OSI Reference Model - Amazon S3

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Oct 23, 2013 (4 years and 18 days ago)

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Spring 2002 Computer Networks, MSU 1
Communication Models
Spring 2002 Computer Networks, MSU 2
Reference Models
 Dealing with complexity
 systematic approach
 long and joint effort
 large volumes of analytical and empirical
data
 common set of principles and concepts
Spring 2002 Computer Networks, MSU 3
OSI Reference model
 The basic ideas of the OSI Model
 partition functionality
 hierarchical structure
 open system
 end system
 sub-network
 intermediate system
Spring 2002 Computer Networks, MSU 4
OSI Reference model
The concept of layering
 Layering is a structured technique
similar to module development in
software engineering
 Divides problem into subproblems
 Principles in defining layers
 Permits a network of Open Systems to
be viewed as logically composed of a
succession of layers
Spring 2002 Computer Networks, MSU 5
OSI Reference model
 Lower layers are isolated from the higher
layers
 Independence between layers
 A layer is actually considered to be made of
subsystems of the same rank in all
interconnected systems.
 Seven layer model
Spring 2002 Computer Networks, MSU 6
Reference model
Spring 2002 Computer Networks, MSU 7
The Layered OSI Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical
Intermediate
System
End
System
Spring 2002 Computer Networks, MSU 8
OSI Standards
 Two families of standards
 services
 functional capabilities
 abstract interface
 protocols
 rules of behavior
 format and contents of the control information
 Levels of abstraction
 RM – reference model
 SD – service definition
 PS – protocol specification
Spring 2002 Computer Networks, MSU 9
OSI Reference model
 The fundamental objective is to standardize the
rules of interaction between interconnected
systems.
 Clear division between external behavior and
internal structure of a system
 internal
 organization
 functions
are transparent to other systems, and they should not
be subject to the same standards.
Spring 2002 Computer Networks, MSU 10
OSI Reference model
 Each subsystem
 is made of entities
 each entity belongs to one system.
 Entities in the same layer are termed as peer entities.
 If we currently observe the Nth layer, then the next one
lower is designated as (N-1) layer and the one higher is
designated as (N+1) layer.
 The idea is that each layer adds value to services
provided by the set of lower levels, so the highest level
receives a set of services, which enable execution of
any application.
Spring 2002 Computer Networks, MSU 11
OSI Reference model
 Independence of each layer by
 defining services provided by the current layer to the
next higher one, from the way how those services are
performed.
 anything can be changed or modified underneath as
long as the required service remains.
 With the exception of the highest layer, there is a
collective work of the distributed entities to provide
for a particular services.
 How does it work?
 (
N) entities add value to the (N-1) service from the (N-
1) layer and offer this value-added or (N) service to
the (N+1) entities.
Spring 2002 Computer Networks, MSU 12
OSI Reference model
 Communication between (N+1) entities rests
exclusively on (N) services. Any communication
between (N+1) entities within the same system
is transparent from an external view of the
system.
 Cooperation between (N) entities is governed by
(N) protocols, which completely specify
how the (N) entities work together using the (N-1)
services to perform the (N) functions which add value
to the (N-1) service in order to provide the (N) service
to the (N+1) entities
Spring 2002 Computer Networks, MSU 13
OSI Reference model
 The places where the (N) services are given
to the (N+1) entities are termed as the (N)
service access points or SAPs.
 Each SAP is a logical interface between the
(N) entities and (N+1) entities.
 What are the relations between SAPs on
different layers?
 an (N) SAP can be served by only one entity
 an (N) SAP can be used by only one (N+1) entity
 one (N) entity can serve several (N) SAPs
 one (N+1) entity can use several (N) SAPs
Spring 2002 Computer Networks, MSU 14
OSI Reference model
 A common service offered by all layers is to
provide the appropriate associations between
peer SAPs (these associations can be used to
transfer data).
 Actually, the (N) layer offers (N) connections
between (N) SAPs as part of the (N) services.
 Types of connections
 point to point
 multiendpoint
 The end of an (N) connection at a (N) SAP is
termed as (N) connection endpoint or (N) CEP.
Spring 2002 Computer Networks, MSU 15
OSI reference model
 Identifiers
 Objects within a layer or a boundary between adjacent
layers must be identifiable in a unique way (simply
there is no way to establish a connection between two
SAPs if there is no possibility to distinguish them from
another objects in the system).
 Each (N) entity is identified with a global name, which
enables recognition from anywhere in the network.
