1. ITC242 – Introduction to Data Communications Internet Operation

2. Last Week
SMTP - transmits messages to appropriate hosts via TCP, attempts to provide error-free transmission.
MIME - Intended to resolve problems with SMTP, provides info about body of message, defines multiple content formats, and encodings
HTTP - Stateless protocol, flexible format handling, Proxy, Gateway, Tunnel, Cache
SIP - Manages real-time sessions over IP, enable Internet telephony/VoIP, HTTP-like request/response transaction model

3. Last Week
Client/server - user-friendly client applications, centralized databases, open and modular applications, the network is fundamental
Intranet - internet-based client/server technology within an organization, immensely successful
Extranets – Extend intranet concept to outside community, e.g customers and suppliers, enables sharing of information between companies, TCP/IP enabled form of EDI.

4. Topic 8 – Internet Operation
Learning Objectives
Describe the characteristics of an Internet Address
Describe the different classes of IP addresses
Explain the purpose of subnet masks.

5. Network Layer transport segment from sending to receiving host
on sending side encapsulates segments into datagrams
on rcving side, delivers segments to transport layer
network layer protocols in every host, router
router examines header fields in all IP datagrams passing through it
application
transport
network
data link
physical
network
data link
physical
application
transport
network
data link
physical

6. Two Key Network-Layer Functions
forwarding: move packets from router’s input to appropriate router output( within a single router)
routing: determine route taken by packets from source to dest.
routing algorithms
analogy:
routing: process of planning trip from source to dest
forwarding: process of getting through single interchange

7. Interplay between routing and forwarding1 2 3
0111
value in arriving
packet’s header
routing algorithm
local forwarding table
header value
output link
0100
0101
1001

8. The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
IP protocol
addressing conventions
datagram format
packet handling conventions
Routing protocols
path selection
RIP, OSPF, BGP
Network
layer
forwarding
table
ICMP protocol
error reporting
router “signaling”
Link layer
physical layer

9. IP protocol: IP Addresses
IP (Version 4) addresses are 32 bits long
IP addresses are hierarchical
They contain a network ID and a host ID
IP addresses are assigned statically or dynamically (e.g. DHCP)
IP (Version 6) addresses are 128 bits long

10. IP protocol: IP Addresses
Interface: connection between host/router and physical link
router’s typically have multiple interfaces
host typically has one interface
IP addresses associated with each interface
Every interface has a unique IP address:
A computer might have two or more IP addresses
A router has many IP addresses =
223 1 1 1

11. IP Address Classes A B C D Originally there were 5 classes: CLASS “A”1 7 24
CLASS “A”
(127): 1-126
Net ID
Host-ID 2 14 16
CLASS “B” :
2^14=16,384 Class B addresses 10
Net ID
Host-ID 3 21 8
CLASS “C” :
2^21=2,097,152
110
Net ID
Host-ID 4 28
CLASS “D”
1110
Multicast Group ID 5 27
CLASS “E”
11110
Reserved A B C D
232-1

12. IP Addresses Examples Class “A” address: www.mit.edu 18.181.0.31
Class “B” address: mekong.stanford.edu
(128<171< => Class B)

13. IP Address Some Problems:
Address classes were too “rigid”. For most organizations, Class C were too small and Class B too big. Led to inefficient use of address space, and a shortage of addresses.
Small organizations wanted Class B in case they grew to more than 255 hosts. But there were only about 16,000 Class B network IDs.

14. Solution ?
Subnetting within an organization to subdivide the organization’s network ID.

15. Subnets CLASS “B” e.g. Company 10 Net ID Host-ID e.g. Site2 14 16 10
Net ID
Host-ID 2 14 16 2 14 16
e.g. Site 10
Net ID
0000
Host-ID 10
Net ID
1111
Host-ID
Subnet ID (20)
Subnet
Host ID (12)
Subnet ID (20)
Subnet
Host ID (12) 2 14 16 2 14 16
e.g. Dept 10
Net ID
000000
Host-ID 10
Net ID
Host-ID
Subnet ID (22)
Subnet
Host ID (10)
Subnet ID (26)
Subnet
Host ID (6)

