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INFO 330 Chapter 1 1

  1. 1. INFO 330 Computer Networking Technology I Chapter 1 Networking Overview Glenn Booker INFO 330 Chapter 1
  2. 2. Computer Networks <ul><li>A network is the structure that allows computer applications to communicate with each other </li></ul><ul><ul><li>The applications could be executed by the user, or part of the operating system </li></ul></ul><ul><li>Not every computer system is designed to allow networking </li></ul><ul><ul><li>Microsoft DOS had no native networking ability; it was added after the need arose </li></ul></ul>INFO 330 Chapter 1
  3. 3. The Internet <ul><li>The Internet is the primary model for understanding networking concepts because, well, nearly every computer and many other things could be connected to it </li></ul>INFO 330 Chapter 1
  4. 4. The Internet <ul><li>Key parts of any network include </li></ul><ul><ul><li>Hosts or end systems , which are the computers and other things with which most people interact </li></ul></ul><ul><ul><ul><li>End user computers, workstations, and servers are all considered hosts </li></ul></ul></ul><ul><ul><ul><li>As of July 2008 there were about 600 million hosts on the Internet! </li></ul></ul></ul>INFO 330 Chapter 1
  5. 5. The Internet <ul><ul><li>Communication links , which are the wired or wireless means used to connect to the network </li></ul></ul><ul><ul><li>Packet switches , which help guide information between hosts </li></ul></ul><ul><ul><ul><li>Routers and link-layer switches are the primary types of packet switches </li></ul></ul></ul>INFO 330 Chapter 1 Graphics are taken from the text’s lecture notes
  6. 6. The Internet <ul><li>The network sends chunks of information called packets along a route or path to get from one host to another </li></ul><ul><ul><li>The speed at which it does so is the transmission rate , typically in bits per second (bps) </li></ul></ul>INFO 330 Chapter 1
  7. 7. The Internet <ul><li>The control over choosing the path is known as packet switching </li></ul><ul><li>End systems connect to the Internet through an Internet Service Provider (ISP) </li></ul><ul><li>ISPs provide many levels of service </li></ul><ul><ul><li>Residential or business service, typically from 56kb dialup to DSL, FIOS, or cable modems </li></ul></ul>INFO 330 Chapter 1
  8. 8. The Internet <ul><li>The packets are defined and handled according to protocols , most notably the Transmission Control Protocol (TCP) and Internet Protocol (IP) </li></ul><ul><li>A protocol is a language for communication </li></ul>INFO 330 Chapter 1
  9. 9. Protocols <ul><li>In order for it to work, both parties (e.g. hosts, switches, etc.) need to speak the same language oder Sie werden einander nicht verstehen or they won’t understand each other </li></ul><ul><li>Some protocols use a handshake concept </li></ul><ul><ul><li>Like saying Hi as a greeting, special messages are defined that request a connection, and reply to accept the connection </li></ul></ul>INFO 330 Chapter 1
  10. 10. Protocols <ul><li>More formally, then, protocols define </li></ul><ul><ul><li>The format of messages (like the spelling of words) </li></ul></ul><ul><ul><li>The order of messages (the syntax of sentences, or else your messages like Yoda will sound) </li></ul></ul><ul><li>Much of understanding networking is understanding how these protocols work </li></ul>INFO 330 Chapter 1
  11. 11. Source of Protocols <ul><li>Internet protocols are defined by the Internet Engineering Task Force (IETF) </li></ul><ul><ul><li>The IETF was created by the Internet Architecture Board (IAB) and also reports to the Internet Society (ISOC) </li></ul></ul><ul><li>The Request For Comments (RFCs) define the actual protocols </li></ul><ul><ul><li>The first RFC was dated April 1969 </li></ul></ul><ul><ul><li>As of September 2009, there are over 5700 RFCs </li></ul></ul>INFO 330 Chapter 1
  12. 12. Internet vs Intranet <ul><li>The Internet (a proper noun, hence is capitalized) is the public network of zillions of computers, toasters, etc. </li></ul><ul><li>An intranet (not a proper noun) is the generic term for a local private network that uses the same protocols as the Internet </li></ul>INFO 330 Chapter 1
  13. 13. Type of Internet Service <ul><li>The Internet runs distributed applications </li></ul><ul><ul><li>The World Wide Web, instant messaging, distributed games, etc. are all distributed applications </li></ul></ul><ul><ul><li>These applications are developed using an Application Programming Interface (API) to connect to the Internet </li></ul></ul>INFO 330 Chapter 1
