OSI Model and Standards

ITNW 1325, Chapter II

Understanding the OSI Model

Understanding the OSI ModelOverview: Open Systems Interconnection (OSI) – a layered

reference model comprised of seven functional layers Developed by the International Organization for

Standardization (ISO) in 1984 – based on their analysis of TCP/IP, IBM SNA, and DECNET protocols

Governed by the ISO Standard 7498 – some vendors build their products according to it (Novell)

Ensures compatibility and solves communication issues among different implementations of network hardware and software

Understanding the OSI ModelOverview (continued): Uses the divide-and-conquer approach to networking

from low-level hardware to the high-level software Constructs a series of independent but interconnected

layers – breaks the big problem of communications into smaller problems that are isolated from each other

Individual layers encapsulate specific independent functions – changes to one layer don’t affect other ones

Implements the “peer communication” principle – only identical remote layers communicate to each other

Understanding the OSI ModelOverview (continued): “Universal” resembles “imperfect” – some network

functions operate at several layers, while some do not require services from every layer

Practical usage is questioned by many because of its complexity and lack of flexibility

The OSI networking model remains a great tool for learning

networks – protocols, devices security, and other models

Understanding the OSI ModelReasons for Layering: Divides communications into a finite number of logical

blocks – simplifies comprehension and use Provides design modularity – allows upgrades to a

specific layer to remain separate from the other ones Allows programmers to specialize in a particular layer

of the networking model, with open set of specifications Encourages interoperability by promoting balance

between different networking models Allows vendors to produce standardized interfaces

Understanding the OSI ModelSeven Layers:

Understanding the OSI ModelFrom a meaningless sequence:

Application (L7)

Presentation

Session

Transport

Network

Data Link

Physical (L1)

To the meaningful phrase:

All

People

Seem

To

Need

Data

Processing

Understanding the OSI ModelFrom a meaningless sequence:

Application (L7)

Presentation

Session

Transport

Network

Data Link

Physical (L1)

To the meaningful phrase:

Away

Pizza

Sausage

Throw

Not

Do

Please

Understanding the OSI ModelPeer Communication, Overview: Each layer is unaware of the activities of all other ones

on the same host – doesn’t acknowledge their services Each layer only communicates logically to an identical

layer on the other side of the communication process – information is passed via headers and trailers added

Headers and trailers added at the sending layer will be read and removed at the peer layer on the other side

Protocol suites combine protocols defined at different layers together to enable network communications

Understanding the OSI ModelPeer Communication, Illustration:

Understanding the OSI ModelPeer Communication, Advantages: Allows convenient distribution of networking functions Permits independent error checking on different layers Simplifies creation of protocols

Peer Communication, Disadvantages: Results in overhead that grows as data traverses the

model from the Application to the Data Link layer Leads to reduced efficiency of network utilization

OSI Layer Functions

OSI Layer FunctionsApplication (L7): Defines network services that software applications

(browsers, e-mail clients, etc) can request from the network and requests the services on their behalf

Accepts data from applications and interprets their formatting and procedures to the network

Interprets data coming from the network and passes it to proper applications

Facilitates multiple important protocols – HTTP, FTP, DNS, Telnet, SMTP, SNMP, etc.

OSI Layer FunctionsPresentation (L6): Receives data from the Application layer and prepares

it for transmission over the network Reformats the incoming data from lower layers for

specific machine/application combination Performs encryption and compression of data for

outbound communications – as well as decryption and decompression of data for inbound communications

The only layer that restructures data – other ones add headers and/or trailers without reconfiguring the data

OSI Layer FunctionsPresentation (continued): Distinguishes between file extensions and coding

schemes – BMP, JPG, WAV, MP3, ASCII, HTML, etc. Example – Presentation layer protocols encode online

music tracks into MP3 format Example – Presentation layer protocols interpret JPG

images so that HTTP is able to understand them Example – Presentation layer protocols encode text

using ASCII and other schemes Example – Presentation layer protocols encode/decode

sensitive data within secure Internet connections

OSI Layer FunctionsSession (L5): Allows senders and receivers to establish and manage

data transmission session – independently of the actual data flow over the network

Detects if the transmission has been cut off, notifies the client software, and restart its at the appropriate point

Determines the order of communication, maximum duration of transmission, and provides clocking or timing for the session

