1. Introduction to Computer Networks
Yan Yunyi, Ph.D
Associate Prof.
Xidian University

2. Chapter 5
The Network Layer

3. Network Layer Design Isues
Store-and-Forward Packet Switching
Services Provided to the Transport Layer
Implementation of Connectionless Service
Implementation of Connection-Oriented Service
Comparison of Virtual-Circuit and Datagram Subnets

4. Store-and-Forward Packet Switching
fig 5-1
The environment of the network layer protocols.

5. Services Provided to the Transport Layer
Goals:
1) The services should be independent of the router technology.
2) The transport layer should be shielded from the number, type, and
topology of the routers present.
3) The network addresses made available to the transport layer should use a
uniform numbering plan, even across LANs and WANs.
Service Kinds
1) Connectionless Service--Internet community
the hosts should accept the fact that the network is unreliable ----Internet
2) Connection-Oriented Service--telephone companies
subnet should provide a reliable, connection-oriented service----ATM

6. Implementation of Connectionless Service
Routing within a diagram subnet.

7. Implementation of Connection-Oriented Service① ② ② ② ① ① ① ①
To: C
Label:1
VC 1:H1H2
From: H1
Label:1
VC 2:H3H2
Label Switching
Routing within a virtual-circuit subnet.

8. Comparison of Virtual-Circuit and Datagram Subnets
5-4

9. Routing Algorithms Properties Desirable for Routing Algorithms
The Optimality Principle
Shortest Path Routing
Distance Vector Routing

10. Properties Desirable for Routing Algorithms
correctness, simplicity, robustness, stability, fairness, and optimality.
Robustness
Once a network starts, it is expected to work continuously without system-wide
failure. All kinds of hardware and software failures cannot affect the network.
Stability
A stable algorithm reaches equilibrium and stays there.
Fairness and optimality

11. Properties Desirable for Routing Algorithms
Conflict between fairness and optimality.

12. Routing Algorithms Two major classes: nonadaptive and adaptive.
Nonadaptive algorithms: do not base their routing decisions on measurements or estimates of the current traffic and topology. Instead, the choice of the route to use is computed in advance, off-line, and downloaded to the routers when the network is booted. This procedure is sometimes called static routing.
Adaptive algorithms: change their routing decisions to reflect changes in the topology, and usually the traffic as well.

13. The Optimality Principle
Optimality Principle: if router J is on the optimal path from router I to router K,
then the optimal path from J to K also falls along the same route.
without regard to network topology or traffic
(a) A subnet. (b) A sink tree for router B.

14. The geographic distance in kilometers
Shortest Path Routing
How to measure path length?
The geographic distance in kilometers
The number of hops
mean queue length
transmission delay

15. Shortest Path Routing
The first 5 steps used in computing the shortest path from A to D. The arrows indicate the working node.

16. Distance Vector Routing
(a) A subnet. (b) Input from A, I, H, K, and the new routing table for J.

17. Distance Vector Routing (2)
NOYES A:
YESNO
The count-to-infinity problem.

18. Congestion Control Algorithms
General Principles of Congestion Control
Congestion Prevention Policies
Congestion Control in Virtual-Circuit Subnets
Congestion Control in Datagram Subnets
Load Shedding
Jitter Control

19. Congestion
When too much traffic is offered, congestion sets in and performance degrades sharply.

20. General Principles of Congestion Control
Congestion Control: Open loop and close loop.
Close loop:
Monitor the system .
detect when and where congestion occurs.
Pass information to where action can be taken.
Adjust system operation to correct the problem.

21. General Principles of Congestion Control
Monitor what:
the percentage of all packets discarded,
the average queue lengths,
the number of packets retransmitted,
the average packet delay,
the standard deviation of packet delay
In all cases, rising numbers indicate growing congestion.

22. General Principles of Congestion Control
How to transfer the information about the congestion:
routers in congestion send a packet to the traffic source or sources, announcing the problem.
b) routers fill in a bit or field reserved in every packet whenever congestion.
c) hosts or routers periodically send probe packets out to explicitly ask about congestion.