  14. 14. Type of Internet Service <ul><li>There are two choices for the type of service provided by an Internet connection </li></ul><ul><ul><li>A connection-oriented, reliable service </li></ul></ul><ul><ul><li>A connection-less, unreliable service </li></ul></ul><ul><li>Neither guarantees how fast a message will get from host A to host B </li></ul>INFO 330 Chapter 1
  15. 15. Connection-oriented, Reliable Service <ul><li>This establishes a loose connection between client and server, but not to the switches between them </li></ul><ul><li>Key traits needed from this are </li></ul><ul><ul><li>Reliable data transfer – every little bit counts </li></ul></ul><ul><ul><li>Flow control to keep from overwhelming hosts </li></ul></ul><ul><ul><li>Congestion control to avoid Internet gridlock </li></ul></ul><ul><li>TCP provides this service (see RFC 793) </li></ul>INFO 330 Chapter 1
  16. 16. Connection-less, Unreliable Service <ul><li>This service has no handshaking – it just sends packets of data </li></ul><ul><ul><li>Don’t know if packets ever got there </li></ul></ul><ul><ul><li>No flow or congestion control </li></ul></ul><ul><li>Handled by User Datagram Protocol (UDP), RFC 768 </li></ul><ul><li>Use when speed is critical, such as video conferencing or Internet telephone </li></ul>INFO 330 Chapter 1
  17. 17. The Edge of the Network <ul><li>Now we’ll examine the contents of the Internet from the outside in – from the “edge” to the “core” </li></ul><ul><li>Hosts (end systems) can be divided into clients and servers </li></ul><ul><ul><li>Clients are computers that request services from Servers </li></ul></ul><ul><ul><li>One computer (host) can be multiple clients and servers at once (esp. in peer-to-peer applications) </li></ul></ul>INFO 330 Chapter 1
  18. 18. Access Networks <ul><li>To get from a host to a distant part of the Internet, you need to pass through the access network </li></ul><ul><li>Access networks get residential, business, and wireless users connected </li></ul><ul><li>Types of connections include </li></ul><ul><ul><li>56 kbps dial-up modem, an analog connection over a voice phone line </li></ul></ul><ul><ul><ul><li>Typically get 40-42 kbps due to line noise </li></ul></ul></ul>INFO 330 Chapter 1
  19. 19. Access Networks <ul><li>Digital subscriber line (DSL) gives a dedicated connection, with different upstream and downstream rates </li></ul><ul><ul><li>DSL uses FDM </li></ul></ul><ul><ul><li>Downstream/upstream rates are typically values like 768k/128k, 3.0M/768k, etc. </li></ul></ul><ul><li>Business connections may use dedicated T1 lines (1.536 Mbps), ISDN connections, and other options </li></ul>INFO 330 Chapter 1
  20. 20. Access Networks <ul><li>Cable modems use hybrid fiber-coaxial cable (HFC) to connect to special cable modems </li></ul><ul><ul><li>HFC is a variant on the same cable used for cable TV service </li></ul></ul><ul><ul><li>HFC is a shared medium – if all your neighbors are online, your connection speed will suffer! </li></ul></ul><ul><li>Dial-up connections are only present when needed; DSL and cable modems are always on (we hope) </li></ul>INFO 330 Chapter 1
  21. 21. Access Networks <ul><li>Fiber to the home (FTTH) is fiber optic Internet connection for residential use </li></ul><ul><li>There are two kinds of FTTH </li></ul><ul><ul><li>Active optical networks (AONs) are switched Ethernet </li></ul></ul><ul><ul><li>Passive optical networks (PONs) are used by Verizon’s FIOS service </li></ul></ul><ul><ul><ul><li>Typically about 100 homes share a connection from the provider’s central office (CO) </li></ul></ul></ul>INFO 320 week 1
  22. 22. Wired access <ul><li>Local area networks (LANs) generally use Ethernet for wired connections </li></ul><ul><li>Ethernet speeds of 10-1000 Mbps are common, up to 10 Gbps for servers and routers </li></ul>INFO 320 week 1
  23. 23. Wireless Access <ul><li>Wireless devices connect through wireless access points (base station) on a LAN </li></ul><ul><ul><li>Then the LAN uses some other access connection to get to the Internet </li></ul></ul><ul><li>Wireless devices use the IEEE 802.11 family of technologies </li></ul><ul><ul><li>802.11a supports up to 54 Mbps @ 5 GHz </li></ul></ul><ul><ul><li>802.11b supports 5.5 and 11 Mbps @ 2.4 GHz </li></ul></ul><ul><ul><li>802.11g supports up to 54 Mbps @ 2.4 GHz </li></ul></ul>INFO 330 Chapter 1
