Re: Request for link layer header type

[Guy Harris <guy () alum mit edu>]

On Feb 8, 2017, at 10:38 PM, Erik de Jong <erikdejong () gmail com> wrote:

> I would like to request a link layer header type for encapsulation of LoRa
> packets. It will be used to encapsulate raw packets as logged directly from
> the air. As usage of this link layer header type will be similar to the
> radiotap header, I've picked the name LoRaTap for the encapsulation.
>
> LoRa is a protocol for IoT, it's a proprietary protocol by Semtech. Specs
> can be found at http://www.semtech.com/wireless-rf/rf-transceivers/sx1272/

Specs can be found there, but I couldn't find many specs relevant to assigning a link-layer header type; all I saw 
there was a bunch of PHY-layer stuff, and that doesn't show up in captures - it's typically the MAC layer.

A *useful* spec can be found at

        https://www.lora-alliance.org/portals/0/specs/LoRaWAN%20Specification%201R0.pdf

Presumably the packets in the capture will look like what's specified in section 4, with the first octet being the 
1-octet MAC header field (bit 0 is the low-order bit of an octet and bit 7 is the high-order bit of an octet, right?), 
followed by the MAC payload, possibly followed by the Message Integrity Code - will packets have the MIC or not?

> My initial idea for the encapsulation header can be found at
> https://github.com/eriknl/LoRaTap

So that header will be at the beginning of the packet, followed immediately by the packet data, beginning with the MAC 
header field?

Are the multi-byte fields in that header big-endian, little-endian, or host-endian?

What are the units of the RSSIs?  dBm?  What is the difference between "packet RSSI" and "receiver RSSI"?

Is the signal/noise ration given as a percentage, some form of floating-point value, or something else?

What are the units of the frequency field?  Hz?  KHz?  Something else?

What is the "SF"?  One of the Semtech specs that might be relevant here is

        http://www.semtech.com/images/datasheet/an1200.22.pdf

which says "SF = spreading factor (7..12)" - is that the "SF"?

Why is there a length field?  A pcap or pcapng file already *has* a packet length, which would be the sum of the length 
of the LoRa metadata header (16, in the example on your site) plus 1 for the MAC header plus the length of the MAC 
payload plus 4 if the MIC is included.  Wouldn't that make a packet length field redundant?
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