Transcript Chapter 2 Protocols and Architecture

PROTOCOLS AND
ARCHITECTURE
Lesson 2
NETS2150/2850
Lesson Outcomes
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The need for a Protocol Architecture
Understand importance of OSI
Reference Model
Overview of TCP/IP Suite – the basis for
the rest of this unit of study!
What’s a protocol?
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The set of rules or conventions governing the
exchange of data between two entities
Protocols define format, order of msgs sent
and received among network entities, and
actions taken on msg transmission, receipt
All communication activity in Internet
governed by protocols
What’s a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
req
Hi
TCP connection
response
Got the
time?
Get http://www.awl.com/
2:00
<file>
time
Key Features of a Protocol
Key features of a protocol are:
 Syntax
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Semantics
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Definitions of each of the packet type and error
codes
Timing
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Formats of the supported packet types
Sequence in which packets are exchanged and use
of timers
But, how to manage many related protocols?!
Need For Protocol Architecture
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Task broken into subtasks
Implemented separately in layers as stack
Functions needed in both systems
Peer layers communicate
E.g. File transfer
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Source must activate comms. path or inform network
of destination
Source must check destination is prepared to receive
File transfer application on source must check
destination file management system will accept and
store file for this user
May need file format translation
Standardized Protocol
Architectures
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Required for devices to interoperate
Vendors can have more marketable products
Customers can insist on standards based
equipment
Two standards:
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OSI Reference model
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Never lived up to early promises
de jure (i.e. by legislation)
TCP/IP Reference model
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Most widely used
de facto (i.e. in reality)
OSI Reference Model
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Open Systems Interconnection
Developed by the International
Organization for Standardization (ISO)
Seven layers
A theoretical system delivered too late!!
TCP/IP is the de facto standard
OSI Reference Model II
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Each layer performs a subset of the required
communication functions
Each layer relies on the next lower layer to
perform certain functions
Each layer provides services through
primitives (operations) to the next higher
layer
Changes in one layer should not require
changes in other layers (modular, info hiding)
OSI Layers
Protocol Data Units (PDU)
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At each layer, there is a protocol
Control data is added to user data at each
layer & certain functions performed
E.g.: transport layer may fragment user data
Each fragment has a transport header added
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Destination SAP (i.e port #)
Sequence number
Error detection code
This gives a transport PDU
The OSI Environment
OSI as Framework for
Standardization
Layer Specific Standards
Elements of Standardization
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Protocol specification
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Operates between the same layer on two systems
May involve different operating systems
Protocol specification must be precise
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Service definition
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Format of data units (i.e. syntax)
Semantics of all fields
allowable sequence of PDUs (i.e timing)
Functional description of what is provided
Addressing
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Used by the higher layer, also known as SAP
OSI Layers (1)
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Physical
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Physical interface between devices
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Mechanical
Electrical
Functional
Procedural
Data Link
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Means of activating, maintaining and deactivating
a reliable link
Error detection and control
Higher layers may assume error free transmission
OSI Layers (2)
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Network
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Transfer of information
Higher layers do not need to know about
underlying technology
Not needed on direct links
Transport
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Exchange of data between end systems
Error free
In sequence
No losses
No duplicates
OSI Layers (3)
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Session
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Presentation
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Provides control structure for communications
Manages sessions between applications
Data formats and coding (big/little-endian)
Data compression
Encryption
Application
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Means for applications to access OSI environment
Contains management functions and mechanisms
to support distributed applications
TCP/IP Protocol Architecture
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Developed by the US Defense Advanced
Research Project Agency (DARPA) for its
packet switched network (ARPANET)
Used by the global Internet
No official model but a working one!
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Application layer
Transport layer
Internet layer or network layer
Network access or data link layer
Physical layer
Physical Layer
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Physical interface between data
transmission device (e.g. computer) and
transmission medium or network
Characteristics of transmission medium
Signal levels
Data rates
Network Access Layer
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Exchange of data between end system
and network
Destination address provision
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Physical address
Invoking services like priority
Internet Layer (IP)
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Systems may be attached to different
networks
Routing functions across multiple
networks
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Provides logical addressing
Implemented in end systems and
routers
Transport Layer (TCP)
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Reliable delivery of data
Ordering of delivery
Provides application process addressing
(called port number)
Application Layer
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Support for user applications
e.g. HTTP, FTP, Telnet, SMTP, SNMP etc
OSI v TCP/IP
Encapsulation/Decap Process
TCP/IP Concepts
Addressing levels
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Level in architecture at which entity is named
Unique address for each end system and
router (i.e. physical address)
Network level address (i.e. logical address)
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IP or internet address (in TCP/IP)
Network service access point or NSAP (in OSI)
Process within the system
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Port number (in TCP/IP)
Service access point or SAP (in OSI)
Some Protocols in TCP/IP Suite
Intro Networking Video
Summary
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The needs for standardized protocols
and protocol architecture
Discussed two layered models:
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The OSI Reference Model
TCP/IP protocol suite
Next: The mechanism of data
transmission