  24. 24. Why Does Frequency Matter? <ul><li>Wireless signals can be interfered with by other devices; when that occurs, they detune their speed </li></ul><ul><ul><li>802.11a has seven (48, 36, 24, 18, 12, 9, and 6 Mbps) </li></ul></ul><ul><ul><li>802.11b has three lower data rates (5.5, 2, and 1 Mbps) </li></ul></ul><ul><ul><li>802.11g has a range of lower speeds </li></ul></ul><ul><li>The 802.11b and 802.11g standards use the 2.4 GHz (gigahertz) frequency range </li></ul><ul><ul><li>This frequency range is used by other networking technologies, microwave ovens, 2.4GHz cordless phones (a huge market), and Bluetooth devices </li></ul></ul><ul><li>The 5 GHz frequency range for 802.11a is relatively clear, so it’s less likely to have interference (so far) </li></ul>INFO 330 Chapter 1
  25. 25. Wireless Network Example INFO 330 Chapter 1
  26. 26. WiMAX <ul><li>The next generations of wireless communication are a battle between advanced cell technologies (3G and 4G protocols) and WiMAX </li></ul><ul><li>WiMAX is IEEE 802.16, and promises 5-10 Mbps speed over ranges of tens of km </li></ul>INFO 320 week 1
  27. 27. Physical Media <ul><li>Physical media used for connecting networks can be guided or unguided </li></ul><ul><ul><li>Guided media use something solid – wires, coaxial cable, fiber-optic cable, etc. </li></ul></ul><ul><ul><li>Unguided media use electromagnetic waves of some kind – wireless LAN signals, satellite channels, etc. </li></ul></ul>INFO 330 Chapter 1
  28. 28. Physical Media <ul><li>Specific kinds of physical media include </li></ul><ul><ul><li>Twisted pair copper wire </li></ul></ul><ul><ul><li>Coaxial cable </li></ul></ul><ul><ul><li>Fiber optics </li></ul></ul><ul><ul><li>Terrestrial radio channels </li></ul></ul><ul><ul><li>Satellite radio channels </li></ul></ul>INFO 330 Chapter 1
  29. 29. Twisted pair copper wire <ul><li>Most common physical medium, has multiple coated wires wrapped around each other </li></ul><ul><ul><li>Includes phone lines, which have four thin wires with RJ-11 plugs on the end </li></ul></ul><ul><ul><li>Ethernet cables have eight wires, and RJ-45 plugs on the end, so they’re wider than phone plugs </li></ul></ul><ul><li>Can handle Gbps speeds over distances of about a hundred yards </li></ul>INFO 330 Chapter 1
  30. 30. Coaxial cable <ul><li>Coaxial (coax) cable has a copper wire core, and a copper cylinder around it – they share the same axis of rotation, hence the name </li></ul><ul><li>Handles multiple Mbps speeds for miles </li></ul><ul><li>There are only two conductors, which is why it’s a shared medium – everyone shares the same resources </li></ul>INFO 330 Chapter 1
  31. 31. Fiber optics <ul><li>Fiber optics use hollow fibers to guide light pulses </li></ul><ul><li>Handles hundreds of Gbps speeds up to 100 km </li></ul><ul><li>Most international phone lines, and the Internet backbone, are fiber optic cables </li></ul><ul><li>Used on high speed LANs – 1 to 10 Gbps </li></ul>INFO 330 Chapter 1
  32. 32. Terrestrial radio channels <ul><li>These include the wireless network channels discussed previously, plus radio signals used to beam networks between buildings </li></ul><ul><li>Can reach long distances with the latter, but signals can be intercepted, bounce, fade, and have interference from other signals </li></ul>INFO 330 Chapter 1
  33. 33. Satellite radio channels <ul><li>Consist of geostationary satellites and low-altitude satellites </li></ul><ul><ul><li>Geostationary satellites hover 24,000 miles above the Earth’s surface, and are used to relay TV channels and parts of the Internet backbone </li></ul></ul><ul><ul><li>Low altitude satellites (LEO, low-Earth orbiting) orbit much faster, so you need several to be able to find one at any given time; are not used for networks </li></ul></ul>INFO 330 Chapter 1
  34. 34. Psst – what Internet Backbone? <ul><li>The Internet is a network of many networks </li></ul><ul><ul><li>It was designed that way to be redundant in the event of war – if one part of it was no longer usable (nice euphemism!), the rest of the network would still work </li></ul></ul><ul><li>At its heart are many Tier-1 ISPs </li></ul><ul><ul><li>Sprint, MCI, WorldCom, AT&T, etc. are all Tier-1 </li></ul></ul><ul><ul><li>They run extremely fast “backbone” connections (622 Mbps to 10 Gbps) </li></ul></ul>INFO 330 Chapter 1
  35. 35. Internet Backbone <ul><li>The Tier-2 ISPs are regional or national in scope, and connect to Tier-1 and Tier-2 ISPs </li></ul><ul><li>Points where ISPs connect to each other are Points Of Presence (POPs) </li></ul><ul><ul><li>Don’t confuse with Post Office Protocol (POP) </li></ul></ul><ul><li>They may also connect at Network Access Points (NAPs) to local telecom companies or Tier 1 ISPs </li></ul>INFO 330 Chapter 1
