Computer Science 101 A Survey of Computer Science

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Computer Science 101A Survey of Computer Science

• Networks
• Communication Protocols

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Telephone Protocol

• Hellopause Hello, Prof Whaley Judy from
  class, sorry power is off.Duh pausepau
  se About our exam pause Toodle, Prof
  WhaleyBye, Judy

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Protocols

• Network Protocol agreed upon set of rules,
  conventions, and agreements for the efficient and
  orderly exchange of information.

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Internet Society

• Internet Society
• Non-profit, non-governmental, professional
  society composed of more than 150 worldwide
  organizations
• Establish and enforce network protocol standards
• www.isoc.org

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Protocol Hierarchy (TCP/IP)

• Internet protocols are in layers, each layer
  addressing an aspect of the communication task.
• Change in operation of layers are independent.

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Physical Layer

• Physical layer protocols rules governing the
  exchange of bits across a physical communication
  channel such as fiber optic cable, copper wire,
  wireless, etc.
• How do we know a bit is present on line?
• How long will it remain on the line?
• Digital or analog?
• What voltage levels for 0 or 1?
• Shape of connector between computer and line?

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Data Link Layer

• Data link protocols Deals with
• Error handling errors in the bit stream
• Framing problem Identify start and end of
  message in the bit stream.
• Two stages
• Layer 2a medium access control
• Layer 2b logical link control

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Data Link Layer Medium Access Control

• Medium Access Control Who has access to shared
  line
• Contention-based approach Most common method
• When node wants to send message
• It listens to see if line is in use, if not it
  sends
• If in use, monitors line until it is idle and
  then transmits

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Contention-based protocol
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Data Link Layer Logical Link Control

• Logical Link Control Ensure that message
  traveling across channel (shared or
  point-to-point) arrives correctly.
• Basic idea is to detect that an error has
  occurred and retransmit the original message.

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ARP Automatic Repeat reQuest Algorithm

• Packet makeup
• Start of packet delimiter
• Packet identifier sequence number
• Message
• Error-check bits
• End of packet delimiter

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ARP Algorithm

• A
• sends packet to B
• keeps copy of packet in memory
• waits
• if acknowledgement received that B received
  packet without error
• discard
• send next packet
• else after some fixed time
• resend packet

• B
• receives packet
• does error test
• if OK
• send acknowledgement to A that received ok

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ARP Algorithm
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Network Layer

• Network layer protocols
• so far just dealt with point-to-point
• two critical responsibilities for network layer
• create universal addressing scheme for all
  network nodes
• deliver messages between any two nodes in the
  network
• glue that holds entire network together
• network layer in internet is called Internet
  protocol or IP

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Network Layer - IP

• Internet Protocol
• each node has 32-bit IP address
• IP address often written as four eight bit
  numbers in range 0-255 (00..00 to 11..11)
• IP address of host with the Alsos project
  137.113.100.111 or10001001 01110001 01100100
  01101111

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Network Layer - DNS

• Domain Name Service
• converts symbolic host names like www.cs.wlu.edu
  into its 32-bit IP address
• Huge database distributed over thousands of
  computers
• When symbolic name is used, it is forwarded to
  local name server.
• If found, IP sent back
• If not, request is forwarded to remote name
  server, etc.

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Network Layer - Routing

• Routing
• Finding best path to use from A to B
• Brute force would take too long
• Dijkstras shortest path algorithm is O(n2) where
  n is number of nodes
• Still a very hard problem
• Topology is constantly changing
• Network failures
• Busy links

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Transport Layer Port Addresses

• Transport layer protocols Ports
• How do we get message to correct program on host?
• Need address, in addition to IP, that identifies
  which program is needed on the machine
• Host machine uses port numbers assigned to
  programs
• There are well-known port numbers so we dont
  always have to know the port number for example
  web servers almost always run on port 80

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Transport Layer TCP

• Transport layer protocols TCP
• Transport Control Protocol create high-quality,
  error free, order preserving end-to-end delivery
  service on top of unreliable IP
• Requires that two programs at source and
  destination create a connection an agreement to
  exchange messages.
• Then an acknowledgement and sequence system is
  used to be sure all messages are received and
  placed in correct order.

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Application Layer

• Application layer protocols
• Deals with the application services, different
  protocols for different killer apps

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Application Layer - HTTP

• Hypertext transfer protocol
• Used for World Wide Web
• Single web page identified by Uniform Resource
  Locator URL protocol//host address/page
  http//www.cs.wlu.edu/whaleyt/classes/101/index.h
  tml
• Connection must be established between browser of
  user and web server running on port 80 of host

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Application Layer - HTTP

• Hypertext transfer protocol continued
• Request message sent from browser to host
• message includes several parts such as protocol,
  page name, host name, language, etc.
• Response message sent from host to browser
• message has information that server was
  successful, length and name of page, content
  type, etc. as well as the content itself.

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Alsos Log File Entry
2003-12-09 (Date) 131109
(Time) 137.113.118.71 (Source
IP) 137.113.100.111 (Host IP) 80 (Port) GET
(Method) /adv_rst.asp (URI
stem) tfieldOakRidgeSelectionKeywordsourceal
lresults10 (Query) 200 (Status) Mozill
a/4.0(compatibleMSIE6.0WindowsNT5.1 .N
ETCLR1.0.3705.NETCLR1.1.4322)
(Browser) http//alsos.wlu.edu/history.asp
(Referer)