23. Congestion Prevention Policies
5-26
Policies that affect congestion.

24. Congestion Control in Virtual-Circuit Subnets
(a) A congested subnet. (b) A redrawn subnet, eliminates congestion and a virtual circuit from A to B.

25. Hop-by-Hop Choke Packets
Congestion Control in Datagram Subnets
Hop-by-Hop Choke Packets
(a) A choke packet that affects only the source. (b) A choke packet that affects each hop it passes through.

26. Load Shedding wine milk old is better than new new is better than old
Drop the packets:
wine
old is better than new
FTP
milk
new is better than old
Multimedia

27. Jitter Control (a) High jitter. (b) Low jitter.
For applications such as audio and video streaming, it does not matter much how long delayed as long as the transit time is constant. But higher jitter is unacceptable.
(a) High jitter (b) Low jitter.

28. Quality of Service Requirements
Techniques for Achieving Good Quality of Service
Integrated Services
Differentiated Services
Label Switching and MPLS

29. How stringent the quality-of-service requirements are.
5-30
How stringent the quality-of-service requirements are.

30. Flows in Four categories
ATM networks classify flows in four broad categories
with respect to their QoS demands as follows:
1) Constant bit rate (e.g., telephony).
2) Real-time variable bit rate
(e.g., compressed videoconferencing).
3) Non-real-time variable bit rate
(e.g., watching a movie over the Internet).
4) Available bit rate (e.g., file transfer).

31. Techniques for Achieving Good Quality of Service
Overprovisioning
Buffering
Traffic Shaping

32. Smoothing the output stream by buffering packets.

33. The Leaky Bucket Algorithm
(a) A leaky bucket with water. (b) a leaky bucket with packets.

34. The Token Bucket Algorithm
5-34
(a) Before (b) After.

35. The Leaky Bucket Algorithm
Token Bucket
(a) Input to a leaky bucket. (b) Output from a leaky bucket. Output from a token bucket with capacities of (c) 250 KB, (d) 500 KB, (e) 750 KB, (f) Output from a 500KB token bucket feeding a 10-MB/sec leaky bucket.
Token Bucket
Token Bucket
Token + Leaky Bucket

36. Internetworking How Networks Differ How Networks Can Be Connected
Concatenated Virtual Circuits
Connectionless Internetworking
Tunneling
Internetwork Routing
Fragmentation

37. Connecting Networks A collection of interconnected networks.
SNA (IBM:System Network Architecture)
FDDI (Fiber Distributed Data Interface)
A collection of interconnected networks.

38. Some of the many ways networks can differ.
How Networks Differ
5-43
Some of the many ways networks can differ.

39. How Networks Can Be Connected
Data Link Layer,
MAC Address,
Not need Network protocol
Network Layer,
IP Address,
Network protocol
(a) Two Ethernets connected by a switch. (b) Two Ethernets connected by routers.

40. Concatenated Virtual Circuits
Internetworking using concatenated virtual circuits.

41. Connectionless Internetworking
A connectionless internet.

42. Tunneling a packet from Paris to London.

43. Tunneling a car from France to England.

44. (a) Transparent fragmentation. (b) Nontransparent fragmentation.

45. Fragmentation (2)
Fragmentation when the elementary data size is 1 byte.
(a) Original packet, containing 10 data bytes.
(b) Fragments after passing through a network with maximum packet size of 8 payload bytes plus header.
(c) Fragments after passing through a size 5 gateway.

46. The Network Layer in the Internet
The IP Protocol
IP Addresses
Internet Control Protocols
OSPF – The Interior Gateway Routing Protocol
BGP – The Exterior Gateway Routing Protocol
Internet Multicasting
Mobile IP
IPv6

47. Design Principles for Internet
Make sure it works.
Keep it simple.
Make clear choices.
Exploit modularity.
Expect heterogeneity.
Avoid static options and parameters.
Look for a good design; it need not be perfect.
Be strict when sending and tolerant when receiving.
Think about scalability.
Consider performance and cost.
Simple design to handle various hardware and facilities.

