1. CSS432 Applications Textbook Ch 9.1 and 9.3.1
Professor: Munehiro Fukuda
CSS432: Applications

2. Applications Client – Server Paradigm
Client: initiates contact with server
Server: provides client with a response
Applications use their corresponding protocol
Defined in RFC by Internet Engineering Task Force (
Implemented on top of TCP or UDP
Client
Server
outlook
browser
mail/web
server
SMTP, HTTP, FTP
SMTP, HTTP, FTP
socket
socket
TCP with
buffers,
variables
TCP with
buffers,
variables
Applications
Protocol
RFC
Port
Name service
DNS
1035 53
SMTP
IMAP
821 25
143
Web
HTTP
2616 80
File transfer
FTP
959 21
Remote terminal access
telnet
854 23
Internet
CSS432: Applications

3. Name Service Hosts Files Users (UID + host IP)
DNS
Name Server
Value
System-provided fixed value
Location dependent
Name
Variable length and mnemonic
Location independent
Hosts
cheltenham.cs.princeton.edu
:23:A8:33:5B:9F
Files
/usr/llp/tmp/foo (server, inode)
Users
Larry Peterson
(UID + host IP)
CSS432: Applications

4. Name service (cont) lpr/a2ps commands Unix system call DNS lpr
struct hostent h = gethostbyname( const char *hostname );
*(struct in_addr *)*h->h_addr_list; // returns IP list
Name
server
lpr
program
User
TCP IP 2
uw1-320-p1 3
lpr –Puw1-320-p1 manual.ps 1 4 5
CSS432: Applications

5. Domain Naming System Hierarchy Name DNS
uw bothell.washington.edu
(Currently, uw uwb.edu)
edu
com
washington …
mit u cs uw
medusa
bothell
cisco
yahoo
nasa
nsf
arpa
navy
acm
ieee
gov
mil
org
net uk fr
homer
goodall
CSS432: Applications

6. Name Servers Partition hierarchy into zones
DNS
Name Servers
administrative
unit
Partition hierarchy into zones
Network Information Center manages
edu
com
washington …
mit u cs uw
medusa
bothell
cisco
yahoo
nasa
nsf
arpa
navy
acm
ieee
gov
mil
org
net uk fr
homer
goodall
Managed by UW C&C
Root
name server
washington
Cisco
bothell cs …
Each zone implemented by two or more name servers
CSS432: Applications

7. DNS
Resource Records
Each name server maintains a collection of resource records
(Name, Value, Type, Class, TTL)
Name/Value: not necessarily host names to IP addresses
Type
A: Value returns IP address
NS: Value returns the corresponding name server’s name
CNAME: Value returns aliases.
MX: Value returns the domain name of this host’s mail
Class: IN (Internet class)
TTL: how long the resource record is valid
CSS432: Applications

8. Name Servers From Root to Bottom Level
DNS
Name Servers From Root to Bottom Level
Find a root name server
dig edu ns
edu a.gtld-server.net NS IN
a.gtld-server.net A IN
Find uwb.edu’s name servers
uwb.edu ns
uwb.edu, dns2.uwb.edu NS IN
uwb.edu, dns4.uwb.edu NS IN
dns2.uwb.edu A
dns4.uwb.edu A
Find mercury.uwb.edu
metis.uwb.edu A
metis.uwb.edu A
CSS432: Applications

9. Name Resolution Client
DNS
Name Resolution
Root
name
server
Princeton CS
Local
Client 1
cicada.cs.princeton.edu 8
princeton.edu,
cicada.cs.princeton.edu,
cs.princeton.edu, 2 3 4 5 6 7
Client
Initialized with its local name server’s address
appends its domain name to a given name before a submission.
Local server
needs to know root at only one place (not each host).
caches recent responses from remote servers.
CSS432: Applications

10. Electronic Mail (SMTP, MIME, IMAP)
user agents (client)
a.k.a. “mail reader”
composing, editing, reading mail messages
e.g., Eudora, Outlook, elm, Netscape Messenger, Thunderbird
mail servers (server)
outgoing, incoming messages stored on server
mail server
outgoing
mail queue
User mailbox
user
agent
SMTP/TCP
SMTP
IMAP
CSS432: Applications

