Usenet 1994 October/usenetsourcesnewsgroupsinfomagicoctober1994disk1.iso/answers/osi-protocols

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From: [e-mail] (Markus Kuhn)
Newsgroups: comp.protocols.iso,comp.answers,news.answers
Subject: comp.protocols.iso FAQ
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Followup-To: comp.protocols.iso
Date: Tue, 14 Dec 1993 20:17:39 +0100
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Summary: Introductory information about OSI, a family of internationally
         standardized computer communication protocols and services.
Keywords: OSI, computer networks, standards, data communication, open systems
Xref: senator-bedfellow.mit.edu comp.protocols.iso:4999 comp.answers:3013 news.answers:15807

Archive-name: osi-protocols
Last-modified: $Date: 93/12/14 20:15:56 $
Version: $Revision: 1.9 $

Frequently Asked Questions about OSI with Answers
-------------------------------------------------

This text is a monthly posting to the USENET group comp.protocols.iso.
Its purpose is to give answers to some of the questions appearing most
often in this group, to offer a minimal starting point in the learning
curve of OSI beginners and to collect interesting information that
appeared in USENET discussions.

Another FAQ (Archive-Name: standards-faq) which is posted to
comp.protocols.iso and comp.std.misc contains information about the
international standardization organizations (ISO, CCITT, ...), where
their documents are available, what Internet recources provide
information about standards etc.

This FAQ is crossposted to news.answers and won't expire there on
well-managed news systems until the next version has been posted. As a
consequence of being crossposted to news.answers, this text will also
be automatically archived on many FAQ servers all over the world (e.g.,
look with anonymous ftp at rtfm.mit.edu in /pub/usenet/news.answers).
You'll also find there many other answers to frequently asked questions.

The author of this FAQ has lost much of his enthusiasm for OSI since
the first version has been posted and won't spend too much time in
maintaining it. If you have much more energy left for this subject and
are interested in continuing with the FAQ, just contact me! I'll
concentrate on this text's brother, the Standards FAQ, which has
received much more interest and contributions.

I hope you enjoy it ...

Markus


Contents
--------

  What is OSI?
  What is the OSI reference model?
  What is ASN.1?
  What OSI standards are there?
  How are OSI and TCP/IP related?
  Which is better: TCP/IP or OSI?
  OSI criticism
  Which free OSI implementations are available?
! New and planned OSI standards
  Books and Journals

A '+' in the first column marks a topic that has been added since this
FAQ has been posted the last time and a '!' marks a change. Trivial
typographic changes are not marked.



What is OSI?
------------
                   "the first successful worldwide attempt to develop a set
                    of comprehensive standards for computer communications"
                                                             (Uyless Black)

OSI is an abbreviation for Open Systems Interconnection. In the early
80's, people in several standardization committees all over the world
felt that the time had come to develop a set of non-proprietary
standard protocols. They hope that one day these protocols will replace
most of the vendor-dependent specifications and that this will make the
way free for easy and flexible world-wide computer communication. It
took nearly a decade before the first results were produced: a
reference model and a set of layer standards from physical cable
definitions up to distributed databases and information systems,
together with management and security tools. The process of
standardization hasn't been finished yet and probably won't be in the
next decade, but we already have a powerful set of protocols for the
most important applications. These specifications have been published
as ISO standards and CCITT recommendations.


What is the OSI reference model?
--------------------------------

A good international standard should be flexible and extensible. In
order to achieve this goal, it seems to be a good idea to separate a
complex structure like a computer protocol in several modules. Each
module should be of manageable size and, if different techniques are
available for a certain aspect of a protocol, then the separation into
modules makes it quite easy to change this part of the whole system
(e.g. the cable specification or the encoding of data) without touching
the rest of the specification.

The OSI Reference Model (RM) defined in ISO 7498 divides the
communication process between two application programs into 7
intermediate layers. Each layer provides a certain kind of service to
the next higher layer. This service is provided by communicating with
the peer entity in same layer of the remote host using the service
provided by the next lower layer. Some of the layer entities may be
implemented in physical devices, some may be part of the operating
system and some may be included in application programs. Most layers
provide their service by forwarding protocol data units to the next
layer together with an added or removed header or by performing other
functions and state changes.

The OSI Reference Model only defines the abstract notion of layers and
does not specify whether the boundaries between the layers have to be
visible and documented in implementations. There are other standards
that define application program interfaces (APIs) between the operating
system and the application and these APIs often correspond to a layer
boundary in the OSI Reference Model.

The classic standard diagram that is normally used to describe the OSI
RM looks like this:


     End System                      End System

     +-------+                       +-------+
     |   7   |                       |   7   |     Application Layer
     +-------+                       +-------+
     |   6   |                       |   6   |     Presentation Layer
     +-------+                       +-------+
     |   5   |      Intermediate     |   5   |     Session Layer
     +-------+         System        +-------+
     |   4   |                       |   4   |     Transport Layer
     +-------+     +-----------+     +-------+
     |   3   |     |     3     |     |   3   |     Network Layer
     +-------+     +-----+-----+     +-------+
     |   2   |     |  2  |  2  |     |   2   |     Data Link Layer
     +-------+     +-----+-----+     +-------+
     |   1   |     |  1  |  1  |     |   1   |     Physical Layer
     +-------+     +-----+-----+     +-------+
         |            |     |            |
         +------------+     +----...-----+


The PHYSICAL LAYER provides the service of transferring bits from one
end of the physical medium to the other. This includes the
specification of the medium (wire, coax cable, fiber optics, radio
signals, laser signals, microwave signals, seismic waves, ... :-) as
well as the connectors, the modulation techniques, environmental limits
etc.

