Models for Open System Protocol Development : A Technical Report (1994) - UDT Series on Data Communication Technologies and Standards for Libraries

As of 22 April 2009 IFLA has a totally redesigned new website

This old website and all of its content will stay on as archive – http://archive.ifla.org

*UDT Series on Data Communication Technologies and Standards for Libraries* Models for Open System Protocol Development : A Technical Report (1994) 13. Protocols Beneath The Application: OSI and TCP/IP The basic OSI model (ISO 7498), defined by ISO/IEC JTC1, describes seven layers of functionality necessary for communication to pass from the physical network up through to the application ("the top"). Different activities essential to the communication are performed at each level. The bottom 4 layers, called the lower layers, are concerned primarily with the physical connection; and the top 3, called the upper layers, are concerned primarily with session control and data representation, in the network. ISO specified a suite of ISO standards for services needed and protocols to support them for each layer in the ISO model. These ISO OSI service and protocol standards have strong functionality but are not yet widely implemented in networks. Instead, another network protocol suite, the Transmission Control Protocol/Internet Protocol (TCP/IP) which originated with the ARPANET of the Advanced Research Project Agency in the United States Department of Defense is used in many networks. These networks are connected into a network of networks called the Internet. The formal standards for the TCP/IP protocol suite are called Requests for Comments (RFC). 13.1 Lower Layers The lower layers in network architectures provide services that make it possible to connect two end-systems, to send data between them, and to perform error checking and resend erroneous data. In other words, the lower layers offer the service of moving data between two systems. The lower layers of the OSI and Internet (TCP/IP) protocol suites can be compared, although the services provided are not placed at exactly the same layers in the two protocols. The table below shows the Internet protocols roughly positioned in the OSI model framework. --------------------------------------------------------------------------- Layer OSI Name and Protocol Internet Name and Protocol --------------------------------------------------------------------------- 1-4 Lower Layers Lower Layers 1 Physical X.21, etc. different physical nets 2 Link HDLC RFC 826, RFC 894, etc. 3 Network X.25 and CNLP Internet IP 4 Transport ISO 8073 Host-to-host TCP ---------------------------------------------------------------------------- Network Lower Layers Physical Layer. Layer 1 is the physical medium used in the network. Services in layer 1 are the actual sending of the electric impulses (bits). Link Layer. Layer 2 in OSI contains the frame definitions (how to interpret the received bit stream) and the physical network addressing. OSI networks use the High-level Data Link Control (HDLC) protocol; the Internet uses TCP/IP RFC's such as 826 and 894. Network Layer. In layer 3 the routing of the messages take place. The CCITT protocol X.25 has been adopted by ISO, even though it contains some of the error-checking facilities that originally were defined in the transport layer within the OSI model. For the Internet protocol, IP covers approximately the CNLP services used in OSI layer 3. Transport Layer. The main task for layer 4 is to guarantee reliable communication. The OSI Transport protocols defined in ISO 8073 are TP0-TP4 (representing 5 classes of transport functionality, 0-4). All classes perform error-checking, but the difference between them is the type and degree of error checking. The transport layer also contains services for dividing the data messages into "packages". Each package must be small enough to fit into the data part of a layer 3 message. And finally, layer 4 contains services for directing the received data to the correct application within a computer. The equivalent functionality in the Internet protocol suite is provided by TCP, which corresponds to OSI TP4. 13.2 Upper Layers While the lower layers in the network concentrate on moving data (without errors) between two systems, the upper layers in a network handle the data processing. The upper layers may provide services such as session control, check-point facilities, dialogue/activity control, recovery, agreement on different syntaxes to use, and agreement on which application protocols to use. Three upper layers are defined in the OSI model, session, presentation and application, while there is only one, application, in the Internet model. The application layer in OSI is itself also conceptually structured. Fewer application services have been defined within the Internet upper layer than for the OSI application layer, although some application protocols can be used over both OSI and TCP/IP. ------------------------------------------------------------------------ Layer OSI Name and Protocol Internet Name and Protocol ------------------------------------------------------------------------ 5-7 Upper Layers