There are two forms of QUIC common header: long and short. Long form packets are used for the initial exchange - until both 1-RTT packet protection can be started AND version negotiation is complete. Short form packets carry the bulk of the data.
This removes a lot of the flexibility that was the source of most of the objections to the current format. Fields are aligned on four octet boundaries. All long-form header variations have the exact same form. The connection ID is in the same place in both short and long form. The long form clearly identifies the role of the sender in the first octet and it identifies the