16. Subnets IP address: What’s a subnet ? subnet part (high order bits)
host part (low order bits)
What’s a subnet ?
device interfaces with same subnet part of IP address
can physically reach each other without intervening router
subnet
network consisting of 3 subnets

17. Subnets
/24
/24
/24
Recipe
To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet.
Subnet mask: /24

18. Subnets & Subnet Masks
Allows for subdivision of internets within an organization
Each LAN can have a subnet number, allowing routing among networks
Host portion is partitioned into subnet and host numbers

19. Subnet Mask Calculations

20. Example of Subnetworking

21. Subnet masks
Source:

22. IP addresses: how to get one?
Q: How does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s address space
ISP's block /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23
a.b.c.d/x where x bits constitute the network portion of
The IP address, and often referred to as the prefix of the address

23. IP addresses: how to get one?
Q: How does host get IP address?
hard-coded by system admin in a file
Wintel: control-panel->network->configuration->tcp/ip->properties
DHCP: Dynamic Host Configuration Protocol: dynamically get address from a server
“plug-and-play”
Goal: allow host to dynamically obtain its IP address from network server when it joins network
Can renew its lease on address in use
Allows reuse of addresses (only hold address while connected an “on”
Support for mobile users who want to join network

24. IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
allocates addresses
manages DNS
assigns domain names, resolves disputes

25. The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
IP protocol
addressing conventions
datagram format
packet handling conventions
Routing protocols
path selection
RIP, OSPF, BGP
Network
layer
forwarding
table
ICMP protocol
error reporting
router “signaling”
Link layer
physical layer

26. The Problem
“A”
“B” R2 R1 R4 R3
How does R1 choose a route to host B?

27. Routing Metrics Metrics
Delay to send an average size packet (Make high speed links attractive, but closeness counts)
Bandwidth
Link utilization
Stability: Is a link (or path) up or down?
Today: about 1/3 of Internet routes are asymmetric

28. Technique 1: Naïve Approach
Flood! -- Routers forward packets to all ports
except the ingress port. R1
Advantages:
Simple.
Every destination in the network is reachable.
Disadvantages:
Some routers receive a packet multiple times.
Packets can go round in loops forever.
Inefficient.

29. Technique 2: Bellman-Ford Algorithm
Objective: Determine the route from (R1, …, R7) to R8
that minimizes the cost.
Examples of link cost:
Distance, data rate, price, congestion/delay, … R1 1 1 4 R2 R4 R6 2 3 2 2 R7 3 R5 2 R3 4 R8

30. In this simple case, solution is clear from inspection
Example network
In this simple case, solution is clear from inspection A R1 1 1 4 R2 R4 R6 2 3 2 2 R7 3 R5 2 R3 4 R8 B

31. So what about this network...!? The public Internet in 1999
Learn more at

32. Technique 3: Dijkstra’s Shortest Path First Algorithm
The algorithm identifies the least costly paths between source and destination, given that costs are assigned to the edges.
Routers send out update messages whenever the state of a link changes. Hence the name: “Link State” algorithm.
Each router calculates lowest cost path to all others, starting from itself.

33. The problem
How to route in the Internet?

34. Internet Routing Protocols
Responsible for receiving and forwarding packets between interconnected networks
Must dynamically adapt to changing network conditions

35. Autonomous Systems (AS)
Key characteristics
Set of routers and networks managed by single organization
group of routers exchanging information via a common routing protocol
connected (in a graph-theoretic sense); that is, there is a path between any pair of nodes

36. Autonomous System Example

37. Directed Graph of Example

38. Routing in the Internet
The Internet uses hierarchical routing
The Internet is split into Autonomous Systems (AS’s)
Within an AS, the administrator chooses an Interior Gateway Protocol (IGP)
Examples of IGPs: RIP (rfc 1058), OSPF (rfc 1247).
Between AS’s, the Internet uses an Exterior Gateway Protocol
AS’s today use the Border Gateway Protocol, BGP-4 (rfc 1771)

