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provided by the CCSDS Member Agency Control Authorities (CA). These CAs become 
focal points for the acquisition of metadata. The data registration procedures establish a 
global data identification mechanism, which combined with standard data labelling and 
conventions for the aggregation of modular data objects, enables the comprehensive self 
identification process needed to support data interchange and data archiving. 
Rather than being a standard format into which one puts his data, the 
SFDU concept focuses on the standard labelling of data to enhance the 
understanding of transmitted and stored data. The data may be in any user 
defined arrangement that can be expressed in a precise way. 
Basic Structure 
A Type-Length-Value Object (TLVO) is a self identified and self delimited data object 
which follows CCSDS labelling rules (figure 2). It consists of a fixed length label followed 
by a variable length Value field. The purpose of the label is twofold: 1) to delimit the 
Value field and 2) to provide the means by which the format and semantics of the Value 
field might be understood. The basic structure of the TLV object is given in the figure 
below. The two fields of the label are: a) the Type field (which includes the referenced 
name of the description of the Value field) and, b) the LENGTH field (which provides the 
length of the Value field). The Value field may contain data elements or embedded 
TLVOs. 
In the Type field, the Control 
Authority Identifier (CAID) identifies 
the CA office which maintains the 
format definition. The VERSION 
field (V) gives the structure of the 
label, the CLASS field (C) gives a 
high level classification of the content 
of the Value field, and SP denotes 
two spare characters. The Data 
Description ID (DDID) uniquely 
defines (within a CA) the data definition information applying to the data in the Value field. 
The CAID and the DDID together (called the Authority and Description Identifier or ADI) 
provide a globally unique reference name for the description of the Value field. 
The Class ID is a one character field that is used to characterize the content of the Value 
field. The Class identification codes along with their current status are defined in reference 
[1]. These classification codes are intended to provide rapid identification of the Type of 
data contained within a data entity to facilitate operations of software utility services. 
Type CAID V C SP DDID 
Length Length of Value 
12 bytes 
8 bytes 
Data Elements 
Value / or / 
more TLV Objects I 
Figure 2: Structured TLV Object