1. CSS432 Applications Textbook Ch 9.1 and 9.3.1
Prof. Athirai Irissappane
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
RFC – document showing detailed description of the protocols
Internet
CSS432: Applications

3. The University of Adelaide, School of Computer Science
11 February 2018
DNS
Domain Name System (DNS)—not an application that users normally invoke explicitly, but rather a service that almost all other applications depend upon.
This is because the name service is used to translate host names into host addresses; the existence of such an application allows the users of other applications to refer to remote hosts by name rather than by address.
Chapter 2 — Instructions: Language of the Computer

4. The University of Adelaide, School of Computer Science
11 February 2018
Name Service (DNS)
While perfectly suited for processing by routers, addresses are not exactly user-friendly.
It is for this reason that a unique name is also typically assigned to each host in a network.
Host names differ from host addresses in two important ways.
First, they are usually of variable length and mnemonic, thereby making them easier for humans to remember.
Second, names typically contain no information that helps the network locate (route packets toward) the host. Addresses can have routing information embedded in them
Chapter 2 — Instructions: Language of the Computer

5. The University of Adelaide, School of Computer Science
11 February 2018
Name Service (DNS)
We first introduce some basic terminology.
First, a name space defines the set of possible names.
A name space can be either flat (names are not divisible into components), or it can be hierarchical (Unix file names are an obvious example).
Second, the naming system maintains a collection of bindings of names to values. The value can be anything we want the naming system to return when presented with a name; in many cases it is an address.
Finally, a resolution mechanism is a procedure that, when invoked with a name, returns the corresponding value. A name server is a specific implementation of a resolution mechanism that is available on a network and that can be queried by sending it a message.
Chapter 2 — Instructions: Language of the Computer

6. Name Service Hosts cheltenham.cs.princeton.edu 192.12.69.17
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
A Unix filesystem is a collection of files and directories that has the following properties −
It has a root directory (/) that contains other files and directories.
Each file or directory is uniquely identified by its name, the directory in which it resides, and a unique identifier, typically called an inode.
CSS432: Applications

7. The University of Adelaide, School of Computer Science
11 February 2018
DNS
Uses a hierarchical name space
the DNS hierarchy can be visualized as a tree, where each node in the tree corresponds to a domain, and the leaves in the tree correspond to the hosts being named
DNS names are processed from right to left and use periods as the separator
Chapter 2 — Instructions: Language of the Computer

8. 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

9. The University of Adelaide, School of Computer Science
11 February 2018
Hierarchy implementation
The complete domain name hierarchy exists only in the abstract.
We now turn our attention to the question of how this hierarchy is actually implemented.
The first step is to partition the hierarchy into subtrees called zones.
Each zone can be thought of as corresponding to some administrative authority that is responsible for that portion of the hierarchy.
For example, the top level of the hierarchy forms a zone that is managed by the Internet Corporation for Assigned Names and Numbers (ICANN).
Each zone implemented by two or more name servers
Each name server, in turn is a program that can be accessed over the internet
Name server: mantains DNS and provides translation services
Chapter 2 — Instructions: Language of the Computer

10. 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
Find the root server
Find Washington server
Find cs server
Retrieve the ip address of lab systems
CSS432: Applications

11. 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, canonical name for hosts
MX: Value returns the domain name of this host’s which runs a mail server
Class: IN (Internet class)
TTL: how long the resource record is valid (servers can cache records)
CSS432: Applications

12. 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

13. (1) Every client knows its local name server’s address
DNS
(1) Every client knows its local name server’s address
Every local name server knows one or more root servers
(2)The local name server then queries the remote root servers on the client’s behalf
(3) the root server replies back with the closest matching name record which can be a NS
(4) the princeton NS is queried replies back with NS of CS
(5) the CS NS is queried for a specific record and replies with IP
CSS432: Applications

14. Name Servers From Root to Bottom Level
DNS
Name Servers From Root to Bottom Level
Use dig command to query Name servers options name type ..)
dig edu ns (find a name server for name=edu)
edu a.gtld-server.net NS IN
a.gtld-server.net A IN
Find uwb.edu’s name servers
uwb.edu ns (find NS with name=uwb.edu)
uwb.edu, dns2.uwb.edu NS IN
uwb.edu, dns4.uwb.edu NS IN
dns2.uwb.edu A
dns4.uwb.edu A
Find metis.uwb.edu
metis.uwb.edu A
metis.uwb.edu A
Dig:  use servers listed in in /etc/resolv.conf
search example.com local.lan nameserver nameserver nameserver nameserver

15. 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

16. The University of Adelaide, School of Computer Science
11 February 2018
In the early days of the Internet, host had the mailbox resided, and the mail reader they invoked was a local application program that extracted messages from the file system.
Today, of course, mailbox resides at the server, users remotely access their mailbox from their laptop or smartphone using yet another protocol, such as the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP);
in many cases the mail traverses one or more mail gateways on its route from the sender’s host to the receiver’s host.
SMTP is sometimes called a "Push" protocol--it pushes the message to its next destination and assumes that a user at the destination will "signon" and read their mail directly– POP/IMAP is a "Pull" protocol, in that the user's mail client pulls down the message to their work station
Chapter 2 — Instructions: Language of the Computer

