Fifth week of the Computer Networking : Principles, Protocols and Practice class at UCL

1. Week 5
Sharing resources
Internet applications

2. Agenda
• Sharing resources
• Medium Access Control
• Congestion Control
• Internet applications

3. ALOHA
N=1;
while ( N<= max) do
send frame;
wait for ack on return channel or timeout:
if ack on return channel
exit while;
else
/* timeout */
/* retransmission is needed */
wait for random time;
N=N+1;
end do
/* too many attempts */

4. CSMA
N=1;
while ( N<= max) do
wait until channel becomes free;
send frame immediately;
wait for ack or timeout:
if ack received
exit while;
else
/* timeout */
/* retransmission is needed */
N=N+1;
end do
/* too many attempts */

5. CSMA/CD
• Basic solution
N=1;
while ( N<= max) do
wait until channel becomes free;
send frame and listen;
wait until (end of frame) or (collision)
if collision detected
stop transmitting;
/* after a special jam signal */
else
/* no collision detected */
wait for interframe delay;
exit while;
N=N+1;
end do
/* too many attempts */

6. Example
A B
Start of frame
1
Frame is propagated on LAN (5 microsecond per kilometer)
2
Frame stops at left side and first bit reaches B
3
4
Frame leaves the LAN

7. Collisions
A B
Frame starts at A and B almost at the same time
1
2
Collision : at this point on the
shared medium, it is impossible
to decode the signal
A detects the collision and
stops transmitting its frame
3

8. Can we detect all
collisions ?
A B
A and B send a small frame almost at the same time
1
The two frame collide in the middle of the medium
2
A did not notice any collision
B did not notice any collision
They both consider that their frames were received correctly
3

9. Worst case
A B
Start of 1 the frame sent by A
3
After t seconds, A’s frame reaches B
A time t-e, B starts to transmit its own frame
B notices the collision immediately and stops transmitting
A detects collision at time t+t-e
2

10. How to detect all
collisions ?
• All nodes must always transmit during at
least the two way delay (2∗τ)
• If bandwidth is fixed => minimum frame
size

11. How to react after
collision ?
• Exponential backoff
• Wait random time after collision
• Divide time in slots Backoff (slot = (2∗τ))
• 1st collision : wait 0 or 1 slot
• 2nd collision :wait 0, 1,2 or 3 slots
• ith collision : wait 0..2i-1 slots

12. CSMA/CD
• Final solution
N=1;
while ( N<= max) do
wait until channel becomes free;
send frame and listen;
wait until (end of frame) or (collision)
if collision detected
stop transmitting;
/* after a special jam signal */
k = min (10, N);
r = random(0, 2k – 1);
wait for r time slots;
else
/* no collision detected */
wait for interframe delay;
exit while;
N=N+1;
end do
/* too many attempts */

13. CSMA/CA
• A MAC for wireless networks
• Cannot always detect collisions
• Improvements to CSMA
• Initial delay to transmit (EIFS)
• Min. delay between frames (DIFS)
• Delay between frame and ack (SIFS)

14. CSMA/CA : receiver
While (true)
{
Wait for data frame;
if not(duplicate)
{ deliver (frame) }
wait during SIFS;
send ack (frame) ;
}

15. CSMA/CA : sender
N=1;
while ( N<= max) do
if (previous frame corrupted)
{ wait until channel free during t>=EIFS; }
else
{ wait until endofframe;
wait until channel free during t>=DIFS; }
send data frame ;
wait for ack or timeout:
if ack received
exit while;
else
/* timeout retransmission is needed */
N=N+1;
end do
/* too many attempts */

16. Example
A B C
DIFS
Data frame
Busy
SIFS
ACK frame
DIFS
Data frame

17. Collisions
A B C
DIFS
DIFS
Data frame
Busy
Busy Data frame
Timeout Timeout
Data frame
Busy
Data frame

18. CSMA/CA : sender
N=1;
while ( N<= max) do
if (previous frame corruped)
{ wait until channel free during t>=EIFS; }
else
{ wait until endofframe;
wait until channel free during t>=DIFS; }
backoff_time = int(random[0,min(255,7*2N-1)])*T
wait(channel free during backoff_time)
send data frame ;
wait for ack or timeout:
if ack received
exit while;
else /* timeout retransmission is needed */
N=N+1; }
end do

