Within data communication protocols, optional information may be encoded as a type-length-value or TLV element inside of the protocol. TLV is also known as tag-length value.[1]
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:
Some advantages of using a TLV representation:
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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":
Here command_c
, makeCall_c
and phoneNumberToCall_c
are 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:
A version 1 system which received a message from a version 2 system would first read the command_c
element and then read an element of typecallingNumber_c
. The version 1 system does not understand ;callingNumber_c
so the length field is read (i.e. 14) and the system skips forward 14 bytes to read
phoneNumberToCall_c
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.
Core TCP/IP protocols (particularly IP, TCP, and UDP) use predefined, static fields.
Common TCP/IP-based protocols such as HTTP, FTP, SMTP, POP3, and SIP use text-based "Field: Value" pairs formatted according to RFC 2822.
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.