Wireshark packet dissector for GSM 12.21 (A-bis OML)

During the last weeks I've been spending some time to start a wireshark dissector plugin for GSM 12.21, which is the Organization and Maintenance protocol between BSC and BTS. Using this protocol, many aspects of a BTS are configured by the BSC.

I have already implemented the BSC side of 12.21 inside OpenBSC, and OpenBSC contains parsing code and debug logs about what is happening on this protocol. However, I think it is much better to remove most of that debug printing code from OpenBSC and move it into wireshark. Whoever needs per-message debugging, can start wireshark and look at the output - with the advantage of extensive filtering capabilities.

The protocol is quite complex and has many different messages with each their own set of attributes. So the current work is far from being complete, but it's already at a point where it is really useful.

I've put a specific focus on implementing the vendor-specific bits for ip.access, since those are hard to figure out and much more difficult to implement for anyone who hasn't spent as many weeks looking at hexdumps from their Abis-IP protocol as me. Parsing standard 12.21 messages is easy, just read the publicly-available spec and add wireshark code for it.

In case you're interested, the plugin is available from this path in the OpenBSC git tree