  36. 36. Internet Backbone <ul><li>There are thousands of lower level ISPs, Tier-3, probably including your local ISP </li></ul><ul><li>For a packet to get from one host to another, it may pass through a variety of Tier-1, Tier-2, and Tier-3 ISPs, NAPs, POPs, etc. </li></ul>INFO 330 Chapter 1
  37. 37. Circuit vs Packet Switching <ul><li>In order to get a packet from host A to host B, two major approaches could be used </li></ul><ul><ul><li>Both approaches send packets over communication lines </li></ul></ul><ul><ul><li>Circuit switching is what a traditional telephone system does </li></ul></ul><ul><ul><ul><li>Reserve a path from A to B which is the circuit messages will follow, until the connection is closed </li></ul></ul></ul><ul><ul><li>Packet switching is used by the Internet </li></ul></ul><ul><ul><ul><li>Dump packets into the network with no reserved path, and make a best effort to get packet to destination </li></ul></ul></ul>INFO 330 Chapter 1
  38. 38. Circuit Switching <ul><li>To link host A and host B, each link between switches along the way must be reserved for the duration of that connection or circuit </li></ul><ul><li>There are two ways to share links with many circuits: </li></ul><ul><ul><li>Frequency-division multiplexing (FDM) </li></ul></ul><ul><ul><li>Time-division multiplexing (TDM) </li></ul></ul>INFO 330 Chapter 1
  39. 39. FDM and TDM <ul><li>FDM acts like FM radio – it divides the link by frequency ranges, and assigns a frequency range to each circuit </li></ul><ul><ul><li>Typical frequency range, or bandwidth, is 4 kHz </li></ul></ul><ul><ul><li>This way one link can handle many circuits </li></ul></ul><ul><li>TDM breaks the link into some number (n) of slots in a frame </li></ul><ul><ul><li>Each slot is dedicated to one circuit, so that circuit has full attention of the link 100/n percent of the time </li></ul></ul>INFO 330 Chapter 1
  40. 40. Bits and Bytes <ul><li>To review basic computer units </li></ul><ul><ul><li>A bit is a binary digit – a 0 or 1 </li></ul></ul><ul><ul><li>Typically eight bits are a byte, the shortest word </li></ul></ul><ul><ul><ul><li>Old ASCII text files may use seven bits per byte, so there are 2 7 = 128 ASCII characters </li></ul></ul></ul><ul><ul><li>Transmission rate of data is given in bits per second (bps), or thousands or millions or billions of bits per second (kbps, Mbps, Gbps) </li></ul></ul><ul><ul><li>Data transfer = rate * time </li></ul></ul><ul><ul><ul><li>Which has units of: bits = bits/sec * sec </li></ul></ul></ul>INFO 330 Chapter 1
  41. 41. Key conversion point <ul><li>In dealing with prefixes k, M, G, etc., in computer science they represent 2^(n*10) </li></ul><ul><ul><li>k = 2^10, M = 2^20, G = 2^30, etc. </li></ul></ul><ul><li>For our purposes, treat prefixes as their base 10 equivalents </li></ul><ul><ul><li>k = 1000, M = 1,000,000, G = 1 billion </li></ul></ul>INFO 320 week 1
  42. 42. TDM Example <ul><li>Suppose you have a 1.536 Mbps TDM connection, and want to send a 1 Mb (megabit) file; the connection has 12 links </li></ul><ul><li>How long does it take? </li></ul><ul><ul><li>Your transmission speed is 1/12 of the 1.536 Mbps, or 0.128 Mbps </li></ul></ul><ul><ul><li>Time = data / rate = 1 Mb / 0.128 Mbps = 7.8125 seconds </li></ul></ul><ul><ul><li>This doesn’t include time to make the connection </li></ul></ul>INFO 330 Chapter 1
  43. 43. Packet Switching <ul><li>Messages are divided into packets before going into the network </li></ul><ul><li>Most packet switches must receive an entire packet before forwarding it to the next switch </li></ul><ul><ul><li>This store-and-forward transmission introduces delays while the switch waits for the entire packet to get there </li></ul></ul><ul><ul><ul><li>If a packet size is L, and the transmission rate is R, the delay to receive one full packet is L/R </li></ul></ul></ul>INFO 330 Chapter 1
  44. 44. Store and Forward Delay <ul><li>Assume 1) no queuing delay, 2) no time to make a connection, and 3) no delay to propagate packets </li></ul><ul><li>Send a packet of L bits across a packet-switched network with Q links, all of which have a transmission rate of R bps </li></ul><ul><ul><li>For each link, the store and forward delay of L/R seconds; this occurs Q times, for a total delay of Q*L/R seconds </li></ul></ul>INFO 330 Chapter 1
