1. Traditional Internet Applications – Email / DNS
Chapter 4
Traditional Internet Applications – / DNS
© Bobby Hoggard, Department of Computer Science, East Carolina University
These slides may not be used or duplicated without permission

2. Email Example: direct email transfer from one device to another
apps on each end running constantly
Sender
Text Editor
New
Message
alert:
you have mail
To:
Receiver
New
Message
connect to abc.com
Transfer Application
Mail Server
Text Reader
New
Message
send
New
Message
abc.com
To:
Other Message
A "mailbox" is just a file on the mail server
Outgoing Queue
userX mailbox

3. Email Protocols Transfer Protocols Access Protocols
Used to transfer from a sending device to a receiving device
Most common: SMTP
Access Protocols
Allows a user to access a mailbox to view and send messages
Most common: POP3, IMAP
Representation Protocols
Specifies the format of the message
Most common: RFC2822, MIME

4. SMTP Simple Mail Transfer Protocol Key Points Uses stream transfer
Port: 25
Simple Mail Transfer Protocol
Key Points
Uses stream transfer
Uses text-based control messages
Sends one copy of given message to server
List users one at a time, then a single copy is sent to all users on the list
Allows sender to check receipient names
Only transfers text messages

5. SMTP Example
Server: example.edu
Client: somewhere.com
client forms a control connection to the server
220 example.edu Ready
HELO somewhere.com
250 OK
MAIL FROM:
Attempt to send from: to: three users at example.edu
250 OK
RCPT TO:
250 OK
RCPT TO:
250 OK
RCPT TO:
550 No such user

6. <CR><LF>.<CR><LF>
SMTP Example
Server: example.edu
Client: somewhere.com
DATA
354 End data with <CR><LF>.<CR><LF>
From: John Doe
To: Bill Smith
Cc:
Date: Tue, 15 January :02: <CR><LF>
Subject: Test Message<CR><LF>
<CR><LF>
Hello There! <CR><LF>
Attempt to send from: to: three users at example.edu
<CR><LF>.<CR><LF>
250 OK
QUIT
221 example.edu closing connection

7. Access Protocols
IMAP Port: 143
POP3 Port: 110
Provides access to a mailbox (a file on the mail server)
Needed because most people don't leave their computers running all the time
Users access through an client program (ex: MS Outlook) connecting to an ISP server
ISP
Mail Server
ISP
Mail Server
User 1
User 2
Client
Client
POP/IMAP
SMTP
POP/IMAP

8. Access Protocols
Some users can access by using a web browser (web based service)
User 1
User 2
Web
Server
Web Server
Web Browser
Web Browser
HTTP
HTTP
IMAP/POP
IMAP/POP
Mail Server
Mail Server
SMTP

9. Representation Standards
RFC2822 (named after the standards document that defines it)
Key Points
Mail message is represented as a text file
Consists of a header, a blank line, and a body
Header lines have the form
Keyword: Information
Examples:
From:
Subject: test
Header lines starting with an uppercase 'X' can be added without affecting anything. Ex:
X-Cutest-Pokemon: Pikachu

10. Representation Standards
MIME (Multi-purpose Internet Mail Extensions)
Key Points
Allows the transfer of non-text data in an message
Specifies how binary data is encoded into printable characters
Adds additional lines in the header to specify how the message is encoded
Messages can be divided into sections, and each section encoded independently
MIME is backward compatable, however a system that does not understand MIME will treat the as a single block of text

11. MIME Ex: An email message with a single attachment Body Attachment
Header
From:
To:
Subject: Test
Cc:
X-Cutest-Pokemon: Pikachu
MIME-Version: 1.0
Content-Type: Multipart/Mixed; Boundary=MIME-SEPARATOR
Body
all
printable
text
MIME-SEPARATOR
Attachment

12. DNS (Domain Name System)
Port: 53
Converts readable names into IP addresses: name resolution
Major steps
User specifies the domain name it wants to connect to
A series of DNS servers are contacted until the name is found
IP address is returned back to the user
User then uses the IP address to contact the device it originally wanted

