Computer Networks CSC 457

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Computer NetworksCSC 457

• Dr. Evelyne Tropper

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NETWORKS INTERNET

• Chapter 1.1

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Router
Router
Router
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Internet

• World-wide connected network of computers,
  Personal, Digital Assistants PDAs
• TVs, automobiles, toasters,
• These end systems are connected by communication
  links made of coaxial cables, copper wire, fiber
  optics and radio spectrum.
• The link transmission rate is called the
  bandwidth.
• End systems are connected to switching devices
  called routers.
• Routing is done by chunks of information called
  packets.
• Messages divided in packets share paths. This is
  called packet switching.

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Whats the Internet?

• protocols control sending, receiving of messages
• e.g., TCP, IP, HTTP, FTP, PPP
• Internet network of networks
• loosely hierarchical
• public Internet versus private intranet
• Internet standards
• RFC Request for comments
• IETF Internet Engineering Task Force

router
workstation
server
mobile
local ISP
regional ISP
company network
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(No Transcript)
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Internet Services

• End systems use Internet Service Providers ISPs
  to access the internet.
• This can be done through
• 56 Kbps dial-up modem
• Cable modem or DSL (dedicated service line)
• LAN (local area network) access
• Wireless access
• The lower-tiered ISPs are connected through
  upper-tiered ISPs, such as UUNet, Sprint,
• Private networks called intranets, are used
  within companies using firewalls.

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Whats a protocol?

• a human protocol a computer network
  protocol

Called handshaking procedure
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Protocols

• To send and receive information, rules called
  protocols are followed. The 2 most important are
  TCP (Transfer Control Protocol) and IP (Internet
  Protocol).
• End systems are called hosts. They can be clients
  (requesting services) or servers (giving
  services).
• There is a connection-oriented reliable service
  and a connectionless unreliable service.

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SERVICES

Chapter 1.2
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Connection-oriented service Transfer Control
Protocol

• The connection is established first before
  sending data. This is done by a handshaking
  procedure.
• A connection establishes buffers and state
  variables in the end systems, nothing in the
  switches (routers).
• It guarantees reliable data transfer (all data
  delivered without error in proper order).
• It guarantees flow control not too fast
  overwhelming or too slow.
• It guarantees congestion-control (congestion can
  overflow buffers lose data) by decreasing
  traffic during congestion.
• The internet connection-oriented service is TCP
  used by Telnet, SMTP, FTP, HTTP.

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Network edge connection-oriented service

• Goal data transfer between end systems
• handshaking setup data transfer ahead of time
• Hello, hello back in human protocol
• set up state in two communicating hosts
• TCP - Transmission Control Protocol
• Internets connection-oriented service

• TCP service RFC 793
• reliable, in-order byte-stream data transfer
• loss acknowledgements and retransmissions
• flow control
• sender wont overwhelm receiver
• congestion control
• senders slow down sending rate when network
  congested

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Connectionless serviceand User Datagram Protocol

• No handshaking.
• Data arrives sooner but with no guarantee.
• No flow control or congestion control.
• The internet connectionless service is UDP
• UDP is used for multimedia applications such as
  internet phone and video conferencing.

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Network edge connectionless service

• Goal data transfer between end systems
• same as before!
• UDP - User Datagram Protocol RFC 768
  Internets connectionless service
• unreliable data transfer
• no flow control
• no congestion control

• Apps using TCP
• HTTP (Web), FTP (file transfer), Telnet (remote
  login), SMTP (email)
• Apps using UDP
• streaming media, teleconferencing, DNS, Internet
  telephony

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PACKET vs CIRCUITSWITCHED NETWORKS

Chapter 1.3
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Circuit Switching

• In circuit-switching, the resources are reserved
  for the duration of the communication.
• There is a dedicated end-to-end connection. For
  links with n circuits, the connection gets 1/n of
  links bandwidth.
• A circuit is implemented with frequency-division
  multiplexing (FDM) or time-division multiplexing
  (TDM).
• An example is the telephone network.
• Transmission is at a guaranteed rate.
• Call set-up required