The DATA LINK LAYER uses the bit-transfer ability of the physical layer
to provide a secured link between two hosts. This includes functions
like error detection and correction, separating data blocks,
controlling media access, etc.

In a network, not every computer has to be connected to each other
directly. Hosts should be able to forward data packets according to an
address field that has been attached to the packet. The NETWORK LAYER
provides the service of routing and delivering a packet to any host in
the network. In intermediate systems, where there's no application
program involved in the communication, the packets are only processed
by the lower three layers.

Each layer (especially 2 and 3) can operate in two different modes. In
the CONNECTION-ORIENTED (CO) mode, the communication goes through
several stages similar to a telephone call: (1) dial, (2) wait until
the connection has been established, (3) communicate, (4) close the
connection, (5) wait until close of the connection has been confirmed.
The connection-oriented mode guarantees that no packets are lost and
that all packets arrive in the same order in which they have been sent.
In contrast, in the CONNECTIONLESS (CL) mode only a datagram service is
provided, similar to the postal letter service. No one guarantees that
the letters arrive in the same order in which they have been sent, and
every intermediate system is allowed to discard a packet if there are
insufficient resources available to deal with it (e.g. buffers are
full). In CL-mode systems, higher layers have to deal with
resequencing, retransmission of lost packets, etc. where a reliable
data stream is required.

There is endless debate about whether the CONS (connection-oriented
network service) or the CLNS should be provided by the network layer
and now both alternatives have been defined in OSI standard protocols.
As most applications want to exchange several packets in sequence, they
need a CO-mode service. The TRANSPORT LAYER has been introduced in
order to hide the differences between several network concepts (CO vs.
CL and other things like maximum packet size and quality of service)
from the connection-oriented transport service (COTS) user. In the case
of a CO network, the COTS is quite simple to provide, but over a CL
network, complete error correction and flow control has to be provided
in the transport layer.

The main advantage of the CLNS is higher efficiency because fewer CPU
cycles for complex flow control algorithms are needed in intermediate
systems (routers). If no connection-oriented transport service is
needed, then the response time may be significantly shorter over a
CLNS. CONS, on the other hand, makes accounting easier for network
providers, as only correctly delivered packets are counted. The headers
of the connection-oriented network protocol packets may be much
shorter, as the full address is not required in each packet, which
might be relevant on slow lines. In addition, many existing nets are
CO, e.g. X.25 nets, the telephone net and ISDN.

The SESSION LAYER offers a mechanism to include synchronization points
in the stream of exchanged packets and to go back to one of the
previous sync points. This might be useful if large files are
transferred and it is possible to recover after a breakdown without
having to retransfer the whole file. The session layer only provides
the means to negotiate a recovery. The recovery has to be done by the
application and not by the software that implements the session layer.
This is the reason why many experts consider the session layer as one
of the darker corners of the OSI model, especially as this part of the
standard is quite complex and difficult to understand. Perhaps it would
have been a better choice to include the session layer functionality
somewhere in the application layer. Don't worry if you don't understand
the exact ideas behind the design of the session layer, you are not
alone. There are even obscure relations with CCITT T.62, although only
very few people seem to understand why compatibility with an old
Teletex protocol was necessary in the session layer design ...

The PRESENTATION LAYER provides for negotiation of the form of
representation (syntax) of the data that will be transferred. This
makes sense if several optional ASN.1 encodings or other syntaxes
(e.g. the X Window System protocol) are possible.

Finally, the APPLICATION LAYER provides an application-specific
service. This is not the application itself, but the application layer
is directly used by an application program. For instance, an electronic
mail user interface program might use the X.400 application layer in
order to send a message to another host. Here the application layer
will deal with things like address resolution, routing decisions,
transferring and converting (even very huge) messages, etc. Other
application layer standards offer services like accessing remote file
systems, controlling video terminals and transaction systems, etc.

In addition to the 7 layers, the OSI Reference Model defines a
management and a security architecture that includes all 7 layers.

The seven layer model seems to have been first mentioned in the
literature in

  "A Tutorial on Protocols" by Louis Pouzin and Hubert Zimmerman in
  Proceedings of the IEEE, Volume 66, Number 11 (November 1978), pages
  1346-1370.


What is ASN.1?
--------------

Most protocols are defined as sets of protocol data units (PDUs) that
might be exchanged between two hosts. A PDU is a sequence of bytes that
one host uses to tell the other one something according to the rules of
the protocol. Quite early in the OSI project, it was recognized that a
formal way of defining the syntax of PDUs was needed, i.e. something
like the well-known context-free Backus-Naur grammars (BNF) are used to
define the syntax of programming languages. ASN.1 (abstract syntax
notation number 1) plays the same role in defining OSI protocols that
BNF plays in the definition of a programming language: it gives a
precise and parseable specification how PDUs are structured and which
structures are allowed, but it says nothing about the meaning of a PDU.
This is still defined in English language in the OSI standards.

ASN.1 specifies only the 'abstract syntax' of a PDU. This means that it
only says that, for example, a PDU consists of 2 integer numbers (of
arbitrary size), one optional UTC time value and one ISO 10646 string
or, alternatively, one ASCII string. ASN.1 doesn't define the 'transfer
syntax', the way in which the values in the abstract syntax (e.g. {2,
42, 1992-10-6 10:09:34, "Hey!"}) are encoded in the sending or
receiving computers, or on the transmission path between them. Several
different transfer syntaxes have been defined. The most widely used one
is BER (basic encoding rules). Other transfer syntaxes are simplified
BER subsets or provide a one-to-one mapping between abstract value and
the bit string for cryptographic applications or are optimized with
regard to CPU time or PDU length.

