Networks II (Course Outline)

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Networks II (Course Outline)

• What this course covers
• Internet Basics
• The ISO layers model. The TCP/IP Protocol Stack.
• Basic Queuing Theory
• Notation Poisson, Deterministic and General
  queues.
• Littles Theorem, Markov Chains, Birth Death
  Processes, Generating Functions.
• Routing
• Network Structures, Dijsktra, Bellman-Ford and
  Frank-Wolfe Algorithms.
• Statistics in Networks
• Traffic Assumptions (Poisson, Heavy-Tail
  Distributions, Long-Range Dependence)

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Networks II (Course Aim)

• By the end of this course you should
• Have a working knowledge of how things find their
  way about the internet.
• Be able to understand the mathematics of queuing
  systems and routing.
• Understand research in the area of Network
  Engineering.
• Know some handy ways to investigate networks.
• This course will not teach you
• The practicalities of wiring networks or
  administrating networked computers.
• How to program networked applications.

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Networks II Recommended Texts

• Data Networks Bertsekas/Gallager
• Becoming out of date but a good introduction to
  networking with a mathematical bent. (Course
  recommended text).
• Computer Networks Tanenbaum
• Well known introductory text, more up to date but
  without the mathematical depth of the previous.
• Queueing Systems (I and II) Kleinrock
• A classic text introducing the heavy duty
  mathematics of Queuing Theory.
• TCP/IP Illustrated (I and II) Stephens
• The classic text if you actually need to
  understand and program using internet based
  protocols.

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This Lecture Internet Basics

• Basic terms we need to understand.
• The OSI/ISO (Open Systems Internconnection/Interna
  tional Standards Office) layers model of
  computer networks.
• The standard model to describe how computer
  networks should work.
• The TCP/IP (Transmission Control
  Protocol/Internet Protocol) Protocol Stack
• The standard model which is how computer networks
  actually work.

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Where to go for more information on this
lectures subjects

• RFCs (Requests for Comments) The protocols
  which define the internet
• http//www.rfc-editor.org/
• RFCs define how things work (but some are
  spurious, some are out of date and some are just
  jokes).
• IETF (Internet Engineering Task Force)
• http//www.ietf.org/
• Course texts
• Bertsekas/Gallager Layers Model Section 1.3
  IP Section 2.8 2.9
• Tanenbaum Layers Model Section 1.4 IP Section
  5.5

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Basic Definitions Protocol

• Protocol A formal specification of how things
  should communicate. In networking a protocol
  defines an interface usually (though not
  necessarily) between one computer and another.
• A simple example of a protocol Knock and Enter
• Knock on the door.
• Wait for someone to say Come in.
• If someone says Come in. then open the door and
  enter.
• If you wait for five minutes then give up.
• We might want to combine this with a protocol for
  saying Come in when you hear a knock.
• Two computers need to use the same protocol to
  talk to one another. The definition of protocols
  is critical to networking.

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Basic Definitions Bit, Byte, Octet, Packet,
Header, Bandwidth

• Bit A 0 or a 1 the basic unit of digital data.
• Byte A short collection of bits (usually assumed
  to be 8 bits but may, rarely, be 7, 16 or 32).
• Octet A collection of 8 bits.
• Packet A collection of bits in order assembled
  for transmission.
• Header Part of packet with info about contents.
• Bandwidth The amount of data which can be sent
  on a channel. Usually bits per second
  sometimes in bytes (octets) per second. (Yes
  this is confusing.)
• KB kilobytes. Kb kilobits.

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Basic Definitions Host, Router, Switch, Source,
Destination

• Host A machine which is a point on a network
  which packets travel through a node in a graph.
• Router A host which finds a route for packets to
  travel down an intermediate point in a journey.
• Switch Often used interchangeably with router
  but implies that the routes are fixed.
• Source Where data is coming from.
• Destination (or sink) Where data is going to.

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A Simple Model of Reliable Internet
Communications.

• To send data to another computer
• Find the address of the computer you are sending
  to.
• Break the data into manageable chunks (packets).
• Put the address on each packet (packet heard) and
  also your own address.
• Send each packet in return to the receiving
  computer.
• Get a receipt for each packet which has been
  sent.
• Resend packets for which we do not have a
  receipt.
• The receiver then reassembles the packets to
  retrieve the data sent.

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Models of the Internet
OSI/ISO Reference Model
TCP/IP Reference Model
Application Transport Internet Host-to-network
Application Presentation Session Transport Network
Data Link Physical
Model Layers
Open Systems Interconnection (International
Standards Office)
Transmission Control Protocol/ Internet Protocol
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1) Physical layer

• Purpose Necessary infrastructure.
• Think "wires in the ground and switches
  connecting them".
• This is the physical hardware of the internet.
• Wires/optical cables/wireless links and other
  technologies provide a way for transmission of
  raw bits (0s and 1s).
• Routers and switches connect these cables and
  direct the traffic.