  45. 45. Packet Switching <ul><li>Each switch typically connects to many links </li></ul><ul><li>For each link, there is an output buffer (or output queue) to hold packets waiting to go on that link </li></ul><ul><ul><li>This introduces queuing delays, while a packet waits its turn </li></ul></ul><ul><ul><li>If the buffer is full, the packet can be lost – packet loss isn’t good! </li></ul></ul>INFO 330 Chapter 1
  46. 46. Statistical Multiplexing <ul><li>Compare circuit to packet switching </li></ul><ul><li>Suppose users are active 10% of the time, sending 100 kbps of data, and not using the connection the other 90% of the time </li></ul><ul><li>If there’s a 1 Mbps connection available: </li></ul><ul><ul><li>TDM circuit switching would need 10 slots to allow each user 100 kbps </li></ul></ul>INFO 330 Chapter 1
  47. 47. Statistical Multiplexing <ul><ul><li>Packet switching could handle 35 users total because the total number of active users will be 11 or more only 0.04% of the time (look up the binomial distribution for details) </li></ul></ul><ul><ul><ul><li>The remaining 99.96% of the time, the total data rate is less than the 1 Mbps capacity of the connection </li></ul></ul></ul><ul><li>Hence sharing resources on demand (which is statistical multiplexing) allows the same performance 99.96% of the time, for over three times the number of users! </li></ul>INFO 330 Chapter 1
  48. 48. Packet-Switched Networks <ul><li>There are two major kinds of packet-switched networks – datagram networks and virtual-circuit networks </li></ul><ul><li>A datagram network forwards packets according to the host destination address </li></ul><ul><ul><li>Hence the Internet is a datagram network </li></ul></ul><ul><ul><li>Routers forward packets to make a best effort to get them to the destination address </li></ul></ul>INFO 330 Chapter 1
  49. 49. Virtual Circuit Networks <ul><li>A virtual circuit network forwards packets according to virtual circuit numbers </li></ul><ul><ul><li>A virtual circuit (VC) is an imaginary connection between the source and destination hosts </li></ul></ul><ul><ul><ul><li>Examples are X.25, frame relay, and asynchronous transfer mode (ATM) </li></ul></ul></ul><ul><ul><li>Each packet has a VC identifier (VC ID) </li></ul></ul><ul><ul><li>Each packet switch indexes its VC translation table, and forwards the packet to the right outbound link </li></ul></ul>INFO 330 Chapter 1
  50. 50. Virtual Circuit Networks <ul><ul><li>A key difference between datagram and VC networks is that VC networks have to maintain state information about connections </li></ul></ul><ul><ul><ul><li>Each new VC means a new entry has to be added to the VC translation table, and then is removed when the connection is ended </li></ul></ul></ul><ul><ul><li>It also needs to keep a table to map VC numbers to output interface numbers </li></ul></ul>INFO 330 Chapter 1
  51. 51. Datagram Networks <ul><li>Datagram networks are like the post office </li></ul><ul><li>The contents of a message (like a letter or box) are only seen by the sender and recipient (we hope), and in between them, the postal service only looks at the recipient’s address, e.g. my address here is: </li></ul><ul><ul><li>306 Rush Hall 3141 Chestnut St Philadelphia, PA 19104 USA </li></ul></ul>INFO 330 Chapter 1
  52. 52. Datagram Networks <ul><ul><li>If a letter is mailed to me from outside the USA, the first thing they need to know is that the letter needs to go to America </li></ul></ul><ul><ul><li>Then a machine reader finds the zip code, and knows the letter needs to go to Philadelphia, since 19104 is entirely within Philly </li></ul></ul><ul><ul><li>The local letter carrier recognizes 3141 Chestnut St as the central location for all Drexel mail </li></ul></ul><ul><ul><li>Someone within Drexel knows where 306 Rush Hall is, and carries the letter there </li></ul></ul>INFO 330 Chapter 1
  53. 53. Datagram Networks <ul><ul><li>And the receptionist in 306 Rush Hall knows that I’m full time faculty, and puts the letter in my mailbox </li></ul></ul><ul><li>Each step along the way, the letter is routed essentially by reading the address backward (USA - 19104 – Philadelphia, PA – 3141 Chestnut St – 306 Rush Hall – Glenn Booker) </li></ul><ul><li>Datagram networks do the same thing – a packet of data is wrapped in layers of addresses, which are used by routers </li></ul>INFO 330 Chapter 1
  54. 54. Datagram Networks <ul><li>Notice that datagram networks do not maintain state information about any packet – they only read the address and decide where to send it based on that address </li></ul><ul><li>Traceroute (in Windows, tracert; see also RFC 1393) is an application that shows you the details of how a packet gets from one host to another </li></ul>INFO 330 Chapter 1
