1. Computer
Networking: A
Top Down
Approach
6th edition
Jim Kurose, Keith Ross
Addison-Wesley
March 2012
Dr Nauman Mazhar
Faculty of Information Technology (FIT)
University of Central Punjab (UCP)
Computer Communications
& Networks
SENS-3523
• Introduction; Edge, Access, Core N/W
Week: 1

2. Instructor
DR NAUMAN MAZHAR
Associate Professor, Faculty of IT
PhD (Computer Engineering)
Computer Networks & Security
CASE, Islamabad, research element in Michigan State University, USA
MS (Computer Engineering)
Computer Networks & Security
IOWA State University, USA
BE (Avionics Engineering)
College of Aeronautical Engg (CAE), PAF Academy Risalpur (NED Univ)
nau.maz@ucp.edu.pk
2

3. Course Intro
• Basic course in Computer Networking
• Provides overview of…
– What are computer networks
– How do they operate
– How are they designed
• Adopts TOP DOWN approach…
– What services distributed applications require from N/Ws
– What N/W services & how they are provided to support operation of
remote applications – layered approach
– Relevant networking protocols/standards/algorithms, both in wired &
wireless domains
3
Computer Network ??
allow remote, distributed
applications to
communicate & provide
variety of services

4. • Computer Networks & the Internet
– Nuts & Bolts description
– Services description
– Network Edge & Core
– Delay, Loss & Throughput in packet switched networks
– Network architectures (TCP/IP, OSI)
• Application Layer
– Network applications, principles/architectures
– Services required by Apps, & made available by Networks
– Application layer protocols
(Web/HTTP, DNS, FTP, SMTP, POP3)
– Apps development; Socket Programming (TCP & UDP)
Course Contents
4

5. • Transport Layer
– Connection less, unreliable transport – UDP
– Reliable Data Transfer (Go-Back-N, Selective Repeat ARQ)
– Connection oriented transport – TCP
– TCP Connection management, Flow control, Congestion Control
• Network Layer
– Network service model – Datagram & VC networks
– Forwarding & routing, What’s inside a router
– IPv4 protocol, Addressing, Subnetting, CIDR
– DHCP, NAT, ICMP, IPv6 protocol
– Routing algorithms – LS/DV routing
Course Contents
5

6. • Link Layer
– Link layer functions
– Multiple access protocols
– Link layer addressing, ARP
– Link layer Technologies…
• Ethernet – IEEE 802.3 standard
• WiFi – IEEE 802.11 standard
Course Contents
6

7. • Network Programming :
– Socket Programming
• Client/Server communications paradigm
• TCP & UDP client-server comm
• Iterative & Concurrent servers
(threads & child processes)
• I/O multiplexing, select()
Unix network programming in ‘C’
(Socket API)
Course Contents
7

8. Books Recommended
Text:
Computer Networking: A Top-Down Approach (6th Ed)
by James F. Kurose & Keith W. Ross
Unix Network Programming – Vol I
by W Richards Stevens
Ref:
Computer Networking: A Systems Approach (5th Ed)
by Larry L. Peterson & Bruce S. Davie
8

9. Course Methodology
Lectures
• 16 power point lectures
• Study relevant sections of Text/Ref books
Assignments
• Assignment done individually or in groups
• To be submitted exactly when due
• 5% marks deduction per day for being late
• After one week – Not Accepted
Exam Pattern
• Exams to include complete material in covered chapters
• MCQs, short answers, problems, design questions
9

10. Grading Policy
Quizzes ………. 04 15%
Assignments ………. 04 15%
Class participation ..….…. 05%
Mid-exam ..….…. 25%
Final-exam .……… 40%
Distribution is tentative and flexible
10

11. • Your work in this class must be your own
• If students are found to have:
– collaborated excessively, or
– copied/shared answers
• For the first infraction…
– all involved, at a min, will receive grades of 0
• Further infractions…
– will result in failure in course
Academic Honesty
11

