Type-length-value

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Within data communication protocols, optional information may be encoded as a type-length-value or TLV element inside of the protocol.

The type and length fields are fixed in size (1-4 bytes) and the value field is of variable size. These fields are used as follows:

type - a numeric code 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 set of bytes which is contains data for this part of the message

Some of the advantages of using a TLV representation is:

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 are typically used in a binary format which makes parsing faster and the data smaller

An example of TLVs in action follows. 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"
where command_c, makeCall_c and phoneNumberToCall_c are integer constants and 4 and 8 are the lengths of the "value" fields.
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 revision 1 system which received a message from a version 2 system would read the "command_c" element and then would read an element of type callingNumber_c. The revision 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.

Other ways of representing data in communication protocols are ASN.1 and CSN.1. More recently XML has been used to implement messaging between differnet nodes in a network.

Type-length-value

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