The addressed safety related message could be variable in length, based on the amount of safety related text. The length should vary between 1 and 5 slots.
| Parameter | Number of bits | Description |
|---|---|---|
| Message ID | 6 | Identifier for Message 12; always 12 |
| Repeat indicator | 2 | Used by the repeater to indicate how many times a message has been repeated. See § 4.6.1, Annex 2; 0-3; 0 = default; 3 = do not repeat any more |
| Source ID | 30 | MMSI number of station which is the source of the message. |
| Sequence number | 2 | 0-3; see § 5.3.1, Annex 2 |
| Destination ID | 30 | MMSI number of station which is the destination of the message |
| Retransmit flag | 1 | Retransmit flag should be set upon retransmission: 0 = no retransmission = default; 1 = retransmitted |
| Spare | 1 | Not used. Should be zero. Reserved for future use |
| Safety related text | Maximum 936 | 6-bit ASCII as defined in Table 47 |
| Maximum number of bits | Maximum 1 008 | Occupies up to 3 slots, or up to 5 slots when able to use FATDMA reservations. For Class B “SO” mobile AIS stations the length of the message should not exceed 3 slots For Class B “CS” mobile AIS stations the length of the message should not exceed 1 slot |
Additional bit stuffing will be required for this message type. For details refer to transport layer, § 5.2.1, Annex 2.
Table 62 gives the number of 6-bit-ASCII characters, so that the whole message fits into a given number of slots. It is recommended that any application minimizes the use of slots by limiting the number of characters to the numbers given, if possible:
| Number of slots | Maximum 6-bit ASCII characters |
| 1 | 10 |
| 2 | 48 |
| 3 | 85 |
| 4 | 122 |
| 5 | 156 |
Addressed messages should have a destination user ID. The source station should anticipate an acknowledgement message (Message 7 or Message 13). If an acknowledgement is not received the station excluding Class B “SO” should retry the transmission. The station should wait 4 s before attempting retries. When a transmission is retried, the retransmit flag should be set to retransmitted. The number of retries should be 3, but it could be configurable between 0 and 3 retries by an external application via the presentation interface. When set to a different value by an external application, the number of retries should default to 3 retries after 8 min. The overall result of the data transfer should be forwarded to above layers. The acknowledgement should be between transport layers in two stations.
Each data transfer packet on the presentation interface should have a unique packet identifier consisting of the message type (binary or safety related messages), the source-ID, the destination-ID, and a sequence number.
The sequence number should be assigned in the appropriate presentation interface message which is input to the station.
The destination station should return the same sequence number in its acknowledgement message on the presentation interface.
The source station should not reuse a sequence number until it has been acknowledged or time-out has occurred.
The acknowledgement should be put first in the data transfer queue both on the presentation interface and on the VDL.
These acknowledgements are applicable only to the VDL. Other means must be employed for acknowledging applications.
