1. Lecture 8:
The Network Layer

2. IP routers
Alice
Bob

3. application transport network link physical router Alice’s machine
message
transport Ht
network Hn
link Hl
physical
router
Alice’s machine

4. application transport network link physical router Bob’s machineHn
link Hl
physical Hl Hn Ht
message
router
Bob’s machine

5. application transport network link physical Bob’s machineHl Hn Ht
message
Bob’s machine

6. Outline Network-layer functions Network-layer services
IP forwarding ++

7. Outline Network-layer functions Network-layer services
IP forwarding ++

8. Alice router router Bob
h.v.=1
Alice
router
router
Bob

9. forwarding table
forwarding table

10. header value
output link
header value
output link 1 3 1 2 1 1 2 2 2 2 3 1 3 4 1 1
h.v. =1 3 2 4 2 3

11. Forwarding
Local router process that determines output link for each packet
How:
read value from packet’s network-layer header
search forwarding table

12. 1 1 3 2 4 2 3 routing manually connection setup 1 3 1 2 1 1 2 2 2 2 3
header value
output link
header value
output link 1 3 1 2 1 1 2 2 2 2 3 3 3 4 1 1 3 2 4 2 3

13. routing algorithm

14. routing algorithm
routing algorithm

15. Routing
Network-wide process that determines end-to-end path for each packet
How:
centralized routing algorithm
distributed routing algorithm

16. Connection setup
Path-wide process that sets up connection state in routers
How:
will see in a moment

17. Network-layer functions
Forwarding = determines output link for each packet
Routing = determines end-to-end path for each packet
Connection setup = sets up connection state in routers

18. Network-layer functions
Forwarding table populated by routing or connection setup
...depending on the type of network layer

19. Outline Network-layer functions Network-layer services
IP forwarding ++

20. Possible network-layer services
Guaranteed (in-order) delivery
Guaranteed maximum delay
Guaranteed minimum throughput
Security (confidentiality, authenticity)

21. Possible network-layer services
Guaranteed (in-order) delivery
Guaranteed maximum delay
Guaranteed minimum throughput
Security (confidentiality, authenticity)

22. 1 1 3 2 4 2 3 1 3 2 1 4 1 setup Alice - Bob 100 Mbps Alice - BobVC
input link
output link VC
input link
output link 1 3 2 1 4 1 1 1
setup 3 2 4 2
Alice - Bob
100 Mbps
Alice - Bob
100 Mbps 3
VC #1
VC #1

23. 1 1 3 2 4 2 3 1 3 2 1 4 1 done Alice - Bob 100 Mbps VC #1 VC
input link
output link VC
input link
output link 1 3 2 1 4 1 1 1 3 2 4
done 2
Alice - Bob
100 Mbps 3
VC #1

24. 1 1 3 2 4 2 3 1 3 2 1 4 1 VC=1 VC #1 VC #1 VC input link output link

25. millions of concurrent connections 2 10 5 3 3 24 4 4 2 5 1 7 6 16 7 7 VC
input link
output link 1 3 2
millions of concurrent connections 2 10 5 3 3 24 4 4 2 5 1 7 6 16 7 7 16 3

26. Virtual-circuit networks
Useful for network-layer guarantees
Forwarding state: per connection
VC #, input link, output link
populated by connection setup

27. Internet network-layer services
Best-effort delivery

28. dest. address
output link
dest. address
output link 1 3 1 2 1 1 2 2 2 2 3 3 3 4 1 1
dest=1 3 2 4 1 2 3

29. dest. address
output link
dest. address
output link 2 1 1 2 3 3
otherwise 1
otherwise 4 1 1
dest=1 3 2 4 1 2 3

30. dest. address range
output link
0 - 3
00** 1
4 - 7
01** 2
8 - 11
10** 3
11** 4
1110
0100

31. dest. address range
output link
0 - 2
00** 1 3
0011
0011 2
4, 6, 7
01**
0100, 0110, 0111 3 5
0101
0101 2
8 - 15
1*** 4
0011
0101
0010

