4. Heartbeat Request and Response Messages

The Heartbeat protocol messages consist of their type and an arbitrary payload and padding.

struct {
   HeartbeatMessageType type;
   uint16 payload_length;
   opaque payload[HeartbeatMessage.payload_length];
   opaque padding[padding_length];
} HeartbeatMessage;

The total length of a HeartbeatMessage MUST NOT exceed 2^14 or max_fragment_length when negotiated as defined in [RFC6066].

type: The message type, either heartbeat_request or heartbeat_response.

payload_length: The length of the payload.

payload: The payload consists of arbitrary content.

padding: The padding is random content that MUST be ignored by the receiver. The length of a HeartbeatMessage is TLSPlaintext.length for TLS and DTLSPlaintext.length for DTLS. Furthermore, the length of the type field is 1 byte, and the length of the payload_length is 2. Therefore, the padding_length is TLSPlaintext.length - payload_length - 3 for TLS and DTLSPlaintext.length - payload_length - 3 for DTLS. The padding_length MUST be at least 16.

The sender of a HeartbeatMessage MUST use a random padding of at least 16 bytes. The padding of a received HeartbeatMessage message MUST be ignored.

If the payload_length of a received HeartbeatMessage is too large, the received HeartbeatMessage MUST be discarded silently.

When a HeartbeatRequest message is received and sending a HeartbeatResponse is not prohibited as described elsewhere in this document, the receiver MUST send a corresponding HeartbeatResponse message carrying an exact copy of the payload of the received HeartbeatRequest.

If a received HeartbeatResponse message does not contain the expected payload, the message MUST be discarded silently. If it does contain the expected payload, the retransmission timer MUST be stopped.