API

One of the success factors for regular TCP and programs using that, is thestandardized socket API. It has well defined functionality and using this APIyou can move programs between many different operating systems as TCP worksthe same.

QUIC is not there. There is no standard API for QUIC.

With QUIC, you need to pick one of the existing library implementations andstick with its API. It makes applications “locked in” to a single library tosome extent. Changing to another library means another API and that mightinvolve a lot of work.

Also, since QUIC is typically implemented in user-space, it can’t easily justextend the socket API or appear similar to how existing TCP and UDPfunctionality do. Using QUIC will mean using another API than the socket API.