Internetworking

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					Lecture 2 - Revision
 IP Addressing




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Internetworking
 Networks provide
   – an effective communications capability and
   – resource sharing.
 Internetworking allows computers
   – from different vendors,
   – with different operating systems,
   – using different transmission media,
   – on different networks,
   – in different continents to co-operate.
                                                  2
Internetworking
 Internetworking devices offer
  communication between local area network
  segments.
 The four primary types of these devices are
  –   repeaters,
  –   bridges,
  –   routers, and
  –   gateways.
                                                3
Internetworking
 Repeaters connect LANs at OSI layer 1
  (physical layer),
 bridges connect LANs at OSI layer 2 (data
  link layer),
 routers connect LANs at OSI layer 3
  (network layer),
 gateways connect LANs at OSI layers 4
  through 7 (transport through application
  layers).                                    4
Internetworking
 Used to connect
  – LAN-to-LAN
  – LAN-to-WAN
  – WAN-to-WAN




                    5
IWUs on the OSIRM
 Application

 Presentation
                Gateway
 Session

 Transport

  Network       Router

 Data link      Bridge

 Physical       Repeater   6
Computers
 Every computer has a unique way of
  identifying itself.
 Every computer whether it is attached to a
  network or not has a physical address.
 No two physical addresses are ever alike.
 Referred to as the Media Access Control
  Address or MAC Address, the physical
  address is located on the Network Interface
  Card or NIC Card.                           7
Computers
 Before it leaves the factory, each NIC card is
  assigned a physical address by the hardware
  manufacturer.
 This address is programmed into a chip on the
  NIC card.
 Since the MAC address is located on the NIC
  card, if the NIC card were replaced in a computer,
  the physical address of the station would change to
  that of the new MAC address
                                                    8
Computers
 MAC addresses are written using
  hexadecimal (base 16) numbers.
 You will see two formats for MAC
  addresses (48 bits):
  – 0000.0c12.3456 or
  – 00-00-0c-12-34-56.
 NIC - layer 2 device

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IWUs
 Repeaters:
  – Simple two-way amplifiers.
  – Clean up, amplify, and pass on bits.
  – Used to extend the length of LANs.
  – Media dependent and protocol independent
  – Functionality at the physical layer of the OSI
    framework.
  – Normally confined to a single building.
                                                     13
IWUs/ Repeaters
 – Since they provide no traffic isolation, you
   must make sure that the entire set of networks
   connected by repeaters can carry the traffic
   from all of the computers on it.
 – Since they generally provide no network
   monitoring tools, you will not want to use
   repeaters for a link that is likely to fail.
 – Maximum 4 Repeaters between source and
   destination (Ethernet)
                                                    14
IWUs/ Repeaters
 A simple repeater
  – operates at a very low level indeed.
  – Its primary purpose is to get around limitations
    in cable length caused by signal loss or timing
    dispersion.
  – allows you to construct somewhat larger
    networks than you would otherwise be able to
    construct.
  – a two-way amplifier.
                                                   15
IWUs/Repeaters
 – passes on individual bits in the signal, without
   doing any processing at the packet level.
 – even passes on collisions.
 – There is a limit to the number of repeaters that
   you can use in a network.
 – The basic Ethernet design requires that signals
   must be able to get from one end of the network
   to the other within a specified amount of time.
   This determines a maximum allowable length.
                                                 16
Repeaters
 A buffered repeater
  – operates at the level of whole data packets.
  – Rather than passing on signals a bit at a time, it
    receives an entire packet from one network into
    an internal buffer and then retransmits it onto
    the other network.
  – It does not pass on collisions.


                                                     17
Repeaters
 – Because such low-level features, as collisions
   are not repeated, the two networks continue to
   be separate as far as the Ethernet specifications
   are concerned.
 – Thus there are no restrictions on the number of
   buffered repeaters that can be used.
 – Indeed there is no requirement that both of the
   networks be of the same type.
 – However the two networks must be sufficiently
   similar that they have the same packet format. 18
Repeaters
  – Generally this means that buffered repeaters
    can be used between two networks of the IEEE
    802.x family, or two networks of some other
    related family.
 Multi-port repeaters are often called hubs.
 Hubs are very common internetworking devices.
  Generally speaking, the term hub is used instead
  of repeater when referring to the device that serves
  as the centre of a star topology network
                                                    19
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IWUs/ Bridges
 Bridges:

  –   Store and transmit packets.
  –   Functionality at DLL.
  –   Media dependent
  –   Protocol independent above the DL layer
  –   Can connect networks with similar Network
      layer different DL Layer. (e.g. 802.x-to-802.x
      and 802.x-to-802.y)
                                                       21
Bridges
 – Can reduce traffic load on the network
 – The use of more repeaters (for Ethernet) is
   made possible




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Bridges
 While bridges can determine whether the
  destination MAC address carried by data is
  not part of the same network segment as its
  source, it makes no determination as to
  what network segment the data should be
  sent to.
 Instead, bridges indiscriminately pass data
  along to all other segments of the network.
                                                25
Bridges
 Forward broadcasts
 may cause broadcast storms