11. SMTP TCP port 25 – Persistent connection Email sending protocol
From User agent to his/her local mail server
From a local to an intermediate/destination mail server (direct transfer)
From an intermediate to a destination server (relay)
Three phases of transfer
Handshaking: HELO
Transfer of messages: MAIL FROM, RCPT TO, and DATA
Closure: QUIT
Command/response interaction
Commands: ASCII text
Response: status code and phrase
CSS432: Applications

12. SMTP Example Email [mfukuda@uw1-320-20]$ telnet smtp.washington.edu 25
Trying …
Connected to smpt.washington.edu ( ).
Escape character is ‘^]’.
220 smtp.washington.edu ESMTP Sendmail UW04.02/ UW04.02; Sat, 21
Feb :00:
HELO mfukuda
250 smtp.washington.edu Hello UW bothell.washington.edu [ ]
, pleased to meet you
MAIL
Sender ok
RCPT
Recipient ok
DATA
354 Enter mail, end with “.” on a line by iteself
Blah blah blah…
…etc. etc. etc. .
i1M40uc Message accepted fro delivery
QUIT
smtp.washington.edu closing connection
Connection closed by foreign host. _
CSS432: Applications

13. RFC 822 Message Format Header A series of line<crlf>s Body
Header
To:
Subject:
Date:
From:
Received:
Body
A blank line
Header
A series of line<crlf>s
Body
ASCII characters only.
Then, what about multimedia information?
CSS432: Applications

14. MIME MIME: multimedia mail extension, RFC 2045, 2056
MIME: multimedia mail extension, RFC 2045, 2056
Additional lines in msg header declare MIME content type
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=“ CA6E2DE4ABCAFBC5”
From:
To:
Subject: promised material
Date: Mon, 01 Apr :34:
CA6E2DE4ABCAFBC5
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Dear CSS432 students,
Here is the jpeg image and draft report I promised.
-- Fukuda
Content-Type: image/jpeg
Content-Transfer-Encoding: base64
… Unreadable encoding of a jpeg figure
Content-Type: application/postscript; name=“draft.ps”
… Readable encoding of a postscript document
Carry more than one data type
Type of the data
How the data is encoded
Definitions for content types
3bytes = 3 * 8bits = 24bits = 4 * 6bit-chars, each mapped onto one of 64 ASCII chars (A~Z,a~z,0~9,+,/)
CSS432: Applications

15. Mail Reader - IMAP Email TCP to 143 Connection established
Server greeting
Server: OK
Server: PREAUTH
Server: BYE
Not authenticated
Client LOGOUT
Client: LOGIN or AUTHENTICATE
Authenticated
Client: CLOSE
Client LOGOUT
Client SELECT or EXAMINE
Selected
Client: FETCH, STORE,
DELETE, EXPUNGE
Client LOGOUT
Logout
Both sides close the connection
CSS432: Applications

16. File Transfer Protocol
FTP
local
file
system
remote
FTP
server
client
user
interface
TCP port 20 for data transfer
(not persistent)
TCP port 21 for control (persistent)
Transfer file to/from remote host
Client/server model
Client: initiates a control TCP connection to a server on port 21.
Client: sends a user ID and password as part of FTP commands.
Server: authorizes the client
Client: opens a data TCP connection to a server on port 20.
Server: maintains state: current directory, earlier authentication.
A ftp client is allowed to initiate a transfer between two ftp servers.
CSS432: Applications

17. FTP Commands <CRLF> delimits each command (and reply).
Commands consist of four uppercase ASCII characters, some with optional arguments:
USER username : sends a user identification to server.
PASS password : sends the user password to the server.
PASV: requests the server to send back its IP and port on which it listens to a data TCP connection from the user.
LIST : ask the server to send back its current directory contents through the data connection.
RETR filename : gets a file from the current remote directory.
STOR filename : stores a file into the current remote directory.
Each command is followed by a reply:
331 Username OK, password required
125 Data connection already open; transfer starting
425 Can't open data connection
452 Error writing file
CSS432: Applications

