Computer Networks 364 OSI Architecture John Morris Computer Science/Electrical Engineering University of Auckland Email: [email protected] URL:

Computer Networks 364Computer Networks 364OSI ArchitectureOSI Architecture

John Morris

Computer Science/Electrical EngineeringUniversity of Auckland

Email: [email protected]: http:/www…..

ContentsContents

► Open Systems► Open Systems Networking► OSI Architecture

The Physical layer The Data link layer The Network layer The Transport layer The Session Layer The Presentation Layer The Application layer Data transmission

Communications SoftwareCommunications Software

► Programming Writing a program that communicates with another machine Tasks

► Decide on communication style eg Message passing

► Decide on protocol Reliability needed?

► Messages acknowledged / repeated► Decide message format

Fixed/variable length? Checksums needed?

► Link to use Physical medium Physical message format

► Header / trailer Physical encoding?

► Form of electrical signal – NRZ, Manchester, etc Do this every time?

► No way!


► Programming Do this every time?

► No way!► Create software and hardware layers

For example► Physical layer

Electrical encoding Formation of bits into packets

► Network layer Machine machine

► Destination addresses► Checksums, etc

► Application layer Your program

► Message format and content► Acknowledgements► Retries for corrupted messages


► Software and hardware layers Many layering schemes possible One example

►Physical layer Electrical encoding Formation of bits into packets

► Network layer Machine machine

► Destination addresses► Checksums, etc

► Application layer Your program

► Message format and content► Acknowledgements► Retries for corrupted messages

Add * automatic acknowledgement,* retry corrupted messageshere


► Software and hardware layers Many layering schemes possible eg

►Physical layer Electrical encoding Formation of bits into packets

►Network layer Destination addresses Checksums, etc

►Application layer Your program Message format and content Acknowledgements Retries for corrupted messages

Add automatic acknowledgement,retry corrupted messageshere


► A standard would be useful!► Open Systems Interconnection (OSI) architecture

Defined by International Standards Organization (ISO)►Seven layers

Application Presentation Session Transport Network Data Link Physical

Open SystemsOpen Systems

► Computer Industry matures Moves away from closed systems towards open systems

► Closed systems Proprietary Generally linked to a single vendor who

►Defines how systems communicate Packet and message format Programming interfaces

Difficult to communicate with other systems (closed or open!)►Additional software

Drivers

►Additional hardware Voltage level shifters Voltage current converters

Closed SystemsClosed Systems

► Software written for a closed environment Not portable

►Cannot run on a different system Without expensive conversion

►Substantial code re-working required► Examples

Classic mainframe OS’s► MVS (IBM) or ► VMS (Digital)► Many other examples from lesser manufacturers

Computer suppliers occasionally created nightmares for themselves!

► Multiple proprietary (non-standard) systems on their own machines!► Only good for ‘job maintenance’ for system admins!

But system admin is a thankless task !


► Manufacturer defines protocols and architectures for connecting machines.

► Connect computers from one manufacturer only Only their software could understand the protocols used Getting information about proprietary protocols

►Difficult►Expensive

Writing software drivers►Time consuming

Documentation usually inadequate

►Frustrating Manufacturer ‘owns’ the protocol Free to change it at any time Hitting moving targets is difficult!


► Manufacturer defines protocols and architectures for connecting machines.

► Connect computers from one manufacturer only Only their software could understand the protocols used

► Connecting multiple vendors’ systems Getting information about proprietary protocols

►Difficult►Expensive

Writing software drivers►Time consuming

Documentation usually inadequate

►Frustrating Manufacturer ‘owns’ the protocol Free to change it at any time Hitting moving targets is difficult!

With only closed systems,today’s Internet wouldnot have been possible!


► Environment in which programs run Not linked to an individual computer manufacturer Fully documented

►Writing software for new systems is feasible

► Portable systems Software written for one system may run on another Avoids ‘locking in’ a user to one manufacturer Most manufacturers now accept

►Open systems Easy interconnectionMore applicationsMore flexibility to build systems with desired capabilities

Combining hardware from multiple sourcesMore salesLarger, rather than smaller, profits!

►


► Pioneer UNIX

►OS software available from the beginning►Porting Unix easy►Curiously …

Initially derided by computer salesmen as an academic ‘toy’► Source was given by AT&T to universities► So most used it

Now they all sell it!