Assists large data transfers – informs the receiver about the beginning/end of the stream that’s broken in pieces

OSI Layer FunctionsSession (continued): Allows information of different streams – that may be

originating from different sources – to be properly combined or synchronized

Facilitates NetBIOS, SQL, RPC, and other protocols

OSI Layer FunctionsTransport (L4): Accepts data from the Session layer services and

provides messaging service for them Facilitates connection-oriented (guarantee of delivery)

and connectionless (delivery not guaranteed) protocols Connection-oriented protocols ensure data delivery –

used for sensitive data transmissions over the Internet Connectionless protocols don’t ensure data delivery –

but impose much lower overhead onto the network Submits data with its header added to the Network layer

for further handling

OSI Layer FunctionsTransport, Connection-Oriented Protocols: Explicitly establish a session (“connection”) before

allowing data to be sent Ensure data delivery by requiring and acknowledgement

(ACK) of the receipt of data packets – retransmit in case an ACK is not timely returned

Negotiate for the highest number of data segments to be sent before an acknowledgement is required

Provide data integrity via checksums – unique character strings attached to data that allow the receiving node to determine if a data unit was modified during delivery

OSI Layer FunctionsTransport, Connection-Oriented Protocols (continued):

OSI Layer FunctionsTransport, Connection-Oriented Protocols (continued): Ensure reliable data delivery by breaking large data

units into multiple smaller segments (segmentation) – with segment size related to the MTU size

The MTU size is the maximum data size that nodes on the way can place into their memory buffers

Identify segments that belong to the same message, determine the order of segments (sequencing), and reconstruct the segmented units (reassembly)

Gauge appropriate rate of transmission based on how fast the recipient can accept data (flow control)

OSI Layer FunctionsTransport, Connectionless Protocols: Do not establish a connection before sending data Do not require acknowledgements for data sent – don’t

ensure the that the data was properly received Define a special term for data carried – datagrams Do not perform error check Much less sophisticated and have less transmission and

processing overhead than connection-oriented ones Used in cases when data needs to be sent quickly Example – streaming video and audio transmissions

over the network

OSI Layer FunctionsTransport, Protocols:

OSI Layer FunctionsNetwork (L3): Accepts data from the Transport layer – wraps

segments into packets that carry addressing information May brake large packets into smaller ones – according

to capacity of the network (fragmentation) Defines protocol-dependent logical addressing schemes

that uniquely identify nodes within interconnected networks and enable network segmentation

Establishes the best delivery path (routing) considering addressing, delivery priorities, network congestion, quality of service, and cost of the paths (routes)

OSI Layer FunctionsNetwork (continued): Implements congestion control by sensing delays

associated with routes and managing how much traffic is sent across them – helpful within busy networks

Internet Protocol (IP) is the most common L3 protocol

OSI Layer FunctionsData Link (L2): Encapsulates packets received from the Network layer

into frames – complete packages to be transmitted Defines the format of the header and/or trailer added to

packets received – depend on the network type in use Common network types are Ethernet and Token Ring –

use different frames and can not be used together Frame format and maximum size map onto the carrying

capacity of the network medium Performs verification of data integrity using checksum

mechanism – to detect transmission errors

OSI Layer FunctionsData Link (continued): Implies error correction upon the receiver’s request for

retransmission in case a frame is dropped or altered Manages point-to-point transmission across the

medium within the same logical or physical cable segment

Splits into two sublayers with separate duties – Logical Link Control (LLC) and Media Access Control (MAC)

OSI Layer FunctionsData Link, Sublayers:

OSI Layer FunctionsData Link, Sublayers, LLC: Interfaces the Network layer – implies intelligence Packages data frames differently for different networks Manages flow control and issues requests for

retransmission for data with errors

Data Link, Sublayers, MAC: Defines a unique physical identifier – MAC address –

for network cards (every frame carries a destination and source MAC addresses)

Defines and manages the access to the physical medium

OSI Layer FunctionsData Link, MAC Addresses: 48-bit non-replaceable, “burned-in” addresses (BIA) -

represented using twelve hexadecimal characters Consist of two parts – a block ID and a device ID A block ID (“Organizational Unit Identifier, OUI”) – a

six-character (24-bit) sequence that uniquely identifies each vendor (managed by IEEE), with large vendors assigned several different block IDs