48. Collection of Subnetworks
The Internet is an interconnected collection of many networks.

49. The IP Protocol The IPv4 (Internet Protocol) header.
Header length in 32 bits
Do not
fragment
More
Fragments
Header +Data 4
Reliability or speed
Fragment belongs to which datagram
Fragment lies in where
TCP or UDP
Recomputed at each hop. why???
to limit lifetime (255s max)
The IPv4 (Internet Protocol) header.

50. The IP Protocol (2)
5-54
Some of the IP options.

51. IP Addresses
IP address formats.

52. IP Addresses (2)
Special IP addresses.

53. A campus network consisting of LANs for various departments.
Subnets
A campus network consisting of LANs for various departments.

54. A class B network subnetted into 64 subnets.
Subnet mask: ( )
For example:
First in Subnet0: ( )
First in Subnet1: ( )
First in Subnet2: ( )
First in Subnet3: ( ) ……
A class B network subnetted into 64 subnets.

55. NAT – Network Address Translation
to assign each company a single IP address for
Internet traffic. Within the company, every computer
gets a unique IP address for routing intramural traffic.
When a packet exits the company, a network address
translation (NAT) takes place.
Three private IP arranges:- Can not be used in Internet
– /8 (16,777,216 hosts)
– /12 (1,048,576 hosts)
– /16 (65,536 hosts)

56. NAT – Network Address Translation
True IP address
Private IP
Placement and operation of a NAT box.

57. NAT (2)
IP:port
:80
2001
:21
2002
:21
2003
:8008
2004 ……
Index
process
IP:port
For example:
Remote host
:5236
NAT
http send： :80
:2001
NAT
http receive：
Remote host
:2001
:80

58. Internet Control Message Protocol
5-61
The principal ICMP message types.

59. ARP– The Address Resolution Protocol
DLL hardware does not understand IP address, IP must be mapped onto certain DLL
address (MAC address)---ARP
Three interconnected /24 networks: two Ethernets and an FDDI ring.

60. Dynamic Host Configuration Protocol
Operation of DHCP.

61. OSPF – The Interior Gateway Routing Protocol
OSPF: Open Shortest Path First
(a) An autonomous system. (b) A graph representation of (a).

62. OSPF (2) The relation between ASes, backbones, and areas in OSPF.
on the backbone
wholly within
one area
connect two
or more areas
talk to routers in other ASes
The relation between ASes, backbones, and areas in OSPF.

63. OSPF (3) OSPF distinguishes four classes of routers:
1)Internal routers are wholly within one area.
2)Area border routers connect two or more areas.
3)Backbone routers are on the backbone.
4)AS boundary routers talk to routers in other ASes.
These classes are allowed to overlap

64. The five types of OSPF messeges.
5-66
The five types of OSPF messeges.

65. BGP – The Exterior Gateway Routing Protocol
BGP: Border Gateway Protocol
(a) A set of BGP routers (b) Information sent to F.

66. The IPv6 fixed header (required).
The Main IPv6 Header
The IPv6 fixed header (required).

67. The IPv6 fixed header (required).
The Main IPv6 Header
The IPv6 fixed header (required).

68. IP Address (v6) 8000:0000:0000:0000:0123:4567:89AB:CDEF::

69. Extension Headers
5-69
IPv6 extension headers.

70. The hop-by-hop extension header for large datagrams (jumbograms).
Extension Headers (2)
The hop-by-hop extension header for large datagrams (jumbograms).

71. Summary Datagram subnet and Virtual Circuit subnet
Shortest path routing and Distance Vector routing
Congestion Control, buffer, leaky bucket, token bucket
Quality of Service (QoS), reliability, delay, jitter, bandwidth
IPv4, IP address, IP subnet
NAT ARP DHCP