Transcript CS412 Computer Networks - Winona State University

CS 313 Introduction to
Computer Networking &
Telecommunication
Introduction
Chi-Cheng Lin, Winona State University
Topics
Introduction
 Metric Units
 Network Hardware
 Network Software
 Reference Models
 Example Networks
 Standards and Standards Organizations

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Network Software

Old computer networks:
HW main concern
SW afterthought
Not working now!

Network SW is now highly structured
Approach: Protocol Hierarchies
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Protocol Hierarchies

What is protocol?
Agreement between communication parties
on HOW communication is processed

Layered architecture
Reduce design complexity: Lower layer
offers service to higher layer
Hiding implementation details
Layer n on one machine talks to layer n on
another
Rules and conventions used in layer n’s
talk: Layer n protocol
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Protocol Hierarchies

Peers
Entities comprising corresponding layers on
different machines
Virtual communication using protocol
Peer process abstraction make network design
becomes that of individual layers

Physical communication
Sender: Data and control passed to layer below
Data transmitted via physical media
Receiver: Data and control passed to layer
above
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Layers, Protocols, and Interfaces
Virtual Communication
Physical Communication
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Protocol Hierarchies

Interface between two adjacent layers
Defines primitive operations and services a
lower layer offers to the upper one
Minimizes amount of information passed
between two layers
Simplifies replacement of implementation
E.g., telephone lines  satellite channels
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Protocol Hierarchies

Network architecture
Set of layers and protocols
Implementation and interface specification
not included

Protocol stack
A list of protocols used by a certain system,
one protocol per layer
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Multilayer Communication - Example

Philosopher-translator-secretary
architecture
It is ok if
Dutch is
replaced by
Finnish
fax is
replaced by
email
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Information Flow - Example

Virtual communication for layer 5
Header: control information
00011100011100001110 …
Layer 1
protocol
00011100011100001110 …
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Key Design Issues for the Layers

Reliability
Error control
Error-detecting
Error-correcting
Routing
Selecting the best path for sending a packet from
one point to another
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Key Design Issues for the Layers

Sender/receiver identification mechanism
Addressing/naming
Sequencing
 Message disassembling, transmitting,
reassembling

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Key Design Issues for the Layers

Resource allocation
Multiplexing
The process of combining signals from multiple
sources for transmission across a single data link
Multiple connections can share the link

Flow control
Needed for fast sender, slow receiver
Congestion control
 Quality of service
 Security

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Connection-Oriented and
Connectionless Services

Two basic types of services
Connection-oriented
Connectionless

Consider reliability …
Reliable
Unreliable

Connection-oriented
Connectionless
Note that: Connection  Reliability
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Connection-Oriented Service
A connection is established first, then
used, and then released when done.
 Works like a pipe:

Sender pushes data in at one end
Receiver takes them out, often in the same
order, at the other end

Analogy
Telephone system
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Connectionless Service
No need to set up a connection first
 Each message carrying full destination
address is routed independently of
others

No guarantees on the order

Analogy
Postal system
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Service Primitives
Service is formally specified by a set of
primitives (e.g., OS’s system calls)
available to users or entities
 Five service primitives for implementing
a simple connection-oriented service.

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Service Primitives

Packets sent in a simple client-server
interaction on a connection-oriented
network.
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Relationship of Services to Protocols

Service
Set of primitives a layer provides to the
layer above it
Define WHAT operations
not HOW implemented

Protocol
Set of rules governing format and meaning
of message exchanged by peer entities
within a layer
Used by entities to implement service
definitions
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Services to Protocols Relationship

The relationship between a service and
a protocol.
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Relationship of Services to Protocols

Analogy: object-oriented languages
Service :: ADT or Object
Users do not know the implementation of a
service
Protocol :: Implementation
The protocol of the service is invisible to users
Do you have to understand http (hypertext
transport protocol) before you can surf the
Internet?
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Reference Models

Two reference models will be discussed
OSI reference model
TCP/IP model
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OSI Reference Model
ISO/OSI (Open Systems Interconnection)
Reference Model
 NOT a network architecture itself

Exact services and protocols are not specified
Just "what should be done" in each layer
However, standards are produced for all
layers
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OSI Reference Model

Seven layers
Layer
Layer
Layer
Layer
Layer
Layer
Layer

7:
6:
5:
4:
3:
2:
1:
application layer
presentation layer
session layer
transport layer
network layer
data link layer
physical layer (lowest)
Diagram of OSI reference model
Note: this is one of the most important figures
in the whole book!!
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Physical medium
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Physical medium
End-to-End
Point-to-Point
Point-to-Point
Host A
Subnet
Point-to-Point
Host B
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Physical Layer
Transmitting raw bits (0s and 1s) over
communication channel
 Design issues

Representation of bits
How is 0/1 represented?
Data rate: number of bits sent per second
How long does a bit last?
Transmission mode (bi-directional?)
Mechanical, electrical, timing interfaces
Underlying physical transmission medium
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Data Link Layer
Takes a raw transmission facility and
transforms it into a line (link) that
appears free of undetected transmission
errors to network layer
 Basic function