19. Backoff
A B C
DIFS
DIFS
Backoff[0,7]
Backoff[0,7]
Channel busy!
Busy
Data frame
Data frame
SIFS
Ack frame
Remaining
backoff

20. HIdden station problem
A
B
Hears A and C
C
Only hears B, not C Hears B, but not A

21. Hidden station
• How to solve it ?
• Sender reserves time slot
• Receiver confirms the time reservation

22. RTS/CTS
A B C
DIFS+Backoff
Busy[
100microsec+
SIFS+
CTS+
SIFS+
ACK
]
RTS [100 microsec]
SIFS
CTS[100microsec]
Data [100 microsec]
SIFS
ACK frame
SIFS

23. Agenda
• Sharing resources
• Medium Access Control
• Congestion Control
• Internet applications

24. Max-min fairness
• Fairness definition for networks
• a max-min allocation of bandwidth is an
allocation of bandwidth which maximises the
allocation of bandwidth to the sources
receiving the smallest allocation
• Property
• a max-min fair allocation is such that in
order to increase the bandwidth allocated to
one source, it is necessary to decrease the
bandwidth allocated to another source
which already receives a lower allocation

25. Max-min fairness
• a max-min allocation maximises the
allocation of bandwidth to the sources
receiving the smallest allocation
• Property
• to increase the bandwidth allocated to
one source, it is necessary to decrease
the bandwidth allocated to another
source which already receives a lower
allocation

26. Example
R
R
D1 D3 D5
R
R
R
1000 Mbps
100 Mbps
S1
S2
S3 S4 S5 S6 S7 S8
• Max-min fair bandwidth allocation
D2
D4
D6
D7
D8
Link1
Link2
Link3
Link4

27. Router output port
Q[1]
Q[2]
Q[3]
Q[N]
Flow identification
Input links
Output link
Flow identification
Identifies the TCP/UDP flow
to which the arriving packet
belongs
Buffer acceptance
accepts or rejects
incoming packets
Queuing strategy
Logical organization of the
router's buffers
Scheduler
Chooses the packet to
be transmitted first on
the output link

28. Round robin
Flow 1
Flow 2
Flow 3
Flow 4
Flow 5
Flow 1
Flow 2
Flow 3
Flow N
Scheduler :
F1
F2
FN
F4 F3

29. Example
F1
F2
F3
Flow1(L=1)
Flow2 (L=2)
Flow3 (L=1) F1
F3 F2
Flow1 and Flow3 send packets of size 1
Flow 2 sends packets of size 2

30. Adapting to different
bandwidth

31. Self-clocking

32. The congestion
problem

33. How to detect
congestion ?
• Packet losses
• Routers add information to packets
• Forward binary feedback
• Backward binary feedback
• Rate feedback

34. Congestion control
• Additive Increase / Multiplicative Decrease
# Additive Increase Multiplicative Decrease
if congestion :
rate=rate*betaC # MD, betaC<1
else
rate=rate+alphaN # AI

35. Window-based
Congestion control
# Initialisation
cwin = 1 # congestion window measured in segments
# Ack arrival
if newack : # new ack, no congestion
# increase cwin by one every rtt
cwin = cwin+ (1/cwin)
else:
# no increase
Congestion detected:
cwnd=cwin/2 # only once per rtt

36. Agenda
• Sharing resources
• Internet applications
• DNS
• Email
• Web
• Remote Procedure Calls

37. Internet
Transport services
• Service provided by UDP
• Connectionless, unreliable
• Service provided by TCP
• Connection-oriented
• reliable bytestream

38. Internet addresses
• IPv4
• 32 bits, written as a.b.c.d, e.g.
130.104.1.1
• IPv6
• 128 bits, written in hexadecimal notation,
e.g. 2001:6a8:3080:1::3

39. UDP service
UDP service
Applic.
2
Applic.
1
Identification
IP address : 2001:4860:a005::68
Protocol : UDP
Port : 53
Identification:
IP address :
2001:6a8:3080:2:217:f2ff:fed6:65c0
Protocol : UDP
Port : 1234
• Identification of an application
• IP address + port number

40. TCP service
TCP service
• Identification of an application
• IP address + TCP + port number
Applic.
2
Applic.
1
Identification
IP address: 2001:6a8:3080:1::3
Protocol : TCP
Port : 80
Identification:
IP address :
2a02:2788:2c4:16f:226:bbff:fe09:266e
Protocol : TCP
Port : 9876

41. Agenda
• Sharing resources
• Internet applications
• DNS
• Email
• Web
• Remote Procedure Calls

42. The Domain Name
System
• Hierarchy of domain names
• Domain Name servers
• DNS resolvers

43. DNS messages
Each DNS request contains a number that will be returned in the
response by the server to allow the client to match the request.
32 bits
Identification Flags
12 bytes Number of questions
Number of answers
Number of authority Number of additional
Questions
(variable number of resource records)
Answers
(variable number of resource records)
Authority
(variable number of resource records)
Additional information
(variable number of resource records)
lQuestion/Response
lRecursive question or not
lAuthoritative answer or not
lPossible error

44. DNS Resource Records
• A
• IPv4 address
• AAAA
• IPv6 address
• NS
• Name server

45. Agenda
• Sharing resources
• Internet applications
• DNS
• Email
• Web
• Remote Procedure Calls

46. Simplified architecture
Alice’s
email server
b.net ‘s
email server
Alice@a.net Bob@b.net
Alice sends her email
to local mail forwarder
Alice’s server sends email
to b.net’s MX
Bob retrieves message
from his server

47. Message format
Exp: ABC S.A.
Rue de Fer 10
5000 Namur
DEF Corp.
Steel street 9
WA78 AX London
Grande Bretagne
From: president@abc.be
To: ceo@def.com
Subject: Hello
Date : 27 Sept. 1999 0901
Dear Sir,
Bla Bla Bla...
Header
Message
body

48. Header format
• At least three lines that end with <CRLF>
• From: sender@domain
• To:recipient@domain
• Date: <creation date of message>
• Optional fields
• Subject: , cc: ,Message-ID:, Received: In-
Reply-To: , ...
• Header ends with empty line (<CRLF>)

49. Email protocols
SMTP
SMTP Email
a.net’s
SMTP server
b.net’s SMTP
server
retrieval
Alice@a.net Bob@b.net

50. Agenda
• Sharing resources
• Internet applications
• DNS
• Email
• Web
• Remote Procedure Calls

51. Simplified architecture
Client
(browser)
Server www.machin.be
Server www.truc.fr
Query
Information

52. Key elements of the
web
• URL : An addressing scheme that allows to
identify any document stored on a server
• HTML : An hypertext language to easily
write documents with hypertext links
• HTTP : An efficient and lightweight
protocol to exchange documents
• Servers
• Clients (browsers)

53. URL
• syntax : <protocol>://<document>
• http is the most common
• document indicates the server and the
location of the document
• <user>:<password>@<server>:<port>/
<path>

54. HTML
<HTML>
<HEAD>
<TITLE>HTML test page</TITLE>
</HEAD>
<BODY>
<IMG SRC="http://www.images.be/logo.gif">
<H1>Web servers from UCL UCL<P></H1>
<HR>
<UL>
<LI><A HREF="http://www.uclouvain.be">UCL</A>
<LI><A HREF="http://www.info.ucl.ac.be">CSE Dept.</A>
<LI><A HREF="http://www.math.ucl.ac.be">Math</A>
</UL>
</BODY>
</HTML>
Header
Body
Image on remote server
First level title
External hypertext link

55. HTTP
Client
Server
Request
Method
Header
CRLF
MIME Document
Method
GET
lPOST
l...
Header contains additional information
about request sent by client
Response
Status line
Header
CRLF
MIME Document
Header contains information about server
and optional parameters specific to response
Success or failure
HTTP is a stateless protocol, server does not maintain any state from
one request to another

56. HTTP : Example
Request
Client
Server
GET /index.html HTTP/1.1 www.info.ucl.ac.be
Host: www.info.ucl.ac.be
CRLF
Response
HTTP/1.1 200 OK
Date: Fri, 10 Sep 1999 14:29:19 GMT
Server: Apache/1.3.0 (Unix) ApacheJServ/1.0b5
Last-Modified: Thu, 02 Sep 1999 11:50:50 GMT
Content-Length: 1224
Content-Type: text/html
CRLF
<HTML>
. . .
</HTML>

57. Agenda
• Sharing resources
• Internet applications
• DNS
• Email
• Web
• Remote Procedure Calls