13. DNS Hierarchy IANA – organization which oversees IP addresses
ICANN – organization which oversees name registrations
Root Server
The highest level domain name servers in existence
13 sets of them located throughout the world
See:
Resolves addresses to Top Level Domain servers
Top Level Domain
The last segment of a domain name
Examples: COM, NET, ORG, EDU, GOV, etc.
See:

14. DNS Name Segments rhoggard.cs.ecu.edu. Administered by ECU
Administered by ICANN
rhoggard.cs.ecu.edu.
Name of ONE computer in CS network
Root Server
Network on
ECU campus
Top Level Domain
Name registered with a TLD Registrar

15. DNS Hierarchy Root Server Top Level Domain Server Authoritative Server
Answers requests for locations to top level domain servers
Top Level Domain Server
Answers requests for organizations registered under a particular TLD registrar
When you register a domain name, your name is registered with a company operating one of these servers
Authoritative Server
Answers request for a specific domain name (a specific machine address)
You, or your hosting provider, will set this up after you register your domain name so that this server will point clients to your website/ server/ftp server etc.
The exact authoritative server could be located under a higher level DNS server in a local network

16. DNS Hierarchy Root Server NET EDU COM ORG ATT SPECTRUM ECU YAHOO
GOOGLE
MICROSOFT
REDCROSS
ITCS CS
MAIL
LOGIN
WWW
WWW
MSDN
WWW
RHOGGARD
COURSEELF

17. DNS Hierarchy
Root Server
Local network administrators can set up their own DNS servers to divide up their networks any way they want
COM
server for
foobar.com
FOOBAR
server for
candy.foobar.com
CANDY
SOAP
PEANUT
WALNUT
ALMOND

18. DNS Hierarchy
Root Server
Local network administrators can set up their own DNS servers to divide up their networks any way they want
COM
server for
foobar.com
FOOBAR
CANDY
SOAP
server for
almond.candy.foobar.com
PEANUT
WALNUT
ALMOND

19. Web Browser request for
Example DNS Lookup
I.ROOT-SERVERS.NET
K.EDU-SERVERS.NET
rhoggard
.cs
.ecu
.edu .
Request to:
Try:
Request to:
Try:
SUNNY.NOC.ECU.EDU
Request to:
DNS Port 53
request for:
rhoggard.cs.ecu.edu
Resolving
Nameserver
(your ISP)
Try:
NS2.CS.ECU.EDU
Request to:
Web Browser request for
rhoggard.cs.ecu.edu
Go to:
Web Server at
HTTP Port 80 request for: default page

20. Web Browser request for
DNS Caching
Root Server
DNS requests are cached to try to speed up the resolution process
authoritative server NOT in cache
authoritative server in cache
Check resolving
server
Resolving
Nameserver
(your ISP)
Ask authoritative server
NS2.CS.ECU.EDU
Web Browser request for
rhoggard.cs.ecu.edu
Go to:
Check
local cache
Have you been
there before?
Check local cache
disney.com
ns2.cs.ecu.edu
maps.google.com
mail.google.com
rhoggard.cs.ecu.edu
Add to cache

21. Types of DNS Entries Type A Type MX Type CNAME
Name/IP combination used to point a domain name to an IP address
Type MX
Name/Name or Name/IP combination used to point a domain name to a mail server
Type CNAME
Name/Name combination, also known as an alias
Used to set up a second name (an alias) that points to an already existing name

22. DNS Entry Examples Type Name Points To Description A corporation.com
points corporation.com to MX
corporation.com
points mail requests for corporation.com
to a different machine than non-mail requests MX
mail.corporation.com
corporation.com
sets up a second name to point to the mail server
CNAME
corporation.com
sets up a second name to point to the main server
CNAME
ecu.corporation.com
ecu.edu
sets up a second name to point to ECU's server A
server1.corporation.com
sets up a name to another server (Ex: FTP server)
CNAME
ftp.corporation.com
server1.corporation.com
sets up an alias for the FTP server