An example:

       NonsenseProtocol ::= CHOICE {
                              testPDU [0] TestPDU,  -- Our example
                              getFile [1] GetFile   -- another possible PDU
                            }

       TestPDU ::= SEQUENCE {
                     aNumber INTEGER,
                     anotherNumber INTEGER,
                     today UTCTime OPTIONAL,
                     theText CHOICE {
                               multilingualText ISO10646String,
                               standardText     VisibleString
                             }
                   }

The BER encoding consists of tag-length-value triples for both composed
types (CHOICE, SEQUENCE, SEQUENCE OF, SET, SET OF, tags, ...) and
primitive types (integers, strings, ...). The tag identifies the type
of the value (e.g. 0 for testPDU and 1 for getFile) and the length
defines the end of the TLV triple. This allows an implementation to
jump efficiently over (perhaps even unknown) values and makes ASN.1
defined protocols very extensible.

As an example the encoding of the value {2, 42, 1992-10-6 10:09:34,
"Hey!"} of type NonsenseProtocol:

  a0 80 ----------------------- Length encoding: indefinite.
   +--------------------------- Explicit tag: [0] means that we took
                                the testPDU alternative of the choice.

  30 80 ----------------------- again indefinite length form, as we don't
   |                            know how long the SEQUENCE will finally be.
   |                            Consequently, the end of the SEQUENCE will
   |                            have to be marked by a 00 00 component.
   +--------------------------- This is a composite type SEQUENCE.

  02 01 02 -------------------- The value 2.
   |  +------------------------ Length of the value: 1 byte.
   +--------------------------- This is an INTEGER value.

  02 01 2a -------------------- The value 42.
   |  +------------------------ Length of the value: 1 byte.
   +--------------------------- This is another INTEGER value.

  17 0d 39 32 31 30 30 36 31 30 30 39 33 34 5a
   |  | --------------------------------------
   |  |                 +------ The encoded date and time.
   |  +------------------------ Length: 13 bytes.
   +--------------------------- Type: UTCTime.

  1a 04 48 65 79 21
   |  | -----------
   |  |      +----------------- The value: 'Hey!' coded in ASCII.
   |  +------------------------ This is 4 bytes long.
   +--------------------------- We used the VisibleString alternative
                                of CHOICE.

  00 00 ----------------------- This is the end of the indefinite length
                                SEQUENCE.

  00 00 ----------------------- This is the end of the explicitly tagged
                                CHOICE alternative TestPDU.

Another very important ASN.1 concept and data type are object
identifiers. These are lists of integers that uniquely identify any
object in a protocol, e.g. a X.400 body part, a DFR document type, a
X.500 attribute type, a public key algorithm, a protocol version, etc.
If one object identifier is yours, then only you are allowed to append
further numbers to your OID and create new object identifiers that only
belong to you. You may give some of your newly created object
identifiers to other people, and, then, they also have the right of
creating their own new subtrees in the OID space. The highest levels in
this tree belong to ISO and CCITT, and they have already reserved OIDs
for many organizations. This allows you, for instance, to define your
own X.400 high-end cyberspace body part, and it won't collide with
someone else's self-defined body part if a prefix of the OID belongs to
you. As many options in OSI protocols are identified by OIDs, it is
very easy for implementors to extend the protocols without getting in
conflict with older implementations which will know that they don't
know this new OID. Many protocols exchange sets of OIDs that identify
their implemented subsets and extensions after the connection has been
established and automatically determine the biggest common subset of
all optional protocol features that both may use.

Today ASN.1/BER are also used in many non-OSI protocols and file
formats, and there are a number of tools (ASN.1 compilers) available
that help to create parsers automatically from ASN.1 specifications.
[See also the section about new standards.]

What OSI standards are there?
-----------------------------

As two organizations (ISO and ITU) have been involved in the OSI
standardization process, many of the specifications have been
published as both ISO standards and CCITT recommendations (CCITT
recommendations will in the future be called ITU-T recommendations).
In these cases, both versions are "technically aligned" or one version
is a subset of the other one.

The OSI Reference Model, as defined in ISO 7498-1 and CCITT
recommendation X.200, describes the details of the seven-layer model.
The three other parts of ISO 7498 describe the Security Architecture,
Naming and Addressing and the Management Framework.

Formal protocol description methods are defined in ISO 8807 (LOTOS,
Language Of Temporal Ordering and Specification), ISO 9074 (ESTELLE)
and ISO 9496 (CHILL). ISO 9646 standardizes methods for conformance
testing e.g. the Tree and Tabular Combined Notation (TTCN) for test
suites. ISO TR 10167 (Application guidelines for ESTELLE, LOTOS and SDL)
includes a number of examples of systems, showing how each of them can
be specified using these three standards.

International registration procedures for things like OSI document
types, object identifiers, virtual terminal profiles and control
objects and application process titles are described in ISO 9834 and
X.660. Registration means that a world-wide unique identifier will be
reserved for your extension to an OSI protocol, which guarantees that
extensions developed by different people won't collide with each other
and possible future ISO extensions.

ISO 8824 and X.208 standardize ASN.1. ISO 8825 and X.209 standardize
the Encoding Rules (BER and soon others) for it. [See also the section
about new standards.]

The details of the Security Framework are defined in ISO 10181, and
ISO 10745 describes the Upper Layers Security Model. Several security
techniques are standardized in ISO 9796 (Digital Signature Scheme
Giving Message Recovery) and ISO 9798 (Entity Authentication
Mechanisms, Symmetric Techniques and Public Key Algorithms). You may
register your own cryptographic algorithms as defined in ISO 9979.