A device ID (“serial number”) – a six-character (24-bit) sequence that uniquely identifies the device (managed by the manufacturer)

OSI Layer FunctionsData Link, MAC Addresses (continued):

OSI Layer FunctionsData Link, Frame Integrity: Before a frame is sent, the sender performs a cyclic

redundancy check (CRC) on all of its fields – generates a unique 4-byte frame check sequence (FCS) code

The FCS code is attached to the frame being sent – to be detached and regenerated by receiver

The generated code is compared to the one received – no error is assumed in case the two codes match and a retransmission request is issued in case of mismatch

OSI Layer FunctionsData Link, Frame Handling: All NICs connected to the same physical segment of the

network receive and process frames sent Only NIC with matching destination MAC address

passes the payload to the Network layer – other nodes would drop the frame

Broadcast frames are sent to and processed by all nodes on the physical segment – costs performance

Reducing the number of nodes on a physical network – segmentation – improves performance by reducing the number of frames sent and processed

OSI Layer FunctionsPhysical (L1): Accepts frames from the Data Link layer and turns

frame bits into the medium pulses on the sending end Transforms pulses to bits and passes them to the Data

Link layer on the receiving end Defines mechanical, electrical, and procedural

characteristics of the network hardware and medium Determines data transmission rates and timing intervals Non-intelligent layer – does not read data handled, adds

no header or trailer, and performs no error correction

OSI Layer Functions

OSI Model at Work

OSI Model at WorkEncapsulation, Overview: Each lower layer accepts data from the layer above and

performs encapsulation – adds a protocol data unit (PDU) composed of layer-specific header and/or trailer

A PDU enables logical communication between a layer at the source computer and the identical layer at the destination computer

Headers are layer-specific labels, trailers carry error-detection/correction information and end-of-PDU flags

The encapsulated data is passed to the layer below

OSI Model at WorkEncapsulation, Layer PDU: Application, Presentation, and Session layer PDUs

come in a variety of types and are referred to as Application, Presentation, and Session PDUs

Transport, Network, and Data Link layer PDUs are referred to as segments, packets, and frames

Physical layer PDUs consist of series of pulses that match bit patterns for Data Link layer frames

OSI Model at WorkEncapsulation, Process: Begins at the at the upper three layers – the data is

converted into a standard networking format Transport layer forms segments by adding a header with

port information – ensure proper delivery The Network layer forms packets by adding a header

with logical addressing information – ensures routing The Data Link layer forms frames by adding a header

with physical addressing information and a trailer The Physical layer encodes frames and transmits them

as pulses along the physical network

OSI Model at WorkEncapsulation, Illustration:

OSI Model at WorkDecapsulation: The receiver’s Physical layer accepts the data from the

physical network – transforms pulses into bits, passes to the layer above where bits are read as a frame

Headers and trailers are removed as data travels up the OSI model’s layers at the destination computer

Ultimately, the original data is passed to the receiving application by the receiver’s Application layer – with no headers or trailers present

OSI Model at WorkEncapsulation/Decapsulation:

OSI Model at WorkRelevance:

1984 Today

Physical Medium(wireless, copper, fiber-optics)

Data Link Ethernet(frame format, access to the medium)

Network IP(packet format, address format)

Transport TCP(segment format, reliable procedures)

Networking Standards

Networking StandardsAdvantages: Creation of competition – everybody may create

technological devices based on a standard, as opposed to proprietary, apart from standards, patented devices

Lower cost for consumers – via lower product startup costs, time due to lower manufacturing costs, and healthy competition

Protection of investment into technology – lower costs and clarity of equipment upgrades due to backward compatibility of newer products

Interoperability – all devices from various vendors

Networking StandardsDisadvantages: International standards – open domestic markets to

competition from countries with lower production costs Political conflicts – can be caused by standards or result

in rejection of standards proposed by a nation by others

The advantages outweigh the disadvantages

Networking StandardsTypes, De Facto: Common practices followed by industry for a variety of

reasons – ease of use, established habits, costs, etc. Primary influencing factor – success in the marketplace Examples – MS Windows, Intel x86 architecture

Types, De Jure: Official, entrusted standards established by a body or an

organization – with different subcommittees overseeing different technologies

Subject to lengthy development and acceptance process Published and accessible to everyone online