39. Routing in the Internet
The Internet uses hierarchical routing
The Internet is split into Autonomous Systems (AS)
aggregate routers into regions, AS
routers in same AS run same routing protocol
“intra-AS” routing protocol
routers in different AS can run different intra-AS routing protocol
Gateway router
Direct link to router in another AS

40. Interconnected ASes 3b 1d 3a 1c 2a
AS3
AS1
AS2 1a 2c 2b 1b
Intra-AS
Routing
algorithm
Inter-AS
Forwarding
table 3c
forwarding table configured by both intra- and inter-AS routing algorithm
intra-AS sets entries for internal dests
inter-AS & Intra-As sets entries for external dests

41. Inter-AS tasks
AS1 must:
learn which dests reachable through AS2, which through AS3
propagate this reachability info to all routers in AS1
Job of inter-AS routing!
suppose router in AS1 receives datagram dest outside of AS1
router should forward packet to gateway router, but which one? 3b 1d 3a 1c 2a
AS3
AS1
AS2 1a 2c 2b 1b 3c

42. Example: Setting forwarding table in router 1d
suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2.
inter-AS protocol propagates reachability info to all internal routers.
router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c.
installs forwarding table entry (x,I) … x 3c 3a 2c 3b 2a
AS3 2b 1c
AS2 1a 1b
AS1 1d

43. Example: Choosing among multiple ASs
now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.
to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x.
this is also job of inter-AS routing protocol! … 3b 1d 3a 1c 2a
AS3
AS1
AS2 1a 2c 2b 1b 3c … x

45. Internet inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto standard
maintain a table of IP networks or 'prefixes' which designate network reachability among AS.
BGP provides each AS a means to:
Obtain subnet reachability information from neighboring ASs.
Propagate reachability information to all AS-internal routers.
Determine “good” routes to subnets based on reachability information and policy.
allows subnet to advertise its existence to rest of Internet: “I am here”

46. BGP basics
pairs of routers (BGP peers) exchange routing info over TCP connections: BGP sessions
when AS2 advertises prefix to AS1:
AS2 promises it will forward any addresses datagrams towards that prefix.
AS2 can aggregate prefixes in its advertisement
eBGP session 3c
iBGP session 2c 3a 3b 2a
AS3 2b 1c
AS2 1a 1b
AS1 1d

47. Distributing reachability info
using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1.
1c can then use iBGP do distribute new prefix info to all routers in AS1
1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session
when router learns of new prefix, creates entry for prefix in its forwarding table.
eBGP session 3c
iBGP session 2c 3a 3b 2a
AS3 2b 1c
AS2 1a 1b
AS1 1d

48. Intra-AS Routing Protocols
OSPF(Open Shortest Path First): A link-state protocal
Link-state updates sent (using flooding) as and when required. A router broadcasts routing information to all other routers in the AS, not just to its neighboring routers.
Every router locally runs Dijkstra’s algorithm to determine a shortest-path tree to all subnets.
Authenticated updates: all OSPF messages authenticated (to prevent malicious intrusion)
Autonomous system may be partitioned into “areas”.
hierarchical OSPF in large domains

49. Hierarchical OSPF

50. Hierarchical OSPF two-level hierarchy: local area, backbone.
Link-state advertisements only in area
each nodes has detailed area topology; only know direction (shortest path) to nets in other areas.
area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.
backbone routers: run OSPF routing limited to backbone.
boundary routers: connect to other AS’s.

51. Topic 9 – LAN architecture and protocols
Learning Objectives
Define the various types of Local Area Networks (LANs)
Discuss the different types of transmission media commonly used in LANs.

