Transcript Chapter II - Austin Community College

OSI Model and Standards
ITNW 1325, Chapter II
Understanding the OSI Model
Understanding the OSI Model
Overview:
 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 Model
Overview (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 Model
Overview (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 Model
Reasons 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 Model
Seven Layers:
Understanding the OSI Model
From 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 Model
From 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 Model
Peer 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 Model
Peer Communication, Illustration:
Understanding the OSI Model
Peer 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 Functions
Application (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 Functions
Presentation (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 Functions
Presentation (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 Functions
Session (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 Functions
Session (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 Functions
Transport (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 Functions
Transport, 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 Functions
Transport, Connection-Oriented Protocols (continued):
OSI Layer Functions
Transport, 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 Functions
Transport, 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 Functions
Transport, Protocols:
OSI Layer Functions
Network (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 Functions
Network (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 Functions
Data 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 Functions
Data 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 Functions
Data Link, Sublayers:
OSI Layer Functions
Data 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 Functions
Data 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 Functions
Data Link, MAC Addresses (continued):
OSI Layer Functions
Data 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 Functions
Data 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 Functions
Physical (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 Work
Encapsulation, 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 errordetection/correction information and end-of-PDU flags
 The encapsulated data is passed to the layer below
OSI Model at Work
Encapsulation, 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 Work
Encapsulation, 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 Work
Encapsulation, Illustration:
OSI Model at Work
Decapsulation:
 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 Work
Encapsulation/Decapsulation:
OSI Model at Work
Relevance:
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 Standards
Advantages:
 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 Standards
Disadvantages:
 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 Standards
Types, 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 Standards
Types, 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 Standards
Types, 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 Groups
Institute 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 Groups
International 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 Groups
American 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 Groups
Electronic 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 Groups
International 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 Groups
Internet 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 Groups
Internet 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 Groups
Internet 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 Groups
Internet 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