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Circuit Switching FDM and TDM
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Network Core Circuit Switching

• Network resources (e.g., bandwidth) divided into
  pieces
• pieces allocated to calls
• resource piece idle if not used by owning call
  (no sharing)
• Dividing link bandwidth into equal pieces by
• frequency division
• time division

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Packet Switching-Internet transmission

• In packet-switching, resources are not reserved.
• Applications exchange messages that can contain a
  control function or data.
• The origin of the message breaks it up into
  packets - reduces delay by 3, or does pure
  message-switching.
• The packets pipeline the message transmission
  (parallelizes). It uses statistical-multiplexing.
• Each packet switch (router), uses
  store-and-forward transmission (switch must
  receive entire packet before forwarding). For L
  bits at R bps, the delay at switch is L/R.
• Each router maintains an output queue creating
  queuing delays.
• If the queue is full, there might be a packet
  loss.
• A mixed mode, with reservation for the
  connection, but no transmission rate guarantee,
  is seen in Asynchronous Transfer Mode ATM.

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Packet Switching Statistical Multiplexing
10 Mbs Ethernet
C
A
statistical multiplexing
1.5 Mbs
B
queue of packets waiting for output link

• Sequence of A B packets does not have fixed
  pattern ? statistical multiplexing.
• In TDM each host gets same slot in revolving TDM
  frame.

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Packet-switching store-and-forward

• packet of L bits on to link of R bps takes L/R
  seconds to transmit (push out)
• Entire packet must arrive at router before it
  can be transmitted on next link store and
  forward
• delay 3L/R

• Example
• L 7.5 Mbits
• R 1.5 Mbps
• delay 15 sec

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Packet Switching Message Segmenting

• Break up the message into 5000 packets

• Each packet 1,500 bits
• 1 msec to transmit packet on one link
• pipelining each link works in parallel
• Delay reduced from 15 sec to 5.002 sec

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Packet switching versus circuit switching

• Packet switching allows more users to use
  network!

• Users share1 Mbit link
• Each user generates
• 100 kbps when active
• active 10 of time
• circuit-switching
• At most 10 users
• packet switching
• with 35 users, probability of 10 active users lt
  .0004

N users
1 Mbps link
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Packet switching withVirtual circuit networks

• The routing switches can use destination
  addresses datagrams (as in internet), or
  virtual circuit (VC) numbers for each link ( as
  in frame relay, Asynchronous Transfer Mode).
• A VC network must replace the VC number on each
  link using a VC-number translation table.
• It must maintain state information.
• Overly complex

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Packet switching with Datagram networks

• Each packet has a header with the packets
  destination
• Each switch has a table that maps forwarding
  addresses
• Switch examines addresses table indices to find
  appropriate outbound link
• Switches do not maintain connection state
  information

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Network Taxonomy
Telecommunication networks

• Datagram network is not either
  connection-oriented or connectionless.
• Internet provides both connection-oriented (TCP)
  and connectionless services (UDP) to apps.

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How do loss and delay occur?

• Packets are queued in router buffers
• packet arrival rate to link exceeds output link
  capacity
• packets queue, wait for turn

A
B
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Four sources of packet delay

• 1. nodal processing
• check bit errors
• determine output link

• 2. queuing
• time waiting at output link for transmission
• depends on congestion level of router

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Delay in packet-switched networks

• 4. Propagation delay
• d length of physical link
• s propagation speed in medium (2x108 m/sec)
• propagation delay d/s

• 3. Transmission delay
• Rlink bandwidth (bps)
• Lpacket length (bits)
• time to send bits into link L/R

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Nodal delay

• dproc processing delay
• typically a few microsecs or less
• dqueue queuing delay
• depends on congestion
• dtrans transmission delay
• L/R, significant for low-speed links
• dprop propagation delay
• a few microsecs to hundreds of msecs

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Queueing delay (revisited)

• Rlink bandwidth (bps)
• Lpacket length (bits)
• aaverage packet arrival rate

traffic intensity La/R

• La/R 0 average queueing delay small
• La/R -gt 1 delays become large
• La/R gt 1 more work arriving than can be
  serviced, average delay infinite!