 In case of local domains, an entity may be provided
with a local name (valid within that specific domain).
Spring 2002 Computer Networks, MSU 16
OSI Reference model
 Each (N) SAP is identified with an (N) address that is
also unique (the (N) SAP is at the boundary between
the (N) layer and the (N+1) layer). The binding
between the (N) entities and the (N-1) SAPs which
they use them to access each other and
communicate are part of the (N) directory, which
establishes correspondence between the global
names of the (N) entities and (N) addresses.
 The (N) mapping function provides for the
correspondence between (N) addresses served by
(N) entity and (N-1) addresses.
Spring 2002 Computer Networks, MSU 17
OSI Reference Model
 Each (N) CEP is uniquely identified within its (N) SAP
by an (N) CEP identifier, used by the (N) entity and
(N+1) entity on both sides of the (N) SAP to denote
the (N) connection (which is necessary since there
might be several connections at the same (N) SAP).
(N – 1) entity
(N) entity(N ) entity
(n – 1) entity
(N-1) name
(N) CEP
(N) layer
(N) services
(N-1) layer
(N) name
(N) address
Spring 2002 Computer Networks, MSU 18
OSI Reference model
 Operations of connections
 Establishment and release
 Assume that (N+1) requests the establishment of an (N)
connection from one (N) SAP to another (N) SAP. In this
case it must provide the local (N) SAP, the (N) address of
the distant (N) SAP. Upon the establishment of the (N)
connection, the (N) CEP identifier is used by (N +1) and
(N) entities respectively to designate the (N) connection.
 The establishment and the release of connections may be
done in a dynamic manner.
 The establishment of the (N) connection assumes the
availability of (N -1) connection. And the same argument
goes downward in the hierarchical manner.
Spring 2002 Computer Networks, MSU 19
OSI Reference model
Mapping between addresses
E
D
D
C
M
L
K
K
Mapping
table
A
B DC E
A B
b
B
c
B
a
L MK
(N) - layer
Spring 2002 Computer Networks, MSU 20
OSI Reference model
Relations between connections
(N) - CEP
(N) - layer
One-to-one (N - 1) - CEPMultiplexing Splitting/Demultiplexing
Spring 2002 Computer Networks, MSU 21
OSI Reference model
 Relationships among data units
(N-1) interface
data units
(N-1) Interface
data
(N-1) interface
control
information
(N) to (N-1)
adjacent
layers
(N) Protocol
data units
(N) user data
(N) protocol
control
information
(N) to (N) peer
entities
Combined
Data
Control
Spring 2002 Computer Networks, MSU 22
OSI Reference models
Data units and their logical relationships
(N) - layer
(N-1) - layer
(N-1) SDU
(N) - PDU
(N-1) - PDU
(N-1) - PCI
Spring 2002 Computer Networks, MSU 23
OSI Reference model
 Three types of constructions
 One to one correspondence, where each (N)
connection is built on one (N-1) connection.
 Multiplexing, where several (N) connections
are built on one single (N-1) connection.
 Splitting, where one (N) connection is built on
several (N-1) connections.
Spring 2002 Computer Networks, MSU 24
OSI Reference model
 Data transfer
 Data units are the fundamental information carriers
between peer entities and the ones that worked with
specific SAP.
 (N) Protocol Control Information – PCI
 information exchanged between two (N) entities using a
(N-1) connection
 (N) User Data - UD
 data transferred between two (N) entities on behalf of
the (N+!) entities for whom they provide the necessary
services
Spring 2002 Computer Networks, MSU 25
OSI Reference model
 (N) Protocol Data Unit – PDU
 is a unit of data which contains (N) PCI and
possibly (N) UD
 (N) Interface Control Information – ICI
 is the information exchanged between (N+1) entity
and an (N) entity in order to coordinate their joint
operation
 (N) Interface Data – ID
 is the information transferred from an (N+1)
entity to an (N) entity for a transmission to a
correspondent (N+1) entity over an (N) connection.
Spring 2002 Computer Networks, MSU 26
OSI Reference model
 (N) Interface Data Unit – IDU
 is information (an unit) transferred across the SAP
between an (N+1) entity and an (N) entity during a
single interaction. The size of the unit might be
variable.
 (N-1) Service Data Unit – SDU
 is the amount of (N-1) interface data whose identity
is preserved from both ends of an (N-1) connection.
 Expedited (N-1) service data unit
 is a small (N-1) service data unit whose service is
expedited.
