Introduction to Networks
Khaled Harras
School of Computer Science
Carnegie Mellon University
15-349 Computer and Network Security
Fall 2012
Some material borrowed from Hui Zhang and Adrian Perrig
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Early Internet



In the beginning there were two hosts connected to
the Internet 
Trustworthy environment, everyone knew everyone
else
Network protocols were designed for such a
trustworthy environment
Internet
2
Current Internet


But then, Internet continued to double in size … for
several years …
Today, about 1 Billion hosts connected to Internet
3
What Is In a Network?


End system
Switch (router)
» Access switch
» Core switch


Access line: linking switch and end systems
Trunk line
» Between switches
» Multiple connections simultaneously
– Multiplexing/demultiplexing
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How are Networks “different”?


Distributed system: Avoids single point of
failure, is fault tolerant
Network Environment
»
»
»
»
»

Anonymity (Who’s behind the machine?)
Automation (Are humans needed?)
Distance (So where is google exactly?)
Opaqueness (Am I talking to the same entity?)
Routing Diversity (How many routes are there to google?)
Network Topology
» The way in which networks are configured
» Boundary, Ownership, Control
» LANs, WANs, internets (not “I”nternet)
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What is a Protocol ?
Friendly greeting



An agreement between
parties on how
communication should take
place.
Protocols may have to
define many aspects of the
communication.
Syntax:
» Data encoding, language, etc.

Muttered reply
Destination?
Semantics:
» Error handling, termination,
ordering of requests, etc.
Pittsburgh
Thank you
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Interfaces

Each protocol offers an interface to its users,
and expects one from the layers on which it
builds
» Syntax and semantics strike again
– Data formats
– Interface characteristics, e.g. IP service model

Protocols build upon each other
» Add value
– E.g., a reliable protocol running on top of IP
» Reuse
– E.g., OS provides TCP, so apps don’t have to rewrite
7
Why do networking
people love layers?
8
The Idea of Layering



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)
9
A Fine Grain Layering Model
The Open Systems Interconnection (OSI) Model.
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Application
Application
Application
Application
6 Presentation
Presentation
Presentation
Presentation
5
Session
Session
Session
Session
4
Transport
Transport
Transport
Transport
3
Network
Network
Network
Network
Network
Network
2
Data
Datalink
link
Data
Datalink
link
Data
Datalink
link
1
Physical
Physical
Physical
Physical
Physical
Physical
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OSI Functions






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(1) Physical: transmission of a bit stream.
(2) Data link: flow control, framing, error
detection.
(3) Network: switching and routing.
(4) Transport: reliable end to end delivery.
(5) Session: managing logical connections.
(6) Presentation: data transformations.
(7) Application: specific uses, e.g. mail, file
transfer, telnet, network management.
Multiplexing takes place in multiple layers
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Simpler Layering
User A
User B
Application
Transport
Network
Link
Host
Host
Layering: technique to simplify complex systems
12
Layer Encapsulation
User A
User B
Get index.html
Connection ID
Source/Destination
Link Address
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Layers and Locations
Application
Presentation
Session
Transport
Network
Data Link
Physical
Host
Switch
Router
Host
14
Power of Layering

Solution: Intermediate layer that provides a single abstraction for
various network technologies
» O(1) work to add app/media
» variation on “add another level of indirection”
Application
SMTP
SSH
NFS
HTTP
Intermediate
layer
Transmission
Media
Coaxial
cable
Fiber
optic
802.11
LAN
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TCP/IP Protocol Suite


The complete TCP/IP protocol suite contains many
protocols.
Not even the following graph is a complete list.
User
Process
User
Process
User
Process
TCP
User
Process
UDP
Application
Layer
Transport
Layer
ICMP
IP
IGMP
Network
Layer
ARP
Hardware
Interface
RARP
Link Layer
Media
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Encapsulation