18. FTP Example FTP [mfukuda@uw1-320-20]$ telnet ftp.tripod.com 21
Trying …
Connected to ftp.tripod.com ( ).
Escape character is ‘^]’.
220 Welcome to Tripod FTP.
USER css432
331 Username set to css432. Now enter your password.
PASS ********
230 User ‘css432’ logged on.
LIST
425 Can’t open data connection for LIST.
PASV
227 Entering Passiv Mode (209,202,240,80,195,210)
// Open another xterm and telnet (=195* )
// Trying …
// Connected to ftp.tripod.com ( ).
// Escape character is ‘^]’.
// drwxr-xr-x 1 css432 Tripod Sep 15 21:22 cgi-bin
// -rw-r--r css432 Tripod Sep 16 18:28 ttcp.c
// -rw-r--r css432 Tripod Sep 15 21:22 index.htm
// drwxr-xr-x 1 css432 Tripod Sep 16 18:33 project
// Connection closed by foreign host.
150 Opening ASCII mode data connection for LIST.
226 Transfer complete.
QUIT
221 Goodbye
Connection closed by foreign host. _
CSS432: Applications

19. FTP passive mode FTP client FTP server
TCP port 21 for control (persistent)
Client request: connect( ), USER, PASS, LIST
Server Reply: 220 server ready, 331 send password, 230 login ok, 425 connection timeout
TCP port 20 for data transfer (one time)
FTP
client
FTP
server
TCP port 21 for control (persistent)
Client request: connect( ), USER, PASS, PASV, LIST
Server Reply: 220 server ready, 331 send password, 230 login ok,
227 Entering Passive Mode (140,142,12,173,195,54), 226 complete
TCP port 195* = for data transfer (one time)
data
CSS432: Applications

20. FTP proxy command ftp> open server1 ftp> proxy open server2
(3’) 227 Entering Passive Mode
(140,142,12,173,195,54)
FTP
client
(1) USER, PASS, SYST
(2) USER, PASS, SYST
(3) TYPE I, PASV
(4) TYPE I, PORT (140,142,12,173,195,54),
STOR file
(5) RETR file
ftp> open server1
ftp> proxy open server2
ftp> proxy get file
…(1)
…(2)
…(3)~(5)
FTP
Server 1
FTP
Server 2
TCP port 195* = for data transfer (one time)
data
CSS432: Applications

21. World Wide Web - HTTP
Web
/public_html
HTTP
server
Netscape
TCP port 80 (one-time connection) IE
HTTP request
HTTP response
Hypertext Transfer Protocol: one-time TCP port 80 connection for each response/reply
client: Browser requests, receives, and displays Web objects
server: Web server sends objects in response to the request
CSS432: Applications

22. HTTP Commands Web Client
Initiates a TCP connection to a server on port 80.
Sends an HTTP request over this TCP connection
OPTIONS: request information about available options
GET: retrieve document identified in URL
HEAD: retrieve document metainformation in URL
POST: give inofrmation (annotation, servlet invocation) to server
PUT: store document under specified URL
DELETE, TRACE, CONNECT
Receives an HTTP response
Server
Accepts a TCP connection from a client on port 80.
Receives an HTTP request
Returns an HTTP response and disconnects the connection.
1xx: Informational
2xx: success
3xx: Redirection
4xx: Client error
5xx: Server error
CSS432: Applications

23. HTTP Example Web [mfukuda@uw1-320-20]$ telnet www.washington.edu 80
Trying
Connected to ( ).
Escape character is '^]'.
GET
HTTP/ OK
Date: Sun, 22 Feb :06:17 GMT
Server: Apache/ (Unix) PHP/4.3.3 mod_perl/1.29 mod_backhand/1.2.2
Connection: close
Content-type: text/html
<!doctype html public “-//W3C/DTD HTML 4.0 Transitional//EN”>
<html><head> … </body></html>
Connection closed by foreign host. _
CSS432: Applications

24. Performance Improvement in Web
Persistent TCP connections
Advantages
Eliminate the connection setup overhead
Eliminate repetitions of slow start phase
Disadvantages
Need to know the time period of connection – server times out the connection.
Caching
Recently access pages can be cached at browsers and ISPs
Reduce the number of HTTP accesses
Need to purge out-of-date version of pages – use expiration date and cache directives to cache or not to cache a given page
CSS432: Applications

25. Exercises: try the following examples by yourself.
Reviews
RFC
Naming service: DNS
SMTP, MIME, and IMAP
File transfer: FTP
WWW: HTTP
Exercises: try the following examples by yourself.
Slide p8 (DNS)
Slide p12 (SMTP)
Slide p18 (FTP)
Slide p21 (HTTP)
CSS432: Applications