Spin Bit

One of the perhaps longest design discussions within the QUIC working groupthat has been the subject of several hundred mails and hours of discussionsconcerns a single bit: the Spin Bit.

The proponents of this bit insist that there is a need for operators andpeople 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, foreach QUIC connection, and they set the spin bit on packets it sends for thatconnection to the appropriate value.

Both sides then send out packets with that spin bit set to the same valuefor as long as one round trip lasts and then it toggles the value. The effectis then a pulse of ones and zeroes in that bitfield that observers canmonitor.

This measuring only works when the sender is neither application nor flowcontrol limited and packet reordering over the network can also make the datanoisy.