Application Layer

1
Application Layer

• CE-143
• CNE-0802

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Applications and application-layer protocols

• Application communicating, distributed processes
• e.g., e-mail, Web, P2P file sharing, instant
  messaging
• running in end systems (hosts)
• exchange messages to implement application
• Application-layer protocols
• one piece of an app
• define messages exchanged by apps and actions
  taken
• use communication services provided by lower
  layer protocols (TCP, UDP)

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App-layer protocol defines

• Types of messages exchanged, eg, request
  response messages
• Syntax of message types what fields in messages
  how fields are delineated
• Semantics of the fields, ie, meaning of
  information in fields
• Rules for when and how processes send respond
  to messages

• Public-domain protocols
• defined in RFCs
• allows for interoperability
• eg, HTTP, SMTP
• Proprietary protocols
• eg, KaZaA

4
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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What transport service does an app 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 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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Aplikasi InternetDNS, WEB, EMAIL
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Domain Name System(DNS)
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DNS Domain Name System

• Internet hosts
• IP address (32 bit) - used for addressing
  datagrams
• name, e.g., ww.yahoo.com - used by humans
• DNS provides translation between host name and
  IP address
• distributed database implemented in hierarchy of
  many name servers
• Distributed for scalability reliability

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DNS Services

• Hostname to IP address translation
• Host aliasing
• Canonical and alias names
• Mail server aliasing
• Load distribution
• Replicated Web servers set of IP addresses for
  one canonical name

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DNS Infrastructure
root DNS server
2

• Host at cis.poly.edu wants IP address for
  gaia.cs.umass.edu
• Infrastructure
• Client resolver
• Local DNS server
• Authoritative DNS Server
• Root DNS Server
• Top-Level Domain DNS Server

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TLD DNS server
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5
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1
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authoritative DNS server Ns1.google.com
requesting host Ce1.poltek.ac.id
www.google.com
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Distributed, Hierarchical Database

• Root servers and TLD servers typically do not
  contain hostname to IP mappings they contain
  mappings for locating authoritative servers.

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Recursive queries

• recursive query
• puts burden of name resolution on contacted name
  server
• heavy load?
• iterated query
• contacted server replies with name of server to
  contact
• I dont know this name, but ask this server

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DNS records

• DNS distributed db storing resource records (RR)

• TypeA
• name is hostname
• value is IP address

• TypeCNAME
• name is alias name for some cannonical (the
  real) name
• www.ibm.com is really servereast.backup2.ibm.com
  
• value is cannonical name

• TypeNS
• name is domain (e.g. foo.com)
• value is IP address of authoritative name server
  for this domain

• TypeMX
• value is name of mailserver associated with name

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DNS protocol, messages

• DNS protocol query and reply messages, both
  with same message format

• msg header
• identification 16 bit for query, reply to
  query uses same
• flags
• query or reply
• recursion desired
• recursion available
• reply is authoritative

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DNS protocol, messages
Name, type fields for a query
RRs in reponse to query
records for authoritative servers
additional helpful info that may be used
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DNS Server Configuration (Sample)

• zone file for example.com TTL 2d 172800 secs
  default TTL for zone _at_ IN SOA ns1.example.com.
  hostmaster.example.com. (
• 2003080800
• se serial number 12h
• ref refresh 15m
• ret update retry 3w
• ex expiry 3h
• min minimum )
• IN NS ns1.example.com.
• IN MX 10 mail.example.net.
• joe IN A 192.168.254.3
• www IN CNAME joe

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Security Issue on DNS

• DNS is plain text ? easy to capture
• No security mechanism to secure request
• Even easy to do Name Server Hijacking
• It is vulnerable for Buffer Overflow Attack

18
World Wide Web
19
WWW Background

• 1989-1990 Tim Berners-Lee invents the World
  Wide Web at CERN
• Means for transferring text and graphics
  simultaneously
• Client/Server data transfer protocol
• Communication via application level protocol
• System ran on top of standard networking
  infrastructure
• Text mark up language
• Not invented by Bernes-Lee
• Simple and easy to use
• Requires a client application to render
  text/graphics

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WWW Components

• Structural Components
• Clients/browsers to dominant implementations
• Servers run on sophisticated hardware
• Caches many interesting implementations
• Internet the global infrastructure which
  facilitates data transfer
• Semantic Components
• Hyper Text Transfer Protocol (HTTP)
• Hyper Text Markup Language (HTML)
• eXtensible Markup Language (XML)
• Uniform Resource Identifiers (URI)
• Uniform Resource Locator (URL)

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Uniform Resource Identifiers

• Web resources need names/identifiers Uniform
  Resource Identifiers (URIs)
• Resource can reside anywhere on the Internet
• URIs are a somewhat abstract notion
• A pointer to a resource to which request methods
  can be applied to generate potentially different
  responses
• A request method is eg. fetching or changing the
  object
• Instance http//www.foo.com/index.html
• Protocol, server, resource
• Most popular form of a URI is the Uniform
  Resource Locator (URL)
• FIND THE DIFFERENCE BETWEEN URL AND URI!!!

