Blaming The Network

A team reports that their upload endpoint is congestion limited and asks for more bandwidth. Their capture of one slow transfer shows:

Observation	Value
Retransmissions	0
Duplicate ACKs	0
ACKs from the receiver	steady, one per two segments
Advertised window in those ACKs	falls from 3.1 MB to 0, then stays near 0
Segments from the sender while window is 0	one 1-byte segment every few seconds, spacing doubling
Link utilisation	4 percent

What is actually limiting the transfer?

The bottleneck router's queue is full, so the sender is pacing itself.

The receiving application is not draining its socket buffer, so the receive window closed and the sender is now sending zero-window probes.

The congestion window collapsed after loss the capture filter missed.

A middlebox stripped the window scale option, capping the window at 65535 bytes.