- Email: [email protected]
Open systems Interconnection
communications
OpenSystems Interconnection Updateon communications standards by JOHN LARMOUTH
T
he IS0 7 layer model is formally described in the draft International Standard (DIS 7498) entitled Information Processing Open Systems
The IS0
7 layer model.
APPLICATIONS
PRESENTATION
Interconnection
-
Basic
This is a ‘draft for public comment’, and is obtainable from the British Standards Institute, and other national Standards bodies. The model partitions the problem of providing defined messages (and rules for sending them) for computer applications to interwork. It forms a framework within which standardization will occur, and does not in itself represent anything to which manufacturers’ offerings will conform.
Reference
Model.
SESSION
TRANSPORT
L
I3lATA LINK
PHYSICAL
Abstract: This paper outlines the work which b inprogress to establish International Standards to enable computer applications to inter-work between different manufacturers’ machines over linked wide area and local area networks, public andprivate. Keywords: data processing, computer networks, computer-computer communications.
LINK John Larmouth is director of computing University of Salford, UK.
vol 24 no 9
november 1982
0011-684W82/090027-03$03.00
0
at the
1982 Butterworth
The model describes seven layers of protocol. Typically, each layer contains at least one message which has a transparent field, and is used to carry messages defined in the next layer above. Each layer effectively provides a progressively richer communications path for the layer above it. The lower layers of the model give rise to the lower layer protocols. At the lowest layer the physical layer bits are - usually unreliably transferred. The link layer defines a protocol (message formats and rules for sending the messages), which provides a service to the layer above; it involves the reliable transfer of blocks of information point-to-point. Link layer protocols have been around for many years, and are usually described as comms rather than networking. The next layer is the network layer which can be loosely equated with the X25 level 3 recommendations; these provide end-to-end connections over PTT-provided public packet-switched data networks (PSS and IPSS in the UK). The IS0 network layer extends above X25 level 3, covering in addition the problems of tandem connections crossing, for example, a private X25
& Co (Publishers)
Ltd.
27
(sub) network, a public (sub) network, a private satellite link, a second public (sub) network, and a Cambridge Ring local area (sub) network. The major problem remaining to be solved in this area is that of addressing. The use of the term sub highlights the Edct that the IS0 network layer is concerned with the worldwide network comprising all linked subnetworks. The way addressing and routeing between subnetworks is to be handled has still to be resolved. The transport layer is the highest of what are usually called the lower layers. The transport layer messages are passed between two end-systems across linked subnetworks using the network service. The purpose of the layer is to provide additional reliability for any application which requires it. If the particular connection involved spans only a FIT supplied network, will probably most applications require no additional reliability; if it crosses less reliable private networks, an elaborate transport protocol may be required.
shared data structure - updated by one or other party, read by the other; these are virtual terminal protocols. The name reflects their intended use by a variety ofterminal handling applications, where the shared data structure represents the common view of what is on the screen. Other applications are, however, possible, the protocols in this area are grouped into classes, based on the complexity of the shared data-structure. In all cases they are concerned with issues of round-trip time. 0 The transfer of potentially large amounts of bulk data, mainly in a single direction, with no real endto-end application interaction during the transfer. This is the file transfer protocol; it can provide for transfer of files, the movement of files to printers and plotters, or even the copying of magnetic tapes between two machines over a network. It is primarily concerned with reliable transfer over system and work breaks, and with checkpoint-
ing to enable a long transfer to be interrupted and resumed part-way through. The organization of continuous activity based on the movement of bulk data using the file transfer protocol, and the control, display, and modification of such activity. This is the secalled job transfer and manipulation protocol; it is primarily concerned with activity involving the movement of documents followed by (or in parallel with) the associated movement of other documents, and so on indefinitely. The submission of a job and disposal of its output is just one simple case of this activity; the primary concern is with maintaining the overall activity across system and network breaks of long duration, and providing the necessary authorization mechanisms and the necessary commitment handshakes for concurrent activity. Electronic mail activity; the form of protocols which will emerge in this area is still unclear.
