Course Introduction

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Course Introduction

• OSI reference model
• services
• interfaces
• protocols
• TCP/IP reference model
• Example protocols

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Where do the layers reside?

• Which layers are implemented by OS?
• The interfaces between OS and applications
• Socket API

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What is the course about?

• Application designs
• Client designs
• Server designs
• Application implementations
• Socket APIs
• Interaction of applications and TCP/IP stack
• Application protocols
• RTP, RTSP, SIP

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Standard Application Protocols

• Standard applications well documented,
  standardized and adopted as official TCP/IP suite
• Examples
• email
• telnet
• ftp
• More?

• Let us look more closely at telnet
• (multiple choice) telnet application can be used
  to
• A. Remotely access terminals
• B. Remotely access services
• C. Send an email
• D. Fetch a web page

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Fetching a web page using telnet

• 1. Telnet to your favorite Web server

Opens TCP connection to port 80 (default http
server port) at cis.poly.edu. Anything typed in
sent to port 80 at cis.poly.edu
telnet cis.poly.edu 80

• 2. Type in a GET http request

By typing this in (hit carriage return twice),
you send this minimal (but complete) GET request
to http server
GET /ross/ HTTP/1.0
3. Look at response message sent by http server!
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Scenario Alice sends message to Bob

• 4) MTA (Message Transfer Agent) in mail server
  sends Alices message over the TCP connection
• 5) Bobs mail server places the message in Bobs
  mailbox
• 6) Bob invokes his user agent to read message

• 1) Alice uses MUA to compose message and to
  bob_at_someschool.edu
• 2) Alices MUA sends message to her mail server
  message placed in message queue
• 3) Client side of SMTP opens TCP connection with
  Bobs mail server

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Sample smtp interaction
S 220 hamburger.edu C HELO crepes.fr
S 250 Hello crepes.fr, pleased to meet
you C MAIL FROM ltalice_at_crepes.frgt
S 250 alice_at_crepes.fr... Sender ok C RCPT
TO ltbob_at_hamburger.edugt S 250
bob_at_hamburger.edu ... Recipient ok C DATA
S 354 Enter mail, end with "." on a line
by itself C Do you like ketchup? C
How about pickles? C . S 250
Message accepted for delivery C QUIT
S 221 hamburger.edu closing connection
More messages?
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Sending an email using telnet

• telnet servername 25
• HELO your own domain name
• MAIL FROM ltyour own email addressgt
• RCPT TO ltrecipient addressgt
• DATA
• QUIT

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What can we learn here?

• telnet goes beyond standard terminal access and
  provides basic interactive communication facility
• telnet protocol provides maximal flexibility
  because it only defines interactive communication
  but not the details of the service accessed
• Users at many sites may want to access the same
  service at the same time. It is important yet
  challenging to provide concurrent services

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Client-server model

• Typical network app has two pieces client and
  server

• Client
• initiates contact with server (speaks first)
• typically requests service from server,
• for Web, client is implemented in browser for
  e-mail, in mail reader
• Server
• provides requested service to client
• e.g., Web server sends requested Web page, mail
  server delivers e-mail

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More on Client Server Model

• Clients initiate requests
• Servers wait for incoming requests and provide
  services
• Servers always up and running
• Operating systems normally start servers
  automatically when they start
• More differences
• Servers access system resources gt special
  privileges
• Servers sometimes handle security issues

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Connectionless vs. Connection-Oriented Protocols

• Connectionless protocols
• UDP
• Fast
• Unreliable
• Connection-oriented protocols
• TCP
• Slower
• Reliable
• Examples

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Stateless vs. Stateful Protocols

• Stateful protocols
• Keeping states in the servers
• Reducing the size and number of messages
• Applications sometimes need session information
• Stateless protocols
• Simple
• Protocol reliability
• example
• Stateless file server
• Stateful file server

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How to keep track of clients?

• Two ways
• End points (IP address, port number)
• What is wrong?
• Handles
• A short identifier allocated by the server,
  stored in servers database and sent to the
  client

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Keeping track of clients handles

• user accounts
• shopping carts
• Web portals
• Advertising
• Secretly collecting users browsing habits
• What to do?

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More on Stateful Protocols

• Keeping correct states is challenging
• What happens when clients crash and reboot?
• What happens when messages arrive out of order,
  get duplicated or get lost?
• Keeping states sometimes poses security risks
• Tracking browsing patterns
• Loss of confidential information (e.g. credit
  card number, ssn)

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Concurrent Processing

• Concurrent Processing
• Why is it needed?
• Achieving concurrent processing
• Time sharing
• multiprocessing
• Concurrency in clients
• no special efforts by client programmers
• Concurrency in servers
• Special efforts needed by server programmers
• Multiple processes
• Multiple threads

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Processes and Threads

• Processes
• Fundamental unit of computation
• An address space and at least one thread of
  execution (instruction pointer)
• Thread
• Light-weight process
• Program vs process
• Static concept

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Sharing of Variables

• Local Variable Rule - When multiple processes or
  threads execute a piece of code concurrently,
  each creates its own independent copy of the
  variables associated with the code (local
  variables)
• Global Variable Rule - Each process creates a
  copy of global variables, but multiple threads
  within a single process share the processs
  global variables

• int x
• for (i1 ilt10 i)
• printf (d\n, ix)

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Procedure Calls

• Procedure Call Rule - When multiple threads
  execute a piece of code concurrently, each has
  its own stack of procedure activation records
• Example - Putting the rules together!

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Example of Concurrent Process Creation

• Example 1
• Sequential processing
• Example 2
• Concurrent version
• Computationally light
• Computationally intensive
• Both processes do the same job!

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Sequential C Example
/ seq.c - A conventional C program that sums
integers from 1 to 5 / include
ltstdlib.hgt include ltstdio.hgt int sum
/ global variable / main() int i
/ local variable / sum 0 for
(i1 ilt5 i) / iterate from 1 to 5 /
printf(The value of i is d\n, i)
fflush (stdout) / flush buffer /
sum i printf (The sum is d\n,
sum) exit (0)
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Concurrent C Example
/ concurr.c - A concurrent C program that sums
integers from 1 to 5 / include
ltstdlib.hgt include ltstdio.hgt int sum
/ global variable / main() int i
/ local variable / sum 0
fork() for (i1 ilt5 i) / iterate
from 1 to 5 / printf(The value of i is
d\n, i) fflush (stdout) / flush
buffer / sum i printf
(The sum is d\n, sum) exit (0)
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Concurrent Processes (II)

• Processes perform different tasks
• Distinguished by the return value of fork()
• fork and execve allow newly created processes to
  execute an independent, separately-compiled
  program

/ sum.c - A concurrent C program that performs
different tasks for two processes / include
ltstdlib.hgt include ltstdio.hgt main() int
pid / local variable /
pidfork() if (pid ! 0) / original
process / printf(The original process
prints this.\n) else /
newly created process / printf (The new
process prints this.\n) exit(0)
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Concurrency in I/O

• Let us take a browser as an example
• It needs to respond to input from users
• Mouse clicks
• Keyboard input
• It needs to respond to input from the network
• Downloaded web pages, images,
• It needs to send request to the network
• Send a HTTP request
• How to handle this scenario?
• Asynchronous I/O (aka non-blocking I/O)
• Concurrency in I/O
• select system call

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Lab Exercises

• Use telnet to send an email to yourself
• Concurrent processing
• Modify concurr.c so that the messages printed
  from parent and child processes are different
• Observe how the messages from parent and child
  are interleaved in the output
• Write and compile the summation procedure as a
  separate program and use execv() to call it in
  the child process