Episode Details

This week on BSD Now, Adrian Chadd on bringing up 802.11ac in FreeBSD, a PFsense and OpenVPN tutorial, and we talk about an interesting ZFS storage pool checkpoint project.

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Headlines

Bringing up 802.11ac on FreeBSD

• Adrian Chadd has a new blog post about his work to bring 802.11ac support to FreeBSD
• 802.11ac allows for speeds up to 500mbps and total bandwidth into multiple gigabits

The FreeBSD net80211 stack has reasonably good 802.11n support, but no 802.11ac support. I decided a while ago to start adding basic 802.11ac support. It was a good exercise in figuring out what the minimum set of required features are and another excuse to go find some of the broken corner cases in net80211 that needed addressing.
802.11ac introduces a few new concepts that the stack needs to understand. I decided to use the QCA 802.11ac parts because (a) I know the firmware and general chip stuff from the first generation 11ac parts well, and (b) I know that it does a bunch of stuff (like rate control, packet scheduling, etc) so I don't have to do it. If I chose, say, the Intel 11ac parts then I'd have to implement a lot more of the fiddly stuff to get good behaviour.
Step one - adding VHT channels. I decided in the shorter term to cheat and just add VHT channels to the already very large ieee80211_channel map. The linux way of there being a channel context rather than hundreds of static channels to choose from is better in the long run, but I wanted to get things up and running. So, that's what I did first - I added VHT flags for 20, 40, 80, 80+80 and 160MHz operating modes and I did the bare work required to populate the channel lists with VHT channels as well.
Then I needed to glue it into an 11ac driver. My ath10k port was far enough along to attempt this, so I added enough glue to say "I support VHT" to the ic_caps field and propagated it to the driver for monitor mode configuration. And yes, after a bit of dancing, I managed to get a VHT channel to show up in ath10k in monitor mode and could capture 80MHz wide packets. Success!

By far the most fiddly was getting channel promotion to work. net80211 supports the concept of dumb NICs (like atheros 11abgn parts) very well, where you can have multiple virtual interfaces but the "driver" view of the right configuration is what's programmed into the hardware. For firmware NICs which do this themselves (like basically everything sold today) this isn't exactly all that helpful. So, for now, it's limited to a single VAP, and the VAP configuration is partially derived from the global state and partially derived from the negotiated state. It's annoying, but it is adding to the list of things I will have to fix later.
the QCA chips/firmware do 802.11 crypto offload. They actually pretend that there's no key - you don't include the IV, you don't include padding, or anything. You send commands to set the crypto keys and then you send unencrypted 802.11 frames (or 802.3 frames if you want to do ethernet only.) This means that I had to teach net80211 a few things:

• frames decrypted by the hardware needed to have a "I'm decrypted" bit set, because the 802.11 header field saying "I'm decrypted!" is cleared
• frames encrypted don't have the "i'm encrypted" bit set
• frames encrypted/decrypted