Within data communication protocols, optional information may be encoded as a type-length-value or TLV element inside a protocol. TLV is also known as tag-length value.
The type and length are fixed in size (typically 1-4 bytes), and the value field is of variable size. These fields are used as follows:
TypeA binary code, often simply alphanumeric, which indicates the kind of field that this part of the message represents;LengthThe size of the value field (typically in bytes);ValueVariable-sized series of bytes which contains data for this part of the message.
Some advantages of using a TLV representation:
- TLV sequences are easily searched using generalized parsing functions;
- New message elements which 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 which makes parsing faster and the data smaller;
- It is easier to generate XML from TLV to make human inspection of the data possible.
Imagine a message to make a telephone call. In a first version of a system this might use two message elements, a "command" and a "phoneNumberToCall":command_c/4/makeCall_c/phoneNumberToCall_c/8/"722-4246"
phoneNumberToCall_care integer constants and 4 and 8 are the lengths of the "value" fields, respectively.
Later (in version 2) a new field containing the calling number could be added:command_c/4/makeCall_c/callingNumber_c/14/"1-613-715-9719"/phoneNumberToCall_c/8/"722-4246"
A version 1 system which received a message from a version 2 system would first read the
command_celement and then read an element of type
callingNumber_c. The version 1 system does not understand ;
so the length field is read (i.e. 14) and the system skips forward 14 bytes to read
which it understands, and message parsing carries on.
An example of usage is the Link Layer Discovery Protocol which allows for the sending of organizational-specific information as a TLV element within LLDP packets. Another example is the RR protocol used in GSM cell phones, defined in 3GPP 04.18.
In the RR protocol, each message is defined as a sequence of information elements.
Many other protocols use TLVs, such as COPS, IS-IS, and RADIUS.
Other ways of representing data
ASN.1 specifies several TLV-based encoding rules (BER, DER), as well as non-TLV based ones (PER, XER).
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.