TagLengthValue

Tag-Length-Value (TLV) encoding is a data representation method in which each data item is encoded as a triplet: a tag, a length, and a value. The tag identifies the data type or class, the length specifies how many bytes follow for the value, and the value contains the actual data. This structure makes TLV data self-describing and easy to parse, and it supports nesting to represent complex, hierarchical data.

Structure and encoding: A TLV element begins with a tag that can convey information such as data type, encoding rules, and whether the value is primitive or constructed. The length indicates the size of the value in bytes. The value is the payload itself. Many TLV schemes use a short form for small lengths (one byte) and a long form for larger lengths (multiple bytes). Some families also define an indefinite length option, though that is not used in canonical DER encodings.

Variants and usage: TLV is used in a wide range of standards and protocols. It underpins ASN.1 encoding rules such as BER and DER, and appears in X.509 certificates, SNMP, smart cards (including EMV), NFC and RFID data formats, and various binary communication protocols. While the exact tag byte representations and length encoding vary by standard, the basic concept remains the same.

Examples: The integer 5 can be encoded as tag 0x02 (INTEGER), length 0x01, value 0x05, producing 02 01 05. The string "AB" can be encoded as tag 0x04 (OCTET STRING), length 0x02, value 0x41 0x42, producing 04 02 41 42.

Advantages and considerations: TLV enables modular, extensible data layouts and easy parsing, with support for nested structures. Potential drawbacks include parsing complexity and, in some contexts, overhead from encoding tags and lengths for small data fields.