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Chapter 7: Application layer


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Title: Chapter 7: Application layer

Chapter 7 Application layer
  • Application Layer
  • Domain name system (DNS)
  • Electronic Mail
  • World Wide Web (WWW)
  • Readings
  • Sections 7.1-7.3

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
  • user services provided by lower layer protocols

Client-server paradigm
  • Typical network app has two pieces client and

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
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
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
Internet apps their protocols and transport
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
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

Distributed, Hierarchical Database
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
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

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.

Iterative Queries
root DNS server
TLD DNS server
  • iterated query
  • contacted server replies with name of server to
  • I dont know this name, but ask this server

authoritative DNS server
requesting host
Recursive queries
recursive query puts burden of name resolution
on contacted name server heavy load?
DNS caching and updating records
  • once (any) name server learns mapping, it caches
  • cache entries timeout (disappear) after some time
  • TLD servers typically cached in local name
  • Thus root name servers not often visited

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
  • is really
  • value is canonical name
  • TypeNS
  • name is domain (e.g.
  • value is IP address of authoritative name server
    for this domain
  • TypeMX
  • value is name of mail server associated with name

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

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
  • outgoing, incoming messages stored on server

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

Electronic Mail SMTP RFC 821
  • uses tcp to reliably transfer email msg from
    client to server, port 25
  • direct transfer sending server to receiving
  • 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

Sample SMTP interaction
S 220 C HELO
S 250 Hello, pleased to meet you
C MAIL FROM ltalice_at_crepes.frgt S 250 Sender ok C RCPT TO
ltbob_at_hamburger.edugt S 250 ... 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 closing connection
try smtp interaction for yourself
  • telnet servername 25
  • see 220 reply from server
  • 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

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
  • smtp server uses CRLF.CRLF to determine end of

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

blank line
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
encoded data
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

Multipart Type
From To
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--
Mail access protocols
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.

POP3 protocol
S OK POP3 server ready C user alice S OK
C pass hungry S OK user successfully logged
  • 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
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

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
  • 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
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

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 User-agent
Mozilla/4.0 Connection close Accept-languagefr
(extra carriage return, line feed)
header lines
Carriage return, line feed indicates end of
HTTP request message general format
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
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 Anything typed in
sent to port 80 at
telnet 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
3. Look at response message sent by HTTP server!
telnet 80 Trying Connected to
( Escape character is ''. GET
/index.html /HTTP/1.1 Host 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//"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
Internet2, CS"gt ltmeta NAME"distribution"
CONTENT"global"gt ltmeta NAME"author"
CONTENT"Kendal Van Dyke"gt
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

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

  • 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
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