enum {
        hello_request_RESERVED(0),
        client_hello(1),
        server_hello(2),
        hello_verify_request_RESERVED(3),
        new_session_ticket(4),
        end_of_early_data(5),
        hello_retry_request_RESERVED(6),
        encrypted_extensions(8),
        request_connection_id(9),           /* New */
        new_connection_id(10),              /* New */
        certificate(11),
        server_key_exchange_RESERVED(12),
        certificate_request(13),
        server_hello_done_RESERVED(14),
        certificate_verify(15),
        client_key_exchange_RESERVED(16),
        finished(20),
        certificate_url_RESERVED(21),
        certificate_status_RESERVED(22),
        supplemental_data_RESERVED(23),
        key_update(24),
        message_hash(254),
        (255)
    } HandshakeType;

    struct {
        HandshakeType msg_type;    /* handshake type */
        uint24 length;             /* bytes in message */
        uint16 message_seq;        /* DTLS-required field */
        uint24 fragment_offset;    /* DTLS-required field */
        uint24 fragment_length;    /* DTLS-required field */
        select (msg_type) {
            case client_hello:          ClientHello;
            case server_hello:          ServerHello;
            case end_of_early_data:     EndOfEarlyData;
            case encrypted_extensions:  EncryptedExtensions;
            case certificate_request:   CertificateRequest;
            case certificate:           Certificate;
            case certificate_verify:    CertificateVerify;
            case finished:              Finished;
            case new_session_ticket:    NewSessionTicket;
            case key_update:            KeyUpdate;
            case request_connection_id: RequestConnectionId;
            case new_connection_id:     NewConnectionId;
        } body;
    } Handshake;

    uint16 ProtocolVersion;
    opaque Random[32];

    uint8 CipherSuite[2];    /* Cryptographic suite selector */

    struct {
        ProtocolVersion legacy_version = { 254,253 }; // DTLSv1.2
        Random random;
        opaque legacy_session_id<0..32>;
        opaque legacy_cookie<0..2^8-1>;                  // DTLS
        CipherSuite cipher_suites<2..2^16-2>;
        opaque legacy_compression_methods<1..2^8-1>;
        Extension extensions<8..2^16-1>;
    } ClientHello;