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HTTP Basics

• Protocol for client/server communication
• The heart of the Web
• Very simple request/response protocol
• Client sends request message, server replies with
  response message
• Stateless
• Relies on URI naming mechanism
• Three versions have been used
• 09/1.0 very close to Berners-Lees original
• RFC 1945 (original RFC is now expired)
• 1.1 developed to enhance performance, caching,
  compression
• RFC 2068
• 1.0 dominates today but 1.1 is catching up

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HTTP Request Messages

• GET retrieve document specified by URL
• PUT store specified document under given URL
• HEAD retrieve info. about document specified by
  URL
• OPTIONS retrieve information about available
  options
• POST give information (eg. annotation) to the
  server
• DELETE remove document specified by URL
• TRACE loopback request message
• CONNECT for use by caches

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HTTP Request Format
request-line ( request request-URI
HTTP-version) headers (0 or more) ltblank
linegt body (only for POST request)

• First type of HTTP message requests
• Client browsers construct and send message
• Typical HTTP request
• GET http//www.cs.wisc.edu/index.html HTTP/1.0

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HTTP Response Format
status-line (HTTP-version response-code
response-phrase) headers (0 or more) ltblank
linegt body

• Second type of HTTP message response
• Web servers construct and send response messages
• Typical HTTP response
• HTTP/1.0 301 Moved Permanently
• Location http//www.wisc.edu/cs/index.html

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HTTP Response Codes

• 1xx Informational request received,
  processing
• 2xx Success action received, understood,
  accepted
• 3xx Redirection further action necessary
• 4xx Client Error bad syntax or cannot be
  fulfilled
• 5xx Server Error server failed

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HTTP Headers

• Both requests and responses can contain a
  variable number of header fields
• Consists of field name, colon, space, field value
• 17 possible header types divided into three
  categories
• Request
• Response
• Body
• Example Date Friday, 27-Apr-01 133001 GMT
• Example Content-length 3001

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HTTP/1.0 Network Interaction

• Clients make requests to port 80 on servers
• Uses DNS to resolve server name
• Clients make separate TCP connection for each URL
• Some browsers open multiple TCP connections
• Netscape default 4, WATCH OUT ON PIPELINING
• Server returns HTML page
• Many types of servers with a variety of
  implementations
• Apache is the most widely used
• Freely available in source form
• Client parses page
• Requests embedded objects

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HTTP/1.1 Performance Enhancements

• HTTP/1.0 is a stop and wait protocol
• Separate TCP connection for each file
• Connect setup and tear down is incurred for each
  file
• Inefficient use of packets
• Server must maintain many connections in
  TIME_WAIT
• ISSUE on HTTP/1.0
• Resulted in HTTP/1.1 specification focused on
  performance enhancements
• Persistent connections
• Pipelining
• Enhanced caching options
• Support for compression

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Persistent Connections and Pipelining

• Persistent connections
• Use the same TCP connection(s) for transfer of
  multiple files
• Reduces packet traffic significantly
• May or may not increase performance from client
  perspective
• Load on server increases
• Pipelining
• Pack as much data into a packet as possible
• Requires length field(s) within header
• May or may not reduce packet traffic or increase
  performance
• Page structure is critical

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HTML Basics

• Hyper-Text Markup Language
• A subset of Standardized General Markup Language
  (SGML)
• Facilitates a hyper-media environment
• Embedded links to other documents and
  applications
• Documents use elements to mark up or identify
  sections of text for different purposes or
  display characteristics
• Mark up elements are not seen by the user when
  page is displayed
• Documents are rendered by browsers
• NOTE Not all documents in the Web are HTML!
• Most people use WYSIWYG editors (MS Word) to
  generate HTML

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HTML Example
ltHTMLgt ltHEADgt ltTITLEgt PBs HomePage
lt/TITLEgt lt/HEADgt ltBODYgt ltCENTERgtltIMG SRC
bad_picture.gif ALT gtltBRgtlt/CENTERgt ltPgtltCENT
ERgtltH1gtUW Computer Science Departmentlt/H1gtlt/CENTER
gt Welcome to my goofy HomePage! ltA HREF
http//www.cs.wisc.edu/pb/mydogs_page.htmlgt
Spots Page lt/Agt lt/BODYgt lt/HTMLgt
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Security Issue on Web