  55. 55. Traceroute Output <ul><li>FROM TO </li></ul><ul><li>traceroute to (, 64 hops max, 44 byte packets </li></ul><ul><li>1 ( 0.554 ms 0.420 ms 0.360 ms </li></ul><ul><li>2 ( 0.561 ms 0.873 ms 0.313 ms </li></ul><ul><li>3 ( 0.372 ms 0.355 ms 0.317 ms </li></ul><ul><li>4 ( 16.992 ms 16.940 ms 16.925 ms </li></ul><ul><li>5 ( 17.748 ms 17.743 ms 17.740 ms </li></ul><ul><li>6 ( 17.817 ms 17.812 ms 20.384 ms </li></ul><ul><li>7 ( 17.832 ms 17.917 ms 17.798 ms </li></ul><ul><li>8 ( 18.178 ms 18.202 ms 18.211 ms </li></ul><ul><li>9 ( 90.064 ms 90.101 ms 97.132 ms </li></ul><ul><li>10 ( 107.404 ms 107.474 ms 107.519 ms </li></ul><ul><li>11 ( 113.379 ms 113.830 ms 113.340 ms </li></ul><ul><li>12 ( 154.871 ms 117.584 ms 117.607 ms </li></ul><ul><li>13 ( 113.757 ms 113.659 ms 113.576 ms </li></ul>INFO 330 Chapter 1
  56. 56. Traceroute Output <ul><li>Each line of output gives you </li></ul><ul><ul><li>The hop number (1, 2, …) </li></ul></ul><ul><ul><li>The name of the server it’s passing through </li></ul></ul><ul><ul><li>The IP address of that server (e.g. </li></ul></ul><ul><ul><li>And times of three attempts to “ping” that server (say Hi to it), given in milliseconds (ms) </li></ul></ul><ul><li>Notice the example goes through servers in the UK and Germany (uk, de), and the ping times go over a hundred milliseconds </li></ul>INFO 330 Chapter 1
  57. 57. Summary of Network Types INFO 330 Chapter 1
  58. 58. Delays and Losses <ul><li>We’ve hinted at several kinds of things that can delay a packet or make it get lost; now we’ll examine those concepts in more detail </li></ul><ul><li>After a packet leaves the host, it typically passes through several routers before getting to its destination </li></ul><ul><li>Each router examines the packet’s header to determine which outbound link it needs to follow, and puts it in a queue for that link </li></ul>INFO 330 Chapter 1
  59. 59. Delays and Losses <ul><li>Four main causes of delay at each router: </li></ul><ul><ul><li>Nodal processing delay </li></ul></ul><ul><ul><li>Queuing delay </li></ul></ul><ul><ul><li>Transmission delay </li></ul></ul><ul><ul><li>Propagation delay </li></ul></ul>INFO 330 Chapter 1
  60. 60. Delays and Losses <ul><ul><li>Nodal processing delay is the time needed for the router to examine the packet’s header and choose the right outbound link </li></ul></ul><ul><ul><ul><li>Also may include time for error checking the packet </li></ul></ul></ul><ul><ul><ul><li>Typically in microseconds for good routers </li></ul></ul></ul><ul><ul><li>Queuing delay is the time for a packet waiting to be transmitted across the outbound link </li></ul></ul><ul><ul><ul><li>Depends mostly on how much traffic got to the router which is waiting for the same link </li></ul></ul></ul><ul><ul><ul><li>Could be microseconds or milliseconds in duration </li></ul></ul></ul>INFO 330 Chapter 1
  61. 61. Delays and Losses <ul><ul><li>Transmission delay is like the store-and-forward delay mentioned earlier; it’s the time to transmit the packet onto the link </li></ul></ul><ul><ul><ul><li>The entire packet has to be pushed onto the link by the router, so the transmission delay is L/R, or (packet size)/(transmission speed) </li></ul></ul></ul><ul><ul><li>Propagation delay is the time for the packet to get to the next router </li></ul></ul><ul><ul><ul><li>Distance = speed * time, so the propagation delay is distance/speed, where speed is 2 or 3x10 8 m/sec (the speed of light is 3x10 8 m/s) </li></ul></ul></ul>INFO 330 Chapter 1
  62. 62. Delays and Losses <ul><li>You might think of Transmission delay and Propagation delay as being like leaving for a trip – transmission delay is the time to pack the car (time to get out of the house), and propagation delay is the time to drive to your destination (travel time) </li></ul><ul><li>Or ignore this analogy if it doesn’t help  </li></ul>INFO 330 Chapter 1
  63. 63. Delays and Losses <ul><li>So the delay at one node, the nodal delay, is given by d nodal = d proc + d queue + d trans + d prop </li></ul><ul><ul><li>Where d proc = Nodal processing delay d queue = Queuing delay d trans = Transmission delay d prop = Propagation delay </li></ul></ul><ul><ul><li>The relative magnitude of these terms can vary widely, depending on the circumstances </li></ul></ul>INFO 330 Chapter 1
