my-server
← Wiki Redirected from Type-length-value

Type–length–value

Within communication protocols, TLV (type-length-value or tag-length-value) is an encoding scheme used for informational elements. A TLV-encoded data stream contains code related to the record type, the record value's length, and finally the value itself.

Details

The type and length are fixed in size (typically 1–4 bytes) or can be otherwise parsed without knowledge of the size (see: LEB128, variable-length quantity), and the value field is of variable size. These fields are used as follows:

Type: A binary code, often simply alphanumeric, which indicates the kind of field that this part of the message represents;
Length: The size of the value field (typically in bytes);
Value: Variable-sized series of bytes that contains data for this part of the message.

Some advantages of using a TLV representation are:

  • TLV sequences are easily searched using generalized parsing functions;
  • New message elements that are received at an older node can be safely skipped, and the rest of the message can be parsed. This is similar to the way that unknown XML tags can be safely skipped;
  • TLV elements can be placed in any order inside the message body;
  • TLV elements are typically used in a binary format and binary protocols, which makes parsing faster and the data smaller than in comparable text-based protocols.

Illustrative example

Imagine a message to make a telephone call. A first version of the system defines the following structure:

When it makes a call, it sends the following data: 00 00 T_COMMAND 00 04 length = 4 00 00 00 20 C_MAKE_CALL 00 10 T_PHONE_NUMBER_TO_CALL 00 08 length = 8 37 32 32 2D ASCII for "722-" 34 32 34 36 ASCII for "4246"

A receiving system would then understand that the message tells it to call "722-4246".

Later (in version 2) a new field containing the calling number could be added:

It would send a message like: 00 00 T_COMMAND 00 04 length = 4 00 00 00 20 C_MAKE_CALL 00 11 T_CALLER_NUMBER 00 0c length = 12 36 31 33 2D ASCII for "613-" 37 31 35 2D ASCII for "715-" 39 37 31 39 ASCII for "9719" 00 10 T_PHONE_NUMBER_TO_CALL 00 08 length = 8 37 32 32 2D ASCII for "722-" 34 32 34 36 ASCII for "4246"

A version&nbsp;1 system which received a message from a version&nbsp;2 system would first read the <code>T_COMMAND</code> element and then read an element of type <code>T_CALLER_NUMBER</code>. The version&nbsp;1 system does not understand <code>T_CALLER_NUMBER</code>, so the length field is read (i.e., 12) and the system skips forward 12 bytes to read <code>T_PHONE_NUMBER_TO_CALL</code>, which it understands, and message parsing carries on.

Real-world examples

Transport protocols

Data storage formats

Other

Other ways of representing data

Core TCP/IP protocols (particularly IP, TCP, and UDP) use predefined, static fields.

Some application layer protocols, including HTTP/1.1 (and its non-standardized predecessors), FTP, SMTP, POP3, and SIP, use text-based "Field: Value" pairs formatted according to . (HTTP represents the length of the payload with a Content-Length header and separates headers from the payload with an empty line and headers from each other with a new line.)

ASN.1 specifies several TLV-based encoding rules (BER, DER), as well as non-TLV based ones (PER, XER, JSON Encoding Rules). The TLV-based rules can be parsed without knowing the possible members of the message, while the non-TLV/static PER cannot. XER uses XML, which also allows for parsing without knowing the possible members of the message; the same applies to the JSON encoding rules.

CSN.1 describes encoding rules using non-TLV semantics.

More recently, XML has been used to implement messaging between different nodes in a network. These messages are typically prefixed with line-based text commands, such as with BEEP.

See also

  • KLV, specific type of type-length-value encoding

References