Implementing Custom Congestion Control
In this lesson, you will learn how to bypass standard TCP congestion algorithms like CUBIC or BBR to implement a custom logic tailored for high-latency or specialized, non-standard network environments.
Why Custom Congestion Control?
Standard algorithms are designed for "fairness" and "efficiency" on the open internet. However, if you are operating in a custom environment—such as a proprietary satellite link with extreme delay or an internal high-speed fabric with zero-loss requirements—the standard "additive increase, multiplicative decrease" (AIMD) approach may cause massive throughput oscillations. By writing a kernel module, you can force the TCP stack to react to congestion signals (like ECN or packet loss) exactly as your specific hardware requires.
The Mechanism
To implement a custom controller, you define a tcp_congestion_ops structure. This structure acts as a callback table that the Linux kernel invokes whenever a TCP connection needs to adjust its congestion window (cwnd).
A simplified implementation skeleton looks like this:
static struct tcp_congestion_ops my_custom_algo = {
.init = my_algo_init,
.cong_avoid = my_algo_cong_avoid, // The core: how to grow the window
.ssthresh = my_algo_ssthresh, // Logic for slowing down
.owner = THIS_MODULE,
.name = "my_custom_algo",
};
static int __init my_algo_register(void) {
return tcp_register_congestion_control(&my_custom_algo);
}
When my_algo_cong_avoid is triggered, the kernel hands you the transmission control block (TCB). Instead of the default standard behavior, you can implement logic that ignores minor packet loss if you know your link is noisy, or aggressively ramps up the window size if your network buffers are known to be massive.
Deployment Note
Once compiled as a kernel module and loaded with insmod, your algorithm becomes available in the system. You can then apply it to specific sockets using setsockopt with TCP_CONGESTION or globally via sysctl -w net.ipv4.tcp_congestion_control=my_custom_algo. This allows you to tune specific traffic flows without affecting the general-purpose system traffic.
Key Takeaway: A custom congestion control module allows you to override kernel-level transmission logic, giving you granular control over how your application reacts to network pressure in specialized environments.