The Internet measurement of round-trip time (RTT) is important as an indication of performance and latency. While the Ping utility can and does measure RTT, widespread use increases overhead. This paper describes a low overhead method of passively measuring RTT. Heretofore, passive RTT could be obtained when access to transmission control protocol (TCP) timestamps or other congestion or flow control algorithms were employed. However, for cases where the packets are encrypted--in this case, the data used for the measurement is hidden within the encrypted payload--these methods are unavailable.

The paper describes an efficient latency spin signal algorithm to passively measure RTT with three bits of information in the transport protocol header. One of the bits is called a spin bit. It is sent in one state (one or “up”) until it is a toggled indication of a measurement start, and a two-bit valid edge counter (VEC) marks packets that carry a valid spin bit.

The paper includes an algorithm and a reference chart to describe the mechanism. The authors provide an implementation in an open-sourced QUIC protocol (QUIC originally stood for “quick UDP Internet connections”). Several experimental results are provided. While the authors use the OUIC protocol, the technique could also be a useful enhancement to TCP.

The Internet Engineering Task Force (IETF) QUIC working group has approved experimentation with the spin bit involving an operating system vendor, a telecom operator, and a network equipment vendor. It will be interesting to see how the theoretical results match the realized results.