► Currently demands for Information

► Internet Electronic transactions Linking computers into networks (LAN, WAN, etc)

mean that interconnection is essential ► Open systems permit easy connection of machines

different manufacturers different operating systems

to facilitate these transactions► As use of computer networks increased

Computer manufacturers themselves recognised that some standards were needed to rationalise the chaos of communications that was beginning to develop

Open Systems Networking - Open Systems Networking - Historical notesHistorical notes

► Computer Manufacturers Initial step

►Define particular protocols as standard within a company for all communications of a certain type

► IBM’s BISYNC All communications with terminal concentrators and multi-

drop lines

Second step ►Devise an overall framework or architecture

within which specific protocols would fit►Examples

IBM’s Systems Network Architecture (SNA) andDEC’s Decnet

►These architectures intended to permit open communication between all products of one company

Open Systems NetworkingOpen Systems Networking

► Public data networks Intended to link different organisations Introduced experimentally in the 70's and As an operational service in the early 80's

► X.25 packet switching Most of these networks use the X.25 protocol Interlinking of X.25 networks in different countries

►Proceeded rapidly during the 80's ►Formed a global network by the end of that decade

Defines three protocol layers concerned with subnet access whereas

► SNA and DECNET Seven layers of protocol Include standardised functions

► file transfer, etc


► Limitations of X.25 Defined by the CCITT CCITT largely made up of telecomm-operators

eg British Telecom

►Only concerned with the protocols used in operating the subnets they planned to build

Any file transfer protocol will operate across an X.25 network CCITT did not feel a need to define protocols for higher level

services

► Efficient use of networks Need standard protocols for higher level services

►Applications file transfer web pages remote login electronic commerce


► Full inter-operability►rather than inter-communication

requires All the communications functions in different systems to

operate with one another but also All applications using the communications functions must

►understand data arriving from remote systems and►generate data understandable by remote systems


► Various parties have collaborated to devise a network architecture it is hoped it will eventually replace manufacturer-specific

architectures

► International Standards Organisation (ISO) has been responsible for co-ordinating the process of devising this model

► ISO works through a series of technical committees They devise standards for a wide variety of products and

services, not just networks


► ISO’s architectural model Intended to allow communication between any systems Called the Open Systems Interconnection (OSI) model.

► Few system implement the OSI model directly Usual computer industry problems with standards adoption

►Legacy systems►Slowness to develop and specify standards►Standards often have problems with new technology

Forms an invaluable (standard) reference for description of any protocol

►Descriptive capabilities generally recognised

OSI ArchitectureOSI Architecture

Network

Link

Physical

Network

Link

Physical

Node Node

Sub-net

End to end peer protocols

Sub-net access protocols

Communications Subnet Boundary

Host AHost A Host BHost B

Application

Presentation

Session

Transport

Network

Link

Physical

Application

Presentation

Session

Transport

Network

Link

Physical


► OSI originally Defined as a connection-oriented architecture

►No data could be exchanged until a logical link had been set up between source and destination

► OSI now Includes both

►connection-oriented and ►connectionless services

but still retains a connection-oriented bias


► Each layer is defined in terms of: A service interface to the layer above

►Each layer may only be used by the layer immediately above and directly use the layer immediately below

► It defines a set of primitive functions available for use by the upper layer

A peer protocol ► A protocol for communicating with the corresponding

layer in another machine


For the upper layers the other machine may only be ►a host machine►end system►DTE

For the lower layers it may be either ►a node of the network or ►a host

The peer protocol carries out functions which are entirely transparent to the layer above

►principally the error recovery mechanism of the link layer


► Communications sub-net The collection of nodes and links in the network, possibly

packet switched exchanges, forms what the OSI model calls a communications sub-net

The attached hosts are not considered part of the communications sub-net

The lowest three layers of the OSI model are concerned with access to the communications sub-net

The protocols of these layers are termed sub-net access protocols

►The functions and their form are strongly influenced by those of X.25 and

►Conform closely to it in terms of the peer protocols


► End-to-end peer protocols Upper four layers

►Concerned with communication via the sub-net Protocols for these layers are termed end-to-end peer

protocols Protocol Data Units (PDU) in these layers are carried

transparently across the communications sub-net


• Peer protocols and Service Interfaces in OSI

Transport Layer(Layer 4)

Lower Layers

Transport Layer(Layer 4)

Lower Layers

Transport Service Interface

Transport Protocol(an end-to-end peer protocol)

Local End system Remote End systemSub-net


► OSI model defines only the functions of each layer.► Separate standards define

► the protocols and ►service interfaces for each layer

For many layers there is more than one protocol defined►eg HTTP, FTP, ...