The higher layers The session layer may, for some applications, do noes, for others it will provide a service of regulating which of the two application programs is sending and which receiving. This two-way-alternate, or half-duplex, mode of operation is sometimes considered easier for a profaner to handle than full-duplex. The presentation layer is responsible for providing for the transfer of abstract entities between machines, rather that the bit patterns which are the characteristics of lower layers. This involves coping with EBCDIC and ASCII machines, or ones parity versus even parity ASCII, and should extend to agreed rules for the transfer of integers and real numbers. The final layer is the app~cation layer, in which protocols will be defined for the following activities: 0 A conversation
28
Honeywell was one of the companies which recently announced its allegiance to IS0 standards.
involving use of a
data processing
communications l
Message passing for transaction processing; this area also is still in its infancy.
Service and protocols The definition of each protocol is split into two parts; the first specifies the service of the layer, and the second specifies a protocol providing the required service. The services of the protocol must not be confused with the actual interfaces provided by an implementation on a particular computer. There is a conscious effort within IS0 not to constrain the way a protocol handler is implemented, nor the actual computer components which mirror the abstract model of the world used for service and protocol definition. This makes it difficult to spell out that some of the more sophisticated or complex applications of the protocol should be implemented. The aim of these Standards is to enable real user-level activity to take place between machines of different manufacturer; the extent to which this will be achieved for all but the simplest application of the protocols remains to be seen. Organizational
view
The IS0 Council has established a number of Technical Committees concerned with major industries (such as the chemical industry, building, and so on). TC97 is concerned with all aspects of information processing. TC97 has established a number of sub-committees of which the most relevant to this discussion are: . SC5 . SC6 l SC16 nection 0 SC18 cessors 0 SC20
Programming languages Low-level communications Open Systems InterconCommunicating
word pro-
Uust established) Security.
SC16 has established working groups. It is at the working group level, and in their constituent subgroups and ad hoc groups, that technical
vol 24 no 9
november 1982
activity takes place, with experts contributing to the work. At the SC16 level, representation is by Member Bodies - Standards Institutions with a formal voting procedure. Working groups operate by concensus among the experts. A working group will meet together for a week about once every nine to 12 months, with ad hoc groups meeting perhaps once inbetween. At these meetings contributions from Member Bodies will be considered by the experts, and one or more output documents produced. These output documents usually represent initial drafts of what will become Standards, and are progressively refined. Between the working group meetings, national committees and subcommittees of Standards Institutes will meet three or four times to consider the output documents and to prepare further national contributions. In practice, significant input also occurs from the liaison bodies of which the European Computer Manufacturer’s Association (ECMA) and CCITT are the most prominent. A work item is the IS0 term for activity which will lead to a Standard. SC16 is best known for the activity of its WGl in producing the 7-layer model. Work on the bottom three layers is assigned to SC6, electronic mail to SC18, security to SC20, and all aspects of programming language interfaces to SCS. Within SC16 there is a working group handling transport and session layers, one handling presentation and application layers (WGS), and a third handling management protocols. WGS has for long time operated as three fairly self-contained subgroups dealing with VTP, FTAMP, and JTMP. Regular attenders at the technical meetings vary from six to eight for the smallest to about 30 for the largest. Current status The 7-layer model is now a Draft International Standard, and seems
likely to progress to a fit11Standard in about 12 months; it is almost entirely based on two-Party communication using the concept of a connection at various levels between two parties. The USA Experts have pressed strongly for the extension of the model to cover connectionless activity in which datagrams and broadcast operation are covered. This extension is now seen as highly probable. Work is also in progress to consider whether the lower layer architecture should be substantially modified for local area or satellite networks. Thus although a DIS is with us, the work is in no way complete. The most recent development was the adoption in Tokyo in June 1982 of the working papers on a transport service and protocol as suitable material for a Draft Proposal. This is the IS0 term for a document which is one stage below a Draft International Standard. The session service and protocol is expected to follow it very quickly in February 1983, although we still await network layer standards. The timescale for high layer protocols is a little further away. There is a target of December 1983 for a DP for an initial presentation layer, although a great deal of work remains to be done, and of October 1983 for an initial file transfer protocol. VTP, JTMP, and management, are all scheduled for early 1984, probably with further work to follow in all areas. While there must remain worries about the rapidity of manufacturer implementation, and of a real interworking capability for complex apphcations (most present proposals have classes and subsets available), there are now grounds for cautious optimism. After a fairly long gestation period, the general form and facilities of most of the protocols is now fairly clear, and there is a noticeable sense of urgency in working groups and subcommittee meetings. We should soon see ,interworking between sites using Standard protocols - fingers crossed! ä)
29
OpenSystems Interconnection Updateon communications standards by JOHN LARMOUTH
T
he IS0 7 layer model is formally described in the draft International Standard (DIS 7498) entitled Information Processing Open Systems
The IS0
7 layer model.
