TLV (Type-Length-Value)

What Is TLV?

TLV (Type-Length-Value) is the binary encoding scheme used to store NDEF messages in NFC tag user memory. TLV provides a container format allowing readers to locate and parse NDEF data without prior knowledge of the exact memory layout.

TLV Block Structure

[Type: 1 byte] [Length: 1 or 3 bytes] [Value: L bytes]

• Type: Identifies the data kind.
• Length: For 0-254 bytes, encoded as 1 byte. For 255-65534 bytes, encoded as 3 bytes (0xFF + 2-byte big-endian).
• Value: The actual data payload.

Standard TLV Types

NDEF Message TLV Example

A URI record for "https://example.com" (18 bytes):

03 12 D1 01 0E 55 04 65 78 61 6D 70 6C 65 2E 63 6F 6D FE
|  |  |---------- NDEF message (18 bytes) ---------|  |
|  Length=18                                           Terminator
Type=NDEF

Parsing Algorithm

1. Read Type byte. If 0x00 (NULL): advance one byte, repeat. If 0xFE (Terminator): stop.
2. Read Length field (1 or 3 bytes). If Type = 0x03, the Value bytes are the NDEF message.
3. Continue at next byte after Value.

Three-Byte Length

For NDEF messages longer than 254 bytes (e.g., on NTAG 216 or DESFire):

03 FF 01 F4 [500 bytes of NDEF data] FE

0xFF signals two-byte length follows (0x01F4 = 500).

Relationship to NDEF

TLV and NDEF are complementary layers. NDEF defines message and record format (how to encode URLs, text, etc.). TLV defines how those bytes are physically stored in tag memory. A properly formatted NFC tag always uses TLV to wrap its NDEF content.