Spring 2002 Computer Networks, MSU 27
RM - Reference Model
Addressing
Security
Management
Application
Presentation
Session
Transport
Network
Data Link
Physical
Directory
FTAM
X.400
ISDN
OSI IP
X.25
LAPD
MAC
LAPB
X.21
LAN
ISDN
Reference
Model
Spring 2002 Computer Networks, MSU 28
SD - Service Definition
PS - Protocol Specification
 Service definition
 declarative
 one per layer
 services
 functions
 facilities
 Protocol specification
 complete interpretation
 implementation via control info and data
Spring 2002 Computer Networks, MSU 29
SD and PS
Layer N
Layer N
(N) - protocol
(N-1) - service
(N) - service
(N) - service
Spring 2002 Computer Networks, MSU 30
Service definitions
 Connection oriented mode
 connection establishment (control info)
 data transfer
 disconnection
 Connectionless
 combination of (control info + data) transfer
 unit data transfer
Spring 2002 Computer Networks, MSU 31
Service definitions
 Primitives
 request (Rq)
 indication (I)
 response (Rp)
 confirm (C)
(N)-Service
User
(N)-Service
User
(N) - Service Provider
Rq IRpC
Conceptual representation of the
interactions between layers
Spring 2002 Computer Networks, MSU 32
The Layer
 Entity
 SAP - Service Access Points
 Addresses
 NSAP
 Presentation (concatenation between NSAP and P, S
and T selectors)
((N+1) entities
N-SAPs
(N((N) entities
Spring 2002 Computer Networks, MSU 33
Construction vs... Reduction
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical
BITS
DLH DATA UNIT DLT
NH DATA UNIT
TH DATA UNIT
DATA UNIT
PH DATA UNIT
AH
DATA
SH
DATA
Construction Reduction
Spring 2002 Computer Networks, MSU 34
The Elements of the Paradigm
 Basic ideas and
principles
 inheritance
 abstraction
 encapsulation
 information hiding
 Objects and Protocols
 operations
 attributes
 accessibility
 value carrying
Spring 2002 Computer Networks, MSU 35
The Lower Layers
 Physical Layer
 Data Link Layer
 Network Layer
 Transport Layer
 Functions
 error control
procedures
 flow and
congestion control
 addressing and
naming
 routing
Spring 2002 Computer Networks, MSU 36
Physical Layer
 The interface
 mechanical
 electrical
 functional
 procedural
 Services
 connections
 data transfer
 sequencing
 management
Spring 2002 Computer Networks, MSU 37
Physical Layer
 Point to point
services
 Data Terminal
Equipment
 Data Circuit
Equipment
DCE DCE
DTE
DTE
Spring 2002 Computer Networks, MSU 38
Physical Layer
 The interfaces
 RS-232
 RS-423/X.26/V.10
 RS-422/X.27/V.11
 RS-449 or X.21
Spring 2002 Computer Networks, MSU 39
Physical Layer
 ISDN and B-ISDN
 combined benefits of digital transmission and
switching
 voice, data and image transfer
 time division multiplexing
 several 64 Kbps channels (Basic Rate Access
and Primary Rate Access)
 aspects (mechanical, electrical, framing and
procedural)
Spring 2002 Computer Networks, MSU 40
Physical Layer
 Local Area Networks (LAN)
 new physical medium types
 co-axial
 fiber optic
 twisted pair
 LAN standards
 PHY
 MAC - Medium Access Control
 LLC - Logical Link Control
Spring 2002 Computer Networks, MSU 41
Physical Layer
 Two topologies (logical) *
 Bus
 Ring
 Characteristics
 each transmission to all stations in the
network
 MAC protocol provides the transmission
rights
(*)
independent of the wiring configuration
Spring 2002 Computer Networks, MSU 42
Data Link Layer
 The objective
 data organization and transformation
 orderly transmission
 error control
 Key protocol
 HDLC (High level Data Link Control)
 OSI Link Standards (LAPB, LAPD, LLC)
Spring 2002 Computer Networks, MSU 43
Data Link Layer
 Purpose
 to pass data over lines (SDU-Service Data Units)
 possible error detection and correction
 Connection mode (link management,
transfer of SDU, report on catastrophic
failures)
 Connectionless mode (no error recovery
and no information about data loss)
Spring 2002 Computer Networks, MSU 44
Data Link Layer
 HDLC (High level Data Link Control)
 Three sub protocols
 NRT (Normal Response Mode)
 ART (Asynchronous Response Mode)
 ABM (Asynchronous Balanced Mode)
 Frame Structure
 fields for flags, address, control,
information, and checking
Spring 2002 Computer Networks, MSU 45
Data Link Layer
 Frame Structure
 address field
 control field
 information field
 checksum (CRC)
11000000001
Address field
Control field
Check field