12. Office Hours
12
Office : Room 11, C-Block
Office Hours: ???
Attendance Policy
Strictly in accordance with the Univ policy…

13. Lets Start the Course…
13

14. Chapter 1: roadmap
1.1 what is the Internet?
1.2 network edge
 end systems (hosts), access networks, links
1.3 network core
 packet switching, circuit switching, network structure
1.4 performance
 delay, loss, thruput in networks
1.5 protocol layers, service models
1.6 networks under attack: security
1.7 history
14

15. What is the Internet: “Nuts & Bolts” view
 Computing devices:
• hosts & end systems
• run network apps
• generate/use data
 Communication links:
• copper, fiber,
radio, satellite
• data rate & distance
 Packet switches:
forward packets
(chunks of data)
 switches & routers
wired
links
wireless
links
router
smartphone
PC
server
wireless
laptop
15
• What basic components make
up the Internet…

16. • Internet: “network of networks”
– hierarchical structure
– interconnected ISPs
(lower tier ISPs, upper tier ISPs, …)
– home N/Ws plug into ISP N/Ws
• Networking “protocols”
– run at end systems & switches
– control sending/receiving of msgs
– HTTP, DNS, TCP, IP, Skype
• Internet standards
– IETF : Internet Engineering Task Force
– RFC : Request for comments
16
What’s the Internet: “Nuts & Bolts” view

17. “Fun” Internet appliances
IP picture frame
http://www.ceiva.com/
Web-enabled toaster +
weather forecaster
Internet phones
Internet
refrigerator
Slingbox: watch,
control cable TV remotely
Tweet-a-watt:
monitor energy use
17

18. What’s the Internet: Services view
• What services are provided to Apps
by Internet…
– Apps: Web, Email, VoIP, E-commerce,
Internet TV/radio, audio/video on demand,
video conf, online games, social nets
– Distributed Apps: involve multiple end
systems that exchange data remotely
– Require different Types of Services:
reliable or unreliable data delivery,
thruput/delay needs, security
– How does N/W provide services to
applications…???
18

19. What’s the Internet: Services view
• End Systems provide an interface
to apps
– Network API
“library functions” that allow
“process” on source end system to:
• “connect” to Internet
• “send/recv” data
– Specifies syntax
• how an appl asks Internet to
“send/receive data”
(provides options for various
types of services …)
19

20.  An important buzzword in computer networking… “Protocol“
“pre-defined set of rules”
 Analogy… Human protocol & Computer Network protocol…
What’s a protocol?
Hi
Hi
Got the
time?
2 pm
TCP connection
response
Get http://www.awl.com/kurose-ross
<file>
time
TCP connection
request
20

21. Why Network protocols?
• machines need coordination to interact with other machines
• require some protocol to operate in coordination
Network protocols define…
format/order of msgs among network entities,
& actions taken on msg Tx/Rx
21
 all communications activity in Internet governed by protocols…
 specific msgs sent
 specific actions taken when msgs sent/recvd, or other events
occur

22. Network Structure
 network edge:
 end systems
 hosts, servers, apps
 access networks
 N/Ws connecting end
systems to edge routers
 wired/wireless comm
links
 network core:
 interconnection of routers
 route pkts from src to dst host
 form network of networks
22

23. The network edge
 End systems (hosts):
 host (run) appl programs
 e.g, Web, email
Client/server model:
 hosts act as clients or servers
 server – always ON powerful machine
 client – request/receive service
 e.g, web browser/server,
email client/server
Peer-peer model:
 all hosts equal
 minimal (or no) use of dedicated servers
 e.g, Skype, BitTorrent, Gnutella
23

24. Access networks
Networks to connect end
systems to edge router
• Several types of access networks
used in various settings…
– home access
– enterprise access
– wireless access
points to note…
• range & data rate of access
network ?
• access is shared or dedicated ?
24

25. Home Access
How do homes connect to
Internet
• Digital Subscriber Line (DSL)
• Cable Internet access
• Fiber to the Home (FTTH)
• Dial up modem
Two most prevalent ones are
DSL & Cable
25

