1. Networking CS 3470, Section 1 Sarah Diesburg
Protocols
Networking
CS 3470, Section 1
Sarah Diesburg

2. Applications Applications need their own protocols
Just like we are writing our network programs with a certain specification so that any two randomly-chosen network programs may work together

3. Applications and Network Protocols
As an example, many browsers can interact with web servers
Firefox IE
Chrome
Opera
Others…

4. Networking Application Protocols
Many specialized network application protocols are defined
For a list, just take a look at /etc/services
Today we will discuss two
Simple Mail Transfer Protocol (SMTP) used to exchange electronic mail
HyperText Transport Protocol (HTTP) is used to communicate between web browsers and web servers

5. Networking Application Protocols
Both HTTP and SMTP have protocols that specify the format of the data that can be exchanged
SMTP
RFC821 – Actual SMTP protocol
RFC822 – Companion protocol that specifies standard message format
HTTP
HTTP is the protocol for fetching web pages
HTML (HyperText Markup Language) is the companion protocol that defines the form of web pages

6. SMTP
Goal of SMTP is to transfer mail reliably and efficiently on TCP port 25
Two main components
User Agents (UAs) prepare the message
Also called mail readers in the text
Mail Transfer Agents (MTAs) transfer the mail across the Internet
Also called mail daemons in the text (where daemon is just another word for process)

7. SMTP UA UA
Internet
MTA
MTA

8. Gateways
Often the sender’s MTA doesn’t get to connect directly to the receiver’s MTA
Mail traverses one or more mail gateways
Stores and forwards addresses
Example: Sending a mail to pizzahut.com first forwards to a global pizzahut gateway, then is sent to MTA specific to location (Cedar Falls area)

9. SMTP Gateways UA UA
MTA/Gateway
MTA/Gateway
MTA
MTA

10. RFC 822 ARPA Internet Text Messages
Defines format of the messages being exchanged
Messages have two parts: header and body
Both parts are represented in ASCII text
Attachments like pictures are encoded
ASCII protocols can be tried by humans running telnet!
$> telnet some.mail.server 25

11. How SMTP works The Basics Keyword Arguments HELO
Sender’s Host Domain Name
MAIL FROM:
Address of sender
RCPT TO:
of Intended recipient
DATA
Body of the message
QUIT

12. Status Codes
The Server responds with a 3 digit code that may be followed by text info
2## - Success
3## - Command can be accepted with more information
4## - Command was rejected, but error condition is temporary
5## - Command rejected, Bad User!

13. Book’s Example
$> telnet mail.cs.princeton.edu 25 HELO cs.princeton.edu 250 Hello MAIL 250 OK RCPT

14. Book’s Example (Cont.)
DATA 354 Start mail input; end with . Blah blah blah… OK QUIT 221 Closing connection…

15. Before we move on…
Use of Computer Resources poicy: 
"Unauthorized or inappropriate use of the University of Northern Iowa computing resources is prohibited and is grounds for sanctions which can include suspension or loss of computing privileges, disciplinary action or, in extreme cases, legal action."

16. Demo Let’s find UNI’s SMTP server…
Can we send from any address (even fake ones)?
Can we send to anybody?
What’s in the headers?

17. HTTP Hypertext Transport Protocol TCP port 80 RFC 1945 ASCII protocol
Protocol used for communication between web browsers and web servers
TCP port 80
RFC 1945
ASCII protocol
This means we can use our good friend, telnet!
$> telnet some.webserver.com 80

18. HTTP - URLs URL Uniform Resource Locator
protocol (http, ftp, news)
host name (name.domain name)
port (usually 80 but many on 8080)
directory path to the resource
resource name

19. HTTP Request Operations
Description
OPTIONS
Request information about available options
GET
Retrieve document identified in URL
HEAD
Retrieve metainformation about document identified in URL
POST
Give information to server (e.g., submitting forms)
PUT
Upload document under specified URL

20. Status Codes 200 OK 201 created 202 accepted 204 no content
301 moved perm.
302 moved temp
304 not modified
400 bad request
401 unauthorized
403 forbidden
404 not found
500 int. server error
501 not impl.
502 bad gateway
503 svc not avail

21. Demo
$> telnet 80 GET …<Lots of HTML code here….>

22. Demo
Can we get a webpage?
Do all the images come along?