APPLICATIONS
PRESENTATION
Interconnection
-
Basic
This is a ‘draft for public comment’, and is obtainable from the British Standards Institute, and other national Standards bodies. The model partitions the problem of providing defined messages (and rules for sending them) for computer applications to interwork. It forms a framework within which standardization will occur, and does not in itself represent anything to which manufacturers’ offerings will conform.
Reference
Model.
SESSION
TRANSPORT
L
I3lATA LINK
PHYSICAL
Abstract: This paper outlines the work which b inprogress to establish International Standards to enable computer applications to inter-work between different manufacturers’ machines over linked wide area and local area networks, public andprivate. Keywords: data processing, computer networks, computer-computer communications.
LINK John Larmouth is director of computing University of Salford, UK.
vol 24 no 9
november 1982
0011-684W82/090027-03$03.00
0
at the
1982 Butterworth
The model describes seven layers of protocol. Typically, each layer contains at least one message which has a transparent field, and is used to carry messages defined in the next layer above. Each layer effectively provides a progressively richer communications path for the layer above it. The lower layers of the model give rise to the lower layer protocols. At the lowest layer the physical layer bits are - usually unreliably transferred. The link layer defines a protocol (message formats and rules for sending the messages), which provides a service to the layer above; it involves the reliable transfer of blocks of information point-to-point. Link layer protocols have been around for many years, and are usually described as comms rather than networking. The next layer is the network layer which can be loosely equated with the X25 level 3 recommendations; these provide end-to-end connections over PTT-provided public packet-switched data networks (PSS and IPSS in the UK). The IS0 network layer extends above X25 level 3, covering in addition the problems of tandem connections crossing, for example, a private X25
& Co (Publishers)
Ltd.
27
(sub) network, a public (sub) network, a private satellite link, a second public (sub) network, and a Cambridge Ring local area (sub) network. The major problem remaining to be solved in this area is that of addressing. The use of the term sub highlights the Edct that the IS0 network layer is concerned with the worldwide network comprising all linked subnetworks. The way addressing and routeing between subnetworks is to be handled has still to be resolved. The transport layer is the highest of what are usually called the lower layers. The transport layer messages are passed between two end-systems across linked subnetworks using the network service. The purpose of the layer is to provide additional reliability for any application which requires it. If the particular connection involved spans only a FIT supplied network, will probably most applications require no additional reliability; if it crosses less reliable private networks, an elaborate transport protocol may be required.
shared data structure - updated by one or other party, read by the other; these are virtual terminal protocols. The name reflects their intended use by a variety ofterminal handling applications, where the shared data structure represents the common view of what is on the screen. Other applications are, however, possible, the protocols in this area are grouped into classes, based on the complexity of the shared data-structure. In all cases they are concerned with issues of round-trip time. 0 The transfer of potentially large amounts of bulk data, mainly in a single direction, with no real endto-end application interaction during the transfer. This is the file transfer protocol; it can provide for transfer of files, the movement of files to printers and plotters, or even the copying of magnetic tapes between two machines over a network. It is primarily concerned with reliable transfer over system and work breaks, and with checkpoint-
ing to enable a long transfer to be interrupted and resumed part-way through. The organization of continuous activity based on the movement of bulk data using the file transfer protocol, and the control, display, and modification of such activity. This is the secalled job transfer and manipulation protocol; it is primarily concerned with activity involving the movement of documents followed by (or in parallel with) the associated movement of other documents, and so on indefinitely. The submission of a job and disposal of its output is just one simple case of this activity; the primary concern is with maintaining the overall activity across system and network breaks of long duration, and providing the necessary authorization mechanisms and the necessary commitment handshakes for concurrent activity. Electronic mail activity; the form of protocols which will emerge in this area is still unclear.
