NDEF data is structured in messages. Each message consists of one or more records, which are made up of a header and the record payload. The Record header contains metadata about, amongst others, the payload type and length. The Record payload constitutes the actual content of the record.
Record header
The NDEF record header consists of the following fields:
| Field | Length | Required | Description |
|---|---|---|---|
| Flags and TNF | 1 byte | yes | See Flags and TNF. |
| Type Length | 1 byte | yes | Specifies the length of the payload type field. Required, but may be zero. |
| Payload Length | 1 or 4 bytes | yes | Specifies the length of the payload. Either 1 byte or 4 byte long, depending on the SR flag. Required, but may be zero. |
| ID Length | 1 byte | no | Required if the IL flag is set. Specifies the size of the Payload ID field. |
| Payload Type | variable | no | Required if the Type Length is > 0. Specifies the type of the NDEF record payload. |
| Payload ID | variable | no | Required if the IL flag is set and the ID Length is > 0. Specifies the ID of the NDEF record payload. |
Flags and TNF
The Flags and TNF byte contains the following flags:
- MB (Message Begin) and ME (Message End): Specify the position of the NDEF record within the message. The MB flag is set for the first record in the message. Similarly, the ME flag is set for the last record in the message. If a record is the only record in a message, both flags are set.
- CF (Chunk Flag): Used for chunked payloads (a payload that is partitioned into multiple records). Set in all chunks of the record except for the last one. Note, however, that chunking is not supported by this library.
- SR (Short Record): Used to determine the size of the payload length field. If the flag is set, the Payload Length occupies 1 byte; otherwise it is 4 bytes long. Note that the NDEF generator supports a Payload Lenght of 4 bytes only at the moment.
- IL (ID Length present): Indicates whether an ID Length field is present in the header. If the flag is set, the ID Length field is present.
- TNF (Type Name Format): Specifies the structure of the Payload Type field and how to interpret it. The following values are allowed (square brackets contain documentation reference related to the specific type):
Value Type Name Format 0x00 Empty 0x01 NFC Forum well-known type [NFC RTD] 0x02 Media-type [RFC 2046] 0x03 Absolute URI [RFC 3986] 0x04 NFC Forum external type [NFC RTD] 0x05 Unknown 0x06 Unchanged 0x07 Reserved
Record payload
The content of the payload is application-specific and related to the type of the record. For example, in Connection Handover LE and EP records, the payload is composed of data required by BLE for successful pairing. In URI records, the payload contains a web address of the page that the polling device should open. In launch app records, it contains the ID of the application that should be launched.
Note that the payload of an NDEF record can contain a nested NDEF message. This nested message must be a full NDEF message, consisting of one or multiple NDEF records with the appropriate setting of MB and ME flags.