Uncategorized – Page 28

TCP Tracing – Part 8 – BBR and Uplink Congestion Avoidance

Typical uplink throughput graph over Android Wi-Fi tethering and a slow uplink

Wi-Fi Tethering on Android is a great thing and has totally changed the way we use the mobile Internet today. One thing Android does not do well today at all, however, are bulk uploads over Wi-Fi tethering, especially under bad LTE radio conditions with low throughput. The screenshot above shows how a big file upload looks like over the course of one minute. Up to 22 seconds into the transmission, the throughput graph looks quite reasonable, throughput is around 4 Mbps in the uplink direction. But at 22 seconds into the upload, the curve suddenly goes crazy. Average throughput stays at 4 Mbps, but things could not be more wrong. Let’s have a closer look why that is so and what can be done about it.

Steam – Giana Sisters – And Linux – 10 Years Later

Giana Sisters on the C64 – many eons ago

The favorite game of my teenage years was undoubtedly ‘Giana Sisters‘. I loved the game and the soundtrack and played it hours on end on my Commodore 64. A couple of decades then followed in which I didn’t play a lot of computer games at all, there were just so many other exciting things to do with computers that kept me engaged. Then, in 2012, Black Forrest Games had a Kickstarter campaign to create a modern successor to the game. I was hooked and once it was released, I bought it on Steam. Unfortunately, my 3 year old notebook I bought in 2009 would not run the game smoothly. I was a bit disappointed, but ok, it was not a gamer notebook. So I shelved the thing for a decade but was recently reminded of it again. So perhaps it would run now with my current media notebook with an Intel 11th generation CPU which still only has an integrated GPU? So lets see I thought…

The LTE Timing Advance

One nice thing about GSM ‘in the old days’ was, that there was a timing advance (TA) value that indicated to the mobile how much earlier it had to start transmitting relative to the network to account for the limited speed of radio waves. In my early ‘mobile days’, the TA was an invaluable help to get an idea of how far cell sites are apart from each other in the city and in the countryside, and if a cell site I stood in front of was actually the one my mobile communicated with or if it was from a different network operator. Unfortunately, the timing advance went away with UMTS, and when it came back with LTE, it was not shown in the ‘engineering menu’ of mobile devices any more. A pity. Also, I thought that the timing advance value was deeply buried in MAC messages, and hence difficult to get to with professional trace tools. Recently, however, I found out that at least the later assumption was not quite true.

Proper Shutdown of VMs on Host Reboot

In the past, I’ve always used KVM’s GUI to properly shut-down remote Virtual Machines when I needed to reboot the host and then re-launch them again after the reboot. Over time, this is a bit of a tiresome process, especially as the frequency of host reboots after security updates seems to increase. So at some point I thought that there must be an easier way to do this:

TCP Tracing – Part 7 – Between Data Centers

Data transfer speed between a VM in Nürnberg and a VM in Helsinki

After the interesting throughput experiences I have made over the last mile, it was time to push the limit a bit and get an idea of how a high speed data transmission between virtual machines looks like that are in different data centers. While most of the public cloud infrastructure I used is located in Hetzner’s Helsinki data center, I do have a couple of virtual machines in Hetzner’s Nürnberg facility as well. In other words, an ideal setup to test inter-data center throughput.

The image at the beginning of the post shows the throughput when transferring large amounts of data from a VM in Nürnberg to a VM in Helsinki. Both VMs are running on servers that are connected by 1GbE links to the backbone. That means that the bandwidth on that link has to be shared by all VMs on the physical server. So at first I was not surprised that I could ‘only’ get a throughput of around 300 to 400 Mbps. Or so I thought…

Zero Trust – The Elevator Pitch

Over the past few months I’ve heard the term “Zero Trust Networking” from time to time but I didn’t quite understand what was meant by the term. So I started to look into it a bit and here’s my elevator pitch for it:

TCP Tracing – Part 6 – Segmentation Offloading

With multi gigabit per second Ethernet and wireless interfaces, CPUs are quite challenged by the sheer number of packets that need to be handled. Let’s say a transmission on a (meager) 1 Gbps Ethernet link is well utilized by large file transfers (let’s say 100 MBbytes per second) and a typical maximum segment size (MSS) of 1460 bytes is used. That’s 68.493 packets per second in one direction, not even counting the TCP ACKs in the other direction! Also, Wireshark starts smoking when it has to look at a 60 seconds trace with 4 million packets inside. But there’s a fix for that with almost no downsides that: Segmentation Offloading.

Kubernetes Intro – Part 12 – Jaeger

It’s been a while since part 11 of my Kubernetes Intro, but new things keep coming. When you have a nice little Kubernetes cluster with dozens of interacting microservices running, you quickly come to the point where you need to debug the system. Today, and particularly in the telecoms networking world, tracing on network interfaces and analyzing the result with Wireshark is one of the most important debugging tools. But that’s a bit of a challenge when your signaling path flows through a Kubernetes cluster.

TCP Tracing – Part 5 – High Speed Wi-Fi and the Receive Window

In part 4 of this series, I’ve compared the TCP behavior of high speed LTE and Wi-Fi to a virtual machine in a dater center over links with round trip delay times of around 25 ms. The results were very insightful but also raised new questions that I will follow up in this post. So here’s a quick recap of what happened so far to set things into context:

For fast Wi-Fi, I went to the local university in Cologne that has an excellent Eduroam Wi-Fi deployed in many areas. There, I was able to get an iperf uplink throughput of around 500 Mbps to a virtual machine in Nürnberg. When analyzing the TCP behavior in more detail, I noticed that my notebook quite frequently hit the default maximum TCP receive window size of 3 MB of the VM in the remote data center. In other words: More would perhaps have been possible with a bigger TCP receive buffer. So I gave this another try to see what would happen.

Get Notified of SSH Logins

Here’s a quick tip of the day that I came across when I needed a way to get a notification on my mobile device when particular users log into a server. Turns out that the Linux PAM (Pluggable Authentication Modules) offers a convenient way to do this: