Compared to other Smart Metering messages, firmware images are large. Further, each image is likely to be applicable to a significant number of Devices. Thus, an end-to-end, unicast Message to each affected Device, with each message containing a copy of the image, is not efficient from a WAN perspective.
This leads to the approach for firmware update process being separated into two stages:
a separate and subsequent ‘activation’ Command to each Device.
The Distribute Firmware Command is not a Critical Command (since it does not affect the operating firmware) and does not need to be unicast.
The Activate Firmware Command is a Critical Command and so must be unicast – as it must be digitally signed and be for one, and only one specified Device. Further, the Activate Command must apply to one, and only one, image and that image must have originated from the same party that signs the Activate Firmware Command (that is, the party responsible for that Device). To meet these requirements:
the Activate Firmware Command is of type SME.C.C and so the Signature and MAC on the Command shall have been verified by the Device prior to the Hash validation (see next bullet); and
a Device receiving an Activate Firmware Command shall calculate a Hash over the Manufacturer Image it holds and ensure the Hash so calculated matches that in the Activate Firmware Command, before the Device attempts to activate the corresponding Manufacturer Image.
The GBCS does not constrain the mechanisms used by Device manufacturers to ensure that only valid Manufacturer Images are activated on Devices manufactured by them. The GBCS does require that the manufacturer information related to a Manufacturer Image is made available, so that the Upgrade Image and the ZigBee Over-The-Air (OTA) Header can be provided when requesting distribution of an image.
In common with other Messages, the GBCS shall not constrain the mechanisms by which the firmware Messages are transported to the Communications Hub. The GBCS constrains HAN transport mechanisms to those provided by ZSE.