Defining a NDEF Message

I am working on a Python project with my Raspberry Pi and the RFID-RC522 board. As NFC tags I use NXP NTAG213. My plan now is to store links on the tags. I can read/write on them without a problem. But I don't understand how to define the NDEF header for the stored data on the tags.

When I write a link on the tags with my smartphone and read it with my program the stored data on the tag looks like this:

The NDEF header for the URL "http://www.gmx.at" is

[3, 11, 209, 1, 7, 85, 1, ... (Data)]

I recognized that some of these parameters change when I write another link, but some still stay the same.

I found this tutorial which describes the different fields of the NDEF header but I still don't get how I need to set them to store a link to a website.

I would be very happy if someone can describe how I need to calculate/define the parameters for a link correctly.

Solution

In order to understand the NDEF format and the way how NDEF formatted data is stored on an NFC Forum Type 2 tag (which is the tag platform that is implemented by the NTAG213), I suggest that you read the following NFC Forum specifications:

The data that you read from the tag is an NDEF Message TLV object containing an NDEF message that consists of one URI record.

NDEF Message TLV:

  0x03             TLV tag = NDEF Message TLV
    0x0B           TLV length = 11 bytes
    0xD1 ... 0x74  TLV value = NDEF message

This means that the tag contains an NDEF message with a length of 11 bytes. The NDEF message is 0xD1 ... 0x74.

NDEF Message:

  0xD1             Record header
                     Bit 7 = MB = 1: first record of NDEF message
                     Bit 6 = ME = 1: last record of NDEF message
                     Bit 5 = CF = 0: last or only record of chain
                     Bit 4 = SR = 1: short record length field
                     Bit 3 = IL = 0: no ID/ID length fields
                     Bit 2..0 = TNF = 0x1: Type field represents an NFC Forum
                                           well-known type name
    0x01           Type length = 1 byte
    0x07           Payload length = 7 bytes
    0x55           Type field = "U" (in US-ASCII) = binary form of type name urn:nfc:wkt:U
    0x01 ... 0x74  Payload field = URI record payload

This means that the NDEF message consists of one URI record (type name urn:nfc:wkt:U) following the URI Record Type Definition.

URI record payload:

  0x01             Identifier byte = URI prefix "http://www."
  0x67 ... 0x74    URI field (UTF-8 encoded) = "gmx.at"

This means that the URI record points to the URI "http://www.gmx.at".