Upgrading my home internet; a story of yak shaving

This has ended up longer than I expected. I’ll write up posts about some of the individual steps with some more details at some point, but this is an overview of the yak shaving I engaged in. The TL;DR is:

• I wanted to upgrade my internet connection, but:
• My router wasn’t fast enough, so:
• I bought a new one and:
• Proceeded to help work on mainline Linux support, and:
• Did some tweaking of my Debian setup to allow for a squashfs root, and:
• Upgraded it to Debian 11 (bullseye) in the process, except:
• It turned out my home automation devices weren’t happy, so:
• I dug into some memory issues on my ESP8266 firmware, which:
• Led to diagnosing some TLS interaction issues with the firmware, and:
• I had an interlude into some interrupt affinity issues, but:
• I finally got there.

The desire for a faster connection

When I migrated my home connection to FTTP I kept the same 80M/20M profile I’d had on FTTC. I didn’t have a pressing need for faster, and I saved money because I was no longer paying for the phone line portion. I wanted more, but at the time I think the only option was for a 160M/30M profile instead and I didn’t need it and it wasn’t enough better to convince me.

Time passed and BT rolled out their GigE (really 900M) download option. And again, I didn’t need it, but I wanted it. My provider, Aquiss, initially didn’t offer this (I think they had up to 330M download options available by this point). So I stayed on 80M/20M. And the only time I really wanted it to be faster was when pushing off-site backups to rsync.net.

Of course, we’ve had the pandemic, and that’s involved 2 adults working from home with plenty of video calls throughout the day. The 80M/20M connection has proved rock solid for this, so again, I didn’t feel an upgrade was justified. We got a 4K capable TV last year and while the bandwidth usage for 4K streaming is noticeably higher, again the connection can handle it no problem.

At some point last year I noticed Aquiss had added speed options all the way to 900M down. At the end of the year I accepted a new role, which is fully remote, so I had a bit of an acceptance about the fact that I wasn’t going back into an office any time soon. The combination (and the desire for the increased upload speed) finally allowed me to justify the upgrade to myself.

Testing the current setup for bottlenecks

The first thing to do was see whether my internal network could cope with an upgrade. I’m mostly running Cat6 GigE so I wasn’t worried about that side of things. However I’m using an RB3011 as my core router, and while it has some coprocessors for routing acceleration they’re not supported under mainline Linux (and unlikely to be any time soon). So I had to benchmark what it was capable of routing. I run a handful of VLANs within my home network, with stateful firewalling between them, so I felt that would be a good approximation of the maximum speed to the outside world I might be able to get if I had the external connection upgraded. I went for the easy approach and fired up iPerf3 on 2 hosts, both connected via ethernet but on separate networks, so routed through the RB3011. That resulted in slightly more than a 300Mb/s throughput. Ok. I confirmed that I could get 900Mb/s+ on 2 hosts both on the same network, just to be sure there wasn’t some other issue I was missing. Nope, so unsurprisingly the router was the bottleneck.

So. To upgrade my internet speed I need to upgrade my router. I could just buy something off the shelf, but I like being able to run Debian (or OpenWRT) on the router rather than some horrible vendor firmware. Lucky MikroTik launched the RB5009 towards the end of last year. RouterOS is probably more than capable, but what really interested me was the fact it’s an ARM64 platform based on an Armada 7040, which is pretty well supported in mainline kernels already. There’s a 10G connection from the internal switch to the CPU, as well as a 2.5Gb/s ethernet port and a 10G SFP+ cage. All good stuff. I ordered one just before the New Year.

Thankfully the OpenWRT folk had done all of the hard work on getting a mainline kernel booting on the device; Sergey Sergeev and Robert Marko in particular fighting RouterBoot and producing a suitable device tree file to get everything up and running. I ended up soldering a serial console connection up to aid debugging, and lightly patching Rob’s u-boot to fix the incorrect RAM size reported by RouterBoot. A few kernel tweaks were necessary to make the networking entirely happy and at that point it was time to think about actually doing a replacement.

Upgrading to Debian 11 (bullseye)

My RB3011 is currently running Debian 10 (buster); an upgrade has been on my todo list, but with the impending replacement I decided I’d hold off and create a new Debian 11 (bullseye) image for the RB5009. Additionally, I don’t actually run off the internal NAND in the RB3011; I have a USB flash drive for the rootfs and just the kernel booting off internal NAND. Originally this was for ease of testing, then a combination of needing to figure out a good read-only root solution and a small enough image to fit in the 120M available. For the upgrade I decided to finally look at these pieces. I’ve ended up with a script that will build me a squashfs image, and the initial rootfs takes care of mounting this and then a tmpfs as an overlay fs. That means I can easily see what pieces are being written to. The RB5009 has a total of 1G NAND so I’m not as space constrained, but the squashfs ends up under 50M. I’ve added some additional pieces to allow me to pre-populate the overlay fs with updates rather than always needing to rebuild the squashfs image.