As data is moving down the protocol stack, each
protocol is adding layer-specific control information.
User data
Application
Application
Header
User data
TCP
TCP Header
IP
Application data
TCP segment
IP Header
Ethernet
Driver
TCP Header
Application data
IP datagram
Ethernet
Header
IP Header
TCP Header
Application data
Ethernet frame
Ethernet
Trailer
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Protocol Demultiplexing

Multiple choices at each layer
FTP
HTTP
NV
TCP
IPX
NET1
TFTP
UDP
Network
IP
Type
Field
Protocol
Field
TCP/UDP
IP
NET2
…
NETn
Port
Number
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Internet Protocol Architecture
FTP
program
FTP protocol
FTP
program
TCP
TCP protocol
TCP
IP
Ethernet
Driver
IP protocol
Ethernet
protocol
Ethernet
Driver
IP
IP protocol
ATM
Driver
ATM
protocol
IP
ATM
Driver
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Internet Protocol Architecture
MPEG Servier
program
UDP
IP
Ethernet
Driver
MPEG Player
program
RTP protocol
UDP protocol
IP protocol
Ethernet
protocol
Ethernet
Driver
IP
UDP
IP protocol
ATM
Driver
ATM
protocol
IP
ATM
Driver
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What are “some”
Internet Challenges?
Which layer is responsible for
addressing this challenge?
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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 IP address configured in OS
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Naming

Humans use readable host names
» E.g. www.cmu.edu
» Globally unique (can correspond to multiple hosts)

Naming system translates to physical
address
» E.g. DNS translates name to IP Address (e.g. 128.2.11.43)
» Address reflects location in network
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Application Layer
» Service: Handles details of application programs.
» Functions:
» Depends on what you want to do???
Application telnet, ftp, email
Layer
www, AFS
Transport
Layer
Network
Layer
TCP, UDP
IP, ICMP, OSPF
RIP, BGP
(Data) Link Ethernet, WiFi
PoS, T1
Layer
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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 the Data is Out of Order?
Problem: Out of Order
ml
inde
x.ht
GET
GET x.htindeml
Solution: Add Sequence Numbers
ml 4
inde 2
x.ht 3
GET 1
GET index.html
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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
6,7,8 21
X
4,5 7
1,2,3 6
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Transport Layer


Service: Controls end-to-end delivery
of data between hosts.
Functions: Connection
Establishment, Termination,
Error control, flow control,
Congestion control, reordering.
Application
Layer
telnet, ftp, email
www, AFS
Transport
Layer
TCP, UDP
Network
Layer
IP, ICMP, OSPF
RIP, BGP
(Data) Link Ethernet, WiFi
PoS, T1
Layer
28
How do I reach x.y.z.k?




Routing is crucial
Forwarding tables at each router populated
by routing protocols
Original Internet: manually updated
Routing protocols update tables based on
“cost”
» Exchange tables with neighbors or everyone
» Use neighbor leading to shortest path

What happens if a router is overwhelmed?
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Network Layers
» Service: Moves packets inside the network.
» Functions: Routing, addressing, switching,
congestion control.
Application telnet, ftp, email
Layer
www, AFS
Transport TCP, UDP
Layer
Network
Layer
IP, ICMP, OSPF
RIP, BGP
(Data) Link Ethernet, WiFi
PoS, T1
Layer
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What if the Data Doesn’t Fit?
Problem: Packet size


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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Data Link Layer
» Service: Transfer of frames over a link.
» Functions: Synchronization,
error control, flow control
Medium Access Control (MAC)
Application telnet, ftp, email
Layer
www, AFS
Transport
Layer
TCP, UDP
Network
Layer
IP, ICMP, OSPF
RIP, BGP
(Data) Link Ethernet, WiFi
Layer
PoS, T1
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Degradation of Internet Security





Internet has become communication
infrastructure for government, economy,
society, even for safety-critical uses
Unfortunately, trust assumptions of the early
Internet do not scale to billions of users
Indeed, hackers, criminals, terrorists, military
all use the Internet for their activities
All this, and so much more, was designed
without considering security issues
What do we do now?????????
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