Network Systems Spring 2002 CSCI 42735273

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Network SystemsSpring 2002CSCI 4273/5273

• Professor Rick Han
• University of Colorado at Boulder
• rhan_at_cs.colorado.edu

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Network Systems

• Goals
• Learn how the Internet works under the covers!
• Learn general computer networking concepts and
  principles
• Obtain practical experience in network
  programming
• Have fun in the process
• Information for CSCI 4273/5273
• http//www.cs.colorado.edu/rhan, then
• follow the Network Systems link at the top

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Announcements

• Textbooks
• Colorado book store had 19 as of 1/7/2002
• CU book stores order of 40 should arrive
  this/next week
• Waiting List
• Students who have not satisfied the OS
  prerequisite should drop
• Admit students from wait list
• Fire regulations limit class size
• Questions so far?
• Next, a preview and Chapter 1

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Historical Perspective on Computer Networking

• 1957 USSR launches Sputnik. In response, US
  forms Advanced Research Projects Agency (ARPA)
  with Department of Defense (DoD)

• 1969 ARPANET commissioned by DoD for research
  into networking.
• Kleinrock, 4 nodes, packet-switched, 50 kbps
  links
• Do you see the L?
• Yes, we can see the L,
• Do you see the O?
• Yes, we see the O

• Then we typed the G, and the system crashed

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Historical Perspective on Computer Networking (2)

• 1971 Logical map of the ARPANET

• 1972 ARPA renamed to DARPA

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Historical Perspective on Computer Networking (3)

• 1971 email invented, _at_ sign chosen
• 1973 ARPANET becomes global with addition of
  nodes in England and Norway.
• 1976 UUCP (Unix-to-Unix copy) developed at ATT
  Bell Labs and distributed with Unix.
• 1981 IBM PC introduced
• 1982 TCP and IP defined as a protocol suite
• 1984 gt 1,000 hosts, DNS introduced, Cisco
  founded
• 1989-90 gt 100,000 hosts, ARPANET decommissioned
• 1991 727,000 unique hosts on Internet, CERNs
  Berners-Lee invents Web (HTML, HTTP, URL)
• 1993 Mosaic developed by Andreessen at NCSA
• 2001 175,000,000 unique hosts on Internet

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Bottom-Up Approach to Computer Networking

• Approach taken by the textbook
• Two hosts want to communicate with each other
• First, how do I send a bit from host A to host
  B?

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Layer 1 The Physical Layer

• Solution Host A encodes the bit into an analog
  signal. Host B decodes the analog signal into a
  received bit.

• Physical Layer, also called Layer 1, ensures that
  host B can decode a digital bit from an analog
  representation of the digital bit sent by host A
• Modem has advanced DSP to achieve 56 kbps

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Layer 2 The Data Link Layer

• Next Problem How do I send a message from Host A
  to Host B?

• Data Link Layer, also called Layer 2, ensures
  that host B can decode a digital message from a
  stream of bits sent by host A

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The Data Link Layer (cont.)

• A Data Link Layer Protocol implements
• Delimiting/framing of a message
• Fragmenting of a long message
• Retransmission of a lost message

1011000
Host A
Host B
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Defining a Protocol

• A protocol is an agreement between two parties or
  endpoints as to how information is to be
  transmitted
• A protocol implements this agreement via
• A Header
• How each endpoint responds to control info in the
  header ( external input)

Host A
Host B
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How Physical and Data-Link Layers Interact
Host A
Host B
1011000
1011000
Layer 2
Layer 2
Layer 1
Layer 1
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Defining a Service
Host A
1011000
Layer 2
Layer 1
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Communication Across a Network of Hosts
Host C
Host A
Host B

• Suppose there is a link connecting A to B, and B
  to C
• How does Host B know that a message from Host A
  should be forwarded to Host C?
• Therefore, the intermediate Host B needs
• Address information
• Topology information

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Layer 3 The Network Layer

• The Network layer is responsible for routing a
  message across an interconnected mesh of hosts

Host C
Host A
Host B
Host F
Host E
Host D

• Network often represented as a cloud
• Routers have specialized hardware OSs, but
  initially were just computer hosts

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Internet Communication Across a Network of
Networks

• The Internet is a network of heterogeneous
  networks

Host A
Host F

• The Internet Protocol, or IP, is an example of a
  Network Layer protocol
• IP routers at entry/exit to cloud ( possibly
  within cloud)

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Design Considerations for the Internet

• Correctness can IP route a packet correctly
  from source to destination?
• Reliability if a router crashes/joins, can the
  Internet adapt to changes in topology?
• Scalability can IP routing scale to support
  millions of nodes?
• Performance can the packet quickly be routed to
  its destination?
• Efficiency how much overhead does the routing
  take?

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Design Philosophies for the Internet

• Early (and winning) design philosophy keep the
  network simple
• Smart clients dumb network Internet
• In contrast, telephone network dumb clients,
  smart network hard to introduce new services
• A second design philosophy share the wealth
• Bandwidth is statistically multiplexed, or shared
• In contrast, telephone network sets up a
  dedicated circuit that is not shared.

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Layer 4 The Transport Layer

• The Transport Layer offers end-to-end delivery of
  packets across a network
• Packets may be lost due to stat mux congestion or
  router failure

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Internet Transport Layers TCP and UDP

• The Transmission Control Protocol, or TCP, is a
  reliable Transport Layer protocol above IP
• Reliable delivery
• In-order or stream delivery
• The User Datagram Protocol, or UDP is an
  unreliable protocol above IP
• Unreliable packet delivery
• Out-of-order packet delivery
• Why would you ever want unreliable out-of-order
  delivery?

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Layer 5 The Internet Application Layer

• Given reliable in-order delivery by TCP, the
  Internet Application Layer builds
  application-level protocols above TCP like
• HTTP Hypertext Transfer Protocol
• SMTP Simple Mail Transfer Protocol
• FTP File Transfer Protocol
• NNTP Network News Transfer Protocol
• The Open Systems Interconnection (OSI) model
  expands its Application Layer into 3 sub-layers
  Application, Presentation, and Session

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The Layered Network Stack
Internet Stack
Application Layer
Transport Layer (TCP/UDP)
Network Layer (IP)
Data Link Layer
Physical Layer