Alert Protocol in TSL

One of the content types supported by the TLS record layer is the
alert type.Alert messages convey the severity of the message
(warning or fatal) and a description of the alert.Alert messages
with a level of fatal result in the immediate termination of the
connection.In this case, other connections corresponding to the
session may continue, but the session identifier MUST be invalidated,
preventing the failed session from being used to establish new
connections.Like other messages, alert messages are encrypted and
compressed, as specified by the current connection state.

enum { warning(1), fatal(2), (255) } AlertLevel;

enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed_RESERVED(21),
record_overflow(22),
decompression_failure(30),
handshake_failure(40),
no_certificate_RESERVED(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),

export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110),
(255)
} AlertDescription;

struct {
AlertLevel level;
AlertDescription description;
} Alert;