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3rd Edition: Chapter 2

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Title: 3rd Edition: Chapter 2 Author: Jim Kurose and Keith Ross Last modified by: Edith Created Date: 10/8/1999 7:08:27 PM Document presentation format – PowerPoint PPT presentation

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Title: 3rd Edition: Chapter 2


1
1DT057 Distributed Information Systems Chapter
2 Application Layer
2
Chapter 2 Application layer
  • 2.1 Principles of network applications
  • 2.2 Web and HTTP
  • 2.3 FTP
  • 2.4 Electronic Mail
  • SMTP, POP3, IMAP
  • 2.5 DNS
  • 2.6 P2P applications

3
Chapter 2 Application Layer
  • Our goals
  • conceptual, implementation aspects of network
    application protocols
  • transport-layer service models
  • client-server paradigm
  • peer-to-peer paradigm
  • learn about protocols by examining popular
    application-level protocols
  • HTTP
  • FTP
  • SMTP / POP3 / IMAP
  • DNS

4
Some network apps
  • e-mail
  • web
  • instant messaging
  • remote login
  • P2P file sharing
  • multi-user network games
  • streaming stored video clips
  • voice over IP
  • real-time video conferencing

5
Chapter 2 Application layer
  • 2.1 Principles of network applications
  • 2.2 Web and HTTP
  • 2.3 FTP
  • 2.4 Electronic Mail
  • SMTP, POP3, IMAP
  • 2.5 DNS
  • 2.6 P2P applications

6
Application architectures
  • Client-server
  • Peer-to-peer (P2P)
  • Hybrid of client-server and P2P

7
Client-server architecture
  • server
  • always-on host
  • permanent IP address
  • server farms for scaling
  • clients
  • communicate with server
  • may be intermittently connected
  • may have dynamic IP addresses
  • do not communicate directly with each other

8
Pure P2P architecture
  • no always-on server
  • arbitrary end systems directly communicate
  • peers are intermittently connected and change IP
    addresses
  • Highly scalable but difficult to manage

9
Hybrid of client-server and P2P
  • Instant messaging
  • chatting between two users is P2P
  • centralized service client presence
    detection/location
  • user registers its IP address with central server
    when it comes online
  • user contacts central server to find IP addresses
    of buddies

10
Processes communicating
  • Client process process that initiates
    communication
  • Server process process that waits to be
    contacted
  • Process program running within a host.
  • within same host, two processes communicate using
    inter-process communication (defined by OS).
  • processes in different hosts communicate by
    exchanging messages

11
Sockets
  • process sends/receives messages to/from its
    socket
  • API (1) choice of transport protocol
  • (2) ability to fix a few parameters
  • (lots more on this later)

controlled by app developer
Internet
controlled by OS
12
ADDRESSING PROCESSES
  • to receive messages, process must have
    identifier
  • host device has unique 32-bit IP address
  • Q does IP address of host suffice for
    identifying the process?

13
ADDRESSING PROCESSES
  • to receive messages, process must have
    identifier
  • host device has unique 32-bit IP address
  • Q does IP address of host on which process runs
    suffice for identifying the process?
  • A No, many processes can be running on same host
  • identifier includes both IP address and port
    numbers associated with process on host.
  • Example port numbers
  • HTTP server 80
  • Mail server 25
  • to send HTTP message to gaia.cs.umass.edu web
    server
  • IP address 128.119.245.12
  • Port number 80
  • more shortly

14
App-layer protocol defines
  • Public-domain protocols
  • defined in RFCs
  • allows for interoperability
  • e.g., HTTP, SMTP
  • Proprietary protocols
  • e.g., Skype
  • Types of messages exchanged,
  • e.g., request, response
  • Message syntax
  • what fields in messages how fields are
    delineated
  • Message semantics
  • meaning of information in fields
  • Rules for when and how processes send respond
    to messages

15
WHAT TRANSPORT SERVICE DOES AN APP NEED?
  • Throughput
  • some apps (e.g., multimedia) require minimum
    amount of throughput to be effective
  • other apps (elastic apps) make use of whatever
    throughput they get
  • Security
  • Encryption, data integrity,
  • Data loss
  • some apps (e.g., audio) can tolerate some loss
  • other apps (e.g., file transfer, telnet) require
    100 reliable data transfer
  • Timing
  • some apps (e.g., Internet telephony, interactive
    games) require low delay to be effective

