink opportunities and therefore adjusting to the delay requirements of the application.