CS 241 Section Week

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CS 241 Section Week 12(04/23/09)
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Outline

• LMP2 Overview
• Brief intro to Networking
• What is a protocol?
• OSI Model
• TCP/IP Model
• UDP and TCP
• Socket Programming Library Functions
• Hypertext Transfer Protocol

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LMP2 Overview
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LMP2 Overview

• LMP2 encodes or decodes a number of files
• It has the following parameters
• the filenames
• the number of bytes (rw_units) for each
  read/write from the file
• But, process files in round robin fashion
• encrypt rw_unit of file1
• encrypt rw_unit of file2
• encrypt rw_unit of file3
• encrypt rw_unit of file1

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LMP2 Overview

• For the output in 2nd part,
• print in get_frame() will suffice
• you may print in my_munmap.
• Most csil machines, use signed char, whose range
  is from -128 to 127. So use unsigned char if you
  compare with 255.

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LMP2 Overview

• For N files, -The Table of File-Mappings has
  exactly N entries -The Virtual Page Table has
  enough entries to fit each file's length divided
  by 4096B -The Physical Memory Frame Table has 16
  entries
• Example 3 files of length 3072bytes, 2048bytes,
  and 8192bytes -The Table of File-Mappings has 3
  entries-The Virtual Page Table has
• (3072/4096gt1) (2048/4096gt1) (8192/4096gt2)
  4 entries -The Physical Memory Frame Table has
  16 entries and four of them are filled

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Brief intro to Networking
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Networking

• Allows computers and other networked devices to
  talk to each other
• How can we tell what the packet we just received
  means?
• Interactions between applications on different
  machines are governed by protocols
• Protocols dictate the format and sequence of the
  information exchange

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Names and Addresses

• A network address identifies a specific computer
  on the network
• Several kinds of addresses exist MAC address
  (for LANs), IP address (for the Internet), etc.
• Domain or DNS names are used for convenience, so
  we dont have to remember numerical addresses

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Ports

• Ports are numbers that represent an end-point for
  communication
• Ports are logical, not physical, entities
• All packets arrive to the same physical
  interface, and are then differentiated based on
  the port number (and other contents of the packet
  header)
• Usually, distinct ports are used for
  communication via different protocols
• E.g., port 80 is for HTTP, 22 is for SSH, etc.
• See /etc/services for a list

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What is a protocol?
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What is a protocol?

• It is a formal description of message formats and
  the rules that two computers must follow in order
  to exchange messages.
• This set of rules describes how data is
  transmitted over a network.

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Why are protocols needed?

• Protocols are needed for communication between
  any two devices.
• In what format will the messages be transmitted?
• At what speed should messages be transmitted?
• What to do if errors take place?
• What to do if parts of a message are lost?

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Network Model

• What is a model? A hypothetical description of
  a complex entity or process.
• Network model - A method of describing and
  analyzing data communications networks by
  breaking the entire set of communications process
  into a number of layers
• Each layer has a specific function

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Open Systems Interconnect (OSI) Model

• Who made
• International Standards Organization (ISO)
• A Model of How Protocols and Networking
  Components Could be Made
• Open means the concepts are non-proprietary
  can be used by anyone.
• OSI is not a protocol. It is a model for
  understanding and designing a network
  architecture that is flexible and robust.

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Network Architecture

• A set of layers and protocols is called a network
  architecture
• It refers to the physical and logical design of a
  network

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7-layer OSI model

• Why so many layers?
• To reduce complexity, networks are organized as a
  stack of layers, one below the other
• Each layer performs a specific task. It provides
  services to an adjacent layer
• This is similar to the concept of a function in
  programming languages function do a specific
  task

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The Layers of the OSI Model
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The Layers of the OSI ModelSome Mnemonics
All People Seem To Need Data Processing
Please Do Not Tell Secret Passwords Anytime
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Physical layer

• Specifications for the physical components of the
  network
• Bit representation encode bits into electrical
  or optical signals
• Transmission rate the number of bits sent each
  second

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Data Link Layer

• Responsible for delivery of data between two
  systems on the same network. Main functions are
• Framing divides the stream of bits received from
  network layer into manageable data units called
  frames.
• Physical Addressing add a header to the frame to
  define the physical address of the source and the
  destination machines.

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Network Layer

• Main functions of this layer are
• Responsible for delivery of packets across
  multiple networks.
• Routing Provide mechanisms to transmit data over
  independent networks that are linked together.

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Transport Layer

• Main functions of this layer are
• Responsible for source-to-destination delivery of
  the entire message.
• Segmentation and reassembly divide message into
  smaller segments, number them and transmit.
  Reassemble these messages at the receiving end.

