The Open System Interconnection (OSI)
protocol suite is comprised of numerous standard protocols that are
based on the OSI reference model. These protocols are part of an
international program to develop data-networking protocols and other
standards that facilitate multivendor equipment interoperability. The
OSI program grew out of a need for international networking standards
and is designed to facilitate communication between hardware and
software systems despite differences in underlying architectures.
The OSI specifications were conceived and
implemented by two international standards organizations: the
International Organization for Standardization (ISO) and the
International Telecommunication Union-Telecommunication Standardization
Sector (ITU-T). This chapter provides a summary of the OSI protocol
suite and illustrates its mapping to the general OSI reference model.
Figure 32-1 illustrates the entire
OSI protocol suite and its relation to the layers of the OSI reference
model. Each component of this protocol suite is discussed briefly in
this chapter. The OSI routing protocols are addressed in more detail in "Open
Shortest Path First (OSPF)."
Figure 32-1: The OSI protocol
suite maps to all layers of the OSI reference model.
The OSI protocol suite supports numerous
standard media-access protocols at the physical
and data link layers. The wide variety of media-access
protocols supported in the OSI protocol suite allows other protocol
suites to exist easily alongside OSI on the same network media.
Supported media-access protocols include IEEE 802.2 LLC, IEEE 802.3,
Token Ring/IEEE 802.5, Fiber Distributed Data Interface (FDDI), and
X.25.
The OSI protocol suite specifies two
routing protocols at
the network layer: End System-to-Intermediate System (ES-IS) and
Intermediate System-to-Intermediate System (IS-IS). In addition, the OSI
suite implements two types of network services: connectionless service
and connection-oriented service.
In addition to the standards specifying
the OSI network-layer protocols and services, the following documents
describe other OSI network-layer specifications:
OSI connectionless network service is
implemented by using the Connectionless
Network Protocol (CLNP) and Connectionless Network Service (CLNS). CLNP
and CLNS are described in the ISO 8473 standard.
CLNP is an OSI network-layer protocol
that carries upper-layer data and error indications over connectionless
links. CLNP provides the interface between the Connectionless Network
Service (CLNS) and upper layers.
CLNS provides network-layer services to
the transport layer via CLNP.
CLNS does not perform connection setup or
termination because paths are determined independently for each packet
that is transmitted through a network. This contrasts with
Connection-Mode Network Service (CMNS).
In addition, CLNS provides best-effort
delivery, which means that no guarantee exists that data will not be
lost, corrupted, misordered, or duplicated. CLNS relies on
transport-layer protocols to perform error detection and correction.
OSI connection-oriented network service
is implemented by using the Connection-Oriented Network
Protocol (CONP) and Connection-Mode
Network Service (CMNS).
CONP
is an OSI network-layer protocol that carries upper-layer data and error
indications over connection-oriented links. CONP is based on the X.25
Packet-Layer Protocol (PLP) and is described in the ISO 8208 standard,
"X.25 Packet-Layer Protocol for DTE."
CONP provides the interface between CMNS
and upper layers. It is a network-layer service that acts as the
interface between the transport layer and CONP and is described in the
ISO 8878 standard.
CMNS performs functions related to the
explicit establishment of paths between communicating transport-layer
entities. These functions include connection setup, maintenance, and
termination, and CMNS also provides a mechanism for requesting a specific
quality of service (QOS). This contrasts with CLNS.
OSI network-layer addressing
is implemented by using two types of hierarchical addresses: network
service access-point addresses and network-entity titles.
A network
service-access point (NSAP) is a conceptual point on the boundary
between the network and the transport layers. The NSAP is the location
at which OSI network services are provided to the transport layer. Each
transport-layer entity is assigned a single NSAP, which is individually
addressed in an OSI internetwork using NSAP addresses.
Figure 32-2 illustrates the format of
the OSI NSAP address, which identifies individual NSAPs.
Figure 32-2: The OSI NSAP
address is assigned to each transport-layer entity.
NSAP Address Fields
Two NSAP Address
fields exist: the Initial
Domain Part (IDP) and the Domain-Specific Part (DSP).
The IDP
field is divided into two parts: the Authority Format Identifier (AFI)
and the Initial Domain Identifier (IDI). The AFI provides
information about the structure and content of the IDI and DSP
fields, such as whether the IDI is of variable length and whether the
DSP uses decimal or binary notation. The IDI
specifies the entity that can assign values to the DSP portion of the
NSAP address.
The DSP is subdivided into four
parts by the authority responsible for its administration. The Address
Administration fields allow for the further administration of addressing
by adding a second authority identifier and by delegating address
administration to subauthorities. The Area field identifies the specific
area within a domain and is used for routing purposes. The Station field
identifies a specific station within an area and
also is used for routing purposes. The Selector field
provides the specific n-selector within a station and, much like the
other fields, is used for routing purposes. The reserved n-selector 00
identifies the address as a network
entity title (NET).
End-System NSAPs
An OSI end
system (ES) often has multiple NSAP addresses, one for each transport
entity that it contains. If this is the case, the NSAP address for each
transport entity usually differs only in the last byte (called the n-selector).
Figure 32-3 illustrates the relationship between a transport entity,
the NSAP, and the network service.
Figure 32-3: The NSAP
provides a linkage between a transport entity and a network service.
A network-entity
title (NET) is used to identify the network layer of a system without
associating that system with a specific transport-layer entity (as an
NSAP address does). NETs are useful for addressing intermediate systems
(ISs), such as routers, that do not interface with the transport layer.