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Packet loss

• Queue preceding link in buffer has finite
  capacity
• When packet arrives to full queue, packet is
  dropped (lost)
• Lost packet may be retransmitted by previous
  node, by source end system, or not retransmitted
  at all

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Residential networks

Modem, DSL, HFC
Chapter 1.4
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Residential access point to point access

• Dialup via modem
• up to 56Kbps direct access to router (often less)
• Cant surf and phone at same time cant be
  always on

• ADSL asymmetric digital subscriber line
• up to 1 Mbps upstream (typically lt 256 kbps)
• up to 8 Mbps downstream (typically lt 1 Mbps)
• FDM 50 kHz - 1 MHz for downstream
• 4 kHz - 50 kHz for upstream
• 0 kHz - 4 kHz for ordinary
  telephone

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Home networks

• Typical home network components
• DSL or cable modem
• router/firewall
• Ethernet
• wireless access point

wireless laptops
to/from cable headend
cable modem
router/ firewall
wireless access point
Ethernet (switched)
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Physical Media coax, fiber

• Fiber optic cable
• glass fiber carrying light pulses of one bit
• high-speed operation
• point-to-point transmission (e.g., 5 Gps)
• low error rate repeaters spaced far apart
  immune to electromagnetic noise

• Coaxial cable
• two concentric copper conductors
• bidirectional
• baseband
• single channel on cable
• legacy Ethernet
• broadband
• multiple channel on cable
• HFC

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Hybrid Fiber Coax residential access cable modems
Diagram http//www.cabledatacomnews.com/cmic/diag
ram.html
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Cable Network Architecture Overview
cable headend
home
cable distribution network (simplified)
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Cable Network Architecture Overview
FDM
cable headend
home
cable distribution network
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PROTOCOL LAYERS

Chapter 1.7
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Protocol of air travel
ticket (purchase) baggage (check) gates
(load) runway takeoff airplane routing
ticket (complain) baggage (claim) gates
(unload) runway landing airplane routing
airplane routing

• a series of steps

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Internet protocol stack

• application supporting network applications
• FTP, SMTP, STTP
• transport host-host data transfer
• TCP, UDP
• network routing of datagrams from source to
  destination
• IP, routing protocols
• link data transfer between neighboring network
  elements
• PPP, Ethernet
• physical bits on the wire

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Protocol layering and data

• Each layer takes data from above
• adds header information to create new data unit
• passes new data unit to layer below

Messages are called layer N protocol data units -
n-PDUs
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Layer functions

• Error control
• Flow control
• Segmentation reassembly
• Multiplexing
• Connection setup

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APPLICATION LAYER

• CHAPTER 2

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Process program in execution

• Web applications protocols are
• HTTP Hyper Text Transfer Protocol (RFC 2616)
• SMTP Simple Mail Transfer Protocol (RFC 2821)

• Host, destination, process ID
• Host destination have IP addresses on internet
• The process has a port number

• The transparent interface between layers
• Sockets are the doors between the application
  transport layers
• Sockets are also called Application Programming
  Interfaces, API

User agents are interface between user network
(browsers)
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Client-server paradigm
Typical network app has two pieces client and
server

• Client
• initiates contact with server (speaks first)
• typically requests service from server,
• Web client implemented in browser 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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Transport service applications need

• Data loss
• some apps (e.g., audio) can tolerate some loss
• other apps (e.g., file transfer, telnet) require
  100 reliable data transfer

• Bandwidth
• some apps (e.g., multimedia) require a minimum
  amount of bandwidth to be effective
• other apps (elastic apps) make use of whatever
  bandwidth they get

• Timing
• some apps (e.g., Internet telephony, interactive
  games) require low delay to be effective

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Transport service requirements of common apps
Time Sensitive no no no yes, 100s msec yes,
few secs yes, 100s msec yes and no
Application file transfer e-mail Web
documents real-time audio/video stored
audio/video interactive games instant messaging
Bandwidth elastic elastic elastic audio
5kbps-1Mbps video10kbps-5Mbps same as above few
kbps up elastic
Data loss no loss no loss no loss loss-tolerant
loss-tolerant loss-tolerant no loss
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Internet transport protocols services