11000000001
Information
Open
Close
Spring 2002 Computer Networks, MSU 46
Data Link Layer
 Frame Types
 I frames for Data (Numbered frames)
 Supervisor frames for error and flow control
 Unnumbered frames for connection
establishment and data transfer (CNLS)
 Frame functions
 divided along the types
Spring 2002 Computer Networks, MSU 47
Data Link Layer
 Finding Errors
 Checksum generation via CRC
 Polynomial code via special bit
 An effective way to detect errors
 all single and double bits
 all of odd number of bits
 all burst whose length is <= n (usually 16)
 high percentage with burst (n+1) and (n+2)
Spring 2002 Computer Networks, MSU 48
Data Link Layer
 Doing away with Errors
 retransmission
 timers
 acknowledgments
Spring 2002 Computer Networks, MSU 49
Data Link Layer
 Local Area Networks
 Ethernet
 Token Ring
 MAC
 Bridges do the job
 Better local effect than global
 Spanning tree, source routing and
routing
Spring 2002 Computer Networks, MSU 50
Network Layer
 Going from local to global by
 routing
 switching
 relaying
 Technology diversity is not an obstacle for
 combining end and intermediate systems
 internetworking
Spring 2002 Computer Networks, MSU 51
Network Layer
 Connection Oriented Network Service- CONS
 basic function: transparent transfer of data between
transport entities
 part of the connection
End system End system
N-service N-service
Access
protocol
Access
protocol
Inter-node protocol
Intermediate systems
Spring 2002 Computer Networks, MSU 52
Network Layer
 CONS
 Service
 data expediency
 reset service
 receipt confirmation
 Additional functions
 multiplexing
 error recovery
Spring 2002 Computer Networks, MSU 53
Network Layer
 Connectionless Network Service- CLNS
 service: transmission of N-SDU
 based on Connectionless Mode Network Protocol
(CLNP) with two functions
 segmentation
 internetworking
Transport Network Transport
N-Unit data
request
IPDU
N-Unit data
indication
Spring 2002 Computer Networks, MSU 54
Network Layer
 Routed protocols
 how to behave
 what to do
 Routing protocols
 where to go
 ISO 9542 (ES-IS)
 IS-IS for large networks
Spring 2002 Computer Networks, MSU 55
Network Layer
 Organization
 three entities
 end systems
 sub-networks
 inter-working
units(IWU)
 Internetworking
 bridges
 routers
 gateways
 IWUs
Spring 2002 Computer Networks, MSU 56
Network Layer
 Naming and addressing
 interplay of number of entities
 names
 addresses
 objects
 mapping or translation
 processes
 Name space defines the world of reference
for a process and its relation to objects
Spring 2002 Computer Networks, MSU 57
Network Layer
 process and its name space
 name spaces co-operate via sockets
<network, host, socket>
Machine A
Machine B
P1
P2
P3
P4
P5
Socket
Data link
Data link
Subnetwork
Spring 2002 Computer Networks, MSU 58
Network Layer
 Addressing and Routing
 address translation (NSAP to subnetwork
addresses) is done hop by hop at ES and IS
 look-up tables and routing algorithms
 the step by step procedures improve reliability and
minimize local effects
 addresses are part of the routed protocols (CLNP is
the responsible entity)
Spring 2002 Computer Networks, MSU 59
Transport Layer
 Objectives
 full communication between two end
systems
 quality of service and cost
 mainly connection oriented
Subnet 1
Subnet 2
Transport
Net 1
Transport
Net 2
Transport protocol
Intermediate
system
Spring 2002 Computer Networks, MSU 60
Transport Layer
 Service and functions
 connection
 mapping: transport address to network
address
 network connection
 how to optimize
 optimal data unite size
 additional functions determination
 types of transport connections
 data transfer
Spring 2002 Computer Networks, MSU 61
Client/Server Paradigm
 The fundamental paradigm of distributed
computing
 The existence of at least two processes
 client
 server
Client
Kernel
Server
Kernel
Request
Response
Network
Spring 2002 Computer Networks, MSU 62
Client/Server Paradigm
 Characteristics
 simple CLNS protocol (request/reply)
 efficient
 smaller protocol stack/conceptually only three
layers
 Saves resources via lightweight processes
 Respect for an autonomous process - the
objective of distribution
Spring 2002 Computer Networks, MSU 63
Transport Layer
 Data transfer
 sequencing
 blocking
 concatenation
 segmenting