An IS can have a single NET or multiple NETs, if it participates
in multiple areas or domains.
OSI Protocols
Transport Layer
The OSI protocol suite implements two
types of services at the transport layer: connection-oriented transport
service and connectionless transport service.
Five connection-oriented transport-layer
protocols exist in the OSI suite, ranging from Transport
Protocol Class 0 through Transport Protocol Class 4. Connectionless
transport service is supported only by Transport Protocol Class 4.
Transport
Protocol Class
0 (TP0), the simplest OSI transport protocol, performs
segmentation and reassembly functions. TP0 requires connection-oriented
network service.
Transport
Protocol Class
1 (TP1) performs segmentation and reassembly and offers basic
error recovery. TP1 sequences protocol data units (PDUs) and will
retransmit PDUs or reinitiate the connection if an excessive number of
PDUs are unacknowledged. TP1 requires connection-oriented network
service.
Transport
Protocol Class 2
(TP2) performs segmentation and reassembly, as well as multiplexing and
demultiplexing data streams over a single virtual circuit. TP2 requires
connection-oriented network service.
Transport
Protocol Class 3
(TP3) offers basic error recovery and performs segmentation and
reassembly, in addition to multiplexing and demultiplexing data streams
over a single virtual circuit. TP3 also sequences PDUs and retransmits
them or reinitiates the connection if an excessive number are
unacknowledged. TP3 requires connection-oriented network service.
Transport
Protocol Class 4 (TP4) TP4 offers basic error
recovery, performs segmentation and reassembly, and supplies
multiplexing and demultiplexing of data streams over a single virtual
circuit. TP4 sequences PDUs and retransmits them or reinitiates the
connection if an excessive number are unacknowledged. TP4 provides
reliable transport service and functions with either connection-oriented
or connectionless network service. It is based on the Transmission
Control Protocol (TCP) in the Internet Protocols suite and is the only
OSI protocol class that supports
connectionless network service.
The session-layer implementation
of the OSI protocol suite consists of a session protocol and a session
service. The session protocol allows session-service
users (SS-users) to communicate with the session service. An SS-user
is an entity that requests the services of the session layer. Such
requests are made at Session-Service Access Points (SSAPs),
and SS-users are uniquely identified by using an SSAP address. Figure
32-4 shows the relationship between the SS-user, the SSAP, the session
protocol, and the session service.
Session service provides four basic
services to SS-users. First, it establishes and terminates connections
between SS-users and synchronizes the data exchange between them.
Second, it performs various negotiations for the use of session-layer
tokens, which must be possessed by the SS-user to begin communicating.
Third, it inserts synchronization points in transmitted data that allow
the session to be recovered in the event of errors or interruptions.
Finally, it allows SS-users to interrupt a session and resume it later
at a specific point.
Figure 32-4: Session layer
functions provide service to presentation layer functions via a SSAP.
Session service is defined in the ISO
8326 standard and in the ITU-T X.215 recommendation. The session
protocol is defined in the ISO 8327 standard and in the ITU-T X.225
recommendation. A connectionless version of the session protocol is
specified in the
ISO 9548 standard.
OSI
Protocols Presentation Layer
The presentation-layer implementation
of the OSI protocol suite consists of a presentation protocol and a
presentation service. The presentation protocol allows
presentation-service users (PS-users) to communicate with the
presentation service.
A PS-user is an entity that requests the
services of the presentation layer. Such requests are made at Presentation-Service
Access Points (PSAPs). PS-users are uniquely identified by using PSAP
addresses.
Presentation service negotiates transfer
syntax and translates data to and from the transfer syntax for PS-users,
which represent data using different syntaxes. The presentation service
is used by two PS-users to agree upon the transfer syntax that will be
used. When a transfer syntax is agreed upon, presentation-service
entities must translate the data from the PS-user to the correct
transfer syntax.
The OSI presentation-layer service is
defined in the ISO 8822 standard and in the ITU-T X.216 recommendation.
The OSI presentation protocol is defined in the ISO 8823 standard and in
the ITU-T X.226 recommendation. A connectionless version of the presentation
protocol is specified in the ISO 9576 standard.
The application-layer implementation
of the OSI protocol suite consists of various application entities. An
application entity is the part of an application process that is
relevant to the operation of the OSI protocol suite. An application
entity is composed of the user element and the application
service element (ASE).
The user element is the part of an
application entity that uses ASEs to satisfy the communication needs of
the application process. The ASE is the part of an application entity
that provides services to user elements and, therefore, to application
processes. ASEs also provide interfaces to the lower OSI layers. Figure
32-5 portrays the composition of a single application process (composed
of the application entity, the user element, and the ASEs) and its
relation to the PSAP and presentation service.
Figure 32-5: An application
process relies on the PSAP and presentation service.
ASEs fall into one of the two following
classifications: Common-Application Service Elements (CASEs) and
Specific-Application Service Elements (SASEs). Both of these might be
present in a single application
entity.
Common-Application
Service Elements (CASEs)
Common-Application Service
Elements (CASEs) are ASEs that provide services used by a wide variety
of application processes. In many cases, multiple CASEs are used by a
single application entity. The following four CASEs are defined in the
OSI specification:
Specific-Application
Service Elements are ASEs that provide services used only by a specific
application process, such as file transfer, database access, and
order-entry, among
others.
An application process is the element of
an application that provides the interface between the application
itself and the OSI application layer. Some of the standard OSI
application processes include the following:
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