A graph of download speeds is shown, with two triangular spikes and declines. Above the graph, the label “8 MB/s” is shown.

A Quick Introduction To TCP Congestion Control

May 25, 2025 by Aaron Beckendorf 11 Comments

It’s hard to imagine now, but in the mid-1980s, the Internet came close to collapsing due to the number of users congesting its networks. Computers would request packets as quickly as they could, and when a router failed to process a packet in time, the transmitting computer would immediately request it again. This tended to result in an unintentional denial-of-service, and was degrading performance significantly. [Navek]’s recent video goes over TCP congestion control, the solution to this problem which allows our much larger modern internet to work.

In a 1987 paper, Van Jacobson described a method to restrain congestion: in a TCP connection, each side of the exchange estimates how much data it can have in transit (sent, but not yet acknowledged) at any given time. The sender and receiver exchange their estimates, and use the smaller estimate as the congestion window. Every time a packet is successfully delivered across the connection, the size of the window doubles.

Once packets start dropping, the sender and receiver divide the size of the window, then slowly and linearly ramp up the size of the window until it again starts dropping packets. This is called additive increase/multiplicative decrease, and the overall result is that the size of the window hovers somewhere around the limit. Any time congestion starts to occur, the computers back off. One way to visualize this is to look at a graph of download speed: the process of periodically hitting and cutting back from the congestion limit tends to create a sawtooth wave.

[Navek] notes that this algorithm has rather harsh behavior, and that there are new algorithms that both recover faster from hitting the congestion limit and take longer to reach it. The overall concept, though, remains in widespread use.

If you’re interested in reading more, we’ve previously covered network congestion control in more detail. We’ve also covered [Navek]’s previous video on IPV5. Continue reading “A Quick Introduction To TCP Congestion Control” →

Modeling Network Latency

January 17, 2024 by Bryan Cockfield 6 Comments

The selfhosting community is an interesting and useful part of the Internet dedicated to removing one’s own services and data from the cloud and hosting it on their own servers, often on hardware that can be physically touched. With that kind of network usage, it’s not uncommon for people to build their own routers, firewalls, and other network support systems from the ground up. And, if you go deep enough, maybe even a home lab dedicated to testing and improving the network’s various layers. This piece of software helps simulate network latency to more accurately assess quality of service, performance, and the optimization of one one’s own networks.

The tool, called Speedbump, allows a network administer to quickly build a test network where characteristics of the network such as base latency and wave shape and size can be set up. From there, a TCP proxy sends the network traffic through the virtual network, adding in a set amount of delay to anything traveling on the network. It can be installed (or built from source) on an existing installation or used from within a Docker terminal, so there are plenty of options depending on preference. It’s also available as a library for any programs written in Go.

While this certainly has applications for home labs where self-hosting services is done at a high level, this could have professional applications as well. For troubleshooting simpler network issues we’d always recommend this tool which allows a more comprehensive network test than the standard “ping” command, and if you haven’t heard of selfhosting before it’s probably time to read this primer on it and build a hobby web server from scratch.

Bufferbloat, The Internet, And How To Fix It

August 18, 2022 by Jonathan Bennett 48 Comments

There’s a dreaded disease that’s plagued Internet Service Providers for years. OK, there’s probably several diseases, but today we’re talking about bufferbloat. What it is, how to test for it, and finally what you can do about it. Oh, and a huge shout-out to all the folks working on this problem. Many programmers and engineers, like Vint Cerf, Dave Taht, Jim Gettys, and many more have cracked this nut for our collective benefit.

When your computer sends a TCP/IP packet to another host on the Internet, that packet routes through your computer, through the network card, through a switch, through your router, through an ISP modem, through a couple ISP routers, and then finally through some very large routers on its way to the datacenter. Or maybe through that convoluted chain of devices in reverse, to arrive at another desktop. It’s amazing that the whole thing works at all, really. Each of those hops represents another place for things to go wrong. And if something really goes wrong, you know it right away. Pages suddenly won’t load. Your VoIP calls get cut off, or have drop-outs. It’s pretty easy to spot a broken connection, even if finding and fixing it isn’t so trivial.