26. Enterprise access networks (Ethernet)
26
 Local Area Network (LAN) – connects end systems to edge router
 Ethernet : most prevalent access N/W technology
 twisted pair copper wire & LAN switches
 dedicated link between host and switch
 typically 100 Mbps to end systems; 1 - 10 Gbps to servers

27. Wireless access networks
• Shared wireless access network connects end system to edge router
– via “access point”or “base station”
Wireless LANs:
 within building (100 m)
 802.11b/g (WiFi): 11, 54 Mbps
transmission rate
Wide-area Wireless access:
 by cellular N/W operator
 range 10’s of kms
 data rate 1 - 10 Mbps
 3G, 4G, LTE,….
 WiMAX
to Internet
to Internet 27

28. • Mesh of interconnected routers
• Fundamental question…
“how is data transferred thru N/W”
– Circuit switching
̶ resources along the path reserved
for Tx duration
e.g, telephone network
̶ guaranteed service
– Packet switching
̶ N/W resources used on demand
e.g, Internet
̶ best effort service
The network core
28

29. Network Architecture
29

30. Network Architecture
• Network communications - a complex task
• To deal with this complexity… SIMPLIFY
– comm task divided into modules
– modules arranged in layers
– each layer performs a subset of comm function
– Forms a Network Architecture
• multiple layers
• each layer has one/more Protocols
• protocols perform specific comm tasks
• provide/obtain services to/from higher/lower layer
30

31. Example of a layered
network system
Network Architecture
Network Architecture
A structured set of protocols to implement the
communications function
application
transport
network
link
physical
31

32. Internet protocol stack
• Application: support applications (network API)
 FTP, SMTP, HTTP, DNS, DHCP
• Transport: process-process data transfer (ports)
 TCP (reliable), UDP (unreliable)
• Network: host-host data transfer, global
addressing (IP addr), routing of pkts from src to
dest
 IPv4, IPv6
• Link: hop-hop data transfer, between
neighboring network elements
 Ethernet, WiFi (802.11), 3G/4G, PPP
• Physical: bits “on the wire”
application
transport
network
link
physical
32

33. TCP/IP Model
• Some of the protocols & networks in TCP/IP protocol stack…
33
Ethernet WiFi PPP
3G/4G
HTTP FTP SMTP DNS

34. Encapsulation
34
 Data generated by Appl layer
 When being sent, each layer
appends its header
 When being received, each
layer removes its header
Principle of Encapsulation &
Decapsulation

35. Why Layered Architecture
• Network Architecture - layered architecture
– provides modularity
• changes in one layer do not require changes in other layers
• simplifies system maintenance & upgradation
– facilitates process of network evolution
• allows to change/improve underlying technologies, with
increase in application demands
35

36. Internet Applications
36

37. Some network apps
• e-mail
• web surfing
• search engine
• P2P file sharing
• text messaging
• social networking
• remote login
• streaming stored video
(YouTube, Netflix)
• IP radio, TV
• multi-user N/W games
• voice over IP
(Skype, viber)
• real-time video
conferencing
• …..
37
How these appls are created…

38. 38
Creating network apps
Write appl programs to run only on
end systems
 appl comm over N/W
 web browser comm with server
No need to write software for
network-core devices…
 network-core devices function at
lower layers
confining appls to end systems
allows rapid appl development &
deployment
application
transport
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical

39. Application architectures
Possible structures of applications:
• client-server
• peer-to-peer (P2P)
39

40. Client-server architecture
Servers
• always-on host
• permanent addr (IP & port)
• rely on fixed infra-structure
• data centers for scaling
Clients
• request services from servers
• intermittent host connections
• dynamic addr (IP & port)
• no direct comm with each other
client/server
40

41. P2P architecture
• NO always-on server
– arbitrary end systems comm directly
• peers request services from other
peers, & also provide services
• Self Scalability
– new peers bring new service demands
– also add new service capacity
• Complex Management
– peers connect intermittently
– change IP addrs
• min, or no reliance, on infra-structure
41
peer-peer