Upper Layers 5 Session ISO 8327 6 Presentation ISO 8823 7 Application ISO 8650 & Application well-known port & SR, ILL, etc. SR, ILL, etc. ------------------------------------------------------------------------- Network Upper Layers Session Layer. The session layer consists of services that establish connections and control communication between systems such as the synchronization of the communication. OSI has a well-defined session protocol, ISO 8327, but TCP/IP does not provide session-related functionality. Presentation Layer. The role of the presentation layer has changed as OSI developed. As more applications have been defined, additional services at the presentation layer have become necessary. Services such as negotiating the presentation context and the encoding/decoding of application messages are offered by the presentation layer. An illustration of the OSI use of presentation is in the structuring of the SR and ILL protocol messages which are described using ASN.1. When a message is sent to the lower OSI layers, it is one string of bytes (octets) constructed by the presentation layer program. The program uses the ASN.1 description and the real data handed down from the application to construct, using BER, a string of fields. (See section 9.2 for a description of ASN.1.) TCP/IP ignores presentation issues and conversion of messages to ASN.1 takes place at the application level. Application Layer. The OSI application layer is itself composed of parts: 1) a general association control part, and 2) general application and the specific application services. The specific services may use one or more of the general application services, and all use the control service, Association Control Service Element (ACSE) ISO 8649 (Service) and ISO 8650 (Protocol). The application layer is still regarded as one layer, however. The application services interact directly with the application on the host machine. The application layer in the Internet environment is a single application protocol with a well-known port. 13.3 Comparison Between OSI and Internet Protocols The OSI model was defined over a decade ago, but the necessary protocols did not become standards until the late eighties. The implementations of pure OSI networks are therefore of recent date, and are few. Some commercial OSI products are on the market, but not many. TCP/IP, meanwhile, has been used for a long time by academic institutions, and now it is also possible for non-academic institutions to connect to the Internet. The mere presence of a working network gives it an advantage over OSI. Since Internet protocol implementations not only exist, but are widespread, while OSI is in its infancy, today's problem is how to make the two types of network interact. 13.3.1 OSI and TCP/IP at the Lower Layers A comparison between the lower layers of the OSI and TCP/IP protocols shows that almost the same basic services are offered. They both provide network routing, end-to-end connection, and send/receive services that guarantee the error-free communication of application messages. The Internet protocols are organized in a layered model like OSI protocols, but the model is simpler than the OSI and the tasks are not divided exactly as they are in the OSI model. Because the two networks have divided the services differently between the layers, and because the structure of the messages used in the communication are different, it is impossible for networks using the two different protocol suites to interoperate directly. It should be emphasized, however, that seen from the end-user's point of view, the TCP/IP and the OSI offer equally "good" network services on the lower level. 13.3.2 OSI and Internet Protocols in the Upper Layers As described above, the lower protocol layers have two distinct but equal protocol stacks in use, OSI and TCP/IP. In the upper layers, Internet protocols do not provide the services specified by the layers 5 and 6 in the OSI model. Neither do they offer the service of identifying the application context as ACSE in OSI layer 7 does. This means that the Internet implementors are left to handle these functions in a different way as part of the application protocol. Other OSI services missing from the Internet protocols include: direction control, check point control and synchronization, dialogue/activity control, recovery, syntax manipulation, and application context. In the application layer, fewer application protocols are defined for the Internet than for OSI, but several new protocols are under development and some of the OSI protocols can be used over TCP/IP. The addressing in the Internet for connecting to another computer (binding) is simpler than in OSI. The addition of another node in the network is far easier in the Internet than in an OSI network due to the global addressing system of the Internet. In the Internet, File Transfer Protocol (FTP), is a simple file transfer compared to OSI's FTAM. But for libraries, the best way to use FTAM is to use unstructured files which reduces the complexity of FTAM. The library protocols SR (and Z39.50) and ILL may be used on top of both OSI and TCP/IP. When using TCP/IP the upper layer