# Spin Bit One of the perhaps longest design discussions within the QUIC working group that has been the subject of several hundred mails and hours of discussions concerns a single bit: the Spin Bit. The proponents of this bit insist that there is a need for operators and people on the path between two QUIC endpoints to be able to measure latency. The opponents to this feature do not like the potential information leak. ## Spinning a bit Both endpoints, the client and the server, maintain a spin value, 0 or 1, for each QUIC connection, and they set the spin bit on packets it sends for that connection to the appropriate value. Both sides then send out packets with that spin bit set to the same value for as long as one round trip lasts and then it toggles the value. The effect is then a pulse of ones and zeroes in that bitfield that observers can monitor. This measuring only works when the sender is neither application nor flow control limited and packet reordering over the network can also make the data noisy. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://http3-explained.haxx.se/en/quic/quic-spinbit.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.