Networking StandardsTypes, De Jure (continued): First step – working groups of industry experts propose

the initial draft that gets published Second step – requests for comments (RFCs) are sought

from all interested developers, users, and specialists Third step – the comments are reviewed and may be

incorporated into a draft of the standard Finally, the entire organization reviews the draft before

it gets published as an official standard A De Facto standard may become De Jure one upon

approval by a committee or other authorized entity

Networking StandardsTypes, Consortia: Introduced by industry-sponsored organizations that

want to promote a specific technology within a short period of time

Example – World Wide Web Consortium (W3C) that involves Microsoft, Sun, and IBM (developed Internet standards such as HTML, CSS, DOM)

Imply membership that may be open or not

Standards can be enforced by the market

De Jure standards are enforced by a regulatory authority

Networking Standards Groups

Networking Standards GroupsInstitute of Electrical and Electronics Engineers (IEEE): World’s largest technical professional society – consists

of 37 smaller societies and councils Developed more than 800 standards in IT and

communication, circuits and devices, control and automation, signal processing, optics, power and energy, etc. since early 1980s

Project 802 develops computer network architecture and technology standards: Ethernet LAN (802.3), Token Ring (802.5), wireless LAN (802.11), etc.

Website – www.ieee.org

Networking Standards GroupsInternational Organization for Standardization (ISO): A collection of more than 17000 standards developed in

more than 157 countries – titled after the Greek word iso than means “equal”

Covers multiple fields – communications, packaging, energy production, banking and financials, etc.

Promotes and facilitates global exchange of information and barrier-free trade

Website – www.iso.org

Networking Standards GroupsAmerican National Standards Institute (ANSI): Established standards for electronics industry, chemical

and nuclear engineering, construction, health and safety Involves industry and government representatives –

represents the US in developing international standards Requires rigorous testing of new technology for

obtaining its approval Compliance with its standards is voluntary but

beneficial – constitutes reliability and compatibility and is beneficial

Website – www.ansi.org

Networking Standards GroupsElectronic Industries Alliance (EIA): A trade organization that involves representatives of

USA electronics manufacturing firms Lobbies for legislation favorable to the growth of

computer and electronics industries Assists writing ANSI standards, sets standards for its

members, and sponsors conferences and exhibitions Its subgroup – Telecommunications Industry

Association (TIA) – focuses on standards for IT Websites – www.eia.org, www.tiaonline.org

Networking Standards GroupsInternational Telecommunication Union (ITU): A United Nations agency that regulates international

communications with members from 191 countries Offers global standards in radio/TV frequencies,

networking, satellite and global communications, etc. Provides developing countries with technical expertise

and telecommunications equipment Actively involved into implementation of worldwide

Internet services Website – www.itu.int

Networking Standards GroupsInternet Corporation for Assigned Names and Numbers

(ICANN): A private nonprofit corporation upon recommendation

of the US Department of Commerce Responsible for Internet Protocol addressing (IP

addressing) and domain name management Assigns rights to use internet addresses and names Website – www.icann.org

Networking Standards GroupsInternet Assigned Numbers Authority (IANA): A nonprofit group that is used to keep records of

available and reserved IP addresses and to determine how they are distributed

Cooperated with three Regional Internet Registries (RIRs) – American Registry for Internet Numbers (ARIN), Asia Pacific Network Information Centre (APNIC), and Reseaux IP Europeens (RIPE)

Performs system administration within ICANN Website – www.iana.org

Networking Standards GroupsInternet Society (ISOC): A professional membership society that establishes

technical standards for the Internet – involves Internet professionals and companies

Addresses Internet’s growth, accessibility, security, addressing services, and open standards

Oversees several active subgroups that carry specific missions

Website – www.isoc.org

Networking Standards GroupsInternet Engineering Task Force (IETF): An ISOC subgroup that manages Internet protocol

standards Openly accepts proposals for standards – performs

reviews, testing, and issues approvals Promotes standards approved in the US internationally

Internet Architecture Board (IAB): A technical advisory group of researchers and

professionals – another ISOC subgroup Oversees Internet’s growth and management strategy,

resolution of technical disputes, and standards

Homework Read the chapter and the summary section, then review

the key terms learned Answer the review questions and verify your answers

with the chapter or lecture slides Complete the hands-on project 2-2 and case projects 2-

2 and 2-3