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2) Data link layer

• Purpose Provides basic connection between two
  logically connected machines.
• Think I stuff packets down a wire to my
  neighbour
• Send raw packets between hosts.
• Basic error checking for lost data.
• In TCP/IP the "Physical layer" and the "Data
  Link" layer are grouped together and called the
  host-to-network layer.

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3) Network Layer/Internet Layer

• Purpose Provide end-to-end communication between
  any two machines.
• Think I try to get a packet to its destination
• Tells data which link to travel down.
• Addresses the problem known as routing.
• Deals with the question "where do I go next to
  get to my destination?"
• Ensures packets get from source A to destination
  B.

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4) Transport Layer

• Purpose Ensure that data gets between A and B.
• Think From the source and destination, I make
  sure that the data gets there.
• Ensures a data gets between source and
  destination.
• If necessary ensure that connection is lossless
  (resend missing data).
• Provides flow control if necessary (send data
  faster or slower depending on the network
  conditions).

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5) Session Layer (not TCP/IP)

• Purpose Provides a single connection for one
  application.
• Think I am in charge of the entire message.
• This connection may be two way or may be
  synchronised.
• Not discussed much as it is never implemented.

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6) Presentation Layer (Not TCP/IP)

• Purpose Provides commonly used functions for
  applications.
• Think I meet internationalisation standards.
• The main job of the presentation layer is to
  ensure that character sets match e.g. that
  Chinese characters are correctly received by the
  sends.
• Again not discussed much as it is never
  implemented.

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7) Application layer

• Purpose The computer programs which actually do
  things with the network.
• Think I deliver the mail, browse the web etc.
• For example, your email client program which will
  talk to the email server at the other end.
• At this layer, we have many protocols (http,
  snmp, smtp, ftp, telnet) which different bits of
  software use.
• We often talk in terms of client and server
  architecture for the software.

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TCP/IP model in summary
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Internet (IP) addresses

• richard_at_manor.york.ac.uk (email)
• http//www.apoptygma.eu.org (www)
• ftp//ftp.uk.debian.org (file transfer)
• telnet//towel.blinkenlights.nl (telnet)
• 144.32.100.24
• 148.122.211.110
• 195.224.53.39
• 62.250.7.101

These are the real IP addresses of the above
sites. IP addresses are 32 bits grouped into 4
octets. (Octet 8 bits a number from 0-255)
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IP Networks(1)

• IP addresses use less significant bits first to
  indicate sub-networks.
• IP address 123.45.67.89
• Netmask255.255.255.0 (no holes allowed)
• If two IP addresses are the same when bitwise
  ANDd against the netmask then they are on the
  same subnet.
• 123.45.67.?? is always on the same subnet in the
  above example.

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IP Networks(2)

• IP networks were originally subdivided into class
  A, B, C, D and E networks.

Start End Networks Hosts/network
A 1.0.0.0 127.255.255.255 126 16 million
B 128.0.0.0 191.255.255.255 16,382 64K
C 192.0.0.0 223.255.255.255 2 million 254
D 224.0.0.0 239.255.255.255 Multicast
E 240.0.0.0 247.255.255.255 Reserved
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Subnet examples

• IP Addresses
• A 132.128.208.32 10000100.10000000.11010000.0010
  0000
• B 132.128.217.63 10000100.10000000.11011001.0011
  1111
• Subnet mask 1 255.255.255.0
• 11111111.11111111.11111111.00000000
• Subnet mask 2 255.255.240.0
• 11111111.11111111.11110000.00000000
• A and B would be on the same subnet if the subnet
  mask was 1 but different subnets if the mask was
  2.

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The IP header

• IP packets all have a header as shown

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About the IP header

• Type of Service (Best efforts, immediate
  delivery etc)
• Total length (of whole packet)
• Identification (number of packet for later
  reassembly)
• Fragment offset sometimes the network splits a
  packet into fragments.
• Flags (information about fragments). DF Dont
  Fragment MF More Fragments to come

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About the IP header (2)

• Time To Live (TTL) reduced by one every hop.
  When it reaches zero packet is killed. (This is
  to ensure that the network doesnt fill up with
  lost packets).
• Protocol identified by a number (usually TCP or
  UDP).
• Checksum to ensure that the packet is not
  corrupted.

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IPv6

• IPv4 allows over 4 billion computers (but not
  really) inefficient subnetting is using these
  up.
• IPv6 allows 16 octet addresses (4 octets in
  IPv4).
• 3x1038 addresses (gt Avogadros number). 7x1023
  IP addresses per square meter of the earths
  surface.
• Why so many? Electrical devices may want IP
  addresses your house could be its own
  subnetwork. Why NOT?
• Better security than current IP(v4).
• Allow roaming hosts.
• Pay more attention to type of service (for real
  time data).

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Next Lecture

• IP tells us how to get a message from A to B.
• However, the IP protocol is lossy (it doesnt
  guarantee that anything will actually get
  there).
• In the next lecture we will look at TCP/IP and
  UDP/IP which sit on top of IP and deal with the
  sending of the messages.