17. The University of Adelaide, School of Computer Science
11 February 2018
POP – downloads mails to local storage, IMAP – maintains copy of mails in the server even after download
Both are PULL protocols: Poll the server for new mails periodically or when active
Why pull? If push, Host must stay up to receive mail
IMAP has notifications feature: Notify receiver for new mails then pulls the actual mails
Nowadays, the protocol used is ActiveSync which supports Push operations
Mails notified when received and can be downloaded instantly
Chapter 2 — Instructions: Language of the Computer

18. 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
Keep alive is a method to allow the same tcp connection for HTTP conversation instead of opening a new one with each new request. More simply put, it is a communication between the web server and the web browser that says "you can grab more than just one file at a time".
 if the sending mail server has several messages to send to the same receiving mail server, it can send all of the messages over the same TCP connection. For each message, the client begins the process with a new HELO crepes.fr and only issues QUIT after all messages have been sent.
CSS432: Applications

19. SMTP Example Email [css432@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
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 itself
Blah blah blah…
…etc. etc. etc. .
i1M40uc Message accepted from delivery
QUIT
smtp.washington.edu closing connection
Connection closed by foreign host. _
CSS432: Applications

20. RFC 822 Message Format Header Body ASCII characters only.
Header
a series of <CRLF>-terminated lines. (<CRLF> stands for carriage-return+ line-feed, ASCII control characters for end of a line)
Each header line contains a type and value separated by a colon
Body
ASCII characters only.
Then, what about multimedia information?
RFC 822 extended: MIME
Header
To:
Subject:
Date:
From:
Received:
Body
A blank line
The RFC series
contains technical and organizational documents about the Internet, including the specifications and policy documents produced by four streams: the Internet Engineering Task Force (IETF), the Internet Research Task Force (IRTF), the Internet Architecture Board (IAB), and Independent Submissions.
CSS432: Applications

21. The University of Adelaide, School of Computer Science
11 February 2018
MIME
allow messages to carry many different types of data: audio, video, images, PDF documents, and so on.
Message Format
MIME consists of three basic pieces.
collection of additional header lines
MIME-Version: (the version of MIME being used),
Content-Description: (a human-readable description of what’s in the message, analogous to the Subject: line),
Content-Type: (the type of data contained in the message), and
Content-Transfer- Encoding (how the data in the message body is encoded).
Chapter 2 — Instructions: Language of the Computer

22. The University of Adelaide, School of Computer Science
11 February 2018
MIME
Definitions for a set of content types (and subtypes). For example,
Type:image; SubType: image/gif and image/jpeg
Type:text; SubType: text/plain, text/richtext (markup, special fonts);
Type:application; SubType:application/postscript, application/msword, representing output of different application programs.
Type:Mutltipart; how messages carrying more than 1 data type are structured SubType:Multipart/mixed (set of independent data pieces, each piece has separate header describing its content type)
Way to encode the various data types so they can be shipped in an ASCII message
Base64 encoding
Every 3 bytes of original binary data into 4 ASCII charcters
Group binary data into 24 bit units (3 bytes)
Break each unit into 4, 6-bit pieces
Map each 6-bit piece into 1 ASCII character
Man is TWFu. Encoded in ASCII, the characters M, a, and n are stored as the bytes 77, 97, and 110, which are the 8-bit binary values  ,  , and  These three values are joined together into a 24-bit string, producing  Groups of 6 bits 6 bits have a maximum of 26 = 64 different binary values) are converted into individual numbers from left to right (in this case, there are four numbers in a 24-bit string), which are then converted into their corresponding Base64 character values , , , represent 19, 22, 54, 6.
In the 64 index table (choose 64 from ASCII based on certain criteria) T, W, F, u
Hence man becomes TWFu
Chapter 2 — Instructions: Language of the Computer

23. 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: CSS432- MIME Sample
Date: Wed, 08 Feb :59:
CA6E2DE4ABCAFBC5
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Hi all,
Attached is the jpeg and draft report.
Cheers!
athirai
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
3bytes = 3 * 8bits = 24bits = 4 * 6bit-chars, each mapped onto one of 64 ASCII chars (A~Z,a~z,0~9,+,/)
Postscript, as the name sort of suggests, is actually a Turing-complete programming language. Directions are written out in a human-readable way, and communicated to the printer, which creates the high quality art from the instructions.
SMTP, by definition (RFC 821), limits mail to lines of 1000 characters of 7 bits each. That means that none of the bytes you send down the pipe can have the most significant ("highest-order") bit set to "1".
The content that we want to send will often not obey this restriction inherently. Think of an image file, or a text file that contains Unicode characters: the bytes of these files will often have their 8th bit set to "1". SMTP doesn't allow this, so you need to use "transfer encoding" to describe how you've worked around the mismatch.
The values for the Content-Transfer-Encoding header describe the rule that you've chosen to solve this problem.
7bit simply means "My data consists only of US-ASCII characters, which only use the lower 7 bits for each character." You're basically guaranteeing that all of the bytes in your content already adhere to the restrictions of SMTP, and so it needs no special treatment. You can just read it as-is.
Note that when you choose 7bit, you're agreeing that all of the lines in your content are less than 1000 characters in length
Definitions
for content types

