Re: Add Sigfox support to libpcap

[Guy Harris <gharris () sonic net>]

On Jan 10, 2019, at 10:17 AM, Jeija <jeija () jeija net> wrote:

> >      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?

Where *else* would it go in a pcap file?

There *could* be additional metadata in the form of options in a pcapng file, but there's no such option for pcap.  
802.11 has multiple link-layer header extensions for radio information, such as radiotap.

> >      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.

So are you saying that the first byte of frame data should have the first 8 bits of the Type, with the next byte having 
4 bits of Type and 4 bits of Flags?

> Thank you two for your work, I'd be very happy to see this being integrated into libpcap :) !

"Integrated into libpcap" or "integrated into the link-layer header types page on tcpdump.org"?

If you're just specifying a link-layer header type, the key part is "integrated into the link-layer header types page 
on tcpdump.org".  Putting into pcap/dlt.h and pcap-common.c just acknowledges the addition to the link-layer header 
type page.
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