Breaks up input data to data frames
Transmits data frames sequentially
Processes acknowledgement frames sent
back from receiver for reliable transmission
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Data Link Layer

Responsibilities
Physical addressing (e.g. MAC address)
Framing
Creating and recognizing frame boundaries
Error control (adjacent nodes, node-to-node)
Errors: damaged, lost, duplicate
Flow control (adjacent nodes, node-to-node)
Traffic regulation between fast sender and slow
receiver
Medium access control
Shared channel access control in broadcast
networks
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Network Layer
Subnet operation control
 Responsibilities

Logical addressing (e.g., IP address)
Routing
Static tables
Determined at the start of conversation
Dynamic
Congestion control
Quality of service
Accounting
Heterogeneous network interconnection
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Transport Layer

End-to-end layer
Talk to destination machine directly (virtually)
Layers 4 through 7 are end-to-end
Layers 1 through 3 are node-to-node (chained)

Basic function
Split data from session layer into smaller units
Pass units to network layer
Ensure units arrive correctly at the other end
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Transport Layer

Determine services provided to session
layer (and ultimately to users)
Error-free point-to-point channel that
delivers messages in the order in which
they were sent
Transport of isolated messages w/o
guarantee about order
Broadcasting
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Transport Layer

Responsibilities include
Service-point addressing (e.g., port number)
Which message belong to which connection
(application):
(End-to-end) Flow control
(End-to-end) Error control
Compare to the
Data Link layer
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Session layer
Session establishment between users
on different machines
 Responsibilities

Dialogue control
Deciding who sends, and when
Token management
Control of same critical operation not to be
performed at the same time
Synchronization
Inserting checkpoints (checkpointing)
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Presentation Layer
Syntax/semantics of information
transmitted
 Responsibilities

Make communication between computers
with different internal data representations
possible
Approach: standard encoding
Convert from data representation used in one
host to the standard abstract data structure
and back
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Application Layer
Provides interface and support for
services to users (human, software,
robots)
 Examples

File transfer
Email
Network news
Hypertext transfer
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TCP/IP Reference Model

Goals
Internetworking
Fault tolerance
Flexible architecture

Four layers of TCP/IP Reference Model
Host-to-network layer
Internet layer
Transport layer
Application layer
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Internet Layer
Packet-switching, connectionless
 Packets injected to network

Independent travel
Out-of-order arrival

Analogy
Mail system

IP (Internet Protocol)
Packet routing
Congestion control
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Transport Layer

Two end-to-end protocols
UDP (User Datagram Protocol)
TCP (Transmission Control Protocol)

UDP (User Datagram Protocol)
Unreliable, connectionless
Widely used for
client-server type request-reply queries
speech, video
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Transport Layer

TCP
Reliable connection-oriented
Incoming byte stream (form application
layer) is fragmented into discrete
messages and passed onto internet layer
Message is reassembled at destination
Flow control
Analogy
A
B
Pipe
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Applications and Host-to-Network
Layers

Application layer
No session and presentation layers
TELNET, FTP, SMTP, DNS, NNTP, HTTP

Link layer
Defines what links must do to meet the
need of connectionless internet layer
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TCP/IP Protocols
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OSI and TCP/IP Models

Correspondence
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OSI and TCP/IP Models

Similarities
Stack of independent protocols
Layer functionality
Transport layer
Application layer
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
OSI
Differences between
OSI and TCP/IP Models
Distinction between services, interfaces, and
protocols (perhaps the biggest contribution)
Better Protocol-Hidden
Model first, then protocols
 Pro: No bias, more general
 Con: Designers did not have
 much experience with the subject
 a good idea of which functionality to put in which layer
No thought given to internetworking
7 layers
Communication
 Connection-Oriented and connectionless in network layer
 Only connection-oriented in transport layer
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Differences between
OSI and TCP/IP Models

TCP/IP:
No clear distinction between services, interfaces, and
protocols
Worse protocol-hidden
Protocol first, then model
 Pro: Protocols fit model perfectly
 Con: Model does not fit any other protocol stacks (not
general)
4 layers
Communication
 Connectionless in network layer
 Both in transport layer (good for request-response
protocols)
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Summary of Reference Models

OSI
OSI model exceptionally useful for
discussing computer networks
OSI protocols not popular

TCP/IP
TCP/IP model practically nonexistent
TCP/IP protocols widely used

Modified framework is used in the text
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Summary of Reference Models

Modified framework is used in the text
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Example Networks

The Internet
Overview of the Internet architecture
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Example Networks
3G mobile networks
 Wireless LANs: 802.11
 RFID and sensor networks

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Standards and Standards Organizations
Why standards?
 Categories

de facto
de jure

Organizations
ITU-T (formerly CCITT)
ISO
ANSI
IEEE
IETF
ATM Forum
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