Chapter 7: Application layer

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Chapter 7 Application layer

• Application Layer
• Domain name system (DNS)
• Electronic Mail
• World Wide Web (WWW)
• Readings
• Sections 7.1-7.3

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Applications and application-layer protocols

• Application communicating, distributed processes
• running in network hosts in user space
• exchange messages to implement app
• e.g., email, file transfer, the Web
• Application-layer protocols
• one piece of an app
• define messages exchanged by apps and actions
  taken
• user services provided by lower layer protocols

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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,
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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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

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

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
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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 financial apps
Data loss no loss no loss loss-tolerant loss-tole
rant loss-tolerant loss-tolerant no loss
Bandwidth elastic elastic elastic audio
5Kb-1Mb video10Kb-5Mb same as above few Kbps
up elastic
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Internet apps their protocols and transport
protocols
Application layer protocol smtp RFC 821 telnet
RFC 854 http RFC 2068 ftp RFC
959 proprietary (e.g. RealNetworks) NFS proprieta
ry (e.g., Vocaltec)
Underlying transport protocol TCP TCP TCP TCP TCP
or UDP TCP or UDP typically UDP
Application e-mail remote terminal access Web
file transfer streaming multimedia remote file
server Internet telephony
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DNS Domain Name System

• 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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Distributed, Hierarchical Database
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DNS Root name servers

• contacted by local name server that cannot
  resolve name
• root name server
• contacts authoritative name server if name
  mapping not known
• gets mapping
• returns mapping to local name server

13 root name servers worldwide
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TLD and Authoritative Servers

• Top-level domain (TLD) servers responsible for
  com, org, net, edu, etc, and all top-level
  country domains cn, ca, fr, jp, uk etc.
• Network solutions maintains servers for com TLD
• Educause for edu TLD
• Authoritative DNS servers organizations DNS
  servers, providing authoritative hostname to IP
  mappings for organizations servers (e.g., Web
  and mail).
• Can be maintained by organization or service
  provider

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Local Name Server

• Each ISP (residential ISP, company, university)
  has one.
• Also called default name server
• When a host makes a DNS query, query is sent to
  its local DNS server
• Acts as a proxy, forwards query into hierarchy.

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Iterative Queries
root DNS server
2
3
TLD DNS server
4

• iterated query
• contacted server replies with name of server to
  contact
• I dont know this name, but ask this server

5
6
7
1
8
authoritative DNS server dns.cs.umass.edu
requesting host cis.poly.edu
gaia.cs.umass.edu
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Recursive queries
recursive query puts burden of name resolution
on contacted name server heavy load?
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DNS caching and updating records

• once (any) name server learns mapping, it caches
  mapping
• cache entries timeout (disappear) after some time
• TLD servers typically cached in local name
  servers
• Thus root name servers not often visited

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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 canonical (the
  real) name
• www.ibm.com is really
• servereast.backup2.ibm.com
• value is canonical 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 mail server associated with name

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• Example
• Aix 86400 IN A 192.168.42.2
• 86400 IN MX 5 aix.unpbook.com.
• 86400 IN MX 10
  mailhost.unpbook.com.
• Aix-4 86400 IN A 192.168.42.2
• ftp 86400 IN CNAME linux.unpbook.com
• www 86400 IN CNAME linux.unpbook.com
• DNS uses UDP to exchange information
• Query is initiated from a system call
  gethostbyname, gethostbyaddr.

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Electronic Mail

• Three major components
• user agents
• mail servers
• simple mail transfer protocol SMTP
• User Agent
• a.k.a. mail reader
• composing, editing, reading mail messages
• e.g., pine, 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 (yet to be
  read) 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 821

• uses tcp to reliably transfer email msg 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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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)
• The current SMTP-based email system cannot verify
  the identity of sender
• Sender Policy Framework/SenderID proposed to
  verify if sender mail server is authorized to
  send email for the sender email address

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smtp final words

• smtp uses persistent connections
• keep-alive connections, more than one
  request-response per TCP/IP connection
• if there are more messages they are sent via a
  persistent TCP connection
• smtp requires that message (header body) be in
  7-bit ascii
• certain character strings are not permitted in
  message (e.g., CRLF.CRLF). Thus message has to be
  encoded (usually into either base-64 or quoted
  printable)
• smtp server uses CRLF.CRLF to determine end of
  message

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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
• Multipurpose Internet Mail Extensions
• Support for non-ASCII messages, non-textual
  messages, multipart messages, non-ASCII message
  headers, etc
• 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
• example 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
boundary98766789 --98766789 Content-Transfer-En
coding quoted-printable Content-Type
text/plain Dear Bob, Please find a picture of a
crepe. --98766789 Content-Transfer-Encoding
base64 Content-Type image/jpeg base64 encoded
data ..... .........................
......base64 encoded data --98766789--
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Mail access protocols
SMTP
POP3 or IMAP
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 (more complex)
• manipulation of stored msgs on server
• HTTP Hotmail , Yahoo! Mail, etc.

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POP3 protocol
S OK POP3 server ready C user alice 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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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

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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)

• Over 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

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HTTP request message

• two types of HTTP messages request, response
• 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-languagefr
(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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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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Trying out HTTP (client side) for yourself

• 1. Telnet to your favorite Web server

Opens TCP connection to port 80 (default HTTP
server port) at www.cs.fsu.edu. Anything typed in
sent to port 80 at cis.poly.edu
telnet www.cs.fsu.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 index.html / HTTP/1.1 Host www.cs.fsu.edu
3. Look at response message sent by HTTP server!
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telnet www.cs.fsu.edu 80 Trying
192.168.23.10... Connected to www.cs.fsu.edu
(192.168.23.10). Escape character is ''. GET
/index.html /HTTP/1.1 Host www.cs.fsu.edu HTTP/1
.1 200 OK Date Wed, 28 Nov 2007 183429
GMT Server Apache/2.0.52 (Scientific
Linux) Last-Modified Mon, 29 Aug 2005 180235
GMT ETag "1defce0-29c5-4cd2a4c0" Accept-Ranges
bytes Content-Length 10693 Connection
close Content-Type text/html charsetISO-8859-1
lthtmlgt ltheadgt lttitlegtComputer Science _at_ Florida
State Universitylt/titlegt ltbase
HREF"http//www.cs.fsu.edu/"gt ltmeta
NAME"resource-type" CONTENT"document"gt ltmeta
NAME"description" CONTENT"Website for the
Computer Science Department
at Florida State
University"gt ltmeta NAME"keywords"
CONTENT"Florida State University, Computer
Science,
Internet2, CS"gt ltmeta NAME"distribution"
CONTENT"global"gt ltmeta NAME"author"
CONTENT"Kendal Van Dyke"gt
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User-server state cookies

• HTTP is stateless.
• two requests are treated independently.
• Why stateless?
• What is the problem with a stateless http?
• E-commence People buy things by making many
  requests. Need the ability to bind the requests
  from the same customer together.
• Solution cookies

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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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• Some issues in HTTP
• Mainly due to its popularity
• Cache support.
• Insufficient in http/1.0, improved in http/1.1
• Intermediate nodes, encoding, etc
• Dynamically generated date
• Not reliable in http/1.0
• Performance
• Persistent or non-persistent TCP connection
• Download the whole file or part of a file
• User preference
• Security