[EN] RTT: The Master Metric of Latency and Flow Control

[EN] RTT: The Master Metric of Latency and Flow Control

Round Trip Time (RTT) is the fundamental metric that defines the response speed of a network, representing the total time elapsed from sending a signal to receiving its acknowledgment. More than a simple number in milliseconds, RTT is the heartbeat of congestion control algorithms and the critical variable for buffer calculations in mission-critical protocols such as SRT, TCP, and QUIC.

Knowledge Architecture	Study First
UDP	The base transport protocol for low-latency measurements.
Propagation Delay	The physical limit imposed by the speed of light in optical fiber.
Concept	Anatomy of Round Trip Time

RTT is not just the "distance" between two points; it is the sum of four main components:

Processing Delay	Time taken by routers to examine the header.
Queuing Delay	Time spent in congested network buffers.
Transmission Delay	Time to push bits into the physical medium.
Propagation Delay	The actual signal travel time.

In unstable networks, RTT fluctuates due to queuing delay, a phenomenon known as Jitter, which RTP and SRT attempt to mitigate through dynamic buffers.

Operation and Internal Structure	RTT in TCP
Engineering Calculation	The 4x RTT Latency Rule
Formula	`Safe Latency = average RTT + (4 RTT Standard Deviation)`.

In a scenario where the average RTT is 50ms and the variation (Jitter) is 10ms, the minimum buffer latency should be configured to at least 90ms to ensure that a NAK has time to go and return before the frame playout. Ignoring standard deviation and looking only at average RTT is the number one mistake causing stuttering in professional broadcasts.

To learn more about the subject:

1. How does the Jacobson algorithm optimize RTT estimation in networks with high variance?

Click here to investigate

2. What is the impact of "Bufferbloat" on the artificial increase of RTT in home routers?

Click here to investigate

3. Why is LEO satellite RTT (Starlink) disruptive compared to traditional GEO satellites?

Click here to investigate

Technical Disclaimer and Intellectual Property Notice This blog presents analyses and facts based exclusively on technical documentation, RFCs, and publicly available materials on the global computer network. Lack of Affiliation: This project is independent and has no official affiliation, endorsement, or link with the developers, companies, or rights holders of the mentioned technologies. Liability: The implementation of any protocol or configuration based on these notes is the sole responsibility of the user. The author disclaims any liability arising from the misuse of this information. Rights and Corrections: We fully respect intellectual property. If you are the rights holder of any material or technology cited here and identify the need for corrections, adjustments, or wish to make official comments, please send a private message directly to the author for immediate resolution.