A QUIC connection is a single conversation between two QUIC endpoints. QUIC's connection establishment combines version negotiation with the cryptographic and transport handshakes to reduce connection establishment latency.
To actually send data over such a connection, one or more streams have to be created and used.
Each connection possesses a set of connection identifiers, or connection IDs, each of which can be used to identify the connection. Connection IDs are independently selected by endpoints; each endpoint selects the connection IDs that its peer uses.
The primary function of these connection IDs is to ensure that changes in addressing at lower protocol layers (UDP, IP, and below) do not cause packets for a QUIC connection to be delivered to the wrong endpoint.
By taking advantage of the connection ID, connections can thus migrate between IP addresses and network interfaces in ways TCP never could. For instance, migration allows an in-progress download to move from a cellular network connection to a faster wifi connection when the user moves their device into a location offering wifi. Similarly, the download can proceed over the cellular connection if wifi becomes unavailable.
QUIC is built atop UDP, so a 16 bit port number field is used to differentiate incoming connections.
A QUIC connection request originating from a client will tell the server which QUIC protocol version it wants to speak, and the server will respond with a list of supported versions for the client to select from.