functionality is provided partially by the initialize service and are partially ad hoc. 13.4 Interoperability Solutions Between OSI and Internet Protocols The Internet has many nodes around the world. According to one source, the Internet consists in 1994 of 1.8 million nodes in several thousand networks, with approximately 10 million users. The protocols employed, TCP/IP, are easily available and the expertise to implement and use them is widespread. On the other hand, there are few implementations of pure OSI networks, although the numbers are increasing. Many institutions have as a goal to migrate to OSI as soon as the OSI protocols become available as commercial products. It is going to be desirable to enable networks using TCP/IP protocols to communicate with those using OSI protocols through gateways that translate an Internet service into an OSI service and vice versa. Several studies have taken place in order to find out the best way to connect OSI networks with the Internet. Gateways. If Internet and OSI networks had the same structure and offered the same set of services (on the same relative level), the task for a gateway would simply be to translate the messages. But as this is not the case, the task for a gateway becomes much more complex. Gateways have been implemented for specific services, e.g., the electronic mail. These gateways makes it possible to send electronic mail between OSI nodes that use the X.400 mail application protocol and Internet nodes using Simple Mail Transfer Protocol (SMTP). Skinny Stacks. Since OSI is general and must suit many different users, the set of services are numerous. For basic communication, experimentation is taking place in a Skinny stack project. The specification of a minimal OSI (mOSI) upper layer stack for basic communication is being worked on by the regional OSI workshop in North America (OIW) and it will be harmonized with EWOS (European Workshop for Open Systems) and AOW (Asian OSI Workshop). In the mOSI, the session, presentation and ACSE part of the application are independent of the transport protocol. This work has started in order to make easier the implementation of OSI and increase interoperability with Internet systems by making mOSI work over the TCP/IP lower layers. ISODE. Another interoperability solution, the system ISODE (ISO Development Environment) is used in many projects. ISODE enables institutions to implement an OSI application over TCP/IP fairly easy. ISODE provides the OSI services for the transport (optional), session and presentation layers as well as ACSE, and it provides an interface to TCP/IP. ISODE is therefore a tool which makes it possible to run mixed OSI and TCP/IP stacks. ISODE is a possible solution to the present situation for library systems, but we do not know how ISODE based systems will behave when the number of transactions in the network increase. Abbreviations ACID � atomicity, consistency, isolation, and durability ACSE � Association Control Service Element AE � Application Entity AEI � Application Entity Invocation ALD � A-profile for Library and Documentation AOW � Asian OSI Workshop APDU � Application Protocol Data Unit ASE � Application Service Element ASN.1 � Abstract Syntax Notation One BER ASN.1 � Basing Encoding Rule CAN/MARC � CANadian MARC format CCF � Common Communication Format CCITT � Comité Consultatif International Télégraphique et Téléphonique (from 1993-03-01 TSB) CCL � Common Command Language CCR � Concurrency Commitment and Recovery CF � control function CLI � Call Level Interface CWIS � Campus Wide Information system DBMS � DataBase Management System DFR � Document Filing and Retrieval DS � Document Store DSA � Directory Service Agent DUA � Directory User Agent EDI � Electronic Data Interchange EDIFACT� Electronic Data Interchange For Administration, Commerce and Trade EWOS � European Workshop for Open Systems FTAM � File Transfer, Access and Management FTP � File Transfer Protocol IEC � International Electrotechnical Commission IFLA � International Federation of Library Associations and Institutions IFOBS � International Forum for Open Bibliographic Systems ILL � Interlibrary Loan IPM � InterPersonal Message IPRL � International Profile Requirement List ISO � International Organization for Standardization ISODE � ISO Development Environment ISP � International Standardized profile JTC1 � Joint Technical Committee 1 (Information Technology) MARC � MAchine Readable Cataloguing MHS� Message Handling System MS � Message Store MTA � Message Transfer Agent MTS � Message Transfer System NORMARC � NORwegian MARC format ODA � Office Document Architecture ODIF � Office Document Interchange Format OID � Object Identifier OIW � OSI Workshop (in North America) OSI � Open Systems Interconnection PDU � Protocol Data Unit PICS � Protocol Implementation Conformance Statement RDA � Remote Database Access RFC � Request for Comments (Internet standards) ROSE � Remote Operations Service Element RPC � remote procedure call RPN � Reverse Polish Notation RTSE � Reliable Transfer Service Element SDIF � SGML Document Interchange Format SGFS � Special Group on Functional Standardization SGML � Standard