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Session Layer

• Main functions of this layer are
• Dialog control allows two systems to enter into
  a dialog, keep a track of whose turn it is to
  transmit.

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Presentation Layer

• Responsibilities of this layer are
• Translation since different computers use
  different encoding systems (bit order translation

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Application Layer

• Contains protocols that allow the users to
  access the network (FTP, HTTP, SMTP, etc)
• DOES NOT include application programs such as
  email, browsers, word processing applications,
  etc.

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Summary of Functions of Layers
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TCP/IP Model

• Layers in the TCP/IP model
• Physical Layer
• Data Link Layer (MAC)
• Network Layer (IPv4, IPv6)
• Transport Layer (UDP, TCP)
• Application Layer (HTTP, SSH)

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Networking

• Each layer is (traditionally) self-contained.
• The data-link layer doesnt care if its running
  IPv4 or IPv6, doesnt care it its TCP or UDP.
• When processing the data-link layer, no other
  layer is (traditionally) considered.

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Networking
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Networking

• Data Link Layer
• Purpose To transfer packets in a flat addressing
  space.
• Multiple packets go into a hub.
• The hub bundles packets together to send them
  upstream if necessary, based only on the MAC
  address.
• Not significantly covered in this class.

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Networking

• Network Layer
• Purpose To transfer packets in a hierarchical
  addressing space.
• Streams of packets go into switches.
• Switches determine if the address is within its
  subnet and will route based on subnets
  (IPv4/IPv6).
• Also, not significantly covered in this class.

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Networking

• Transport Layer
• After the data link and network layer, packets
  have arrived at the destination computer.
• The transport layer
• Provides port multiplexing, allowing for
  multiple programs all communicate without
  receiving every packet.
• Protocol-specific assurances.

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UDP

• User Datagram Protocol (UDP)
• The simplest commonly used transport protocol.
• Provides
• Port multiplexing
• Error-checking (via simple checksum)

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UDP

• User Datagram Protocol (UDP)
• Does NOT provide
• Ordering
• Packet A may be sent before Packet B, but Packet
  B may arrive after Packet A
• Reliability
• The sender has no way to know that Destination
  received some packet sent by Sender (assured by
  retransmissions)
• Flow / Congestion Control
• Theres no mechanism to tweak packet size

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UDP

• User Datagram Protocol (UDP)
• Only a few parameters are needed for UDP
• Source Port
• Destination Port
• From the network layer (IP), you have two more
  parameters
• Source IP
• Destination IP

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UDP

• UDP Packet

Source Port
Destination Port
Data Length
Checksum
Data
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TCP

• Transmission Control Protocol
• TCP provides an alternative to UDP to allow for
  more assurances
• Provides
• Port multiplexing
• Ordered data / no duplicates
• Error-free transmission
• Flow control
• Congestion control

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TCP

• TCP Packet

Source Port
Destination Port
SEQ Number
ACK Number
Offset / Flags
Window Size
Checksum
Urgent Pointer
Optional Fields
Data
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Socket Programming
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Socket

• Standard APIs for sending and receiving data
  across computer networks
• Introduced by BSD operating systems in 1983
• POSIX incorporated 4.3BSD sockets and XTI in 2001
• include ltsys/socket.hgt

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Typical TCP Server-Client
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Typical UDP Server-Client
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Programming Sockets

• Lucky for us, the OS does most the work between
  UDP and TCP!
• TCP Socket
• socket(PF_INET, SOCK_STREAM,
  IPPROTO_TCP)
• UDP Socket
• socket(PF_INET, SOCK_DGRAM, 0)

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Programming Sockets

• To create a socket in C, you need to run two
  commands
• socket()

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socket

• int socket(int domain, int type, int protocol)
• Creates a communication endpoint
• Parameters
• domain AF_INET (IPv4)
• type SOCK_STREAM (TCP) or SOCK_DGRAM (UDP)
• protocol 0 (socket chooses the correct protocol
  based on type)
• Returns a nonnegative integer corresponding to a
  socket file descriptor if successful, -1 with
  errno set if unsuccessful

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Programming Sockets

• To create a socket in C, you need to run two
  commands
• socket()
• bind()

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bind

• int bind(int socket, const struct sockaddr
  address, socklen_t address_len)
• Associates the socket with a port on your local
  machine
• struct sockaddr_in used for struct sockaddr
• sa_family_t sin_family / AF_INET /
• in_port_t sinport / port number /
• struct in_addr sin_addr / IP address /
• Returns 0 if successful, -1 with errno set if
  unsuccessful

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Programming Sockets

• Since UDP is packet-based and TCP is
  connection-based, you need to establish a
  connection in TCP
• Server listen(), accept()