• TCP service
• connection-oriented setup required between
  client and server processes
• reliable transport between sending and receiving
  process
• flow control sender wont overwhelm receiver
• congestion control throttle sender when network
  overloaded (multimedia?)
• does not provide timing, minimum bandwidth
  guarantees

• UDP service
• Unreliable data transfer between sending and
  receiving process
• May not arrive in order
• Does not provide connection setup, reliability,
  flow control, congestion control, timing, or
  bandwidth guarantee
• Q why bother? Why is there a UDP?

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Internet Applications and their Application
Transport Protocols
Application layer protocol SMTP RFC
2821 Telnet RFC 854 HTTP RFC 2616 FTP RFC
959 proprietary (e.g. RealNetworks) proprietary (
e.g., Dialpad)
Underlying transport protocol TCP TCP TCP TCP TCP
or UDP typically UDP
Application e-mail remote terminal access Web
file transfer streaming multimedia Internet
telephony
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Processes communicating across network

• process sends/receives messages to/from its
  socket
• socket analogous to door
• sending process shoves message out door
• sending process assumes transport infrastructure
  on other side of door which brings message to
  socket at receiving process

• process sends/receives messages to/from its
  socket
• socket analogous to door
• sending process shoves message out door
• sending process assumes transport infrastructure
  on other side of door which brings message to
  socket at receiving process

controlled by app developer
Internet
controlled by OS

• API (1) choice of transport protocol
• (2) ability to fix a few parameters

• API (1) choice of transport protocol
• (2) ability to fix a few parameters

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2.2 HTTP overview
HTTP hypertext transfer protocol

• Webs application layer protocol
• client/server model
• client browser that requests, receives,
  displays Web objects
• server Web server sends objects in response to
  requests
• HTTP 1.0 RFC 1945
• HTTP 1.1 RFC 2068

HTTP request
PC running Explorer
HTTP response
HTTP request
Server running Apache Web server
HTTP response
Mac running Navigator
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HTTP overview (continued)

• Uses TCP
• client initiates TCP connection (creates socket)
  to server, port 80
• server accepts TCP connection from client
• HTTP messages (application-layer protocol
  messages) exchanged between browser (HTTP client)
  and Web server (HTTP server)
• TCP connection closed

• HTTP is stateless
• server maintains no information about past client
  requests

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HTTP connections

• Non-persistent HTTP
• Server closes connection after each object
  transmission
• Response time is 2 RTTs plus transmission
• HTTP/1.0 uses non-persistent HTTP

• Persistent HTTP
• Server leaves TCP connection open with timeout.
• Multiple objects can be sent over single TCP
  connection
• HTTP/1.1 uses persistent pipelined connections in
  default mode

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Nonpersistent HTTP
URL enteredwww.someSchool.edu/someDepartment/hom
e.html
1a. HTTP client initiates connection to server
(process) at www.someSchool.edu on port 80
1b. HTTP server at host www.someSchool.edu
waiting for connection at port 80. accepts
connection, notifying client
2a. Client sends request with URL into TCP
connection socket indicates it wants object
someDepartment/home.html
2b. Server receives request, forms response
message containing requested object, and sends it
into its socket
4. Client receives response containing html file,
displays it. Parsing html file, it finds 10
referenced jpeg objects
3. HTTP server closes TCP connection.
time
5. Steps 1-5 repeated for each of 10 jpeg objects
or other
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Response time modeling

• Definition of RTT time to send a packet from
  client to server and back.
• Response time
• 1 RTT to initiate TCP connection
• 1 RTT for request and first few bytes of HTTP
  response to return
• file transmission time
• total 2RTT transmit time

3-way handshake
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Persistent HTTP

• Persistent without pipelining
• client issues new request only when previous
  response has been received
• one RTT for each referenced object
• Persistent with pipelining
• default in HTTP/1.1
• client sends requests as soon as it encounters a
  referenced object
• as little as one RTT for all the referenced
  objects