  64. 64. Traffic Intensity <ul><li>Consider if all packets were the same size L bits, and arrive at a router at a rate of ‘a’ packets per second </li></ul><ul><ul><li>The rate of data arriving at the router is L*a bits per second </li></ul></ul><ul><li>The output rate from the router is its transmission rate, R bits per second </li></ul><ul><li>The traffic intensity is L*a/R </li></ul><ul><ul><li>Want traffic intensity < 1 – why? </li></ul></ul>INFO 330 Chapter 1
  65. 65. Traffic Intensity <ul><li>Average queuing delay grows exponentially as traffic intensity approaches one </li></ul><ul><ul><li>This is the router equivalent of gridlock! </li></ul></ul><ul><li>It was assumed that the router could hold an infinite amount of packets in its queue </li></ul><ul><ul><li>A dropped or lost packet occurs when a packet arrives at a router with its outbound link queue full </li></ul></ul><ul><ul><li>Fraction of lost packets is a key measure </li></ul></ul>INFO 330 Chapter 1
  66. 66. End-to-end Delay <ul><li>So far we focused on one router </li></ul><ul><li>Now consider the total delays getting from host to host – the end-to-end delay </li></ul><ul><li>If we assume </li></ul><ul><ul><li>1) there are N-1 routers between hosts, </li></ul></ul><ul><ul><li>2) queuing delays are negligible, and </li></ul></ul><ul><ul><li>3) processing delays are the same for each router and the source host, </li></ul></ul><ul><ul><li>4) transmission rates are all R bits/sec, and </li></ul></ul><ul><ul><li>5) propagation delays are all equal </li></ul></ul>INFO 330 Chapter 1
  67. 67. End-to-end Delay <ul><li>Then the total delay from source host to destination host is d end-end = N*(d proc + d trans + d prop ) </li></ul><ul><ul><li>And d trans is L/R, with L the packet size </li></ul></ul><ul><li>So why is it N instead of (N-1)? </li></ul>INFO 330 Chapter 1
  68. 68. Internet Throughput <ul><li>Much of the Internet core has more capacity than currently needed (it is over-provisioned) </li></ul><ul><li>As a result, the limit of getting data through the Internet is the speed of your access link (ISP connection) and your destination’s access link </li></ul>INFO 330 Chapter 1
  69. 69. Layered Architecture <ul><li>As hinted at in the syllabus, the layers of networking are a key concept </li></ul><ul><ul><li>Why use layers? </li></ul></ul><ul><ul><ul><li>To solve a big problem, break it into little problems </li></ul></ul></ul><ul><ul><ul><li>Each layer has a small, focused amount of work it needs to accomplish; each layer provides services to the layer above it </li></ul></ul></ul><ul><li>Disadvantages are: possible duplication of work (error recovery on multiple layers), and violating the scope of a layer’s services </li></ul>INFO 330 Chapter 1
  70. 70. Layered Architecture <ul><li>The layers are seen at right </li></ul><ul><ul><li>The application layer is where user-visible software exists – HTTP, SMTP, FTP, etc. protocols </li></ul></ul><ul><ul><li>The transport layer is home to the TCP and UDP protocols </li></ul></ul><ul><ul><li>The network layer is home to the Internet Protocol, IP, and the protocols used by routers </li></ul></ul>INFO 330 Chapter 1
  71. 71. Layered Architecture <ul><li>The link layer is key for local routing; includes Ethernet and Point-to-Point Protocol (PPP) </li></ul><ul><li>The physical layer moves the bits of data (frames, as we’ll see shortly) across the guided or unguided media discussed earlier </li></ul><ul><ul><li>Each medium has protocols for how data is encoded and decoded </li></ul></ul>INFO 330 Chapter 1
  72. 72. But Wait Professor Booker! <ul><li>Aren’t we missing the Presentation and Session layers ?!? </li></ul><ul><ul><li>Yes, the OSI reference model has them between the application and transport layers, but they aren’t directly relevant here </li></ul></ul><ul><ul><ul><li>The presentation layer includes coding and conversion functions that are applied to application layer data – such as MPEG, QuickTime, JPG, GIF, TIFF </li></ul></ul></ul><ul><ul><ul><li>The session layer opens and closes communication sessions; AppleTalk is a familiar protocol here </li></ul></ul></ul>INFO 330 Chapter 1
  73. 73. Layered Architecture <ul><li>To make it more confusing, the packet we’ve been discussing has different names as it descends the layers </li></ul><ul><ul><li>Terms may vary from vendor to vendor </li></ul></ul><ul><li>A packet becomes </li></ul><ul><ul><li>A message in the application layer </li></ul></ul><ul><ul><li>A segment in the transport layer </li></ul></ul><ul><ul><li>A dataframe (or datagram) in the network layer </li></ul></ul><ul><ul><li>A frame in the link and physical layers </li></ul></ul>INFO 330 Chapter 1
