TCP Graphs

The report contains graphs with multiple TCP statistics. We will explain the different statistics in more detail here.

This is an example TCP graph:

Goodput indicates the rate at which data is arriving at the application layer. It's what a web browser refers to as "download speed" when downloading a file.

Throughput indicates the rate of data that is transferred by the TCP protocol. This includes application payload, TCP header size, and TCP retransmissions. In practice, throughput is very close to goodput because the size of the TCP header is only a fraction of the payload and retransmissions don't happen very often.

A large gap between goodput and throughput indicates a high retransmission rate.

The round-trip time indicates how long it takes on average for a transmitted TCP segment to be acknowledged.

Looking at the evolution of the round-trip time is more useful than focusing on its absolute value. For example, a steadily rising RTT usually indicates that network buffers are filling up resulting in longer queuing delays. A sudden rise in RTT may indicate that a competing flow started transmitting data.

The transmit window determines how much data there can be in-flight at a given point in time. Its value is derived from the congestion window and receive window used by TCP internally. A high transmit window is needed for high throughput.

A drop in the transmit window usually indicates that packet loss has occurred.