Many OSI standards have been divided in two documents, the service
definition and the protocol specification, in order to separate the
description of the functionality from the description of the protocol
realization.

Layer seven has been further subdivided into several modules (service
elements), as some of them may be useful for more than one application.
This Application Layer Structure is defined in ISO 9545 and CCITT
X.207. The Association Control Service Element (ACSE) (ISO 8649|X.217
service definition and ISO 8650|X.227 protocol definition) manages the
establishment of a connection between two remote applications. CCR
(Commitment, Concurrency and Recovery) is a service element that
provides services needed for keeping distributed databases consistent
(ISO 9804/9805|X.861/871). ISO 9066 and CCITT X.218/228 define the
Reliable Transfer Service Element (RTSE) that transfers huge messages
safely from one node to the next one with recovery, etc. (e.g. used by
e-mail). The Remote Operations Service Element (ROSE) allows a client
to execute operations at a server process and to receive the results or
error indications.

Now, it's time to come to the most interesting part of the whole OSI
story, the application services and protocols:

ISO 9595 and CCITT X.710 define the Common Management Information
Service (CMIS) and ISO 9596|X.711 the Common Management Information
Protocol (CMIP) that may be used for controlling all parts of a
computer network, e.g. routers, queues, environmental sensors,
accounts, security logs, installed software versions, clocks, etc. ISO
10164 describes these various aspects of System Management, and ISO
10165 standardizes the Management Information Model used for
controlling all these devices.

FTAM (File Transfer, Access and Management) is a protocol defined in
ISO 8571 that has been designed for both simple file transfers a la
kermit and DoD ftp and for directly accessing remote file systems like
NFS, etc.

The Virtual Terminal (VT) service and protocol specified in ISO 9040
and ISO 9041 allow a host application to control a terminal with screen
and keyboard and similar devices like printers. In addition, not only
application-terminal, but also the less common application-application
and terminal-terminal communication is supported. Today, only the Basic
Class VT, which covers character-oriented terminals has been specified.
This service is comparable to DoD Telnet and the old CCITT
X.3/X.28/X.29 PAD protocol, but much more powerful. It also includes
control of cursor movement, colors, character sets and attributes,
access rights, synchronization, multiple pages, facility negotation,
etc. This means that the huge number of classic terminal type
definitions (e.g. in UNIX termcap or terminfo) are unnecessary at each
host in the net, as the VT protocol includes the corresponding commands
for one abstract virtual terminal that only have to be converted by the
local implementation to the actual terminal control sequences.
Consequently, the use of VT means not every host needs to know every
type of terminal.

As with most ISO standards that require general consensus amongst
participating members, the OSI VT has many optional capabilities, two
modes of operation and an almost infinite number of implementation-
specific options. Profiles may help in reducing the optionality present
(e.g., there exists a Telnet profile for VT).  But it is doubtful that
the OSI VT can completely put an end to the 'm x n' terminal
incompatibility problem that exists in a heterogeneous computer
network.

ISO 8831 and ISO 8832 define the Job Transfer and Manipulation (JTM)
protocol and service that may be used to control the disposition of the
programs and files and the execution of processes on remote hosts, to
query their status and to retrieve the results.

The X.400 electronic mail system, also known as Message Handling System
(MHS) is specified in CCITT X.400-X.440 and in ISO 10021. The system
consists of Message Transfer Agents (MTAs), that store and forward
messages through the network and User Agents (UAs) that present the
mail to the end user and allow him/her to send mail. The X.400 P1
protocol which is used between MTAs is based on the RTSE. The MTAs
transfer the messages to and from the UAs using the P3 protocol or by
local means (e.g. the file system if both processes run on the same
host). The Message Transfer System (MTS) that is formed by all MTAs may
be used for Interpersonal Messaging (IPM) or for other purposes (e.g.
system management). The MTS is used for sending mail to other people
directly or via Distribution Lists (DLs) and to transport delivery
reports (after the message has been delivered or discarded) and
receipt/non-receipt notifications (after the message has been read by
the recipient or deleted without ever having been seen by the
recipient) back to the sender. MTAs may be capable of converting
message body part types (e.g. different character sets or voice data
encodings) and rerouting if default links are unavailable.

X.500-X.521 and ISO 9594 specify the OSI Directory Service (DS). The DS
is a distributed database used for storing information about people,
organizations, application processes (e.g. MTAs or public DFR servers),
routing tables, etc.  X.500 may be used to search the X.400 addresses
of people interactively as well as other attributes like their phone
number, postal address, preferred delivery method (fax, mail, etc.),
photo and public keys. X.509 specifies a distributed authentication
framework based on the DS. The X.500 directory information base is
structured hierarchically, i.e. the entries that represent real world
objects (countries, persons, servers, management information, news
groups, distribution lists, groups of persons, organizational roles,
etc.) are connected to a world wide tree. Each entry has a number of
standardized or locally defined attributes and a unique Distinguished
Name that describes its location in the tree. Generally, there are
nodes for countries under the root and nodes for organizations under
each country and these may be further subdivided and contain person
entries, etc. But also other tree stuctures are possible. Powerful
search queries allow the implementation of easy-to-use Directory User
Agents (DUAs, the clients) that provide users the ability to find
information stored in the interconnected Directory System Agents (DSAs,
the servers). The DSAs talk to each other using the Directory System
Protocol (DSP). DUAs send their queries and modification commands to
the DSAs using the Directory Access Protocol (DAP).