  74. 74. Layered Architecture <ul><li>With each layer, headers are added to the message to describe the address information needed by that layer </li></ul><ul><li>This process is called encapsulation , as we put the message in bigger and bigger boxes </li></ul><ul><li>Routers and switches typically look at the link or network layer information </li></ul><ul><ul><li>Like a letter carrier, they don’t read your mail </li></ul></ul>INFO 330 Chapter 1
  75. 75. Layered Architecture INFO 330 Chapter 1
  76. 76. Network Security <ul><li>While security is covered in detail in INFO 331, we’ll mention a couple of key concepts </li></ul><ul><li>Malware is a generic term for software that does harm (malicious software) </li></ul><ul><ul><li>It could enroll your computer in a botnet , where it helps distribute spam or help attack other computers </li></ul></ul><ul><ul><li>Much malware is self-replicating, so it can spread very quickly </li></ul></ul>INFO 330 Chapter 1
  77. 77. Network Security <ul><li>Viruses are malware that require the user to activate it somehow, but it could be disguised as a web link </li></ul><ul><li>Worms can enter your computer without user activation </li></ul><ul><li>Trojan horses enter via a legitimate application, such as a simple game </li></ul>INFO 330 Chapter 1
  78. 78. Network Security <ul><li>Threats can keep a host from getting legitimate network traffic – this is a denial of service (DoS) attack </li></ul><ul><ul><li>Types of DoS attacks include exploiting a vulnerability in the OS or an application, flooding the bandwidth leading to the host, or making the host establish phony network connections </li></ul></ul><ul><li>Herds of computers can participate in a distributed DoS attack (DDoS) </li></ul>INFO 330 Chapter 1
  79. 79. Network Security <ul><li>Network data can be read using packet sniffers </li></ul><ul><ul><li>We’ll use one for our labs, WireShark </li></ul></ul><ul><li>Or people can fake who they are on the network, and impersonate you (IP spoofing) or intercept a network connection (man in the middle attack) </li></ul>INFO 330 Chapter 1
  80. 80. A Little History <ul><li>The concept of packet switching was developed in the early 1960’s by MIT and the Rand Institute, in order to make it possible to share really expensive computer time efficiently </li></ul><ul><li>The first packet switches were called interface message processors (IMPs) </li></ul><ul><li>ARPAnet, the Internet predecessor, was proposed in 1967 </li></ul>INFO 330 Chapter 1
  81. 81. A Little History <ul><li>By 1969, four computers were on ARPAnet, and RFCs were being published </li></ul><ul><li>By 1972 there were 15 nodes on ARPAnet, and it was first seen publicly </li></ul><ul><li>The first email program was written in 1972 </li></ul><ul><li>A microwave network was developed in Hawaii, and various packet switching networks were developed by the mid 1970’s </li></ul>INFO 330 Chapter 1
  82. 82. A Little History <ul><li>As the number of similar networks grew, connecting them to aid researchers became an obvious direction </li></ul><ul><li>Vint Cerf helped establish the core Internet protocols by the end of the 1970’s – TCP, IP, and UDP </li></ul><ul><li>Robert Metcalfe defined Ethernet in 1976 </li></ul><ul><li>By 1983, ARPAnet switched to TCP/IP </li></ul>INFO 330 Chapter 1
  83. 83. A Little History <ul><li>The French installed Minitel, a public packet-switched network, in the early 1980’s, a decade before the US caught on to the Internet </li></ul><ul><li>DNS wasn’t invented until the late 1980’s (RFC 1034) </li></ul><ul><li>The World Wide Web was invented between 1989 and 1991 by Tim Berners-Lee, based on work as far back as 1945 </li></ul>INFO 330 Chapter 1
  84. 84. A Little History <ul><li>At the end of 1992 there were ~200 web servers in the world </li></ul><ul><li>In 1994 Mosaic was formed, later known as Netscape, and much of the world was introduced to the Internet </li></ul><ul><li>By the late 1990’s, peer-to-peer file sharing, instant messaging, email, and the Web formed the ‘killer apps’ that launched the world we see today </li></ul>INFO 330 Chapter 1
  85. 85. A Little History <ul><li>The dot-Com bubble burst by 2001, but a few companies survived </li></ul><ul><li>Through the 1990’s, issues such as security and handling of streaming video became urgent, as e-commerce became as common as a 7-11 </li></ul><ul><li>Now more devices are connected – phones, PDAs – and we can’t imagine not having the Internet at our disposal </li></ul>INFO 330 Chapter 1