Scott’s Weblog

Thoughts on libtrace wireless API and radiotap

April 25th, 2007

With libtrace 3.0 we included an API for extracting wireless metadata from packets. So for example, you can call trace_get_wireless_signal_strength_dbm() on a libtrace packet and get it’s absolute signal strength. This is done by decoding the Radiotap monitoring header if present. This is all fine for physical layer attributes, such as signal and noise levels, but the abstraction starts to get fuzzy when it comes to link-layer specific stuff. For example, libtrace 3.0 released with trace_get_wireless_fcs() which extracted the 802.11 FCS from the Radiotap header (even though this was a non-standard field and has since been removed). The problem is, trace_get_wireless_* shouldn’t be specific to certain MAC layer protocols. What if a CRC-16 is used instead of a CRC-32? trace_get_wireless_fcs() has since been removed, but the point also applies to some of the other functions.

So some of the functions as they exist now extract physical layer attributes, and others extract MAC layer attributes. Since libtrace was released, the Radiotap standard has been updated to include a couple of extra fields, such as the number of retries a packet had. Should a new accessor be added to libtrace to extract this? I’d say no, even though it’s a very interesting piece of data. Keep the trace_get_wireless_* functions as generic ways to get physical layer attributes of wireless frames. Let the user decode the Radiotap header in full if they want the 802.11 specific stuff.

Turns out I’m one of those users, so I’ve created a stand-alone Radiotap decoder in C which can extract all the Radiotap fields. If a new Radiotap field is added that describes an interesting physical layer attribute, then maybe an accessor can be added to libtrace for it, but for MAC layer specific fields a stand-alone Radiotap decoder should be used. This should hopefully keep libtrace as generic as possible.

Download version 0.1 of my C Radiotap decoder if you’re interested. Maybe I’ll get around to uploading it to Google Code Hosting at some point in the future.

Entry Filed under: libtrace, radiotap