16
TRANSPORT SERVICE REQUIREMENTS OF COMMON APPS
Data loss (no loss / loss-tolerant) loss-tolerant
Time Sensitive (yes / no) no
Application file transfer e-mail Web
documents real-time audio/video stored
audio/video interactive games instant messaging
Throughput elastic elastic elastic audio
5kbps-1Mbps video10kbps-5Mbps same as above few
kbps up elastic
17
INTERNET TRANSPORT PROTOCOLS SERVICES
  • UDP service
  • unreliable data transfer between sending and
    receiving process
  • does not provide connection setup, reliability,
    flow control, congestion control, timing,
    throughput guarantee, or security
  • Q why bother? Why is there a UDP?
  • 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
  • does not provide timing, minimum throughput
    guarantees, security

18
INTERNET APPS APPLICATION, TRANSPORT PROTOCOLS
Transport protocol (TCP / UDP)
Application layer protocol
Application e-mail remote terminal access Web
file transfer streaming multimedia Internet
telephony
19
Chapter 2 Application layer
  • 2.1 Principles of network applications
  • app architectures
  • app requirements
  • 2.2 Web and HTTP
  • 2.3 FTP
  • 2.4 Electronic Mail
  • SMTP, POP3, IMAP
  • 2.5 DNS
  • 2.6 P2P applications

20
Web and HTTP
  • First some jargon
  • Web page consists of objects
  • Object can be HTML file, JPEG image, Java applet,
    audio file,
  • Web page consists of base HTML-file which
    includes several referenced objects
  • Each object is addressable by a URL
  • Example URL

21
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 request
PC running Explorer
HTTP response
HTTP request
Server running Apache Web server
HTTP response
Mac running Navigator
22
HTTP overview (continued)
  • HTTP is stateless
  • server maintains no information about past client
    requests
  • 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

23
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
24
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
25
HTTP RESPONSE STATUS CODES
In first line in server-gtclient response
message. 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

26
User-server state cookies
  • Example
  • Susan always access Internet always from PC
  • visits specific e-commerce site for first time
  • when initial HTTP requests arrives at site, site
    creates
  • unique ID
  • entry in backend database for ID
  • Many major Web sites use cookies
  • Four components
  • 1) cookie header line of HTTP response message
  • 2) cookie header line in HTTP request message
  • 3) cookie file kept on users host, managed by
    users browser
  • 4) back-end database at Web site

27
COOKIES KEEPING STATE (CONT.)
client
server
cookie file
backend database
one week later
28
Cookies (continued)
aside
  • Cookies and privacy
  • cookies permit sites to learn a lot about you
  • you may supply name and e-mail to sites
  • What cookies can bring
  • authorization
  • shopping carts
  • recommendations
  • user session state (Web e-mail)
  • How to keep state
  • protocol endpoints maintain state at
    sender/receiver over multiple transactions
  • cookies http messages carry state

29
WEB CACHES (PROXY SERVER)
Goal satisfy client request without involving
origin server
  • user sets browser Web accesses via cache
  • browser sends all HTTP requests to cache
  • object in cache cache returns object
  • else cache requests object from origin server,
    then returns object to client

origin server
Proxy server
client
client
origin server
30
More about Web caching
  • cache acts as both client and server
  • typically cache is installed by ISP (university,
    company, residential ISP)
  • Why Web caching?
  • reduce response time for client request
  • reduce traffic on an institutions access link.
  • Internet dense with caches enables poor
    content providers to effectively deliver content
    (but so does P2P file sharing)

31
Chapter 2 Application layer
  • 2.1 Principles of network applications
  • 2.2 Web and HTTP
  • 2.3 FTP
  • 2.4 Electronic Mail
  • SMTP, POP3, IMAP
  • 2.5 DNS
  • 2.6 P2P applications
  • 2.7 Socket programming with TCP
  • 2.8 Socket programming with UDP
  • 2.9 Building a Web server
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