                               26
IWUs/ Routers
 Routers:

  – Used when routing is required.
  – Operate at Network layer.
  – Connect networks with similar transport layers
    and dissimilar network layers.
  – media independent
  – protocol dependent above DL layer (if designed
    to process IP packets cannot process IPX
    packets)                                       27
Routers
 more complex & expensive than bridges
 require more configuration
 Routable protocols: TCP/IP, IPX/SPX.
 Non-routable: NetBEUI




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Routers
 Routers base their decisions on network and
  sub-network addresses rather than node
  addresses.
 This is done through ANDing a note
  address with the corresponding subnet mask
  to obtain the network/subnet address
 e.g. 158.94.16.1 AND 255.255.240.0 gives
  158. 94.16.0
                                            31
Routable and routing protocols
 A routable (routed) protocol is a protocol
 that contains enough network layer
 addressing information for user traffic to be
 directed from one network to another one.
 Routable protocols define the format and
 use of fields within a packet. Packets are
 delivered between networks.

                                               32
Routable protocols
 Examples of routable protocols
  – the Internet protocol (IP),
  – Novell's IPX,
  – AppleTalk,
  – DECNet, and
  – XNS.
  – Protocols such as DEC's local area transport
    (LAT) and Microsoft's NetBEUI are not
    routable.                                      33
Routing protocols
 A routing protocol supports routed protocols to
  carry messages between networks.
 Routing protocols are used exchange information
  between routers, but they do not carry any user
  traffic.
 The exchange of information between routers is
  used to update routing tables maintained by
  routers and calculate the best path for packet
  transmission.
                                                    34
Routing Protocols
 Best known examples of routing protocols
 (Autonomous Systems)
  – Routing Information Protocol (RIP),
  – Interior Gateway Routing Protocol (IGRP),
  – Extended Interior Gateway Routing Protocol
    (EIGRP), and
  – Open Shortest Path First (OSPF).


                                                 35
Routing
 Static Routes
   – manually defined by the system administrator
     as the next hop to a destination.
   – useful for security and traffic reduction.
   – May contain alternative routes.
 Default Routes
   – Manually defined by the system administrator
     as the path to take when no route to the
     destination is known.
 Dynamic Routing                                   36
Routing Protocols
 Interior gateway protocols (IGPs): Routing
  protocols that run inside an enterprise.
  – Examples: RIP, IGRP, EIGRP, and OSPF.
 Exterior gateway protocols (EGPs): Protocols
  that run outside an enterprise, or between
  autonomous systems (AS).
  – Used to exchange routing information between
    ISPs, or in some cases between a customer's AS
    and the provider's network.
  – Border Gateway Protocol, version 4 (BGP4), is the
    most common EGP and is considered the Internet
    standard.                                       37
Interior routing protocols
 RIP:
  – A distance-vector routing protocol (also known
    as Bellman-Ford algorithms).
  – originally specified in RFC 1058.
  – Key characteristics:
      • Hop count is used as the metric for path selection.
        The maximum allowable hop count is 15. Routing
        updates are broadcast every 30 seconds by default.
   – Most widely used protocol on the Internet
                                                              38
Interior routing protocols
 IGRP:
   – Cisco’s distance-vector routing protocol. The
     distance-vector routing approach determines
     the direction (vector) and distance to any link in
     the internetwork.
   – IGRP sends routing updates at 90-second
     intervals that advertise networks for a particular
     autonomous system.

                                                     39
IGRP
 some key characteristics:
   – automatically handling indefinite, complex
     topologies.
   – Flexibility for segments having different
     bandwidth and delay characteristics.
   – Scalability to function in very large networks.



                                                       40
IGRP
 The IGRP routing protocol uses a
  combination of variables to determine a
  composite metric.
  –   Bandwidth
  –   delay
  –   load
  –   reliability
  –   maximum transmission unit (MTU) - the
      maximum packet size, in bytes, that a particular
      interface can handle.                          41
OSPF
 OSPF:
  – A link-state routing protocol.
  – The link-state (also called shortest path first)
    approach recreates the exact topology of the
    entire internetwork (or at least of the partition
    in which the router is situated).



                                                        42
EIGRP
 EIGRP:
  – A balanced hybrid routing protocol.
  – The balanced hybrid approach combines
    aspects of the link-state and distance vector
    algorithms.




                                                    43
Routing algorithms
 Distance-vector routing:
  – Problem:
     • Routing Loops: Routing loops can occur if the
       network’s slow convergence on a new configuration
       causes inconsistent routing entries.




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Routing
 Routing loops
  – symptom: count to infinity
      • solution: define a maximum
      • solution: split horizon
      • solution: hold down timers




                                     46
BGP
 Used to advertise network addresses
  between ISPs and ASs
 In general, it is necessary to use BGP to
  connect to an ISP only when you have
  different policy requirements than the
  ISP.