The Document Filing and Retrieval (DFR) service and protocol are
defined in ISO 10166. A document store may contain a large number of
documents (ASCII texts, word processing files, hypertext files,
software packages, etc.) together with attributes like title, name of
the author, date of creation and latest modification, version number,
pointers to other copies of the same document, pointers to older
versions, language, summary, keyword list, access rights, lock
semaphores and so on. Documents may be arranged in a tree and described
with attributes very similar to the X.500 database. DFR allows the
implementation of an easy to use distributed information retrieval
system similar to the Internet gopher. In addition to the gopher
protocol, the DFR protocol provides the ability to store and modify
documents, access control, searches on attributes, a version control
mechanism and other things. Documents may also be represented by links
to other DFR servers and a unique identifier is assigned to each
document by the system, so that the user need not be aware that he/she
accesses several servers while browsing through the document (and
perhaps hypertext) space. Documents may be of various types (e.g. ODA,
SGML and HyTime, X.400 IPM messages, binary).

The Bibliographic Search, Retrieval and Update Service and Protocol
(SR) are defined in ISO 10162/10163. It is based on NISO Z39.50.

An Interlibrary Loan Service and Protocol are specified in ISO
10160/10161 for library applications.

ISO 10031 defines Distributed Office Applications (DOA).

The Manufacturing Message Specification (MMS) Service and Protocol have
been defined for controlling and integrating industrial automation
systems (ISO 9506).

An application layer protocol for Remote Database Access (RDA) is
specified in ISO 9579 and a Distributed Transaction Processing Model,
Service and Protocol is defined in ISO 10026 and X.850/860/870.

This was a very short overview on the application layer. Now let's dive
into the deeper layers.

The OSI presentation layer service and protocol are defined in ISO
8822/8823 and X.216/226. A connectionless mode protocol version is
specified in ISO 9576.

The OSI session layer service and protocol are defined in ISO 8326/8327
and CCITT X.215/225. The connectionless variant is specified in ISO
9548.

The OSI transport layer service and protocol are defined in ISO
8072/8073 and X.214/224. These standards define 5 different flavors of
one transport protocol for the connection-oriented transport service.
TP0 is the simplest version, and assumes that the network service is
reliable enough for the application. TP1 provides error recovery, i.e.,
after a network failure, the transport layer implementation is capable
of automatically reestablishing the connection without notifying the
higher layers. TP1 won't detect errors that are not signaled by the
network layer, e.g., by a disconnect message. TP2 provides the ability
to multiplex several transport connections over a single network
connection and TP3 provides both error recovery and multiplexing
(TP1+TP2=TP3). Finally, TP4 provides all this, plus error detection,
error correction and packet resequencing. TP4 is intended to be used
over unreliable connectionless networks. ISO 8602 defines a protocol
for the connectionless transport service (CLTS) and ISO 11570 a
transport protocol identification mechanism.

The network service, and the secrets of OSI network service access
point (NSAP) addresses, are defined in ISO 8348 and X.213. ISO 8648
defines the internal organization of the network layer in 3 sublayers
and ISO 8880 describes protocol combinations for the network service.
ISO 10028 specifies the relaying function of an intermediate system.

The connectionless network service may be provided by the
connectionless network protocol (CLNP, also known as ISO IP) defined in
ISO 8473. In combination with CLNP, the End System to Intermediate
System Routeing Information Exchange Protocol (ESIS) defined in ISO
9542, the Intermediate System to Intermediate System Routeing
Information Exchange Protocol (ISIS) defined in ISO 10589 and the
Protocol for Exchange of Inter-Domain Routeing Information (ISO 10747)
may be used. CLNP is normally used together with TP4 as the transport
protocol.

The connection-oriented network service (CONS) may be provided by
X.25 (ISO 8202) as defined in ISO 8878|X.223. ISO 8881 extends X.25 for
use in local area networks, ISO 10732 describes some details of using
X.25 for providing the CONS over the telephone network and ISO 10588
describes how to use X.25 over X.21/X.21bis lines for providing the
CONS. ISO 10177 defines an intermediate system using X.25/CONS and ISO
10030 is the ESIS version for X.25/CONS. Using ISDN (the new digital
telephone network) for providing the CONS is specified in ISO 9574.

And another layer: the Data Link Service is defined in ISO 8886|X.212.
Most data link layer protocols are based on the High-level Data Link
Control (HDLC) family of error correction protocols. These are
described in the standards ISO 3309/4335/7478/7809/8471/8885. One
popular HDLC protocol is the subset LAPB defined in ISO 7776, which is
often used with X.25 on point-to-point lines. ISO 8802 (also known as
IEEE 802) defines several LAN systems, e.g. ISO 8802-3 is the well
known Ethernet (CSMA/CD) specification. ISO 8802-2 defines the data
link layer protocol for LANs Logical Link Control (LLC). LLC1 is the
connectionless and LLC2 the connection-oriented version. The Fiber
Distributed Data Interface (FDDI) is a fiber optic LAN system for 100
Mbit/s specified in ISO 9314.

The Physical Layer Service Definition is defined in ISO 10022|X.211.
The physical layers of LANs are, in most cases, defined in the same
documents as the layer 2 descriptions. There are also many ISO and IEC
standards for all kinds of plugs and connectors.