OSI - Physical LayerOSI - Physical Layer

► This layer defines Electrical signalling on the transmission channel How bits are converted into electrical current, light pulses or

any other physical form and Ability to detect signalling errors in the network media

► Examples serial line ethernet cable Low Voltage Digital Signalling (LVDS)

► Network device functioning at this layer only is called a repeater


► Electrical signalling Typical questions are:

►How many volts or amps represent 1 and 0 ?►How many microseconds a bit lasts?►Transmission simultaneously in both directions?

Full or half duplex

►How many pins the network connector has … Signal lines Control signals Grounds

Design issues ►mechanical, electrical and procedural interfaces►physical transmission medium

V.24 / RS-232 is an example of a physical layer standard


► Electrical signalling Examples

NRZ

Clock

Manchester

Bits 0 0 0 0 0 0 0 0 01 1 1 1 1 1 1

NRZI

OSI - Data Link LayerOSI - Data Link Layer

► Defines how the network layer packets are transmitted as bits

► Sender breaks the input data up into a data frame (a few hundred

bytes), transmits the frames sequentially and processes any acknowledgement frame sent back by the

receiver

► Data link layer creates and recognises frame boundaries


► Frame formatting and the Cyclic Redundancy Check (CRC), which checks for errors in the whole frame, are incorporated at this layer

► Main task of the data link layer take a raw transmission facility (physical layer) and transform it into a line that appears free of undetected transmission

errors to the network layer

► Physical layer Accepts and transmits a stream of bits without regard to meaning or

structure

► Data link layer creates and recognises frame boundaries Accomplished by

►special bit patterns to the beginning and end of a frame►signals on separate lines

(less common in modern high bandwidth systems)


► Noise Noise can corrupt or destroy a frame completely The data link layer software on the source machine should

retransmit the frame Multiple transmission of a frame

►possibility of duplicate frames Link layer required to solve problems caused by damaged, lost

and duplicate frames


► Service classes Data link layer may offer several different service classes to

the network layer Each with a different quality and price.

► Examples of data link layer protocols are Point to Point Protocol (PPP) , High-Level Data Link Control (HDLC) and Ethernet

► Bridges and switches provide operations up to the data link layer

OSI - Network LayerOSI - Network Layer

► Network layer defines how information from the transport layer is sent over

networks and how different hosts are addressed

► Each host must have at least one unique address► A host’s network address allows other machines to

address their messages to that host► Using the network address, network layer controls the

forwarding of messages between stations. It determines how packets are routed from source to

destination


► Routers and Gateways Packets of data are transmitted to other networks though

routers (sometimes called gateways in Internet technology)

► The network layer controls the operation of the subnet► Key design issue

Determine how packets are routed from source to destination


► Routing Routes can be

►stored in static tables wired into the network and rarely changed.

►determined at the start of each conversation, for example a terminal session.

►highly dynamic - determined anew for each packet reflecting the current network load or state If too many packets are present in the subnet, they will interfere

with each other, forming a bottleneck Control of subnet congestion belongs to the network layer Network layer overcomes these problems and allows

connection of heterogeneous networks

► Packet layer of X.25 is a standard Network Layer protocol

OSI - Transport LayerOSI - Transport Layer

► Provides for end-to-end data transmission Network devices between sender and receiver do not

change the data

► It takes care of data transfer, ensuring integrity of data, if desired, by the upper layers

► guarantee correct sequencing delivery in the same order as it was sent

► transmission in a timely manner splitting data from the session layer into smaller units

►Frames to be handled by the network layer

OSI - Transport LayerOSI - Transport Layer

► Determines Types of services provided to the session layer Who will use the network

► Two typical connection modes in this layer Connection-oriented

►Establishes logical connection between applications on two hosts

►Point-to-point Each connection involves two hosts only

Connectionless►Broadcast and multi-cast transmissions

One-to-many transmissions

Most popular type of transport connection ►Error-free point-to-point model that delivers messages in order

OSI - The Transport LayerOSI - The Transport Layer

► Basic function Accept data from the session layer, (if necessary) split it up into smaller units pass these to the network layer and ensure that the pieces all arrive correctly at the other end

► Additional requirements This must be all done efficiently and in a way that isolates the upper layers

from inevitable changes in the hardware technology

OSI - Transport Layer FunctionsOSI - Transport Layer Functions

► Too many functions performed to examine them all! ► Key functions are:

Splitting Re-sequencing Multiplexing

► Some functions always required all types of sub-net

► Others are optional Needed for certain types of sub-net


► SPLITTING Always required Every type of sub-net has some restriction on data unit size

►Examples 128 bytes in X.25 packet switched networks 1500 bytes in Ethernet LANs 48 bytes in ATM networks

