Re: Add Sigfox support to libpcap

[Jeija <jeija () jeija net>]

Author of the "renard" Sigfox specifications here.
Just to give you and everyone else reading this some background: These specifications are not "official" documentation 
by Sigfox, but the result of my protocol analysis that I recently presented at 35C3 ("Hunting the Sigfox: Wireless IoT 
Network Security").

>       1) Would the link-layer header include any radio metadata?  For 802.11, there are various forms of radio 
> metadata headers, such as the radiotap header:
>
>               http://www.radiotap.org
>
>          If so, what would the format of the radio metadata be?

I would suggest something similar to what the LoRa people did with LoRaTap: https://github.com/eriknl/LoRaTap
For the uplink, except for version / padding / length of course, I don't think we need anything other than the uplink 
frequency (Sigfox uses a continuous uplink band) and an RSSI value.
For the downlink we should again have frequency and RSSI, but maybe also some metadata (sequence number / device ID) 
about the corresponding uplink (Sigfox is an uplink-initiated protocol, i.e. downlinks are only transmitted after they 
have been requested by a corresponding uplink and their scrambling depends on the uplink metadata, something that is 
described in patent EP3259864A1).
But I'm not sure whether the link-layer header is the correct place for this kind of information?

>       2) The 2.2.2 "Implementation by Sigfox" section of the spec shows, on page 14, a frame structure.
>
>          In the captures, is the first byte of frame data (following the radio metadata, if there is radio metadata, 
> or at the beginning of the packet, if there is no radio metadata), the first byte of the Preamble/Type?

IMO the first byte of frame data should be the first byte of the "Type", since the "Preamble" is constant and just 
something that is used to detect the presence of a Sigfox uplink inside the uplink band and to synchronize the receiver.
At least, that is how I currently handle it with renard-phy.

>       3) Is the CRC-16 included with the frames?

Yes, it's included in the uplink transmission.

Thank you two for your work, I'd be very happy to see this being integrated into libpcap :) !
If you happen to have any further questions, feel free to ask!

- Florian Euchner (Jeija)
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