OSI standards define a lot of options, and they may be combined in a
huge variety of ways in order to be suitable for all kinds of
requirements (and sometimes for political reasons :-( ). Consequently,
if you have two boxes that claim to be OSI conforming, this doesn't
mean that you can plug them together and they will work together,
because they can use different lower layer protocols and different
higher layer options. In order to create standards that allow that
conforming and compatible products will interoperate, profiles have
been defined. A profile is a set of options and combinations selected
from the protocol standards such that it is guaranteed that
implementations conforming to the (same!) profile will be able to
communicate. Technical report ISO TR 10000 gives the ISO profile
framework. 'A' profiles have been defined for application layer
protocols (e.g. AFT11 in the international standardized profile ISO ISP
10607 for a simple FTAM file transfer subset and AOM12 in ISO ISP 11183
is an enhanced management protocol subset), 'T' profiles define layer
1-4 stacks (ISO ISP 10609) and 'F' profiles are for file formats (e.g.
FOD36 is a de luxe version of ODA).

A good list of all OSI related standards and their current status is
printed twice a year in the ACM SIGCOMM journal Computer Communication
Review. A list of CCITT standards is available from the Teledoc mail
server described in the standards FAQ where you'll also find a short
list of other ISO standards relevant to computing. For more
information, order the orange ISO Catalogue from your national ISO
member body.


How are OSI and TCP/IP related?
-------------------------------

TCP/IP is a suite of protocols that has been developed by the US
Department of Defense and that are used on the Internet. Software
supporting TCP/IP is part of nearly every UNIX distribution today.
TCP/IP is not a OSI protocol and does not fit in the OSI reference
model. However, the service provided by IP is very similar to the
connectionless network service provided by CLNP, and so, IP is
generally called a layer 3 protocol. Similarly, TCP may be compared
with TP4 and can be seen as a layer 4 protocol in the reference model.
The major differences are the address space, which is a 4-byte sequence
in IP and up to 20 bytes in OSI, and the fact that TCP is a
stream-oriented protocol that doesn't provide any protocol data unit
boundaries. Other details are a few missing features in TCP/IP like
quality of service negotiation and routing restrictions.

The Internet standard RFC 1006 defines a method of providing the
connection-oriented transport service (COTS) for OSI over TCP. This is
done using a packet length indicator and the TP0 protocol. RFC 1006 is
only an interim solution, and there are long term plans to introduce
CLNP in the Internet. OSI Services that are used today on the Internet
(e.g. the X.400 and X.500 pilots) use RFC 1006 over TCP/IP.

The higher layer protocols in the DoD/Internet suite may be mapped as
follows:

       DoD/Internet                   OSI
     ----------------------------------------------------------------
       ftp                            FTAM
       RFC822, SMTP, MIME             X.400
       Telnet, rlogin                 VT
       Gopher+, WWW                   DFR
       WAIS                           SR    (both are based on Z39.50)
       SNMP                           CMIP
       USENET, NNTP                   X.gc
       X Window System                (will be included in OSI)
       NTP (network time protocol)    (under consideration by CCITT)
       OSPF, ISIS                     ISIS

Internet Protocols with no OSI equivalent today:

       Relay Chat, IRC                OSI has only a standard name for this
                                      so far: synchronous group communication
       Talk                           might perhaps be defined as a VT profile

OSI protocols with no TCP/IP equivalent:

       X.500                          perhaps finger :-)
       JTM
       parts of DFR
       parts of X.gc
       RDA
       MMS
       Interlibrary Loan Protocol


Which is better: TCP/IP or OSI?
-------------------------------

This question periodically results in flame wars in comp.protocols.iso.
If you missed the last one, here is one opinion:

  Neither.  In theory, OSI has the more advanced feature set,
  including significantly more sophisticated application protocols
  including some for services not available at all in the TCP/IP suite.
  In practice, TCP/IP is much more widely implemented and deployed, so
  you are much more likely to find TCP/IP products to suit your needs,
  and typically at much lower prices than equivalent OSI products.

                                                  don provan
                                                  [e-mail]

[Others are welcome!]


OSI criticism
-------------

The OSI protocols still suffer from not being used outside communities
where their use is enforced by regulations. Many other protocols (e.g.
TCP/IP, SNA and Novell) are much more popular than the official standards
of computer networking and many people think that the following reasons
have contributed to this situation:

  - OSI protocols haven't been tested widly before having been
    standardized and are not based on existing practice in large
    scale computer networking (e.g. Arpanet).

  - OSI standards are (compared to Internet standards and RFCs) very
    expensive and difficult to obtain.

  - The OSI reference model is too complex and has too much layers.
    The session layer should better be a part of the application layer
    and isn't of much use at the current position. Although it is
    possible to implement just a minimal kernel functionality of
    both the session and presentation layers, even having to
    understand the very difficult documents isn't very motivating.

  - Promising new network technologies like ATM networks don't fit in the
    OSI reference model very well and many important techniques like
    LANs, RPC and stateless protocols became popular after the OSI RM
    had been standardized.

  - Having two completely incompatible alternative protocols (CLNP and
    X.25) at the network layer (and consequently many different transport
    layers that try to compensate the differences) isn't what helps you
    in building up a fully interconnected easy to use and maintain
    global network. There is broad agreement among knowledgeable
    people that a connectionless network layer is technically superior
    to the X.25/TP0 approach (which in practice provides NO reliable
    end-to-end transport service, because recovery from intermediate
    node failures is impossible).

Christian Huitema <[e-mail]> explained the various problems
with OSI quite well when he posted suggestions about explaining the
design principles behind OSI to students in network classes:

"Once the class is reasonably at ease with the "basics", it is relatively easy
to introduce the OSI model -- as a baroque left over of the 70's. You have to
do it with an historical perspective; a few tips:

 * the model was invented before local networks, and also before RPC.

 * the layer decomposition dates back from the time when you had the transport
   programmed in a "front end" and a "session control" programmed in a
   "channel manager".