42. Architectural Challenges
Client Server
• Infra-structure intensive
• Cost of server hardware, software & access network B/W
• System management needs/costs
• May become a bottleneck
P2P
• Not ISP friendly; require high upload B/W
• Security issues
• Incentives; users need to volunteer storage, bandwidth &
computation resource
42

43. How do Appls comm
• Appls within same host
– processes usually comm
using IPC
(Inter Process Comm)
– Pipes, FIFOs, shared
memory
43
Applications are processes running on hosts…
 Appls in distant hosts
 processes comm by
exchanging messages
 Sockets, RPC

44. • Remote processes comm thru sockets…
Socket : “software interface between process & N/W”
• Process sends/receives msgs to/from its socket
– process reads/writes the socket to receive/send msgs
44
Sockets

45. Process Address
• to receive msgs, process must
have identifier
• host device has unique
IP address (32 bits)
but many processes may run
on same host
 need another level of
identifier – Port No (16 bits)
• identifier includes both
IP address & port number
associated with a process
on a host…
– Socket address
IP addr + Port No
45
to send HTTP msg to web server gaia.cs.umass.edu…
IP address: 128.119.245.12 port number: 80

46. Socket Programming
• Allows network applications to communicate across an Internet
• Socket API mainly provides
Transport layer service
interface
• Stream-sockets for TCP
• Datagram-sockets for UDP
• Based on client/server
architecture
Socket Interface
TCP UDP
IP
Network Access
46

47. Client/Server functions
• Client / Server may run on same or different hosts
• Client makes Request…
– sends message to server to perform a task
• Server Responds…
– performs task & sends back reply
Client
process
Server
process
1. Client sends request
2. Server
handles
request
3. Server sends response
4. Client
handles
response
Resource
47

48. Servers
• Servers : long-running application processes (daemons)
– typically created at boot-time by OS
– run continuously in background
– web server, or mail server
• Server waits for requests on a well-known port associated with
a particular service
– Port 7: echo server
– Port 23: telnet server
– Port 25: mail server
– Port 53: DNS server
– Port 80: HTTP server
/etc/services
provides list of available
services
(Linux machine)
48

49. Clients
• Client – appl launched to access some service, mostly on remote
system
– web browser, ftp client, telnet client, ssh client
• Client does not need well known port
– usually assigned ephemeral port by kernel
– can also be selected by application
Server - need not know client location
Client - needs to know server location
(port + IP address)
49

50. Socket
• Socket is an endpoint of bidirectional comm…
– identified by socket descriptor
• Clients & Servers comm with each other thru sockets
– open a socket
– write data to socket
– read data from socket
App
socket
3 2 1
Dest.
50

51. Socket Address
• The pair IP Address + Port -– makes up a “socket-address”
Server
Client
Client socket address
128.2.194.242:3479
Server socket address
208.216.181.15:53
Client IP address
128.2.194.242
Server IP address
208.216.181.15
3479 is an
ephemeral port
allocated by
kernel (unix)
53 is a well-known
port associated with
DNS servers
51

52. TCP & UDP Ports
• TCP and UDP port numbers ranges:
– values 0 – 216 (65,536 ports)
– Internet Assigned Numbers Authority (IANA)
• Well Known Ports (0 - 1023)
– used by system processes for well known services
– HTTP: 80, E-mail: 25, DNS: 53
• Registered Ports (1024 - 49151)
– used by vendors for common applications
– Web Proxy: 8080, IPSec: 1293, Kaaza: 1214
• Dynamic or Ephemeral Ports (49152 - 65535)
– used by clients
– automatically allocated by kernel on temporary basis
52

53. Summary
1.1 what is the Internet?
1.2 network edge
 end systems, access networks, links
1.3 network core
 packet switching, circuit switching, network structure
1.4 delay, loss, throughput in networks
1.5 protocol layers, service models
1.6 networks under attack: security
1.7 history
53