Transcript Application Layer Puzzles

Chabot College
ELEC 99.05
Application Layer Puzzles
CISCO NETWORKING ACADEMY
Puzzle
• When I run a network application, I see
results on my screen.
• Where are those events happening?
CISCO NETWORKING ACADEMY
Application Layer
• Host relationships:
– Client
• typically a user PC
• the source of a transaction
– Server
• a shared host on the network
• the destination of the transaction
• Network applications usually are
“client-server”
CISCO NETWORKING ACADEMY
Application Layer
• Connection types
– Make-Break
– Session
CISCO NETWORKING ACADEMY
Make-Break Connections
• Typically involve a single file transfer.
• Examples
– HTTP
• HyperText Transfer Protocol
• web file
• TCP port 80
– SMTP
• Simple Mail Transfer Protocol
• E-mail file
• TCP port 25
CISCO NETWORKING ACADEMY
Session Connections
• Maintained until “session” is closed.
• Examples
– telnet
• terminal emulation
• passes keystrokes from client to server
• TCP port 23
– FTP
• File Tranfer Protocol
• manages transfer of many files
• TCP port 21
CISCO NETWORKING ACADEMY
telnet Examples
• telnet://library.uark.edu
• telnet://locis.loc.gov
CISCO NETWORKING ACADEMY
Running telnet (DOS)
• At the DOS Prompt, type
>telnet hostname
or
>telnet Ipaddress
• Try it! Demo
Telnet to library.uark.edu
• Be sure to choose “Q” to logoff when
done.
CISCO NETWORKING ACADEMY
Running telnet (Windows)
• Start | Programs | Accessories | Communications |
Hyperterminal
• Double-click Hypertrm.exe
• Enter a name for this connection
• At the bottom of the next dialog box, for “Connect
Using”, select TCP/IP (winsock)
• Add the destination host name or IP address
• (Notice the port number in this dialog)
• Try it! Demo
Telnet to library.uark.edu
• Be sure to choose “Q” to logoff when done.
• When asked if you wish to save the session, say “No”
CISCO NETWORKING ACADEMY
Running telnet (browser)
• Browsers use helper applications to provide
certain functions.
• By default, Internet Explorer launches Hyperterm
when it encounters a URL that specifies the
telnet protocol
• Try it! Demo
• Point your browser at
telnet://library.uark.edu
• Be sure to choose “Q” to logoff when done.
• When asked if you wish to save the session, say “No”
CISCO NETWORKING ACADEMY
telnet
• In Semester 2, you will use telnet
extensively to connect your PC to
routers.
CISCO NETWORKING ACADEMY
DNS Puzzler
• In the following URL,
what does “WWW” refer to?
• http://www.clpccd.cc.ca.us
• Answer:
It is the hostname (computer name) portion of a
“fully qualified domain name”
It does not specify the protocol HTTP (web)!
This computer just happens to be named “www”.
CISCO NETWORKING ACADEMY
Fully Qualified Domain Name
www.clpccd.cc.ca.us
Host name
CISCO NETWORKING ACADEMY
Domain name
Top-Level
Domain name
Let’s try it!
• Point your browser at:
• http://209.129.156.3
• http://www.clpccd.cc.ca.us
• http://www
• http://porter.clpccd.cc.ca.us
• http://porter
CISCO NETWORKING ACADEMY
Let’s try it!
• For the host at 209.129.156.3, Chabot’s
DNS servers list
• porter as the host name
• www as a name alias (a canonical name)
• We will study the structure of DNS tables
in Semester 2.
CISCO NETWORKING ACADEMY
DNS Puzzler
• Why is Chabot-Las Positas assigned a
domain name of clpccd.cc.ca.us ?
• Why aren’t we clpccd.edu ?
• Answer:
RFC 1480
• A recent change permits community
colleges to move to the .edu top-level
domain.
CISCO NETWORKING ACADEMY
Browser URL
• Universal Resource Locator
• Example:
http://www.cisco.com
protocol
hostname
domain
top-level
domain
• Browser uses the text before :// to
select protocol and port number.
CISCO NETWORKING ACADEMY
Let’s try it!
• Point your browser at:
• telnet://library.uark.edu
• http://library.uark.edu
CISCO NETWORKING ACADEMY
Browser URL
• Universal Resource Locator
• Example:
http://www.cisco.com
• If protocol:// is omitted, then the
protocol is not specified.
• In that case, the browser tries to “guess”
the protocol from the hostname:
– hostname www defaults to http (port 80)
– hostname ftp defaults to ftp (port 21)
CISCO NETWORKING ACADEMY
HTTP Puzzler
• When you view a web page, where is it?
• Answer:
On your PC!
• Let’s look at the contents of
C:\windows\temporary internet files demo
• Note that one web page may be made up
of many files of various types.
• Note that one web page may contain files
loaded from several servers!
CISCO NETWORKING ACADEMY
E-mail Puzzler
• When you view your E-mail, where is it
stored?
• Answer
It depends on the protocol!
– Post Office Protocol (POP)
– Web mail (e.g. yahoo mail)
CISCO NETWORKING ACADEMY
E-mail Clients
• POP Clients
–
–
–
–
E-mail is transferred from the mail server to your PC
Port 110
Client examples” Eudora, Outlook Express, Netscape
Most ISPs offer POP E-mail
• Web mail clients
– E-mail remains on the server for viewing by any browser in the
world.
– Port 80
– Browser is the client.
– Offered by Yahoo, Hotmail, GroupWise Web Access
CISCO NETWORKING ACADEMY
POP E-mail
SMTP Server/
POP Server
POP (110)
SMTP Server/
POP Server
SMTP (25)
POP (110)
E-mail passes through the post office
server but does not remain there.
E-mail is delivered to client PC.
Once delivered, it can only be accessed at that PC.
CISCO NETWORKING ACADEMY
Web E-mail
SMTP Server/
HTTP Server
HTTP (80)
SMTP Server/
HTTP Server
SMTP (25)
HTTP (80)
E-mail is delivered to the Web mail
server and remains there.
Web E-mail can be viewed from any web browser.
CISCO NETWORKING ACADEMY