Transcript Ch3_OSI_1

Protocols
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Used for communications between entities in a system
Must speak the same language
Entities
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User applications
E-mail facilities
Terminals
Systems
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Computer
Terminal
Remote sensor
Protocol Cont
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Protocol:
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A set of rules and conventions
Governs how computers exchange information over a
shared/network medium.
Used for communications between entities in a system
Defines :
 What is communicated
 How it is communicated
 When it is communicated
Varieties of communication protocols exist.
 LAN protocols, WAN protocols, Network protocols,
Routing protocols etc.
Key Elements of Protocol
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Syntax:
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Semantics:
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Refers to structure or format of the data (the order they
are presented)
Example - data format, coding
Refers to the meaning of each section of the bits
Example - data information, control information
Timing:
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Refer to synchronous
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When data should be sent
How fast they can be sent
Example sequencing, speed/delay matching
Protocol Architecture
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Protocols in networking follows ‘layered’ architecture
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Purpose of this layered architecture:
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functions necessary for transfer of data are grouped into
layers.
a set of/each task(s) is separated from others.
several protocols at each layer, each suitable for a
different situation.
work that must be done in networking is broken down
into manageable units.
Protocol Architecture
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Task of communication broken up into modules, for
example file transfer could use three modules
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File transfer application
Communication service module
Network access module
A simplified architecture for file
transfer
Three-layered Model
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Network Access Layer
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Exchange of data between the computer and the
network
Sending computer provides address of destination
computer.
May invoke levels of service
Dependent on type of network used (LAN, packet
switched etc.)
Cont…
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Transport Layer
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Reliable data exchange
Independent of network being used
Independent of application
Application Layer
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Support for different user applications
E.g. e-mail, file transfer
Protocols in simplified
architecture
Addressing Requirements
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Two levels of addressing required
Each computer needs unique network address
Each application on a (multi-tasking) computer
needs a unique address within the computer
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The service access point or SAP
The port on TCP/IP stacks
Protocol Data Unit
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At each layer, protocols are used to communicate
Control information is added to user data at each
layer
Transport layer may fragment user data
Each fragment has a transport header added
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Destination SAP
Sequence number
Error detection code
This gives a transport protocol data unit
Protocol Data Units
Network PDU
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Adds network header
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Network address for destination computer
Facilities requests
Standardized Protocol
Architectures
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Required for devices to communicate
Vendors have more marketable products
Customers can insist on standards based equipment
Two standards:
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OSI Reference model
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TCP/IP protocol suite
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Never lived up to early promises
Most widely used
Also: IBM Systems Network Architecture (SNA)
OSI Model
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The OSI Model
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A layered framework for network systems design
allows for communication across all types of
computer systems
 consists of 7 layers
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each layer defines a segment of process of moving
information
Layered Architecture
Application
Presentation
Session
Transport
Network
Data Link
Physical
Please Do Not Touch Steve’s Pet Alligator
Peer-to-Peer Processes
Application
Presentation
<---peer-to-peer process ---> Application
Presentation
provides services Session
for
Transport
Session
uses services
provided by
Network
Network
Data Link
Data Link
Physical
Physical
machine A
Transport
machine B
The OSI Environment
Functions of the Layers
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Physical Layer
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Transmits a bit stream over a physical link
Deals with the mechanical and electrical
specifications of the interface and transmission
medium
 Specification Responsibilities : defines
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interface the devices and the transmission medium
 data rates : # of bits sent each second
 synchronisation of bits : synchronises both side clocks
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Physical Layer (Cont.)
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Specification Responsibilities : defines (cont.)
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Line configuaration :
concerns with the connection of devices to the medium
 point-to-point configuration
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multipoint configuaration
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2 devices are connected together through a link
a link is shared between several devices
physical topology, defines
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how devices are connected (mesh, star, etc.)
Physical Layer (Cont.)
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Specification Responsibilities : defines (cont.)