 * ASN.1 was *NOT* part of the original model. If you go back to the early
   ideas, there should not have been an "application standard". Rather, the
   presentation would specify some "common document format", common to all
   applications. The session would be used to ensure that a "consistent" copy
   of this common document was kept at both end.

 * the session itself cannot be understoud if you don't present T.62 -- or
   rather, 1980's S.62. Why modern applications would have to bear the load of
   compatibility with a still born super-telex service is indeed a matter of
   interesting debate.

 * explaining the relation between an application syntax, a presentation
   context, an application context, an application entity, an application
   process and a PSAP address is great fun.

 * indeed, the use of session through the presentation by ACSE, RTSE and CCR
   is also amazing.

Then you end up the class explaining that all this is not very serious..."


Which free OSI implementations are available?
---------------------------------------------

ISODE (pronounce it ISO-D-E, but don't call it the ISO Development
Environment) implements ISO layers 4-7 on UNIX systems. Among the
supported lower layer stacks are RFC1006/TCP/IP and various X.25 and
CLNP implementations. With ISODE you get ASN.1 tools and
implementations of X.500 (QUIPU) and FTAM. Version 8.0 is the last
public domain version and an ISODE Consortium has been founded that
will coordinate further development on a commercial basis. A free Patch
No. 1 with bug fixes for ISODE 8.0 has been provided by the ISODE
Consortium which can be ftped from isode.com.

The ISODE and QUIPU mailing list archives are available via anonymous
ftp and gopher from info.educom.edu. The isode and quipu are in
/pub/isode and /pub/quipu respectively.

PP is a X.400 message transfer agent and a gateway to other e-mail
systems based on ISODE. The current public domain version is 6.0 and
further commercial development will be done by the ISODE Consortium.

Both ISODE and PP are available from many anonymous ftp servers, e.g.
from cs.ucl.ac.uk in directory src.

OSIMIS is UCL's OSI management system. It does not intend to provide a
comprehensive set of OSI management facilities but rather to show how
the rich OSI Management functionality can be exploited and to provide
facilities of a generic OSI management platform. In particular,
extended support through an object-oriented (C++) Application Program
Interface (API) is provided to implementors of management applications
in both agent and manager roles, which hides the details of management
information access through the OSI management service/protocol CMIS/P.
The system has been developed using ISODE 7.0 and GNU C++ and is
available with anonymous ftp from cs.ucl.ac.uk [128.16.5.31] in the
directory osimis.

[Posted by Peter Kay <[e-mail]>:]

The clients of ISODE 8.0 have now been ported to OS/2. (pepsy, pepy,
posy, rosy, isoc, imisc, ftam, dish, de and sd). They are available by
anonymous ftp from cc-vms1.massey.ac.nz (130.123.1.4), in the files
os2isode80.zip (for those who want to rebuild the system) or
os2isode-runtime.zip (for those who only want the executables). I
suggest you copy and read the file os2readme first.

The port was based on Essex Systems Inc TCP/2 package. It will NOT work
with any others. To run the programs you will need the TCP/2 package.
To rebuild the system you will also need its Developers Kit and the
Microsoft C v6.0 compiler.

[Posted by Graham Wheeler <[e-mail]>:]

An MS-DOS-based ESTELLE compiler/interpreter should be on simtel, in
the Networks directory, with the name PEW2_1.ZIP.


New and planned OSI standards
-----------------------------

A group is working on an 'asynchronous group communication' standard
based on X.400, X.500 and DFR. This will include a news system similar
to but more sophisticated than USENET and facilities for joint editing
of documents and voting/polling. The name of the draft document is
currently X.gc.

X.400(1988) has been extended to include voice, Electronic Data
Interchange (EDI) and file transfer in the message body. Also many
bugs in the specification have been fixed.

X.500(1993) has been finished and will define a new replication
protocol, extended search methods, access rights, attribute
inheritance, ISO 10646 strings and other things. It will be published
in late 1993.

An addendum 1 (filestore management) to FTAM now defines a hierarchical
file system with subdirectories, links, paths, etc. Previously, paths
have been long file names and were OS dependent (e.g. / in Unix, \ in
MS-DOS) and directories have been treated like files (e.g. '.').

HDLC has been extended to include an asynchronous mode that defines how
to use the layer 2 framing with start/stop bits and byte stuffing on
the serial ports that are part of every PC. Check ISO 3309 addendum
1-3.

ISO is working on an OSI-based remote procedure call standard (ISO CD
11578).

A new version of ASN.1 is about to be released. The new ASN.1 standard
consists of a 4-part document:

        ITU-T Rec. X.680 | ISO/IEC 8824-1
        ITU-T Rec. X.681 | ISO/IEC 8824-2
        ITU-T Rec. X.682 | ISO/IEC 8824-3
        ITU-T Rec. X.683 | ISO/IEC 8824-4

The first part is very similar to 1990 ASN.1, except that there is no
more macro notation or ANY DEFINED BY.  Also, there are niceties such
as automatic tagging and support for multi-byte characters in
the new ASN.1. The last 3 parts replace the macro notation and
ANY DEFINED BY. On the encoding rules side there is:

        ITU-T Rec. X.690 | ISO/IEC 8825-1
        ITU-T Rec. X.691 | ISO/IEC 8825-2

The first contains the good old Basic Encoding Rules (with support for
multi-byte characters, etc.), plus the Canonical Encoding Rules and the
Distinguished Encoding Rules. The second is the Packed Encoding Rules.


