ITC242 Introduction to Data Communications Internet Operation

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

6
Two Key Network-Layer Functions

• forwarding move packets from routers 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 forwarding
8
The Internet Network layer

• Host, router network layer functions

Transport layer TCP, UDP
Network layer
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

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IP protocol IP Addresses

• Interface connection between host/router and
  physical link
• routers 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
223.1.2.9
223.1.1.4
223.1.1.3
223.1.1.1 11011111 00000001 00000001 00000001
223
1
1
1
11
IP Address Classes
Originally there were 5 classes
24
1
7
CLASS A 00000000-01111111(127) 1-126
Host-ID
0
Net ID
16
2
14
CLASS B 100000000-10111111 128-191 21416,384
Class B addresses
Host-ID
10
Net ID
8
3
21
CLASS C 11000000-11011111 192-223 2212,097,1
52
110
Net ID
Host-ID
4
28
CLASS D
1110
Multicast Group ID
5
27
CLASS E
11110
Reserved
A
B
C
D
0
232-1
12
IP AddressesExamples
Class A address www.mit.edu 18.181.0.31
(18lt128 gt Class
A) Class B address mekong.stanford.edu 171.
64.74.155
(128lt171lt12864 gt 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 organizations network ID.

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Subnets
16
2
14
CLASS B e.g. Company
Host-ID
10
Net ID
16
16
2
14
2
14
e.g. Site
Host-ID
0000
Host-ID
1111
10
Net ID
10
Net ID
Subnet ID (20)
Subnet Host ID (12)
Subnet ID (20)
Subnet Host ID (12)
16
16
2
14
2
14
e.g. Dept
10
Net ID
Host-ID
Host-ID
1111011011
10
Net ID
000000
Subnet ID (26)
Subnet Host ID (6)
Subnet ID (22)
Subnet Host ID (10)
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Subnets
223.1.1.1

• IP address
• subnet part (high order bits)
• host part (low order bits)
• Whats a subnet ?
• device interfaces with same subnet part of IP
  address
• can physically reach each other without
  intervening router

223.1.2.1
223.1.1.2
223.1.2.9
223.1.1.4
223.1.2.2
223.1.1.3
223.1.3.27
subnet
223.1.3.2
223.1.3.1
network consisting of 3 subnets
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Subnets

• 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
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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

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Subnet Mask Calculations
20
Example of Subnetworking
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Subnet masks
Source http//zdnetasia.com/insight/network/0,390
44847,39372217,00.htm
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IP addresses how to get one?

• Q How does network get subnet part of IP addr?
• A gets allocated portion of its provider ISPs
  address space

ISP's block 11001000 00010111 00010000
00000000 200.23.16.0/20 Organization 0
11001000 00010111 00010000 00000000
200.23.16.0/23 Organization 1 11001000
00010111 00010010 00000000 200.23.18.0/23
Organization 2 11001000 00010111 00010100
00000000 200.23.20.0/23 ...
..
. . Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23 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-gtnetwork-gtconfiguration-gttcp
  /ip-gtproperties
• 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

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The Internet Network layer

• Host, router network layer functions

Transport layer TCP, UDP
Network layer
Link layer
physical layer
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The Problem
A
B
R2
R1
R4
R3
How does R1 choose a route to host B?
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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.

• 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.

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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,
1
1
4
R1
R6
R4
R2
2
3
2
2
R7
3
R5
2
R3
4
R8
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Example network
In this simple case, solution is clear from
inspection
A
1
1
4
R1
R6
R4
R2
2
3
2
2
R7
3
R5
2
R3
4
R8
B
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So what about this network...!?The public
Internet in 1999
Learn more at http//www.lumeta.com
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Technique 3 Dijkstras 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.

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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

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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

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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
  (ASs)
• Within an AS, the administrator chooses an
  Interior Gateway Protocol (IGP)
• Examples of IGPs RIP (rfc 1058), OSPF (rfc
  1247).
• Between ASs, the Internet uses an Exterior
  Gateway Protocol
• ASs today use the Border Gateway Protocol, BGP-4
  (rfc 1771)

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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

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Interconnected ASes

• 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

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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?

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
3a
3b
2a
AS3
AS2
1a
AS1
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!

x
44
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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
iBGP session
3a
3b
2a
AS3
AS2
1a
AS1
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
iBGP session
3a
3b
2a
AS3
AS2
1a
AS1
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 Dijkstras 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

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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 ASs.

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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.

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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
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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

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Types of Twisted Pair

• STP (shielded twisted pair)
• the pair is wrapped with metallic foil or braid
  to insulate the pair from electromagnetic
  interference

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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

70
Fiber Optic Types

• single-mode 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.

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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.