COMS W3156: Software Engineering, Fall 2001

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COMS W3156Software Engineering, Fall 2001

• Lecture 5 Tools and Networking
• Janak J Parekh
• janak_at_cs.columbia.edu

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Administrativia

• Recitation tomorrow most likely early afternoon
• Permanent time yet to be set (hopefully by
  classtime)
• Late group proposals
• Well have to return assignments tomorrow
• Requirements will be out tomorrow night, barring
  unforeseen problems
• Notes going out between noon and 1pm, soon 9am

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Next class

• We go object-oriented in a big way
• Read UML chapters 3-5, Schach chapter 12
• Object-oriented specification using UML
  technologies what you will be doing for the next
  milestone
• Were skipping around to prepare for milestones
• Might want to consider reading chapters 1-2 of
  UML pretty short, useful intro

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Todays class

• Finish up chapter 5 analytical tools
• Networking basics

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Stepwise refinement (I)

• Based on the principle of functional abstraction
• Worry about the high-level details first, smaller
  problems can be dealt with later
• Useful for specification, design and
  implementation
• In implementation, though, you sometimes need the
  smaller pieces first

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Stepwise refinement (II)

• Purchase order system
• Add purchase orders
• Edit/delete purchase orders
• Print purchase orders
• Fulfill purchase orders
• Now, drill down one level

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Stepwise refinement (III)

• Add purchase orders
• Get input from user
• Validate input
• Compute totals on PO
• Add to database
• Edit/delete similar to previous
• Print purchase orders
• Request which to print from user
• Format for printout
• Print

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Stepwise refinement (IV)

• Fulfill purchase orders
• Identify purchase order to fulfill
• Have user enter quantity shipped
• Determine if purchase order is completely
  fulfilled
• Now, one drills down, and down, and down
• I dont particularly like Schachs example, but
  the process is valid

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Cost-benefit analysis

• Is it worth computerizing? Not always
• Not straightforward intangible benefits
• Need to make assumptions
• Complex science, mostly outside of the scope of
  this course well touch upon it during chapter 9

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Software metrics (I)

• Many different kinds
• Lines of code (LOC), or thousand-lines-of-code
  (KLOC)
• MTBF
• Person-months of work, turnover
• etc.

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Software metrics (II)

• Five fundamental metrics
• Size (KLOC others in chapter 9)
• Cost ()
• Duration (months)
• Effort (person-months)
• Quality ( of faults detected)
• Must be measured per each phase
• Note Metrics are controversial

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Networking

• Introduction to networking essentials
• History and background
• Layers of networking abstraction
• LAN networks Ethernet
• WAN networks Internet
• TCP/IP addressing, routing, transmission, DNS
• Socket programming
• See http//www.novell.com/info/primer/primer.html
  
• How much do you know?

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History and background

• We define modern computer networks to be
  packet-based networks encapsulate data in
  small packets that are sent out one-at-a-time
• Compare to a telephone, which has a complete
  circuit you can include many more parties in the
  conversation
• This stemmed out of DARPA research ARPAnet in
  the 70s and 80s
• TCP/IP eventually made public, led to Internet

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Networking abstraction

• Similar to functional abstraction, but slightly
  different
• Many different models OSI reference model
• Most include (example)
• physical/hardware electric signals (Ethernet)
• network deal with nodes addressing (IP)
• transport transmit data efficiently and reliably
  (TCP)
• application allow programs to communicate
  (sockets)
• Protocol language on many different levels
  www.ietf.org

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LAN Ethernet

• Physical and electronic transmission medium
• Very cheap and popular, available in 10, 100, and
  1000Mbps varieties
• Communication via CSMA/CD
• Shout the message to everyone else
• If confusion, shut up for a bit, then try again
• Many, many others

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WAN Internet

• Really not analogous to Ethernet
• Once known as the ARPAnet
• Internet comprised of many local networks
  interconnected using large telecommunication
  links
• Links generally digital-circuit-based, over which
  packet-based networks are run
• http//www.cs.bell-labs.com/who/ches/map/gallery/i
  sp-ss.gif

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TCP/IP

• Transmission Control Protocol/Internet Protocol
• Its a misnomer
• IP is solely responsible for addressing and
  routing computers across the Internet
• TCP is a means of transmitting data over IP, and
  not the only one (UDP, ICMP, GRE, )
• IP is the most popular protocol today
• Current version is IPv4, but new one out (IPv6)

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IPv4 (I)

• Addressing and routing protocol
• Defines packets to be delivered to or routed
  between machines
• Each node on an IPv4 network has a 4-byte address
• Commonly represented using quad notation, e.g.
  www.xxx.yyy.zzz

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IPv4 (II)

• Each packet contains source IP address,
  destination IP address, checksum, length, and
  other fields
• Gruesome details at http//www.ietf.org/rfc/rfc079
  1.txt?number0791
• This is a request-for-comment (not anymore for
  IP) all standard Internet protocols are defined
  by an RFC
• John Postel father of the Internet
• IP will attempt to deliver this packet for you to
  the destination node

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Subnets and routing (I)

• How can IP deliver? Two possibilities
• Target machine is locally reachable, i.e.,
  connected via LAN
• Give that machine the packet
• Target machine is not local
• Give the packet to a router, i.e., a machine that
  knows where it should go
• Subnetting determines which machines are local
  and which arent

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Subnets and routing (II)
Janakscomputer
Other Columbia networks
CS network (128.59.16.0-128.59.23.255)
Router
Router
Router
Columbias network (128.59.x.y)
Big-ass router
Big-ass router
note this isstrictly approximate
OC3(155Mbps)
OC3(155Mbps)
Applied Theory andthe Internet
Internet2(other research inst)
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Subnets and routing (III)

• tracert/traceroute will do what its name implies
• Uses packet time-to-live field increments it and
  sees how far it goes

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TCP

• Transmission Control Protocol
• Mechanism of handling reliable transmission of
  data over IP connection
• Packets delivered in order
• Retransmission if one is not received on the
  other side
• Ports to handle many connections

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UDP

• User Datagram Protocol
• Not connection-oriented send the packet and
  forget about it if the other side doesnt
  receive it, or its out of order, oh well, let
  the program worry about it
• Also has port construct

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Servers and services

• Both UDP and TCP allow for listening to ports
  for incoming data (TCP inbound connections)
• Common ones include
• 21 ftp
• 23 telnet
• 25 smtp (mail)
• 80 www
• 110 pop3
• Most OSs allow 65,535 ports!
• Try telnet www.columbia.edu 80

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Naming computers DNS

• Domain Name System
• We have lots of nodes with IP addresses
• Can you remember 128.59.23.10?
• Instead, we name each machine (hostname), and
  then have servers that keep track of a
  name-to-IP-address mapping
• Distributed

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So, how do you do all this?

• Well, you could write the packets yourself
• Instead, we use the operating system (or
  language)s API to automate things
• Sockets (they plug into ports ?)
• http//web2.java.sun.com/docs/books/tutorial/netwo
  rking/sockets/index.html for Java
• http//world.std.com/jimf/papers/sockets/sockets.
  html for C/C (or Google) - quite a bit trickier

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Philosophy of socket programming

• Server
• Establish a server socket
• Listen and accept new incoming connections
• Talk
• Hangup
• Repeat
• Client
• Create a client socket (or socket)
• Try to connect to server
• Talk
• Hangup
• Often send strings of data over sockets

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Lets do it!