Выглядит это примерно вот так:
* P_CONTROL_V1 -- Control channel packet (usually TLS ciphertext).
*
* P_CONTROL* and P_ACK Payload: The P_CONTROL message type
* indicates a TLS ciphertext packet which has been encapsulated
* inside of a reliability layer. The reliability layer is
* implemented as a straightforward ACK and retransmit model.
*
* P_CONTROL message format:
*
* local session_id (random 64 bit value to identify TLS session).
* HMAC signature of entire encapsulation header for integrity
* check if --tls-auth is specified (usually 16 or 20 bytes).
* packet-id for replay protection (4 or 8 bytes, includes
* sequence number and optional time_t timestamp).
* P_ACK packet_id array length (1 byte).
* P_ACK packet-id array (if length > 0).
* P_ACK remote session_id (if length > 0).
* message packet-id (4 bytes).
* TLS payload ciphertext (n bytes) (only for P_CONTROL).