EUE - Busy client reducing server transfer speed

EUE Score - Busy Client Reducing Server Transfer Speed
This happens when the client is overwhelmed with processing incoming data and cannot keep up with the server's transmission speed. As a result, the client deliberately signals the server to slow down. Here’s how it works:

TCP flow control: 

TCP uses a mechanism called a receive window (part of the TCP header) to indicate how much data the receiver can handle.
If the client is busy or its buffer is full, it will reduce the size of the advertised receive window. This smaller window tells the server to send less data, effectively slowing the server down.
Analogy: Imagine the server is pouring water into a cup (the client's buffer). If the client’s cup is almost full, it signals the server to pour slower to avoid spilling.

Client-Side Zero Window
A "zero window" occurs when the client’s buffer is completely full and it cannot accept any more data temporarily. The client sets the receive window size to zero in the TCP header, signaling the server to stop sending data until it can process what it already has.

Key Points:
The server pauses data transmission when it receives a zero-window notification.
The client periodically sends a window update (usually in the form of a zero-window probe acknowledgment) when it has cleared enough buffer space, allowing the server to resume sending data.
Analogy: If you’re a waiter serving food to a customer (client), the customer might raise their hand to say “Stop, my plate is full.” When they’ve eaten some of the food, they’ll let you know they’re ready for more.

If you run into packets diagnosed as Zero Window you can tell that your receiving node is not able to process incoming data fast enough. That is the typical performance problem that everybody blames the network for, while it is in fact a software/hardware/performance problem on the receiving node.