Transcript Computer Networks

Computer Networks
(Graduate level)
Lecture 1: Introduction
University of Tehran
Dept. of EE and Computer Engineering
By:
Dr. Nasser Yazdani
Univ. of Tehran
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Objectives of course
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Understand the state-of-the-art in network
protocols, architectures and applications
Understand how networking research is done
and prepare student for research in
networking.
How is class different from undergraduate
networking course
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Training network programmers vs. training
network researchers
They are really different courses!
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Course Materials
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Course Web page
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Research papers
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visit regularly
Printed papers or pdf/ps version on the Web
page
~40 papers, Combination of classic and
recent work. Some Optional reading!
Recommended textbook
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“Computer Networks, A system approach.
Peterson & Davie 3rd edition.
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Grading
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Homework assignments, around %20
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Paper review
Problems and hands-on assignments
Severe late penalties!
Class discussion, participation around %5
point.
Project, ~ %30
Exam(s), Midterm and final, ~%45.
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Covered Topics (we try!)
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Traditional
Layering
Interconnecting (LAN)
Internet architecture
Routing (IP)
Switching
Transport (TCP)
Queue management
(FQ, RED)
Naming (DNS)
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Recent Topics
Multicast
Wireless/Mobility
Active networks
QOS
Security
Network
measurement
Ad-hoc Networks
Sensor Networks
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A tour of networking
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Goal and objective
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needs
design
requirement
Whirlwind tour of networking
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Information, Computers and
Networks
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Information: anything that is represented in
bits
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Form (can be represented) vs substance (cannot)
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Properties:
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Infinitely replicable
Computers can “manipulate” information
Networks create “access” to information
Potential of networking:
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move bits everywhere, cheaply, and with desired
performance characteristics
Break the space barrier for information
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Objective of Networking ?
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Direct or indirect access to every other node in
the network
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Connectivity is the magic needed to communicate
if you do not have a link.
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Must understand many connection factors
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Traffic data rate
Traffic pattern (bursty or constant bit rate)
Traffic target (multipoint or single destination,
mobile or fixed)
Application requirements, Delay sensitivity Loss
sensitivity.
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Another view
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Building a network to support diverse
ranges of applications
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Distributed computing.
Multimedia.
Telecommunication.
E-commerce, etc.
What kind of technology do we need?
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Hardware.
Software.
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What is a Network?
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What is computer Network?
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Different views.
Differences from other networks, Its
generality.
What is requirements? Different
perspective:
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Network provider
Network designer
Application programmer
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Design goals
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Connectivity
Scalability
Simplicity
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Efficiency
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For designers.
Most importantly for users.
cost
performance
Support for common user services.
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Levels of Networking
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Communicating across a link, LANs.
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Connecting together multiple links (Bridges)
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Finding and routing data to nodes on
Internet.
Communicating on the application level,
matching application requirements
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A First Step
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Creating a link between nodes
Link: path followed by bits
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Wired or wireless
Broadcast or point-to-point (or both)
Node: any device connected to a link
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Types of Links
Point-to-Point
Multiple Access
…
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Packet Transmission Modes
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Unicast
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Broadcast
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Transmission to all network nodes
Multicast
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Transmission to single specific receiver
Transmission to specific subset of nodes
Anycast
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Transmission to one of a specific subset of
nodes
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What are Switched Networks?
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Switch: moves bits
between links
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Switched Network
Packet switching
Circuit switching
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Back in the Old Days…
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Then Came TDM…
• Synchronous time division multiplexing
Multiplex (mux)
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Demultiplex (demux)
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TDM Logical Network View
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Packet Switching (Internet)
Packets
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Packet Switching
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Interleave packets from different sources
Efficient: resources used on demand
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General
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Statistical multiplexing
Multiple types of applications
Accommodates bursty traffic
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Addition of queues
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Statistical Multiplexing Gain
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1 Mbps link; users require 0.1 Mbps when
transmitting; users active only 10% of the
time
Circuit switching: can support 10 users
Packet switching: with 35 users,
probability that >=10 are transmitting at
the same time < 0.0017
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Characteristics of Packet
Switching
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Store and forward
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Packets are self contained units
Can use alternate paths - reordering
Contention
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Congestion
Delay
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Second Step: Internet[work]
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A collection of
interconnected
networks
Host: network
endpoints (computer,
PDA, light switch, …)
Router: node that
connects networks
Internet vs. internet
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Internet[work]
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Challenge
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Many differences between networks
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Address formats
Performance – bandwidth/latency
Packet size
Loss rate/pattern handling
Routing
How to translate between various
network technologies
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Third Step: How To Find
Nodes?
internet
Computer 1
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Computer 2
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Naming
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Humans use readable host names
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E.g. www.cmu.edu
Globally unique (can correspond to multiple
hosts)
Naming system translates to physical
address
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E.g. DNS translates name to IP Address (e.g.
