science

Type–length–value

Type–length–value is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Type–length–value rather than just read about it. In short: 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.

Key takeaways

Type–length–value belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Type–length–value to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Type–length–value from memory before moving on to harder problems.

Reference excerpt

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

Real-world examples

Transport protocols TLS (and its predecessor, SSL) use TLV-encoded messages. SSH COPS IS-IS RADIUS Link Layer Discovery Protocol allows for the sending of organizational-specific information as a TLV element within LLDP packets Media Redundancy Protocol allows organizational-specific information Dynamic Host Configuration Protocol (DHCP) uses TLV-encoded options RR protocol used in GSM cell phones (defined in 3GPP 04.18). In this protocol, each message is defined as a sequence of information elements.

Data storage formats IFF Matroska uses TLV for markup tags QTFF (the basis for MPEG-4 containers)

Other ubus used for IPC in OpenWrt

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 RFC 2822. (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

Worked examples

Example 1 — a first encounter with Type–length–value

Start with the simplest possible case. Write down what Type–length–value claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Type–length–value before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Type–length–value ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Type–length–value

In research: Type–length–value appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Type–length–value in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Type–length–value is common in secondary-school and first-year university syllabi. It links to neighbouring topics Data serialization formats, Data transmission, Internet Standards, so understanding it makes those chapters shorter.
In everyday life: Look for Type–length–value outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Type–length–value in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Type–length–value means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Type–length–value out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Type–length–value in simple terms?

Why does Type–length–value matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Type–length–value?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Type–length–value.