52. Backend & Storage Area Networks
“Computer room networks”
High data rate
High-speed interface
Distributed access
Limited distance
Limited number of devices

53. Storage Area Network (SAN)
A separate network to handle storage needs
Decouples storage tasks from specific servers
Creates a shared storage facility across a high-speed network

54. High-Speed Office Networks
Increased processing and transfer requirements in many graphics-intensive applications now require significantly higher transfer rates
Decreased cost of storage space leads to program and file bloat, increased need for transfer capacity
Typical office LAN runs at 10Mbps, high-speed alternatives run at 100Mbps, 1 Gbps, 10Gbps

55. Backbone Local Networks
Used instead of single-LAN strategy
Better reliability
Higher capacity
Lower cost

56. Factory Networks High capacity
Ability to handle a variety of data traffic
Large geographic extent
High reliability
Ability to specify and control transmission delays

57. Tiered LANs
Cost of attachment to a LAN tends to increase with data rate
Alternative to connecting all devices is to have multiple tiers
Multiple advantages
Higher reliability
Greater capacity (less saturation)
Better distribution of costs based on need

58. Tiered LAN Diagram

59. The Media
The Transmission Media is the physical path between transmitter and receiver
Can be classified as guided or unguided
For both transmission is with electromagnetic waves.
Guided Media – waves are guided along a solid medium, e.g. cables
Unguided Media – wireless transmission

60. Guided Media
Twisted Pair Wires
Coaxial Cable
Fibre Optic Cable

61. Twisted Pair Wires
Consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs
Often used at customer facilities and also over distances to carry voice as well as data communications
Low frequency transmission medium

62. Types of Twisted Pair STP (shielded twisted pair)
the pair is wrapped with metallic foil or braid to insulate the pair from electromagnetic interference

63. Types of Twisted Pair UTP (unshielded twisted pair)
each wire is insulated with plastic wrap, but the pair is encased in an outer covering

64. Ratings of Twisted Pair
Category 3 UTP
data rates of up to 16mbps are achievable
Category 5 UTP
data rates of up to 100mbps are achievable
more tightly twisted than Category 3 cables
more expensive, but better performance
Category 5e UTP – 1Gbps
Category 6 UTP- Up to 10 Gbps
STP
More expensive, harder to work with

65. Twisted Pair Advantages
Inexpensive and readily available
Flexible and light weight
Easy to work with and install

66. Twisted Pair Disadvantages
Susceptibility to interference and noise
Attenuation problem
For analog, repeaters needed every 5-6km
For digital, repeaters needed every 2-3km

67. Coaxial Cable (or Coax)
Used for cable television, LANs, telephony
Has an inner conductor surrounded by a braided mesh
Both conductors share a common center axial, hence the term “co-axial”
Traditionally used for LANs, but growth of twisted pair for local nets and optical fiber for larger nets has reduced coax use

68. Fiber Optic Cable
Fiber optic cable is used for modular light transmission. Instead of transmitting electrical signals, it transmits pulses of light that represent bits.
Advantages
Greater capacity
Smaller size/lighter weight
Lower attenuation
Electromagnetic isolation
Operate in the range of about 1014 to 1015 Hz; (portions of the infrared and visible spectrums)

69. Fiber Optic Layers consists of three concentric sections
plastic jacket
glass or plastic
cladding
fiber core

70. Fiber Optic Types single-mode fiber multimode fiber
A single-mode cable uses lasers to generate light. It allows just one mode of light to pass through it at a time, but is capable of greater bandwidth and greater distances than multimode cable. It is more expensive than multimode cable, and has a maximum cable length of 60 kilometers
multimode fiber
Multimode cable allows multiple light modes to pass along its fibers. Favored in workgroup applications, multimode cable uses light emitting diodes (LEDs) to generate light. A multimode fiber optic cable cannot exceed 2 kilometers.

71. Fiber Optic Signals fiber optic multimode step-index
fiber optic single mode

72. Comparison of Media
Twisted pair cable is a common cable type - it is available as shielded twisted pair (STP) or unshielded twisted pair (UTP). STP cable combines the techniques of twisting wires and shielding. UTP cable is a copper wire-based cable used in a variety of networks. Coaxial cable operates over relatively large distances, and transmits data at speeds of up to 100 Mbps. Installing coaxial cable is more expensive than installing twisted pair cable. Fiber optic cable transmits bits in the form of modulated light data. Light is refracted along the cable and can go around bends. Fiber optic cables are available as single-mode or multimode cable. Wireless signals are radio frequencies and infrared waves that can travel through air. They are a growth area in network communications and represent the future of communication media.