32. dest. address range
output link
0 - 1
000* 1
2 - 3
001* 2
4 - 7
01** 3 8
1000
1000 2
9 - 15
1*** 4
prefixes
1000
1010

33. Datagram networks Appropriate for best-effort service
Forwarding state: per destination prefix
destination prefix, output link
populated by routing process

34. Why the datagram approach?
It scales better
no per-connection state in routers
It keeps the network simpler
no support for connection setup and teardown in routers

35. dest. address range
output link
0 - 3
00** 1
4 - 7
01** 2
8 - 11
10** 3
11** 4

36. dest. address range
output link
0 - 2
00** 1 3
0011
0011 2
4, 6, 7
01**
0100, 0110, 0111 3 5
0101
0101 2
8 - 15
1*** 4

37. dest. address range
output link
0 - 1
000* 1 2
0010
0010 2 3
0011
0011 3
4, 6, 7
01**
0100, 0110, 0111 2 4 5
0101
0101
8 - 15
1*** 1 10
1010
1010 3

38. Location-dependent addresses
Address embeds location information
address proximity implies location proximity
Significantly reduces forwarding state
per destination prefix
(otherwise, it would be per destination)

39. Outline Network-layer functions Network-layer services
IP forwarding ++

40. IP address format IP address = number from 0 to 232-1

41. IP prefix format IP prefix = range of IP addresses 223.1.1.0 / 24 mask
/ 24
mask
******** *

42. IP prefix format IP prefix = range of IP addresses 223.1.1.74 / 24
/ 24
mask
******** *

43. IP prefix format IP prefix = range of IP addresses 223.1.1.74 / 24
/ 24
mask
/ 24
/ 24 *

44. IP prefix format IP prefix = range of IP addresses 223.1.1.0 / 8 mask
/ 8
mask
******** ******** ********
223.*.*.*

45. IP prefix format IP prefix = range of IP addresses 223.1.1.0 / 12 mask
/ 12
mask
**** ******** ********
from:
from:
to:
to:

46.
/24
IP subnet
/24
/24
/24
/24
/24

47. /24
dest=
dest=
/24
/24

48. IP subnet
(Informal) Contiguous network area that does not “include” any routers
All its end-systems and incident routers have IP addresses from the same IP prefix

49.
/24
IP subnet
/24
/24
/24
/24
/24

50. IP address assignment Each organization obtains IP prefixes
from its ISP or from regulatory body
Network operator assigns IP addresses
to router interfaces: manually
to end-systems: manually or through DHCP

51. 223.1.1.0/24 10.0.0.0/24 private address space IP dst=223.1.9.2
IP dst=
IP src=
IP dst=
IP src=
/24
private address space
IP dst=
IP src=

52. Network Address Translation (NAT)
IP address
origin TCP port
756
954
new TCP port
/24
IP dst=
IP src=
TCP dst port=954
IP dst=
IP src=
TCP dst port=756
IP dst=
IP src=
TCP src port=954
NAT gateway
/24
Network Address Translation (NAT)
IP dst=
IP src=
TCP src port=756

53. Network Address Translation
IP subnet uses private address space
Attached to NAT gateway with public IP
rewrites private source IP address with its own
encodes original source into source port

54. Network Address Translation
Resolves IP address depletion problem
Requires per-connection state

56. IP fragmentation & reassembly
When packet size exceeds MTU
router fragments it
adds an identifier to each fragment
The end-system reassembles the fragments
before passing the packet to the transport layer

57. IP forwarding Choose the output link Perform IP fragmentation
look up destination IP address in forwarding table
perform longest prefix matching
Perform IP fragmentation
when packet size exceeds MTU
reassembly happens at the destination end-system

58. Network Address Translation
IP subnet uses private address space
Attached to NAT gateway with public IP
rewrites private source IP address with its own
encodes original source into source port