• Nonpersistent HTTP issues
• requires 2 RTTs per object
• OS must work and allocate host resources for each
  TCP connection
• browsers often open parallel TCP connections to
  fetch referenced objects
• Persistent HTTP
• server leaves connection open after sending
  response
• subsequent HTTP messages between same
  client/server are sent over connection

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HTTP request message

• HTTP request message
• ASCII (human-readable format)

request line (GET/ POST, HEAD commands)
GET /somedir/page.html HTTP/1.1 Host
www.someschool.edu User-agent
Mozilla/4.0 Connection close Accept-languageEn
(extra carriage return, line feed)
header lines
Carriage return, line feed indicates end of
message
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HTTP request message general format
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Uploading form input

• Post method
• Web page often includes form input
• Input is uploaded to server in entity body

• URL method
• Uses GET method
• Input is uploaded in URL field of request line

www.somesite.com/animalsearch?monkeysbanana
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Method types

• HTTP/1.0
• GET reqd object in URL
• POST usually for forms
• HEAD
• asks server to leave requested object out of
  response
• Often used for debugging

• HTTP/1.1
• GET, POST, HEAD
• PUT
• uploads file in entity body to path specified in
  URL field
• DELETE
• deletes file specified in the URL field

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HTTP response message
status line (protocol status code status phrase)
HTTP/1.1 200 OK Connection close Date Thu, 06
Aug 1998 120015 GMT Server Apache/1.3.0
(Unix) Last-Modified Mon, 22 Jun 1998 ...
Content-Length 6821 Content-Type text/html
data data data data data ...
header lines
data, e.g., requested HTML file
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HTTP response status codes
A few sample codes

• 200 OK
• request succeeded, requested object later in this
  message
• 301 Moved Permanently
• requested object moved, new location specified
  later in this message (Location)
• 400 Bad Request
• request message not understood by server
• 404 Not Found
• requested document not found on this server
• 505 HTTP Version Not Supported

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Trying out HTTP (client side)

• 1. Telnet to your favorite Web server

Opens TCP connection to port 80 (default HTTP
server port) at www.eurecom.fr. Anything typed in
sent to port 80 at www.eurecom.fr
telnet www.eurecom.fr 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/index.html HTTP/1.0
3. Look at response message sent by HTTP server!
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User-server interaction authorization (user
password used)

• Authorization control access to server content
• authorization credentials typically name,
  password
• stateless client must present authorization in
  each request
• authorization header line in each request
• if no authorization header, server refuses
  access, sends
• WWW authenticate
• header line in response

server
client
usual http request msg
401 authorization req. WWW authenticate
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Cookies keeping state

• Four components
• 1) cookie header line in the HTTP response
  message
• 2) cookie header line in HTTP request message
• 3) cookie file kept on users host and managed by
  users browser
• 4) back-end database at Web site

• Example
• Susan always accesses Internet from same PC
• She visits a specific e-commerce site for first
  time
• When initial HTTP requests arrives at site, site
  creates a unique ID and creates an entry in
  backend database for ID

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Cookies keeping state (cont.)
server creates ID 1678 for user
entry in backend database
access
access
one week later
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Cookies (continued)
aside

• Cookies and privacy
• cookies permit sites to learn a lot about you
• you may supply name and e-mail to sites
• search engines use redirection cookies to
  learn yet more
• advertising companies obtain info across sites

• What cookies can bring
• authorization
• shopping carts
• recommendations
• user session state (Web e-mail)

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Conditional GET client-side caching
server
client

• Goal dont send object if client has up-to-date
  cached version
• client specify date of cached copy in HTTP
  request
• If-modified-since ltdategt
• server response contains no object if cached
  copy is up-to-date
• HTTP/1.0 304 Not Modified

HTTP request msg If-modified-since ltdategt
object not modified
HTTP request msg If-modified-since ltdategt
object modified
HTTP response HTTP/1.0 200 OK ltdatagt