The document outlines a computer networking course led by Olivier Bonaventure, covering essential networking principles, protocols, and practical exercises. It includes an overview of evaluation methods such as programming projects, web analysis, and exams. Key topics discussed are the physical layer of networking, error handling, and data transmission methods, including various protocols and their functionalities.

1. Computer networking
Olivier Bonaventure
http://perso.uclouvain.be/olivier.bonaventure

2. Teaching material
• Third edition, work in progress
https://beta.computer-networking.info
• Feedback/Fixes/Suggestions/Comments
• http://github.com/cnp3/ebook

3. Question
• What are the topics that you expect to
learn during this course ?

4. Course structure
• Principles first
• reliable transfer, building a network,
sharing resources, serving applications,
security
• Protocols in details
• HTTP, SMTP, DNS, TLS, TCP, SCTP, UDP,
IPv6, MPLS, RIP, OSPF, BGP, Ethernet, WiFi
• Practice
• Learning by doing

5. Course organisation
• First pass on the week topic (Thursday)
• Students read book and prepare exercises
• Inginious questions
• Multiple choice questions
• Some weeks : practical exercises as well
• Discussion with teaching assistants (Wed/Fri)
• Second pass on the week topics (Thursday)

6. Course evaluation
• Programming project (20%)
• implement one protocol due end March
(15%)
• Review of code (5%)
• Web site Analysis (15%)
• Analyze the optimizations of a website (-
>May)
• Exam
• Both exercises and theory (65%)

7. Question
• During this course, we will describe all the
components of the Internet, but when
was this network created?

8. ARPANet
First ARPANet message, Oct. 29, 1969
https://en.wikipedia.org/wiki/ARPANET

9. Question
• When (year) were the first public web
servers launched ?

10. https://home.cern/science/computing/birth-web/short-history-
web

11. Internet versus web
https://www.reddit.com/r/pcmasterrace/comments/b086lz/tim_bernerslee_and_vint_cerf_w
earing_funny_shirts/

12. Question
• What is the average size of a web page
(in bytes) ?

13. https://httparchive.org/reports/page-weight

14. Connecting two hosts

15. Question
• What are the technologies that allow to
connect two computers together ?

16. Question
• What is the bandwidth that you can
achieve on a telephone line over a several
kilometers ?

17. VDSL and G.Fast
https://techzine.alcatel-lucent.com/fixed-access-stay-ahead-ultra-broadband-
curve

18. Question
• What is the bandwidth that you can
achieve using current wireless (4G/5G,
Wi-Fi) networks ?

19. Question
• What is the bandwidth that you can
achieve on an optical fiber ?

20. Optical fiber
http://www.nature.com/nphoton/journal/v7/n5/fig_tab/nphoton.2013.94_F1.html

21. Question
• What is the minimum delay (in msec)
required to transmit one byte from
Louvain-la-Neuve to Barcelone (+-1000
km) ?

22. The physical layer
• Transmitting bits over a wire
• Time sequence diagram

23. The physical layer
service
• unreliable service
• transmission errors
• loss/creation of bits
Physical layer Physical layer
Physical transmission medium
Bits
010100010100010101001010

24. Time sequence
diagram
• How to represent the operation of the
physical layer ?
A B
Data.request(0)
0
Data.ind(0)
Data.request(1)
Data.ind(1)
Data.request(1)
Data.ind(1)
1
1

25. Time sequence
diagram
• How to represent the unreliablity of the
physical layer ?
A B
Data.request(0)
0
Data.ind(0)
Data.request(1)
Data.ind(1)
Data.request(1)
Data.ind(1)
0
1

26. Time sequence
diagram
• How to represent the fact that the
physical layer can loose bits ?
A B
Data.request(0)
0
Data.ind(0)
Data.request(1)
Data.request(1)
Data.ind(1)
1

27. Time sequence
diagram
• How to represent the fact that the
physical layer can create bits ?
A B
Data.request(0)
0
Data.ind(0)
Data.request(1)
Data.ind(1)
1
Data.ind(1)
Data.request(0)
0
Data.ind(0)

28. Exchanging frames
• From a software viewpoint, it is much
easier to consider that hosts exchange
frames
Payload
Header Trailer

29. The framing problem
• How to extract variable length frames
from a sequence of bits ?
• Bit stuffing
• Character stuffing

30. How to reliably
transfer data ?
• Hypothesis
• Reliable physical layer service
A B
Data.request(a) Frame(a)
Data.ind(a)
Data.request(b) Frame(b)
Data.ind(b)
Data.request(c) Frame(c)
Data.ind(c)

31. Does this always work
?

32. Question
• What do we need to add to our “reliable”
transfer to allow a datacenter to adapt its
transmission rate to a raspeberry pi ?