128.2.11.43)
Address reflects location in network
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Domain Name System
What’s the IP address for www.cmu.edu?
It is 128.2.11.43
Computer 1
Local DNS Server
DNS server address manually configured into OS
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Packet Routing/Delivery
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Each network technology has different
local delivery methods
Address resolution provides delivery
information within network
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E.g., ARP maps IP addresses to Ethernet
addresses
Local, works only on a particular network
Routing protocol provides path through
an internetwork
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Network:Address Resolution
Protocol
Broadcast: who knows the
Ethernet address for 128.2.11.43?
Ethernet
Broadcast: Yes, it is
08-00-2c-19-dc-45
Ethernet
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Internetwork: Datagram
Routing
Routers send
packet to next
closest point
H
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R: Routers
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Routing
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Forwarding tables at each router
populated by routing protocols.
Original Internet: manually updated
Routing protocols update tables based on
“cost”
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Exchange tables with neighbors or everyone
Use neighbor leading to shortest path
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Fourth Step: Application
Demands
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Reliability
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Corruption
Lost packets
Flow and congestion control
Fragmentation
In-order delivery
Etc…
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What if the Data gets
Corrupted?
Problem: Data Corruption
GET index.html
Internet
GET windex.html
Solution: Add a checksum
0,9 9
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X
4,5 7
Computer Network
1,2,3 6
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What if the Data gets Lost?
Problem: Lost Data
GET index.html
Internet
Solution: Timeout and Retransmit
GET index.html
Internet
GET index.html
GET index.html
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What if Network is
Overloaded?
Problem: Network Overload
Solution: Buffering and Congestion Control
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Short bursts: buffer
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What if buffer overflows?
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Packets dropped and retransmitted
Sender adjusts rate until load = resources
Called “Congestion control”
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What if the Data Doesn’t Fit?
Problem: Packet size
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On Ethernet, max IP packet is 1.5kbytes
Typical web page is 10kbytes
Solution: Fragment data across packets
ml
x.ht
inde
GET
GET index.html
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What if the Data is Out of
Order?
Problem: Out of Order
ml
inde
x.th
GET
GET x.thindeml
Solution: Add Sequence Numbers
ml 4
inde 2
x.th 3
GET 1
GET index.html
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Network Functionality
Summary
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Link
Multiplexing
Routing
Addressing/naming (locating peers)
Reliability
Flow control
Fragmentation
Etc….
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What is Layering?
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Modular approach to network functionality
The idea of divide and conquer
Use abstraction to hide complexity.
Example:
Application
Application-to-application channels
Host-to-host connectivity
Link hardware
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Protocols
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Module in layered structure
Set of rules governing communication
between network elements (applications,
hosts, routers)
Protocols define:
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Interface to higher layers (API)
Interface to peer
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Format and order of messages
Actions taken on receipt of a message
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Protocols
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Building blocks of a network architecture
Each protocol object has two different
interfaces
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service interface: operations on this protocol
peer-to-peer interface: messages exchanged
with peer
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Term “protocol” is overloaded
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specification of peer-to-peer interface
module that implements this interface
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Layering Characteristics
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Each layer relies on services from layer
below and exports services to layer above
Interface defines interaction
Hides implementation - layers can change
without disturbing other layers (black
box)
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Layering
User A
User B
Application
Transport
Network
Link
Host
Host
Layering: technique to simplify complex systems
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Layer Encapsulation
User A
User B
Get index.html
Connection ID
Source/Destination
Link Address
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Protocol Demultiplexing
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Multiple choices at each layer
FTP
HTTP
NV
TCP
UDP
IPX
NET1
TFTP
Network
IP
Type
Field
Protocol
Field
TCP/UDP
IP
NET2
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NETn
Computer Network
Port
Number
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E.g.: OSI Model: 7 Protocol
Layers
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Physical: how to transmit bits
Data link: how to transmit frames
Network: how to route packets
Transport: how to send packets
end2end
Session: how to tie flows together
Presentation: byte ordering, security
Application: everything else
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OSI Layers and Locations
Application
Presentation
Session
Transport
Network
Data Link
Physical
Host
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Router
Host
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Example: Transport Layer
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First end-to-end layer
End-to-end state
May provide reliability, flow and
congestion control
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Example: Network Layer
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Point-to-point communication
Network and host addressing
Routing
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Is Layering Harmful?
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Sometimes..
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Layer N may duplicate lower level
functionality (e.g., error recovery)
Layers may need same info (timestamp,
MTU)
Strict adherence to layering may hurt
performance
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Class Coverage
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Brief coverage of physical and data link
layer
Focus on network to application layer
We will deal with:
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Protocol rules and algorithms
Investigate protocol trade-offs
Why this way and not another?
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Next Lecture: Links
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How to make computers talk across a
wire
Assigned reading
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Chap. 2 of book (Recommended!)
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