Higher layers should not need to be aware of these differences►Application programs should work - without change - on all

types of network


► Splitting … Transport layer

►accepts data units of any size from the layer above and►breaks them up into smaller data units

size acceptable to the sub-net

At the destination ►reassembles these small data units into their original form

and ►delivers the assembled data unit across the transport

service interface to the higher layer The higher layer is unaware that the data unit was transmitted

across the sub-net in pieces


Transport Layer Transport Layer

Transport Layer Splitting

Data Unit

Split data unit

(Reassembled) Data Unit


► Re-Sequencing Connectionless (Datagram) type sub-nets cannot guarantee

that a sequence of packets transmitted across the sub-net will arrive in the same order as they were sent

Data units in a datagram network do not have their own sequence numbers that indicate the order in which data units were sent

► TL’s responsibility to put the received packets in order Transport Layer introduces its own sequence numbers to

permit re-sequencing to be carried out


3 2 1

1

3

1

31

3

1

2

2

1

1 3 2

Effect of different routes on order of packet arrival


► MULTIPLEXING Two possible types Upward multiplexing

►A number of calls may share a single network layer connection to a given destination and

► thus avoid the need and cost of establishing a number of network layer calls across the sub-net between the same source and destination pair

Transport Layer

Network Layer

Upward Mutiplexing


Downward multiplexing►A single network layer connection cannot provide the

required data rate ►A single transport layer connection may be connected to

multiple network layer connections ►Each carries a proportion of the traffic in order to achieve

the desired data rate

Transport Layer

Network Layer

Downward Mutiplexing

OSI - Session LayerOSI - Session Layer

► Responsible for establishing and terminating network connections arranging sessions into logical parts name-to-station address translation

► A Session allows ordinary data transport (as does the transport layer)

but also provides some enhanced services

►useful in some applications.


► Synchronisation Some operations do not allow both sides to undertake the

same operation at the same time►mutual exclusion required

A locking control in the session layer provides flags that can be exchanged

Only the host holding the lock may perform the critical operation

Prevent inconsistent data updates in the application


► Another synchronisation example Transferring a large file Two hour file - aborted after, say, one hour of transmission Session layer adds check points into the data stream After a crash, only the data transferred after the last check

point have to be repeated

OSI - Presentation LayerOSI - Presentation Layer

► Responsible for data translation Formatting Encryption and Decryption

► Typical example of a presentation service Encoding data in a standard, agreed way

►e.g. ASCII, GB, BIG5, etcPossible for computers with different internal data formats to

communicate ►Data structures to be exchanged can be defined in an

abstract way►Presentation layer translates common format used for

transmission

OSI - Application LayerOSI - Application Layer

► Top layer Interface between the application and the user Used for those applications that are specifically written to run

over the network Specifies the protocols to be used between the application

programs

OSI - Application LayerOSI - Application Layer

► Examples File transfer

►Different file systems have different naming conventions, ways of representing text lines, ...

►Transferring a file between two different systems requires handling these incompatibilities

email remote job entry, directory lookup, web serving and various other general and special purpose facilities

OSI Architecture - Data TransmissionOSI Architecture - Data Transmission


► Sending process has some data it wants to send using the OSI model It gives the data to the application layer,

►which attaches the application header, AH (may be null), to the front of it and

gives the resulting item to the presentation layer Presentation layer may transform the data in various ways

►eg character set conversion►and possibly add a header to the front,

gives the result to the session layer …. Process is repeated until the data reaches the physical layer,

where it is actually transmitted to the receiving machine


► Receiving machine Headers are stripped off one by one as the message

propagates up the layers until it finally arrives at the receiving process.

Key idea Although actual data transmission is vertical in the figure, each layer is programmed as though it were horizontaleach layer communicates with its peerin the same layer on the receiving machine


► Nodes in the communications sub-net contain at least two, possibly identical, stacks of protocols.

► Data is copied between these stacks at the highest layer implemented in the node.

► There is no communication within the node between the lower layers of protocol.

► This process of copying between stacks within a node is termed relaying and employs an appropriate form of routing to select which stacks are connected together.


► The reason for the choice of the term 'relay' is the obvious analogy with relay races.

► The packet may be viewed as the 'baton' with the individual links and their associated protocol stacks and protocols correspond to the runners in the race who carry the baton from the start (source) to the finish (destination).

► User process only communicate with the OSI layer via the Service Interface provided by the highest layer - the application layer.

Documents

Computer Networks 364 OSI Architecture John Morris Computer Science/Electrical Engineering University of Auckland Email: [email protected] URL: