Third lesson of the computer networks course given at UCL on the basis of the 'Computer Networking : Principles and Practice' ebook

1. Week 3
Building a network
Supporting applications

2. Agenda
• Building a network
• Network basics
• Distance Vector routing
• Link State routing
• Supporting applications

3. Addresses
• What are the advantages/drawbacks of
hierarchical versus flat addresses ?

4. Port-station tables
• What are the advantages/drawbacks of this
approach ?
• How to deal with mobile devices ?
• What about multihomed devices ?

5. Label switching
• What are the advantages/drawbacks of this
approach ?
• How to create the virtual circuits
automatically ?

6. Source routing
• What are the advantages/drawbacks of this
approach ?

7. Datagram mode
• Early work on this concept that lead to the
current Internet was done in France
• Cyclades project lead by Louis Pouzin
http://goo.gl/lwdlwS

8. Agenda
• Building a network
• Network basics
• Distance Vector routing
• Link State routing
• Supporting applications

9. Distance vectors
l How to deal with link failures ?
C
A B C
D E
D E

10. How to react ?
All routes that use a failed link are advertised with
an infinite cost
C
Routing table
B : 0 [Local]
A : ¥
C : 1 [East]
E : 1 [South]
D : 2 [South]
D E
Routing table
A : 0 [ Local ]
D : 1 [South]
B : ¥
C : ¥
E : ¥
A B C
D E
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
A : 2 [West]
B : 1 [West]
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : 1 [East]
C : 2 [East]
B : 2 [North]
A=0;B=¥;C=¥;D=1;E=¥

11. Reception of DV
Received(Vector V[],link l)
{ /* received vector from link l */
for each destination=d in V[]
{
if (d isin R[])
{ if ( ((V[d].cost+l.cost) < R[d].cost) OR
( R[d].link == l) )
{ /* better route or change to current route */
R[d].cost=V[d].cost+l.cost;
R[d].link=l;
}
}
else
{ /* new route */
R[d].cost=V[d].cost+l.cost;
R[d].link=l;
}
}
Why ?

12. Example
Routing table
A : 0 [ Local ]
D : 1 [South]
B : ¥
C : ¥
E : ¥
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
Routing table
B : 0 [Local]
A : ¥
C : 1 [East]
E : 1 [South]
D : 2 [South]
A B C
A=0;B=¥;C=¥;D=1;E=¥
D E
A : 2 [West]
B : 1 [West]
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : 1 [East]
C : 2 [East]
B : 2 [North]
D must removes all the routes learned from
North that are announced with an ∞ cost

13. Example
C
Routing table
B : 0 [Local]
A : ¥
C : 1 [East]
E : 1 [South]
D : 2 [South]
D E
Routing table
A : 0 [ Local ]
D : 1 [South]
B : ¥
C : ¥
E : ¥
A B C
D E
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
A : 2 [West]
B : 1 [West]
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : 1 [East]
C : 2 [East]
B : ¥
D=0;B= ¥;A=1;C=2;E=11

14. Example
C
D E
Routing table
A : 0 [ Local ]
D : 1 [South]
B : ¥
C : 3 [South]
E : 2 [South]
A B C
D E
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
A : 2 [West]
B : 1 [West]
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : 1 [East]
C : 2 [East]
B : ¥
Routing table
B : 0 [Local]
A : ¥
C : 1 [East]
E : 1 [South]
D : 2 [South]
B=0;A=¥;C=1;E=1;D=2
B=0;A=¥;C=1;E=1;D=2

15. Example
C
D E
Routing table
A : 0 [ Local ]
D : 1 [South]
B : ¥
C : 3 [South]
E : 2 [South]
A B C
D E
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
A : ¥¥
B : 1 [West]
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : 1 [East]
C : 2 [East]
B : ¥
Routing table
B : 0 [Local]
A : ¥
C : 1 [East]
E : 1 [South]
D : 2 [South]
E=0;A=2;D=1;C=1;B=1
E=0;A=2;D=1;C=1;B=1
E=0;A=2;D=1;C=1;B=1

16. Example
C
D E
Routing table
A : 0 [ Local ]
D : 1 [South]
B : ¥
C : 3 [South]
E : 2 [South]
A B C
D E
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
A: 3 [South-West]
B : 1 [West]
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : 1 [East]
C : 2 [East]
B : 2 [East]
Routing table
B : 0 [Local]
A : 3 [South]
C : 1 [East]
E : 1 [South]
D : 2 [South]
A=1;B=2;C=2;D=1;E=1
• Everything is ok ?

17. Another failure
C
D E
Routing table
A : 0 [ Local ]
D : 1 [South]
B : 3 [South]
C : 3 [South]
E : 2 [South]
A B C
D E
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
A: 3 [South-West]
B : 1 [West]
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : 1 [East]
C : 2 [East]
B : 2 [East]
Routing table
B : 0 [Local]
A : 3 [South]
C : 1 [East]
E : 1 [South]
D : 2 [South]
A=1;B=¥;C=¥;D=1;E=¥

18. Another failure
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
A: 3 [South-West]
B : 1 [West]
C
Routing table
B : 0 [Local]
A : 3 [South]
C : 1 [East]
E : 1 [South]
D : 2 [South]
A B C
A=0;D=1;B=3;C=3;E=2
D E
Routing table
A : 0 [ Local ]
D : 1 [South]
B : 3 [South]
C : 3 [South]
E : 2 [South]
D E
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : ¥
C : ¥
B : ¥
But A could also send its distance vector before D

19. Another failure
D updates its routing table
C
D E
Routing table
A : 0 [ Local ]
D : 1 [South]
B : 3 [South]
C : 3 [South]
E : 2 [South]
A B C
D E
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
A: 3 [South-West]
B : 1 [West]
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : 3 [North]
C : 4 [North]
B : 4 [North]
Routing table
B : 0 [Local]
A : 3 [South]
C : 1 [East]
E : 1 [South]
D : 2 [South]
D=0;A=1;E=3;C=4;B=4

20. Another failure
C
D E
Routing table
A : 0 [ Local ]
D : 1 [South]
B : 5 [South]
C : 5 [South]
E : 4 [South]
A B C
D E
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
A: 3 [South-West]
B : 1 [West]
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : 3 [North]
C : 4 [North]
B : 4 [North]
Routing table
B : 0 [Local]
A : 3 [South]
C : 1 [East]
E : 1 [South]
D : 2 [South]
A=0;D=1;B=5;C=5;E=4

21. Another failure
C
D E
Routing table
A : 0 [ Local ]
D : 1 [South]
B : 5 [South]
C : 5 [South]
E : 4 [South]
A B C
D E
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
A: 3 [South-West]
B : 1 [West]
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : 5[North]
C : 6 [North]
B : 6 [North]
Routing table
B : 0 [Local]
A : 3 [South]
C : 1 [East]
E : 1 [South]
D : 2 [South]
A=1;D=0;B=6;C=6;E=5
• This problem is called counting to infinity
• How can we prevent it ?

22. Another failure
Split horizon
Split horizon with poisoning

23. Split horizon
A B C
E
Routing table
A : 0 [ Local ]
D : 1 [South]
B : 3 [South]
C : 3 [South]
E : 2 [South]
D E
• A will not pollute D’s routing table
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
A: 3 [South-West]
B : 1 [West]
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : 1 [East]
C : 2 [East]
B : 2 [East]
Routing table
B : 0 [Local]
A : 3 [South]
C : 1 [East]
E : 1 [South]
D : 2 [South]
A=0

24. Split horizon (2)
D can also send its distance vector
C
D E
Routing table
A : 0 [ Local ]
D : 1 [South]
E : ¥
C : ¥
B : ¥
A B C
D E
Routing table
C : 0 [Local]
E : 1 [South-West]
D : 2 [South-West]
A: 3 [South-West]
B : 1 [West]
Routing table
E : 0 [Local]
D : 1 [West]
A : 2 [West]
C : 1 [North-East]
B : 1 [North]
Routing table
D : 0 [Local]
A : 1 [North]
E : ¥
C : ¥
B : ¥
Routing table
B : 0 [Local]
A : 3 [South]
C : 1 [East]
E : 1 [South]
D : 2 [South]
D=0;B=¥;C=¥;E=¥
Does split horizon allows to avoid all counting to
infinity problems ?

25. Limitations
C
E
Routing table
A : 0 [ Local ]
B : ¥¥
C : ¥¥
E : ¥¥
A B C
E
Routing table
C : 0 [Local]
E : 1 [South-West]
A : 2 [West]
B : 1 [West]
Routing table
E : 0 [Local]
A : 2 [North]
C : 1 [North-East]
B : 1 [North]
Routing table
B : 0 [Local]
A : 1 [West]
C : 1 [East]
E : 1 [South]
A=2;B=1; C=0;E=¥
A=¥;B=0; C=1;E=¥
A=¥;B=0; C=1;E=¥

26. Limitations
C
E
E will send its own distance vector
B will discover a new route
towards A via E and will advertise
it to C
New count to infinity problem
Routing table
A : 0 [ Local ]
B : ¥¥
C : ¥¥
E : ¥¥
A B C
E
Routing table
C : 0 [Local]
E : 1 [South-West]
A : 2 [West]
B : 1 [West]
Routing table after B’s vector
E : 0 [Local]
A : ¥
C : 1 [North-East]
B : 1 [North]
Routing table after C’s vector
E : 0 [local]
A : 3 [North-East]
C : 1 [North-East]
B : 1 [North]
Routing table
B : 0 [Local]
A : 1 [West]
C : 1 [East]
E : 1 [South]
A=¥;B=0; C=1;E=¥
A=2;B=1; C=0;E=¥

27. Operational issue
Operation
At each expiration of its 30-sec timer, each router
sends its distance vector and restarts its timer
Problem
After a power failure, all routers might restart at
same time and have synchronised timers
Each router will need to process bursts of DV
messages

28. Agenda
• Building a network
• Network basics
• Distance Vector routing
• Link State routing
• Supporting applications

29. How to set link costs ?
• By manual configuration
• Based on link bandwidth
• By measurements

30. What is the role of
• Sequence number in link state packets
• How to deal with wrap around ?
• Age field in link state packets
• CRC in link state packets

31. Two way connectivity
check
C
Links
A-B, B-A : 1
B-E, E-B : 1
B-C, C-B : 1
E-D, D-E : 1
E-C, C-E : 1
A-D, D-A : 1
LSPs
E-0 [D:1];[B:1];[C:1]
A-0 [D:1];[B:1]
B-0 [A:1] [C:1] [E:1]
C-0 [B:1] [E:1]
D-0 [A:1] [E:1]
Links
A-B, B-A : 1
B-E, E-B : 1
B-C, C-B : 1
E-D, D-E : 1
E-C, C-E : 1
A-D, D-A : 1
A B C
D E
D E
Links
A-B, B-A : 1
B-E, E-B : 1
B-C, C-B : 1
E-D, D-E : 1
E-C, C-E : 1
A-D, D-A : 1
LSPs
E-0 [D:1];[B:1];[C:1]
A-0 [D:1];[B:1]
B-0 [A:1] [C:1] [E:1]
C-0 [B:1] [E:1]
D-0 [A:1] [E:1]
LSPs
E-0 [D:1];[B:1];[C:1]
A-0 [D:1];[B:1]
B-0 [A:1] [C:1] [E:1]
C-0 [B:1] [E:1]
D-0 [A:1] [E:1]
Links
A-B, B-A : 1
B-E, E-B : 1
B-C, C-B : 1
E-D, D-E : 1
E-C, C-E : 1
A-D, D-A : 1
Links
A-B, B-A : 1
B-E, E-B : 1
B-C, C-B : 1
E-D, D-E : 1
E-C, C-E : 1
A-D, D-A : 1
LSPs
E-1 [D:1];[C:1]
A-0 [D:1];[B:1]
B-0 [A:1] [C:1] [E:1]
C-0 [B:1] [E:1]
D-0 [A:1] [E:1]
LSPs
E-0 [D:1];[B:1];[C:1]
A-0 [D:1];[B:1]
B-0 [A:1] [C:1] [E:1]
C-0 [B:1] [E:1]
D-0 [A:1] [E:1]
LSP : E-1 [D:1];[C:1]
LSP : E-1 [D:1];[C:1]
• A link is only considered useable only if
both directions have been advertised

32. Transient problems
when a link fails
A B
C 100 D
A's FIB
B : East
C : South
D : East
D's FIB
A : North
B : North
C : North
C's FIB
A : North
B : North
D : North
B's FIB
A : West
C : West
D : South
A and B are not yet
aware of failure
Packets will be lost

33. A detects the failure
A B
C 100 D
A's FIB
B : East
C : South
D : East
D's FIB
A : North
B : North
C : North
C's FIB
A : North
B : North
D : North
B's FIB
A : West
C : West
D : South
B not yet aware of failure
A cannot reach B/D
Packets continue to be lost

34. A recomputes Dijkstra
and sends LSP
A B
A:1 C=1
C 100 D
A's FIB
B : South
C : South
D : South
D's FIB
A : North
B : North
C : North
C's FIB
A : North
B : North
D : North
B's FIB
A : West
C : West
D : South
B not yet aware of failure
A loops towards B/D
Packets continue to be lost

35. B detects the failure
A B
A:1 C=1
C 100 D
A's FIB
B : South
C : South
D : South
D's FIB
A : North
B : North
C : North
C's FIB
A : North
B : North
D : North
B's FIB
A : West
C : West
D : South
B cannot reach A/C
A loops towards B/D
Packets continue to be lost

36. B recomputes Dijkstra
and sends its LSP
A B
B:1 D=1
C 100 D
A's FIB
B : South
C : South
D : South
D's FIB
A : North
B : North
C : North
C's FIB
A : North
B : North
D : North
B's FIB
A : South
C : South
D : South
B cannot reach A/C
A loops towards B/D
Packets continue to be lost

37. C recomputes Dijkstra
A B
C 100 D
A's FIB
B : South
C : South
D : South
D's FIB
A : North
B : North
C : North
C's FIB
A : North
B : East
D : East
B's FIB
A : South
C : South
D : South
B loops towards A/C
A can reach everyone
Packets continue to be lost

38. D recomputes Dijkstra
A B
C 100 D
A's FIB
B : South
C : South
D : South
D's FIB
A : West
B : North
C : West
C's FIB
A : North
B : East
D : East
B's FIB
A : South
C : South
D : South
B can reach everyone
A can reach everyone
Network has been updated

39. Router failures
• Planned failures
• Router reboot to upgrade OS
• Sudden failure
• due to power outage, software or
hardware crash

40. Agenda
• Building a network
• Supporting applications
• Transport services
• Reliable transport protocols

41. Connectionless
Source Provider Destination
DATA.request(S,D,"M")
DATA.indication(S,D,"M")
Time

42. Connection oriented
Source Network provider Destination
CONNECT.request
CONNECT.indication
CONNECT.confirm
Source considers
connection open
CONNECT.response
Destination considers
connection open

43. Data transfer
Message mode
Source Provider Destination
CONNECT.request
CONNECT.indication
CONNECT.confirm
CONNECT.response
DATA.request("A") DATA.ind("A")
DATA.request("BCD") DATA.ind("BCD")
DATA.request("EF") DATA.ind("EF")

44. Data transfer
Stream mode
Source Provider Destination
CONNECT.request
CONNECT.indication
CONNECT.confirm
CONNECT.response
DATA.request("AB") DATA.ind("A")
DATA.request("CD")
DATA.ind("B")
DATA.request("EF") DATA.ind("C")
DATA.ind("DEF")

45. Abrupt release
Source Provider Destination
Connection opened Connection opened
DISCONNECT.req(abrupt)
DISCONNECT.indication
DATA.request("A")
DATA.request("B")
DATA.indication("A")
DATA.request("C")

46. Graceful release
Source Provider Destination
Connection opened Connection opened
DISCONNECT.req(graceful)
DATA.request("C")
DISCONNECT.ind(graceful)
DATA.request("A")
DATA.request("B")
DATA.indication("A")
DATA.indication("B")
Source->Destination
connection closed
DATA.indication("C")
DATA.request("D")
DATA.indication("D")
DISCONNECT.req(graceful)
DISCONNECT.ind(graceful) Connection closed
Connection closed

47. Transport services
useable on Internet
• TCP : Connection oriented, bytestream
• UDP : Connectionless

48. Agenda
• Building a network
• Supporting applications
• Transport services
• Reliable transport protocols

49. Multiplexing
• How to multiplex data coming from
multiple applications ?

50. Reliable transport
• How to establish a connection ?

51. Reliable transport
protocols
• How to reliably transport data ?

52. Reliable transport
• How to terminate a transport connection ?