Zero window
The receiver told the sender to stop. This is the finding that most often moves a ticket off the network team's desk, because the network did its job perfectly.
What it looks like
An ACK advertising a window of zero, then the sender pausing, then window probes while it waits for room. The report grades it Degraded and says “The far end stopped reading”.
What it means
Every TCP receiver advertises how much buffer space it has left. Zero means the data arrived correctly, was acknowledged, and is sitting in a socket buffer nobody is reading. TCP then does exactly the right thing and stops sending. The transfer looks slow, and every packet was delivered perfectly.
What causes it
- The receiving process is blocked on something else — disk, a lock, a database query, a garbage collection pause.
- It is CPU-starved and not being scheduled often enough to drain the socket.
- It is single-threaded and busy with another client.
- Deliberate back pressure. Some software slows a producer this way on purpose.
What to do
Go to the host that advertised the zero window and look at what the receiving process is doing. Not the network, not the sender, not the path. Nothing you change in the network will help, because nothing in the network is wrong — and if the window recovers a moment later, you are watching a pause in the application, timed for you to the microsecond.