That’s an obvious problem. What if you have a non-obvious problem? Sites load, but just a little slower than it seems like they used to. You know how to use a command line, so you try a ping test. Huh, 15.0 ms off to Google.com. Let it run for a hundred packets, and essentially no packet loss. But something’s just not right. When someone else is streaming a movie, or a machine is pushing a backup up to a remote server, it all falls apart. That’s bufferbloat, and it’s actually really easy to do a simple test to detect it. Run a speed test, and run a ping test while your connection is being saturated. If your latency under load goes through the roof, you likely have bufferbloat. There are even a few of the big speed test sites that now offer bufferbloat tests. But first, some history. Continue reading “Bufferbloat, The Internet, And How To Fix It” →

GSM Module Does More Than Advertised

March 25, 2020 by Bryan Cockfield 13 Comments

For many projects, a WiFi connection is overkill, too complicated, or too far away to work properly. Even though it’s relatively ubiquitous, sometimes the best choice for getting data to or from the real world is a connection to the cellular network, which can be done with the M590 module for about a dollar each. For that price, lots of people have had the opportunity to explore the module itself, and [marcrbarker] shows some of the extra, unadvertised, features it has.

Acting as a GSM module that can send and receive SMS messages is just the tip of the iceberg for this tiny device which we saw once before for a DIY GPS tracker. With a USB TTL serial data module, a lot more is on the table including answering voice calls and responding with DTMF tones, operate as a dial-up modem, connect with TCP, and even has some FTP capabilities. [marcrbarker] also suggests that it could do “call pranking” where it can send signals without being charged for a call.

There are a lot of details on the project site about all of this newfound functionality, and it reminds us of a time when it was discovered that not only was the ESP8266 a cheap WiFi module, but it could also run custom programs on its own. While the M590 probably can’t do all of that, it does seem to have a lot more locked away than most of us had thought before.

Bread Online Is A Bread Maker For The Internet Of Things

July 7, 2015 by Rick Osgood 17 Comments

An engineering student at the University of Western Macedonia has just added another appliance to the ever-growing list of Internet enabled things. [Panagiotis] decided to modify an off-the-shelf bread maker to enable remote control via the Internet.

[Panagiotis] had to remove pretty much all of the original control circuitry for this device. The original controller was replaced with an Arduino Uno R3 and an Ethernet shield. The temperature sensor also needed to be replaced, since [Panagiotis] could not find any official documentation describing the specifications of the original. Luckily, the heating element and mixer motor were able to be re-used.

A few holes were drilled into the case to make room for the Ethernet connector as well as a USB connector. Two relays were used to allow the Arduino to switch the heating element and mixer motor on and off. The front panel of the bread maker came with a simple LCD screen and a few control buttons. Rather than let those go to waste, they were also wired into the Arduino.

The Arduino bread maker can be controlled via a web site that runs on a separate server. The website is coded with PHP and runs on Apache. It has a simple interface that allows the user to specify several settings including how much bread is being cooked as well as the desired darkness of the bread. The user can then schedule the bread maker to start. Bread Online also comes with an “offline” mode so that it can be used locally without the need for a computer or web browser. Be sure to check out the video demonstration below. Continue reading “Bread Online Is A Bread Maker For The Internet Of Things” →

Propeller-Android Communications Using Debug Mode

March 29, 2011 by Mike Szczys 17 Comments

Here’s a new way to connect an Android phone and a Propeller microcontroller. It’s called the PropBridge and uses a very simple circuit with a voltage regulator, a couple of transistors, and a few resistors. The trick to this method lies in creative use of software features that already exist on Android hardware, the Android Debug Bridge (ADB). The ADB was added with development in mind, but since it provides low-level control of certain parts of these devices it was just waiting to be incorporated into a hack.

The Propeller itself uses firmware to make Android think it is one of two different externally connected hardware devices. It can act like a PC running the ADB client or it can mimic a TCP connection. There’s still plenty of room on the uC to add your own firmware, and the majority of the I/O pins are unneeded for the basic connection. Check out the video after the break for a quick overview of the system.

If you need a little help with Android programming before you’re able to use this in your own projects, check out our Android development series.

Continue reading “Propeller-Android Communications Using Debug Mode” →

Avoiding OS Fingerprinting In Windows

October 4, 2008 by Eliot 12 Comments

[Irongeek] has been working on changing the OS fingerprint of his Windows box. Common network tools like Nmap, P0f, Ettercap, and NetworkMiner can determine what operating system is being run by the behavior of the TCP/IP stack. By changing this behavior, you can make your system appear to be another OS. [Irongeek] started writing his own tool by checking the source of Security Cloak to find out what registry keys needed to be changed. His OSfuscate tool lets you define your own .os fingerprint file. You can pretend to be any number of different systems from IRIX to Dreamcast. Unfortunately this only works for TCP/IP. Other methods, like Satori‘s DHCP based fingerprinting, still work and need to be bypassed by other means. Yes, this is just “security through obscurity”, but it is something fun to play with.