BS 7090-2:1989 download.Open Systems Interconnection: file transfer, access and management Part 2: Virtual filestore definition.
o Introduction
ISO 8571 is one of a set of International Standards produced to facilitate the interconnection of computer systems. It is related to other International Standards in the set as defined by the Reference Model for Open Systems Interconnection (ISO 7498). The Reference Model subdivides the area of standardization for interconnection into a series of layers of specification, each of manageable size.
The aim of Open Systems Interconnection is to allow, with a minimum of technical agreement outside the interconnection standards, the interconnection of computer systems
a) from different manufacturers,
b) under different managements,
c) of different levels of complexity,
d) of different ages.
ISO 8571 defines services for file transfer, access and management. It also specifies a protocol available within the application layer of the Reference Model. The service defined is of the category Application Service Element (ASE). It is concerned with identifiable bodies of information which can be treated as files, stored and managed within open systems, or passed between application processes.
ISO 8571 defines a basic file service. It provides sufficient facilities to support file transfer, file access, and management of files stored on open systems. ISO 8571 does not specify the interfaces to a file transfer, access or management facility within the local system.
ISO 8571 consists of the following four parts:
— Part 1: General introduction;
— Part 2: Virtual Filestore definition;
— Part 3. File Service definition;
— Part 4: File Protocol specification.
The definition in this part of ISO 8571 is used in the subsequent parts of ISO 8571 which specify services and protocols. They reference the filestore definition in order to assign meaning to the various descriptive data items which they manipulate. This definition will also be used by protocol implementors when
choosing a mapping from the protocol items onto their real storage mechanism.
This part of ISO 8571 contains the following annexes which form part of the standard:
Annex A — File access structure constraint sets; Annex B — Document types;
5 Basic concepts
A virtual filestore is handled by one or more application entities; an initiator may communicate with these by initializing an FTAM regime, in which the file service is provided. Each application entity that is capable of operating as a responder in an FTAM regime supports a single virtual filestore. Thus an application entity title serves to identify a virtual filestore.
NOTE Mechanisms for allocation of unambiguous application entity titles and for the corresponding directory services to provide addresses for communication with the entity are outside the scope of ISO 8571.
A filestore may contain an arbitrary number (greater than or equal to one) of objects (see Figure 1).
The properties of each object are defined by the values of a set of object attributes. These attributes are global; at any one time, a single attribute value is available to all initiators. Different object types may have distinct types of attributes, as well as types of attributes in common.
Each file is either empty or has some contents and a structure. Some of the file attributes identify structural aspects of the contents.
Each file-directory maintains a parenthood relationship with zero or more subordinate objects. Some of the file-directory attributes may identify access control information to subordinate objects.
Each reference maintains a link to exactly one other object. The referent is either a file or a file-directory. The identity of the referent is available as an attribute of the reference, in the form of a (possibly incomplete) primary pathname. This attribute can not be changed. Other reference attributes may identify the object type and access control information to the linked object.. if the identity of the referent changes, the corresponding reference ceases to exist.
There is a set of activity attributes associated with each FTAM regime. The activity attributes are of two types.
The first are in one to one correspondence with the object attributes, and indicate the active value of those attributes as perceived by the initiator.
The second type of activity attributes are current values of state information concerning the exchanges taking place within an FTAM regime, and the facts established about the initiator. These are usually derived from corresponding parameters in protocol exchanges. An initiator has available the description of only the FTAM regimes in which it is participating.
The File Access Data Units (FADUs), being in one-to-one correspondence with the subtrees, can be identified in the same manner as subtrees (that is, by their root nodes). Likewise, data units are always associated with a node and may be identified by the identification of the node. The general hierarchical structure can represent a wide range of different practical file structures. However, real systems can only support a limited range of structures, and there are restrictions on the way files can be modified. To express this, the concept of a constraint set is introduced. A constraint set defines limitations on the range of structures allowed, and expresses the way in which the basic access actions can modify the structure, Constraint sets reflecting certain common file types are defined in this part of ISO 8571 (see Annex A), but other constraint sets may subsequently be defined and registered.
NOTE It is expected that a registration authority will be established to maintain a register of constraint sets.
7.2 Abstract structure definition
The access structure of the hierarchical file model is specified in Figure 4, using ASNI. The data unit contents may be expressed in ASN.1, as in Figure 3, or in some other abstract syntax notation.
Files are accessed in terms of File Access Data Units (FADU), which are equivalent to the subtrees in the general hierarchical model. The smallest amount of data to be specified for access is one Data Unit (DU). For the purpose of transfer with checkpointing, the Data Units may be divided into smaller parts called data elements (DE). It is not possible to access the individual data elements of a Data Unit by means of the filestore actions defined in ISO 8571; the data unit is accessed as a whole.
7.3 Abstract syntax definition
For the purpose of providing access to the structure of files in the FTAM hierarchical file model BS 7090-2 assigns the ASNi object identifier value
{ iso standard 8571 abstract-syntax (2) ftam-fadu (2)
as an abstract syntax name for the set of presentation data values, each of which is a value of the ASN.I type ISO 857 1-FADU. Structuring- Data- Element.
The corresponding object descriptor value shall be
‘FTAM F’ADU”
The ASN.1 object identifier and object descriptor values
joint-iso-ccitt rnrnl (1) basic-encoding (1) and
“Basic Encoding of a single ASN.1 type” (assigned to an information object in ISO 8825) can be used as a transfer syntax name with this abstract syntax.