33. Finite State Machine
• Sender
• Receiver

34. Control frames
A B
Data.req(b) C(OK) Data.ind(a)
D(a)
Data.req(a)
D(b)
Data.ind(b)
C(OK)
Payload
Header Trailer

35. Question
• Two hosts are connected with a 1 Gbps
(10^9 bits/sec) link. It takes one msec to
send a frame from host A to host B. What
is the bandwidth that this protocol will
achieve (in bits per second) ?

36. How to deal with
transmission errors ?
• The receiver must be able to detect that
the content of a frame has been modified
• Checksum, CRC

37. Question
• How should a host react when it receives
a frame with an incorrect checksum ?

38. Coping with errors
A B
Data.req(b) C(OK) Data.ind(a)
D(a)
Data.req(a)
D(b)
D(b)
Data.ind(b)
start timer
timer expires

39. Coping with errors
A B
Data.req(b) C(OK) Data.ind(a)
D(a)
Data.req(a)
D(b)
D(b)
Data.ind(b)
start timer
timer expires
C(OK) Data.ind(b)

40. Alternating Bit
Protocol
A B
Data.req(b) C(OK0) Data.ind(a)
D(0,a)
Data.req(a)
Duplicate detected
D(1,b)
Retransmission timer
D(1,b)
Data.ind(b)
Data.req(c)
Lost
D(0,c)
C(OK1)
D(1,b) recvd
C(OK1)
D(1,b) recvd
Retransmission timer
Data.ind(c)
D(0,c)

41. Pipelining
• How many frames can A send ?
A B
Data.ind(a)
Data.req(a)
...
D(0,a)
...
D(4,e)
Data.req(b)
Data.req(e)
Data.ind(e)

42. Pipelining example
A B
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8 Data.req(a)
Data.ind(a)
D(0,a)
0 1 2 3 4 5 6 7 8 Data.req(b)
Data.ind(b)
D(1,b)
0 1 2 3 4 5 6 7 8
Data.req(c)
Data.ind(c)
D(2,c)
C(OK0)
C(OK1)
C(OK2)
0 1 2 3 4 5 6 7 8
Data.req(d)
Data.ind(d)
D(3,d)
0 1 2 3 4 5 6 7 8
Data.req(e)
D(4,e)
Sending window

43. Sequence numbers
can be reused
A B
0 1 2 3
0 1 2 3
0 1 2 3
0 1 2 3
0 1 2 3
Data.req(a)
Data.ind(a)
D(0,a)
Data.req(b)
Data.ind(b)
D(1,b)
Data.req(c)
Data.ind(c)
D(2,c)
C(OK0)
C(OK1)
C(OK2)
Data.req(d)
Data.ind(d)
D(3,d)
Data.req(e)
D(0,e)
Sending window
0 1 2 3
0 1 2 3
0 1 2 3
0 1 2 3

44. How to deal with
errors/frame losses ?
• Go-back-n
• Selective repeat

45. Go-back-n
A B
0 1 2 3
0 1 2 3
0 1 2 3
0 1 2 3
0 1 2 3
Data.req(e)
Data.req(a)
Data.ind(a)
D(0,a)
Data.req(c)
D(2,c)
C(OK,0)
C(OK,0)
Sending window
Data.req(b)
D(1,b)
Lost
Not expected seq num,
discarded
Retransmission
timer expires
Sending window is full
0 1 2 3
0 1 2 3
D(1,b)
Data.ind(b)
Data.req(d)
D(3,d)
Data.ind(d)
D(2,c)
Data.ind(c)

46. Selective repeat
A B
0 1 2 3
0 1 2 3
0 1 2 3
0 1 2 3
Data.ind(b)
Data.req(a)
Data.ind(a)
D(0,a)
0 1 2 3
Data.req(c)
D(2,c)
C(OK,0)
C(OK,0)
Sending window
Data.req(b)
D(1,b)
Lost frame
Frame stored
D(1,b)
Retransmission
timer expires
0 1 2 3
0 1 2 3
0 1 2 3
0 1 2 3
0 1 2 3
Data.ind(c)
Data.req(d)
D(3,d)
Data.ind(d)
0 1 2 3
0 1 2 3
C(OK,2)
C(OK,3)
Rec. window
0 1 2 3
0 1 2 3