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Transmission mode, defines
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direction of transmission between 2 devices
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simplex, half-duplex, full-duplex
Data Link Layer
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node-to-node delivery
fix the errors for the next layer
Specification Responsibility
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framing
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physical addressing
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add headers (souce address, destination address)
flow control
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devides the stream of bits into frames
prevent overflow transmission
error control
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add mechanism for detecting damages
Data Link Layer
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Specification Responsibility (cont.)
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access control
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for multipoint configuaration
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determines which device has control over the link at any given
time
Responsible the delivery of packets between 2
systems on the same network link
Network Layer
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Responsible for the source-to-destination
delivery of packet across mulitple network links
ensures that a packet gets from it origin to
destination
Specification Responsibilities
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logicall addressing
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add source/destination addresses
routing
Network Layer (cont.)
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Responsible for end-to-end delivery of individual
packet
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each packet is treated independently
gets the packet to the correct computer
Transport Layer
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Responsible for source-to-destination (end-toend) delivery of entire message
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ensure that the whole message arrives intact and in
order
Specification Responsibilities
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service-point addressing
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port address
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gets the entire message to the correct program on the
computer
Transport Layer (cont.)
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Specification Responsibilities (cont.)
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segmentation and reassembly
devides message into segment (packet)
 addes a sequence number to each segment
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flow control
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similar to the data link flow control
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perform end to end (machine) rather than across a link
error control
makes sure that the entire message arrives at the receiver
without error (damage, loss, etc.)
 retransmission mechanism
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Session Layer
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The network dialog controller
Establishes, maintains, synchronise the
interaction between communication systems
Specification Responsibilities
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dialog control
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agree to take either simplex, half-duplex, full-duplex mode
synchronisation
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allows a process to add checkpoints (synchronisation
points in to a data stream)
Session Layer (Cont.)
synchronisation
a message of 1320 bytes
100
100
checkpoint
100
100
…...
Presentation Layer
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Concerns with the syntax and semantics of
information exchanged between 2 systems
Specification Responsibilities
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translation
character string
 numbers, etc.
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different computers use different encoding systems
Presentation Layer (Cont.)
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Specification Responsibilities (cont.)
responsible for interoperability between these
different encoding methods
 encryption
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for sensitive information
 encode/decode information
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compression
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reduce the number of bits to be transmitted
Application Layer
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Enables the user (human/software) to access the
network
Specification Responsibilities
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Applications : examples
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network virtual terminal
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to remote login to a remote machine
file transfer
 mail services
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TCP/IP Protocol Suite
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Used in the Internet
Developed before the OSI Model
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do not match exactly with OSI Model
5 layers
Application
 Transport
 Internetworking
 Network
 Physical
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TCP/IP Protocol Suite (Cont.)
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Two main protocols
Transmission Control Protocol
 Internetworking Protocol
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Many other protocols
Hierarchical protocol architecture made up for
interactive modules
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each upper-level protocol is supported by one or
more lower-level protocols
TCP/IP Protocol Suite (Cont.)
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At the Transport layer, TCP/IP defines two
protocols
TCP
 UDP (User Datagram Protocol)
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At the Network layer
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IP
OSI vs TCP/IP
TCP/IP and the OSI Model
Application
Presentation
Session
Transport
Application
SMTP
FTP
DNS
SNMP NFS
TCP
ICMP IGMP
Network
TELNET
UDP
IP
ARP RARP
Data Link Protocols defined by the underlying
Physical networks
Addressing level
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Level in architecture at which entity is named
Unique address for each end system (computer)
and router
Network level address
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IP or internet address (TCP/IP)
Network service access point or NSAP (OSI)
Process within the system
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Port number (TCP/IP)
Service access point or SAP (OSI)
Trace of Simple Operation
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Process associated with port 1 in host A sends
message to port 2 in host B
Process at A hands down message to TCP to
send to port 2
TCP hands down to IP to send to host B
IP hands down to network layer (e.g. Ethernet)
to send to router J
Generates a set of encapsulated PDUs
PDUs in TCP/IP
Standard Organizations
ISO
ITU
ANSI
IEEE
State Department
EIA
Carriers
Other org.