When Chrome makes a request to an origin it has never spoken to before, it does not know that the origin supports QUIC and so it will send the first request over TCP. The server’s response will indicate that it also supports QUIC via the Alternate-Protocol HTTP response header. (For example, the response. ”Alternate-Protocol: 443:quic” tells Chrome that the origin supports QUIC on port 443.). Now that Chrome knows the origin (scheme, host, port) speaks QUIC, it can attempt to use QUIC for the next request. When the request is made, Chrome will race a QUIC connect job with a TCP connect job. (These jobs establish a connection but do not issue the request.) Since the first request went out over TCP, the TCP job will win the race almost immediately and the second request will go out over TCP. At some subsequent point, the QUIC connection will succeed. Once this happens, all future requests will be sent over the QUIC connection.