Encrypted Message Acknowledgement

Introduction/Additional information:

This Message 25 payload is used to acknowledge reception of addressed, encrypted STEDS messages.

Usage Notes

Acknowledgement linkage back to the original message is based upon source MMSI, source FI, and source Message Linkage ID.
If the received FID 17 (Encrypted Text Message), has its “ACK flag” field set to zero, then this acknowledgement shall not be sent. Otherwise, all encrypted, addressed STEDS messages receive an ACK.
If the sending PI does not receive an ACK after 4 seconds, the sending PI shall retry transmission. The sending PI shall retry a maximum of three times, waiting 4 seconds each time for an ACK. This indicates a maximum of four transmitted messages: one initial and three retries.

Permitted as from:

12/05/2016

Registrant:

USCG

Physical link:

AIS

Message number (0=VDES-VDE):

DAC or VPFI:

366

FI / Message ID:

Version:

State:

in force

Number of slots (max):

Last modified:

29/01/2021 - 23:19

Used by:

USCG

Details:

	Parameter	# of Bits	Description
	Parameter	# of Bits	Description
Standard Message Header	Message ID	6	Identifier for Message 25; always 25.
	Repeat Indicator	2	Indicates how many times a message has been repeated. 0 – 3; 0 = default; 3 = do not repeat any more. Set to zero (default).
	Source MMSI	30	MMSI number of source station. Varies according to the transmitter ID.
	Destination Indicator	1	0 = broadcast (no Destination ID field) 1 = Addressed (30 bit Destination ID field). Set to 0 (broadcast).
	Binary Data Flag	1	0 = unstructured binary data (no Application Identifier bits used) 1 = binary data coded as defined by the 16-bit AI. Set to zero (unstructured).
	Destination ID	0	MMSI number of destination station. Not used.
	Spare	0	Not used.
Binary Application Data (encrypted)	Function Identifier	6	Function identifier. Set to 9.
	Version	3	Sequential number used to indicate the message version in steps of 1. 0 = test message = default; 1 – 7 = message version. Set to zero.
	Original Message Source	30	MMSI of the source of the message being acknowledged.
	Original Message DAC	10	DAC of the message to be acknowledged. Set to 0 if no DAC (AIS message 25/26 messages).
	Original Message FI	6	Functional Identifier of message being acknowledged. 0 – 63.
	Original Message MLID	10	MLID of the source message to be acknowledged.
	Original Message Checksum	16	16-bit CRC of the source message to be acknowledged.
	UTC Hour	5	UTC Hour of ACK. 0 – 23; 24 = UTC hour not available = default; 25 - 31 = reserved.
	UTC Min	6	UTC Minute of ACK. 0 – 59; 60 = UTC minute not available = default; 61 - 63 (reserved for future use).
	UTC Sec	6	UTC Second of ACK. 0 – 59; 60 = UTC second not available = default; 61-63 (reserved for future use).
	Encryption Bit Padding	14	Sufficient spare bits to ensure that the binary data is a multiple of 128 bits for block encryption (128). Set to 0.
	Checksum	16	16-bit CRC calculated as per EAIS Specification
Total bits		168	1 Slot Binary Message

Attachment:

eais_idd_v5_4_8may17.pdf [1]