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listen

• int listen(int socket, int backlog)
• Puts the socket into the passive state to accept
  incoming requests
• Internally, it causes the network infrastructure
  to allocate queues to hold pending requests
• backlog number of connections allowed on the
  incoming queue
• bind should be called beforehand
• Returns 0 if successful, -1 with errno set if
  unsuccessful

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accept

• int accept(int socket, struct sockaddr restrict
  address, socklen_t restrict address_len)
• Accepts the pending requests in the incoming
  queue
• address is used to return the information about
  the client making the connection.
• sin_addr.s_addr holds the Internet address
• listen should be called beforehand
• Returns nonnegative file descriptor corresponding
  to the accepted socket if successful, -1 with
  errno set if unsuccessful

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Programming Sockets

• Since UDP is packet-based and TCP is
  connection-based, you need to establish a
  connection in TCP
• Server listen(), accept()
• Client connect()

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connect

• int connect(int socket, const struct sockaddr
  address, socklen_t address_len)
• Establishes a link to the well-known port of the
  remote server
• Initiates the TCP 3-way handshake
• Cannot be restarted even if interrupted
• Returns 0 if successful, -1 with errno set if
  unsuccessful

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Programming Sockets

• In both TCP and UDP, you send and receive by
  using the same calls
• recv() / recvfrom()

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recv and recvfrom

• int recv(int socket, void buf, int len, int
  flags)
• int recvfrom(int socket, void buf, int len, int
  flags, const struct sockaddr from, socklet_t
  fromlen)
• receives data into the buffer buf
• recvfrom is used for unconnected datagram
  sockets. If used in connection-mode, last two
  parameters are ignored.
• Returns the number of bytes actually read if
  successful, -1 with errno set if unsuccessful

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Programming Sockets

• In both TCP and UDP, you send and receive by
  using the same calls
• recv() / recvfrom()
• send() / sendto()

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send and sendto

• int send(int socket, const void msg, int len,
  int flags)
• int sendto(int socket, const void msg, int len,
  int flags, const struct sockaddr to, socklet_t
  tolen)
• sends data pointed by msg
• sendto is used for unconnected datagram sockets.
  If used in connection-mode, last two parameters
  are ignored.
• Returns the number of bytes actually sent out if
  successful, -1 with errno set if unsuccessful

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close and shutdown

• int close(int socket)
• int shutdown(int socket, int how)
• close
• Prevents any more reads and writes
• same function covered in file systems
• shutdown
• provides a little more control
• how
• 0 Further receives are disallowed
• 1 Further sends are disallowed
• 2 same as close
• Returns 0 if successful, -1 with errno set if
  unsuccessful

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Using Sockets in C

• include ltsys/socket.hgt
• include ltsys/types.hgt
• include ltnetinet/in.hgt
• include ltunistd.hgt
• On csil-core
• gcc o test test.c
• On some systems, e.g., Solaris
• gcc o test test.c lsocket -lnsl

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TCP Client/Server Example

• Run the provided test-server and test-client
  executables in two separate windows.
• test-client sends the string Hello World! to IP
  address 127.0.0.1 port 10000
• test-server listens on port 10000 and prints out
  any text received
• Next week, we will reproduce this behavior with
  codes client.c and server.c

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HyperText Transfer Protocol
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HTTP

• Hypertext Transfer Protocol
• Delivers virtually all files and resources on the
  World Wide Web
• Uses Client-Server Model
• HTTP transaction
• HTTP client opens a connection and sends a
  request to HTTP server
• HTTP server returns a response message

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HTTP (continued)

• Request
• GET /path/to/file/index.html HTTP/1.0
• Other methods (POST, HEAD) possible for request
• Response
• HTTP/1.0 200 OK
• Common Status Codes
• 200 OK
• 404 Not Found
• 500 Server Error

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Sample HTTP exchange

• Scenario
• Client wants to retrieve the file at the
  following URL (http//www.somehost.com/path/file.h
  tml)
• What a client does
• Client opens a socket to the host
  www.somehost.com, port 80
• Client sends the following message through the
  socket
• GET /path/file.html HTTP/1.0
• From someuser_at_uiuc.edu
• User-Agent HTTPTool/1.0
• blank line here

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Sample HTTP exchange

• What a server does
• Server responds through the same socket
• HTTP/1.0 200 OK
• Date Mon, 17 Apr 2006 235959 GMT
• Content-Type text/html
• Content-Length 1354
• lthtmlgt
• ltbodygt
• (more file contents)
• .
• .
• .
• lt/bodygt
• lt/htmlgt

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Reference

• Beej's Guide to Network Programming
• http//beej.us/guide/bgnet/

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Summary

• Protocols
• Socket Programming
• Library Functions
• Hypertext Transfer Protocol
• Request
• Response