 splitting
 flow control
 error detection and recovery
 efficiency
 SDU delimitation
 connection identification
Spring 2002 Computer Networks, MSU 64
Transport Layer
 Disconnection
 the reason for disconnecting
 what connection is released
 some data transfer (very limited)
 The Transport Protocol
 rather conventional and similar to l2&l3 in X.25
 based on the exchange of Transport Protocol Data
Units (TPDU)
Spring 2002 Computer Networks, MSU 65
Session Layer
 The Concept of Conversation
 activities
 interactions
 synchronization
 The Objectives
 the need to disassociate control information from an
application
 standardization of control information
Spring 2002 Computer Networks, MSU 66
Session Layer
 The Services
 connection
 addresses
 The interaction with the Transport layer
 open connection
 close connection
 data transfer takes place
Spring 2002 Computer Networks, MSU 67
Session Layer
 Complete services
 Similar to the Transport layer
 connection establishment
 connection release and/or abort
 data exchange
 Specific
 interaction management
 session process synchronization
 activity management
 unexpected events reporting
Spring 2002 Computer Networks, MSU 68
Session Layer
 Functions
 connections
 data transfer
 expedited data
 disconnection
 Concept of tokens
 Protocols
 Session protocol Data Units (SPDUs)
 An extensive use of concatenation
Spring 2002 Computer Networks, MSU 69
Presentation Layer
 The objective
 identifying and implementing the common
principles for presenting information
 support of data presentations
 Data representation differs due to
 syntax differences in the OE in ES
 improvement in security and transmission
 Syntax (abstract, transfer and context)
Spring 2002 Computer Networks, MSU 70
Presentation Layer
 Operations
 two levels (abstract and
detailed)
 abstract syntax
 data value
 concrete syntax
 transfer syntax
 local concrete syntax
Spring 2002 Computer Networks, MSU 71
Presentation Layer
Application 1
Application 2
Presentation 1
Presentation 2
Data Values
Data Values
Abstract syntax
Transfer syntax
Service primitives
Spring 2002 Computer Networks, MSU 72
Presentation Layer
PDV1 PDV2
EBS EBS
Application layer
software
Representation of values
in transfer syntax
Encoding in local
concrete syntax
Syntax
transformation
in the Presentation
Layer
Session user
data parameter
EBS - encoded
bit string
Spring 2002 Computer Networks, MSU 73
Presentation Layer
 Services
 syntax selection
 presentation context
 define context set (DCS)
 default context
 Functions
 functional units
 kernel
 context management
 context restoration
Spring 2002 Computer Networks, MSU 74
Presentation Layer
 syntax transformation
 presentation context
 define syntax
 compile code
 The Presentation Protocol
 similar to the session protocol
 PPDUs
 associated with the presentation primitives
 parameters of the primitive
Spring 2002 Computer Networks, MSU 75
Application Layer
 The objective: support of the user programs
based on system independent applications
 Application process (application implemented
on a single end system)
 the user application
 the application entity (AE)
 vendor independence
 a concept of a virtual device
Spring 2002 Computer Networks, MSU 76
Application Layer
 The Model of a Layer
User
Real Device
User application
Application entity
services defined in terms of
virtual devices
Application
Process
Spring 2002 Computer Networks, MSU 77
Application Layer
Application
entity
a
Control
information
ASE 1
ASE 3
ASE 4
ASE 5
Application service
object (ASO)
The Structure
– process
– entity
– process title
– entity title
– association
– service element
– context
– object
– control function
A
P
P
L
I
C
A
T
I
O
N
Spring 2002 Computer Networks, MSU 78
Application Layer
 Remote operations
 interactive processes
 co-operation between remote systems
 examples
 X.400-P3
 CMIP (management)
 RDA (database access)
 Remote Operation Service Element (ROSE)
Spring 2002 Computer Networks, MSU 79
Application Layer
 Remote operations
 BIND (establishes association with an application)
 A set of OPERATIONS, each one indicates a list of
ERRORS
 UNBIND (disassociates from application)
 Classes and modes of operations
- asynchronous
- synchronous
Classification parameter:
yes or no report on the result