TCP Window Scaling: Solving the 64KB Limit

The original TCP specification only allowed a maximum window size of 65,535 bytes (64KB). On a high-speed fiber link with 100ms of latency, this small window creates a massive bottleneck.

The LFN Problem

In "Long Fat Networks" (LFNs) — connections with high bandwidth and high latency — the sender would spend most of its time waiting for acknowledgments because the window was too small to keep the pipe full.

Window Scaling to the Rescue

RFC 1323 introduced a "scaling factor" that shifts the window size left by up to 14 bits. This allows the window to grow up to 1 Gigabyte, enabling modern high-speed data transfers across continents.

Without this simple extension, the Gigabit internet we enjoy today would be physically limited to just a few megabits over long distances.