Generalized Markup Language SMTP � Simple Mail Transfer Protocol SQL � Structured Query Language SR � Search and Retrieve TC46 � Technical Committee 46 (Information and Documentation) TC46 SC4 � TC46, Subcommittee 4 (Computer Applications in Information Technology) TC46 SC4 WG4 � TC46, SC4, Working Group 4 (Protocols and Formats) TCP/IP � Transmission Control Protocol/Internet Protocol TP � Transaction processing TSB � Telecommunication Standardization Bureau UA � User Agent UKMARC � UK MARC format UNIMARC � Universal MARC format UPI � Unique Permanent Identifier USMARC � USMARC format Standards Referenced ISO 2709: Information and Documentation -- Format for Information Exchange ISO/IEC 7498: Information Processing Systems -- Open Systems Interconnection -- Basic Reference Model ISO/IEC 8073: Information Processing Systems -- Open Systems Interconnection -- Connection Oriented Transport Protocol Specification ISO/IEC 8326: Information Processing Systems -- Open Systems Interconnection -- Basic Connection Oriented Session Service Definition ISO/IEC 8327: Information Processing Systems -- Open Systems Interconnection -- Basic Connection Oriented Session Protocol Specification ISO 8459: Information and Documentation -- Bibliographic Data Element Directory ISO/IEC 8571: Information Processing Systems -- Open Systems Interconnection -- File Transfer, Access and Management (FTAM) ISO/IEC 8613: Information Processing -- Text and Office Systems -- Office Document Architecture (ODA) and Interchange Format ISO/IEC 8649: Information Processing Systems -- Open Systems Interconnection -- Service Definition for Association Control Service Element ISO/IEC 8650: Information Processing Systems -- Open Systems Interconnection -- Protocol Specification for Association Control Service Element ISO 8777: Information and Documentation -- Commands for Interactive Text Searching (CCL) ISO/IEC 8822: Information Processing Systems -- Open Systems Interconnection -- Connection Oriented Presentation Service Definition ISO/IEC 8823: Information Processing Systems -- Open Systems Interconnection -- Connection Oriented Presentation Protocol Specification ISO/IEC 8824: Information Processing Systems -- Open Systems Interconnection -- Specification of Abstract Syntax Notation One (ASN.1) ISO/IEC 8825: Information Processing Systems -- Open Systems Interconnection -- Specification of Basic Encoding Rules for Abstract Syntax Notation One (BER) ISO/IEC 8879: Information Technology -- Text and Office Systems -- Standard Generalized Markup Language (SGML) ISO/IEC 9069: Information Technology -- Text and Office Systems -- SGML Support Facilities -- SGML Document Interchange Format (SDIF) ISO/IEC 9072-1: Information Processing Systems -- Text Communication -- Remote Operations ISO/IEC 9075: Information Technology -- Database Languages -- SQL ISO/IEC 9545: Information Processing Systems -- Open Systems Interconnection -- Application Layer Structure ISO/IEC 9579: Information Processing Systems -- Open Systems Interconnection -- Remote Database Access (RDA) ISO/IEC 9594 (and CCITT X.500): Information Processing Systems -- Open Systems Interconnection -- The Directory. ISO/IEC 9735: Electronic Data Interchange for Administration Commerce and Transport (EDIFACT) --Application Level Syntax Rules ISO/IEC 9804: Information Processing Systems -- Open Systems Interconnection -- Service Definition for the Commitment, Concurrency, and Recovery Service Element (CCR) ISO/IEC 9805: Information Processing Systems -- Open Systems Interconnection -- Protocol Specification for the Commitment, Concurrency, and Recovery Service Element (CCR) ISO/IEC 9834: Information Processing Systems -- Open Systems Interconnection -- Procedures for the Operation of OSI -- Registration Authorities ISO/IEC TR 10000-1: Information Technology -- Framework and Taxonomy of International Standardized Profiles. Part 1: Framework ISO/IEC TR 10000-2: Information Technology -- Framework and Taxonomy of International Standardized Profiles. Part 2 Taxonomy of Profiles ISO/IEC 10021 (and CCITT X.400): Information Technology -- Text Communication -- Message Oriented Text Interchange System (MOTIS) ISO/IEC 10026: Information Technology - Open Systems Interconnection - Distributed Transaction Processing (TP) ISO 10160: Information and Documentation -- Open Systems Interconnection -- Interlibrary Loan Application Service Definition (ILL) ISO 10161: Information and Documentation -- Open Systems Interconnection -- Interlibrary Loan Application Protocol Specification (ILL) ISO 10162: Information and Documentation -- Open Systems Interconnection -- Search and Retrieve Application Service Definition (SR) ISO 10163: Information and Documentation -- Open Systems Interconnection -- Search and Retrieve Application Protocol Specification (SR) ISO/IEC 10166: Information Technology -- Text and Office Systems -- Document Filing and Retrieval (DFR) ANSI/NISO Z39.50: Information Retrieval Application Service Definition and Protocol Specification for Open Systems Interconnection


Latest Revision: April 27, 1995	Copyright © 1995-2000 International Federation of Library Associations and Institutions www.ifla.org