• By User
• Defacing
• SQL or Shell Inject
• DoS
• By Owner
• Phissing (try this http//www.kilkbca.com)
• Malware (Nice app to be downloded, contain
  malware)
• Trojan/Virus/Worm (via sec hole in the Browser)

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EMAIL
35
EMail protocols
SMTP
POP3/ IMAP/ WebMail
receivers mail server

• SMTP delivery/storage to receivers server
• Mail access protocol retrieval from server
• POP Post Office Protocol RFC 1939
• authorization (agent lt--gtserver) and download
• IMAP Internet Mail Access Protocol RFC 1730
• more features (secure, database intragration,
  broad authentication, etc
• HTTP Hotmail , Yahoo! Mail, etc.

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Electronic Mail

• Three major components
• user agents
• mail servers
• simple mail transfer protocol SMTP
• User Agent (mail reader)
• composing, editing, reading mail messages
• e.g., Eudora, Outlook, elm, Netscape Messenger
• outgoing, incoming messages stored on server

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Electronic Mail mail servers

• Mail Servers
• mailbox contains incoming messages for user
• message queue of outgoing (to be sent) mail
  messages
• SMTP protocol between mail servers to send email
  messages
• client sending mail server
• server receiving mail server

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Electronic Mail SMTP RFC 2821

• uses TCP to reliably transfer email message from
  client to server, port 25
• direct transfer sending server to receiving
  server
• three phases of transfer
• handshaking (greeting)
• transfer of messages
• closure
• command/response interaction
• commands ASCII text
• response status code and phrase
• messages must be in 7-bit ASCII

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Scenario Alice sends message to Bob

• 4) SMTP client 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 UA to compose message and to
  bob_at_someschool.edu
• 2) Alices UA 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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2
6
3
4
5
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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
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Try SMTP interaction for yourself

• telnet servername 25
• see 220 reply from server
• enter HELO, MAIL FROM, RCPT TO, DATA, QUIT
  commands
• above lets you send email without using email
  client (reader)

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SMTP final words

• SMTP uses persistent connections
• SMTP requires message (header body) to be in
  7-bit ASCII
• SMTP server uses CRLF.CRLF to determine end of
  message

• Comparison with HTTP
• HTTP pull
• SMTP push
• both have ASCII command/response interaction,
  status codes
• HTTP each object encapsulated in its own
  response msg
• SMTP multiple objects sent in multipart msg

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Mail message format

• SMTP protocol for exchanging email msgs
• RFC 822 standard for text message format
• header lines, e.g.,
• To
• From
• Subject
• different from SMTP commands!
• body
• the message, ASCII characters only

header
blank line
body
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Message format multimedia extensions

• MIME multimedia mail extension, RFC 2045, 2056
• additional lines in msg header declare MIME
  content type

MIME version
method used to encode data
multimedia data type, subtype, parameter
declaration
encoded data
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MIME typesContent-Type type/subtype parameters

• Text
• example subtypes plain, html
• Image
• example subtypes jpeg, gif
• Audio
• exampe subtypes basic (8-bit mu-law encoded),
  32kadpcm (32 kbps coding)

• Video
• example subtypes mpeg, quicktime
• Application
• other data that must be processed by reader
  before viewable
• example subtypes msword, octet-stream

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Multipart Type
From alice_at_crepes.fr To bob_at_hamburger.edu
Subject Picture of yummy crepe. MIME-Version
1.0 Content-Type multipart/mixed
boundaryStartOfNextPart --StartOfNextPart Dear
Bob, Please find a picture of a
crepe. --StartOfNextPart Content-Transfer-Encoding
base64 Content-Type image/jpeg base64 encoded
data ..... .........................
......base64 encoded data --StartOfNextPart Do
you want the reciple?
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POP3 protocol
S OK POP3 server ready C user bob S OK
C pass hungry S OK user successfully logged
on

• authorization phase
• client commands
• user declare username
• pass password
• server responses
• OK
• -ERR
• transaction phase, client
• list list message numbers
• retr retrieve message by number
• dele delete
• quit

C list S 1 498 S 2 912
S . C retr 1 S ltmessage 1
contentsgt S . C dele 1 C retr
2 S ltmessage 1 contentsgt S .
C dele 2 C quit S OK POP3 server
signing off
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POP3 (more) and IMAP

• More about POP3
• Previous example uses download and delete mode.
• Bob cannot re-read e-mail if he changes client
• Download-and-keep copies of messages on
  different clients
• POP3 is stateless across sessions

• IMAP
• Keep all messages in one place the server
• Allows user to organize messages in folders
• IMAP keeps user state across sessions
• names of folders and mappings between message IDs
  and folder name