Books and Journals
------------------

[Posted by Even Splett <[e-mail]> and others with
comments collected by Khac Binh Su <[e-mail]> and
others:]

Data communications, Computer Networks and OSI
Halsall, Fred
549 p.
Addison-Wesley
ISBN 0-201-18244-0

Networking in Open Systems; international seminar, Oberlech, Austria,
Muller, Gunter. Blanc, Robert P.
Berlin, Springer-Verlag, c1987
Lecture notes in computer science; 248
441 p.
QA76.L42 v.248 1987
ISBN 3540177078

OSI Explained; end-to-end computer communication standards.
Henshall, John,  Shaw, Sandy.
Ellis Horwood Ltd.
distributed by John Wiley & Sons Ltd.
217 p.
TK5105.5.H47
ISBN 0745802532  Ellis Horwood
ISBN 0470211008  Halsted Press

  Very good on transport and above, including a consolidation by 3
  examples that shows how the upper layers work together to establish an
  association, transfer data, and release it. It also has a chapter each
  on FTAM, X.400, and X.500, but does very little with ASN.1, a real
  weakness in an otherwise excellent book.  Its other virtue: it is under
  250 pages.

Open Systems; the Basic Guide to OSI and its Implementation.
Judge, Peter J.
QED Information Sciences
Authorized reprint of -- Sutton, Surrey, Computer Weekly. (ISBN 1853840092)
184 p.
TK5105.5.J83  1989
ISBN 0894353101

Open Systems
Karial, Henry S.
JA71.K32 1969

OSI: a Model for Computer Communications Standards
Black, Uyless D.
Prentice Hall
TK5105.5.B5656  1991
ISBN 0-13-637133-7

  An excellent OSI introduction.

The Open Book : A Practical Perspective on OSI
Rose, Marshall T
Tk5105.5.R67  1989
Prentice Hall
ISBN 0-13-643016-3

  It is fun reading, and it gives you the general idea about the whole
  stuff. One of its most interesting topics is transition: how to migrate
  to OSI from a TCP/IP oriented world. Excellent, but not cheap. In
  Germany it is 150,-- DM.

  I started with Marshall T. Rose's The Open Book - a practical
  perspective on OSI by Prentice-Hall. It came out in 1991 so it is quite
  up to date, but not cheap (at least in Finland).

  It is not an exclusive book on the upper layers of the OSI.
  Nevertheless, his presentation of the upper layers in a couple of
  chapters is sufficient enough to start.

  I am not such a fan of Marshall Rose's "The Open Book" as I once was.
  At this point, the book is getting a bit dated (the publication date is
  1990 on my edition; a lot has changed in that time), and it emphasizes
  the upper layers to the detriment of the lower layers. I have found the
  book by  Uyless Black superior in several respects: it is newer, it is
  technically denser, and it refrains from excessive editorializing.

Complete guide to ISO protocols Vol 1
Thomas, Steve
Springer Verlag
ISBN 0387970231

Handbook of computer communications standards, Vol 1
The Open Systems Interconnection (OSI) model
Stallings, William   1989
ISBN 0-672-22664-2 Howard W. Sams & Company
ISBN 002948071x    Macmillan

  This book is OK, but lacks the finer details.

An introduction to Open Systems Interconnection
MacKinnon, Dennis
254 p.
Computer Science Press  1990
TK 5105 M3335  1990
ISBN 0716781808

Standards for Open Systems Interconnection
Knowles
Blackwell Scientific Publications
ISBN 0632018682

OSI in Microcomputer LANs
Strom, Jim.
125 p.
Manchester: NCC Publications, 1989.
TK 5105.5 S87  1989
ISBN 0850127114 (pbk)

ASN.1, The Tutorial and Reference
Douglas Steedman
Technology Appraisals, 82 Hampton Road,
Twickenham, TW2 5QS, U.K.
(Fax [phone])
ISBN 1 871802 06 7

Open Systems Interconnection
by Gary Dickson and Alan Lloyd
Prentice Hall Australia  (1992)
ISBN 0 13 640111 2

ISODE 8.0 Manual

  The ISODE  manual is a nice reference material if you are looking for
  implementation details of the upper layers. In fact, looking at ISODE
  code is probably the best way to see how the "theoretical" stuff is
  applied.

Teleinformatique by Nussbaumer,
published by Presses polytechniques
romandes (french version)

  The vol 3&4 are the best books available in french for the upper
  layers.

Computer Networks by Andrew Tannenbaum

  This book explains accurately the general aspects of problems up-to
  layer 6. Layer 7's description is a bit vague.

  A very nice introduction in computer communication that also covers
  OSI. Highly recommended for computer science students and others
  searching a good general networking introduction.

Communication Network Protocols - OSI Explained
Brian W. Marsden
Chartwell-Bratt (UK)
1991 (3rd edition)
ISBN 0-86238-276-9

  It is extremely idiosyncratic, and should not be read except in
  conjunction with another book. Also, only some of it is about OSI. It
  is, however (a) very cheap (15 UK pounds), and (b) the only book I know
  of which talks about the UK Coloured Book protocols. These had a major
  impact on some parts of OSI, particularly JTM, as well as being
  interesting in their own right.

Networking Standards
A Guide to OSI, ISDN, LAN, and MAN Standards
Stallings, William
Addison-Wesley 1993
627 pages
ISBN 0-201-56357-6

  Provides an overview of basic OSI standards, then a detailed
  technical discussion of leading-edge OSI-related standards, including
  internetworking and routing, ISDN and BISDN, LAN and MAN,  network
  management, security, and international standardized  profiles.

[End of comp.protocols.iso FAQ]

--
Markus Kuhn, Computer Science student ½░o░╗ University of Erlangen, Germany
Internet: [e-mail]   |   X.500 entry available