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Encrypted Message Acknowledgement

Introduction/Additional information: 

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

    1.  

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): 
25
DAC or VPFI: 
366
FI / Message ID: 
9
Version: 
0
State: 
in force
Number of slots (max): 
1
Last modified: 
29/01/2021 - 23:19
Used by: 
USCG
Details: 

 

 

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: 
PDF icon eais_idd_v5_4_8may17.pdf