The higher layers The session layer may, for some applications, do noes, for others it will provide a service of regulating which of the two application programs is sending and which receiving. This two-way-alternate, or half-duplex, mode of operation is sometimes considered easier for a profaner to handle than full-duplex. The presentation layer is responsible for providing for the transfer of abstract entities between machines, rather that the bit patterns which are the characteristics of lower layers. This involves coping with EBCDIC and ASCII machines, or ones parity versus even parity ASCII, and should extend to agreed rules for the transfer of integers and real numbers. The final layer is the app~cation layer, in which protocols will be defined for the following activities: 0 A conversation
28
Honeywell was one of the companies which recently announced its allegiance to IS0 standards.
involving use of a
data processing
communications l
Message passing for transaction processing; this area also is still in its infancy.
Service and protocols The definition of each protocol is split into two parts; the first specifies the service of the layer, and the second specifies a protocol providing the required service. The services of the protocol must not be confused with the actual interfaces provided by an implementation on a particular computer. There is a conscious effort within IS0 not to constrain the way a protocol handler is implemented, nor the actual computer components which mirror the abstract model of the world used for service and protocol definition. This makes it difficult to spell out that some of the more sophisticated or complex applications of the protocol should be implemented. The aim of these Standards is to enable real user-level activity to take place between machines of different manufacturer; the extent to which this will be achieved for all but the simplest application of the protocols remains to be seen. Organizational
view
The IS0 Council has established a number of Technical Committees concerned with major industries (such as the chemical industry, building, and so on). TC97 is concerned with all aspects of information processing. TC97 has established a number of sub-committees of which the most relevant to this discussion are: . SC5 . SC6 l SC16 nection 0 SC18 cessors 0 SC20
Programming languages Low-level communications Open Systems InterconCommunicating
word pro-
Uust established) Security.
SC16 has established working groups. It is at the working group level, and in their constituent subgroups and ad hoc groups, that technical
vol 24 no 9
november 1982
activity takes place, with experts contributing to the work. At the SC16 level, representation is by Member Bodies - Standards Institutions with a formal voting procedure. Working groups operate by concensus among the experts. A working group will meet together for a week about once every nine to 12 months, with ad hoc groups meeting perhaps once inbetween. At these meetings contributions from Member Bodies will be considered by the experts, and one or more output documents produced. These output documents usually represent initial drafts of what will become Standards, and are progressively refined. Between the working group meetings, national committees and subcommittees of Standards Institutes will meet three or four times to consider the output documents and to prepare further national contributions. In practice, significant input also occurs from the liaison bodies of which the European Computer Manufacturer’s Association (ECMA) and CCITT are the most prominent. A work item is the IS0 term for activity which will lead to a Standard. SC16 is best known for the activity of its WGl in producing the 7-layer model. Work on the bottom three layers is assigned to SC6, electronic mail to SC18, security to SC20, and all aspects of programming language interfaces to SCS. Within SC16 there is a working group handling transport and session layers, one handling presentation and application layers (WGS), and a third handling management protocols. WGS has for long time operated as three fairly self-contained subgroups dealing with VTP, FTAMP, and JTMP. Regular attenders at the technical meetings vary from six to eight for the smallest to about 30 for the largest. Current status The 7-layer model is now a Draft International Standard, and seems
likely to progress to a fit11Standard in about 12 months; it is almost entirely based on two-Party communication using the concept of a connection at various levels between two parties. The USA Experts have pressed strongly for the extension of the model to cover connectionless activity in which datagrams and broadcast operation are covered. This extension is now seen as highly probable. Work is also in progress to consider whether the lower layer architecture should be substantially modified for local area or satellite networks. Thus although a DIS is with us, the work is in no way complete. The most recent development was the adoption in Tokyo in June 1982 of the working papers on a transport service and protocol as suitable material for a Draft Proposal. This is the IS0 term for a document which is one stage below a Draft International Standard. The session service and protocol is expected to follow it very quickly in February 1983, although we still await network layer standards. The timescale for high layer protocols is a little further away. There is a target of December 1983 for a DP for an initial presentation layer, although a great deal of work remains to be done, and of October 1983 for an initial file transfer protocol. VTP, JTMP, and management, are all scheduled for early 1984, probably with further work to follow in all areas. While there must remain worries about the rapidity of manufacturer implementation, and of a real interworking capability for complex apphcations (most present proposals have classes and subsets available), there are now grounds for cautious optimism. After a fairly long gestation period, the general form and facilities of most of the protocols is now fairly clear, and there is a noticeable sense of urgency in working groups and subcommittee meetings. We should soon see ,interworking between sites using Standard protocols - fingers crossed! ä)
29