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BGP
 BGP supports two types of sessions,
 each with slightly different configuration
 requirements:
  – Externa BGP (EBGP) session
  – Internal BGP (IBGP) session




                                          49
EBGP
 Occurs between routers in two different
 autonomous systems. These routers
 are usually adjacent to one another,
 sharing the same medium and a subnet




                                            50
IBGP
 Occurs between routers in the same AS, and
  is used to coordinate and synchronize routing
  policy within the AS
 Neighbours may be located anywhere in the
  AS, even several hops away from one
  another.
 An IBGP session typically occurs between
  routers in the same AS in an ISP

                                              51
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IWUs/ Gateways
 Gateways:
  – Also known as protocol converters.
  – May operate up to the Application layer, mostly
    at the upper three layers of the OSI model
  – Connect any type of a network, even when one
    might not be OSI.
  – example: can translate between Simple Mail
    Transfer Protocol (SMTP) and the Message
    Handling System (MHS)
                                                 53
Gateways
  – example: translate TCP/IP to and from
    IPX/SPX
  – processor intensive and slow, creates a
    bottleneck
  – difficult to install and maintain
 Note: The Internet community uses the term
 gateway to refer to a router

                                              54
 OSI model for Repeater
End system                End system
Application               Application
Presentation              Presentation
Session                        Session
Transport                   Transport
Network                      Network
Data Link      Repeater     Data Link
Physical       Physical       Physical



                                         55
OSI model for Bridge
End system                 End system
Application                Application
Presentation               Presentation
Session                         Session
Transport                    Transport
Network         Bridge        Network
Data Link      Data Link     Data Link
Physical       Physical        Physical




                                          56
An OSI Model of a Router
End system                 End system
Application                Application
Presentation               Presentation
Session                         Session
Transport       Router       Transport
Network        Network        Network
Data Link      Data Link     Data Link
Physical       Physical        Physical




                                          57
An OSI model of a Gateway
End system      Gateway       End system
Application    Application    Application
Presentation   Presentation   Presentation
Session          Session           Session
Transport       Transport       Transport
Network         Network          Network
Data Link       Data Link       Data Link
Physical         Physical         Physical




                                             58
IWU: Selection Criteria
 Bridges and gateways should be placed
 sufficiently frequently to break your
 network into pieces for which the traffic
 volume is manageable. You may want to
 place bridges or gateways in places where
 traffic would not require them for network
 monitoring reasons.

                                              59
 Because bridges must pass broadcast
 packets, there is a limit to the size network
 you can construct using them. It is probably
 a good idea to limit the network connected
 by bridges to a hundred systems or so. This
 number can be increased somewhat for
 bridges with good facilities for filtering.

                                            60
 Because certain kinds of network
  misbehaviour will be passed, bridges should
  be used only among portions of the network
  where a single group is responsible for
  diagnosing problems.




                                            61
 Do not use a bridge between networks
  owned by different organisations.
 Portions of your network where
  experiments are being done in network
  technology should always be isolated from
  the rest of the network by gateways.


                                              62
 For many applications it is more important
  to choose a product with the right
  combination of performance, network
  management tools, and other features than
  to make the decision between bridges and
  gateways.


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    158.94.50                             158.94.51                    158.94.52




                158.94.50.1       158.94.51.1         158.94.51.2    158.94.52.1
A                             Gateway 1                         Gateway 2          B
                                                                                   B




                                                                                       64
Configuring TCP/IP (Windows)
 Win 9x does not install support for TCP/IP
  by default
 Install TCP/IP if needed
 Configure TCP/IP




                                               65
Configuring TCP/IP
 Allocate an IP address
  – Obtain an IP address automatically. A
    Dynamic Host Configuration Service (DHCS)
    is required
  – Specify and IP address: Enter IP address and
    Subnet Mask



                                                   66
TCP/IP (WINS)
 Using the Windows Internet Name Service
 (WINS) (can resolve TCP/IP host names)
  – Disable WINS Resolution: No WINS server or
    client is not required to use WINS server
  – Enable WINS Resolution: Enter IP address for
    primary WINS server and Secondary WINS
    server (if applicable)


                                                   67
TCP/IP
 TCP/IP Gateway (refers to a router)
  – add/remove gateways used by the Win 95 client
  – Enter IP address for the default gateway
  – Add IP addressesfor other gateways if
    applicable




                                                68
TCP/IP
 Binding the TCP/IP Protocol to Clients and
  Services
  – specify which clients and services use the
    TCP/IP transport protocol




                                                 69
TCP/IP
 Setting TCP/IP as default protocol
 Using the Domain Name Service to Resolve
  IP Addresses
  – enable/disable the use of DNS
  – DNS translates character based addresses (e.g.
    nw.mdx.ac.uk) to numeric IP addresses (e.g.
    158.95.59.200)


                                                     70
Internetworking protocols in
context
                  MIME


 BGP   FTP HTTP SMTP     TELNET
                                  SNMP



            TCP                   UDP

                                         ICMP   OSPF




                          IP

                                                       71
 MIME: Multi-purpose Internet Mail
  Extension
 BGP: Border Gateway Protocol (for routers)
 ICMP: Internet Control Message Protocol
 OSPF: Open Shortest Path First Protocol
  (router protocol)


                                           72
Principles of Internetworking
 Providing a link between networks
 Providing for routing and delivery of data
 Providing an accounting service
 Provided the services listed above without
  modifying the network architecture



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