With that done I decided to try it out on the RB3011; I tweaked the build script to be able to build for armhf (the RB3011) or arm64 (the RB5009) and to deal with some slight differences in configuration between the two (e.g. interface naming). The idea here was to ensure I’d got all the appropriate configuration sorted for the RB5009, in the known-good existing environment. Everything is still on a USB stick at this stage and the new device has an armhf busybox root meaning it can be used on either device, and the init script detects the architecture to select the appropriate squashfs to mount.

A problem with ESP8266 home automation devices

Everything seemed to work fine - a few niggles with the watchdog, which is overly sensitive on the RB3011, but I got those sorted (and the build script updated) and the device came up and successfully did the PPPoE dance to bring up external connectivity. And then I noticed that my home automation devices were having problems connecting to the mosquitto MQTT server. It turned out it was only the ESP8266 based devices that were failing, and examining the serial debug output on one of my test devices revealed it was hitting an out of memory issue (displaying E:M 280) when establishing the TLS MQTT connection.

I rolled back to the Debian 10 image and set about creating a test environment to look at the ESP8266 issues. My first action was to try and reduce my RAM footprint to try and ensure there was enough spare to establish the connection. I moved a few functions that were still sitting in IRAM into flash. I cleaned up a couple of buffers that are on the stack to be more correctly sized. I tried my new image, and I didn’t get the memory issue. Instead I progressed a bit further and got a watchdog reset. Doh!

It was obviously something related to the TLS connection, but I couldn’t easily see what the difference was; the same x509 cert was in use, it looked like the initial handshake was the same (and trying with openssl s_client looked pretty similar too). I set about instrumenting the ancient Mbed TLS used in the Espressif SDK and discovered that whatever had changed between buster + bullseye meant the EPS8266 was now trying a TLS-DHE-RSA-WITH-AES-256-CBC-SHA256 handshake instead of a TLS-RSA-WITH-AES-256-CBC-SHA256 handshake and that was causing enough extra CPU usage that it couldn’t complete in time and the watchdog kicked in. So I commented out MBEDTLS_KEY_EXCHANGE_DHE_RSA_ENABLED in the config_esp.h for mbedtls and rebuilt things. Hacky, but I’ll go back to trying to improve this generally at some point.

A detour into interrupt load

Now, my testing of the RB3011 image is generally done at weekends, when I have enough time to tear down and rebuild the connection rather than doing it in the evening and having limited time to get things working again in time for work in the morning. So at the point I had an image ready to go I pulled the trigger on the line upgrade. I went with the 500M/75M option rather than the full 900M - I suspect I’d have difficulty actually getting that most of the time and 75M of upload bandwidth seems fairly substantial for now. It only took a couple of days from the order to the point the line was regraded (which involved no real downtime - just a reconnection in the night). Of course this happened just after the weekend I’d discovered the ESP8266 issue.

This provided an opportunity to see just what the RB3011 could actually manage. In the configuration I had it turned out to be not much more than the 80Mb/s speeds I had previously seen. The upload jumped from a solid 20Mb/s to 75Mb/s, so I knew the regrade had actually happened. Looking at CPU utilisation clearly showed the problem; softirqs were using almost 100% of a CPU core.

Now, the way the hardware is setup on the RB3011 is that there are two separate 5 port switches, each connected back to the CPU via a separate GigE interface. For various reasons I had everything on a single switch, which meant that all traffic was boomeranging in and out of the same CPU interface. The IPQ8064 has dual cores, so I thought I’d try moving the external connection to the other switch. That puts it on its own GigE CPU interface, which then allows binding the interrupts to a different CPU core. That helps; throughput to the outside world hits 140Mb/s+. Still a long way from the expected max, but proof we just need more grunt.

Success

Which brings us to this past weekend, when, having worked out all the other bits, I tried the squashfs root image again on the RB3011. Success! The home automation bits connected to it, the link to the outside world came up, everything seemed happy. So I double checked my bootloader bits on the RB5009, brought it down to the comms room and plugged it in instead. And, modulo my failing to update the nftables config to allow it to do forwarding, it all came up ok. Some testing with iperf3 internally got a nice 912Mb/s sustained between subnets, and some less scientific testing with wget + speedtest-cli saw speeds of over 460Mb/s to the outside world.

Time from ordering the router until it was in service? Just under 8 weeks…