24. Mail Reader – IMAP (persistent connection)
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

25. 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)
CSS432: Applications

26. FTP Commands
FTP
FTP: Control connection (TCP)- commands and responses, data connection (TCP) – data transfer
ACTIVE MODE: client establishes the command channel, server establishes the data channel 
Client opens up command channel from client port 35762 to server port 21.
Client sends PORT 35763 to server and server acknowledges on command channel.
Server opens up data channel from server port 20 to client port 35763
Client acknowledges on data channel.
Passive mode: the client establishes both channels
Client opens up command channel from client port 35763 to server port 21
Client issues PASV command
Server sends back (on command channel) port number 46005 after starting to listen on that port.
Client opens up data channel from client  to server port
Server acknowledges on data channel.
Port 20/21 are used traditionally, i.e., 21 for command channel and 20 for data channel
CSS432: Applications

27. 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

28. FTP Example FTP [css432@uw1-320-20]$ telnet ftp.tripod.com 21
Trying …
Connected to ftp.tripod.com ( ).
Escape character is ‘^]’.
220 Welcome to Tripod FTP.
USER css432w17
331 Username set to css432. Now enter your password.
PASS ********
230 User ‘css432w17’ logged on.
LIST
425 Can’t open data connection for LIST.
PASV
227 Entering Passiv Mode (209,202,252,54,127,31)
// Open another xterm and telnet (=127*256+31)
// Trying …
// Connected to ftp.tripod.com ( ).
// Escape character is ‘^]’.
150 Opening ASCII mode data connection for LIST.
226 Transfer complete.
// -rw-r--r css432w17 Tripod Feb 10 17:29 index.htm
// drwxr-xr-x 1 css432w17 Tripod Feb 10 17:39 cgi-bin
// drwxr-xr-x 1 css432w17 Tripod Feb 10 17:54 project
// Connection closed by foreign host.
QUIT
221 Goodbye
Connection closed by foreign host. _
CSS432: Applications

29. 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)
SYST: displays system name, TYPE, ie. Type of file to be transfered
UNIX Type: L8 ; FTP clients use this information to correctly interpret and display server response. Different server’s can have different response code
An FTP server reply is an acknowledgement sent from an FTP server to user in response to FTP commands. A reply is composed of a 3-digit reply code followed by a text string. Different implementations of FTP server can supply different text string for the same reply code, since it is usually intended for human users.
RETR: RETR remote-filename
Begins transmission of a file from the remote host. Must be preceded by either a PORT command or a PASV command to indicate where the server should send data.
STOR remote-filename Begins transmission of a file to the remote site. Must be preceded by either a PORT command or a PASV command so the server knows where to accept data from.
PROXY
Execute an ftp command on a secondary control connec- tion. This command allows simultaneous connection to two remote ftp servers for transferring files between the two servers.
Close: closes the current FTP connection with server   but still leaves you within the FTP environment.
Quit: to exit the FTP environment
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

30. FTP proxy command / FTP client (Transfer files from s2 to s1)
(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),
(6)RETR file
(7) QUIT
(5) STOR file
(8) QUIT
ftp> open server1
ftp> proxy open server2
ftp> proxy put file
ftp>quit
…(1)
…(2)
…(3)~(6)
…(7)~(8)
FTP
Server 1
FTP
Server 2
SYST: displays system name, TYPE, ie. Type of file to be transferred
UNIX Type: L8
RETR: RETR remote-filename
Begins transmission of a file from the remote host. Must be preceded by either a PORT command or a PASV command to indicate where the server should send data. RETR filename gets file from server in the data connection (Retrieve/gets a file from the server, fork a child and start reading the file before sending the retr command)
STOR remote-filename Begins transmission of a file to the remote site. Must be preceded by either a PORT command or a PASV command so the server knows where to accept data from. “There is a file which is going to be transmitted to you. Open a file with the “remote-filename”, get the contents being transmitted after this command and store it in the opened file”
PROXY
Execute an ftp command on a secondary control connec- tion. This command allows simultaneous connection to two remote ftp servers for transferring files between the two servers.
TYPE:Sets the transfer mode (ASCII/Binary).
A - ASCII text
E - EBCDIC text
I - image (binary data)
L - local format
Close: closes the current FTP connection with server   but still leaves you within the FTP environment.
Quit: to exit the FTP environment
PORT
Syntax: PORT a1,a2,a3,a4,p1,p2 Specifies the host and port to which the server should connect for the next file transfer. This is interpreted as IP address a1.a2.a3.a4, port p1*256+p2.
Close only -> send QUIT and return
Close & come out of ftp -> send
TCP port 195* = for data transfer (one time)
data
CSS432: Applications

31. 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

32. HTTP Commands
Web
HTTP
The core idea of hypertext is that one document can link to another document, and the protocol (HTTP) and document language (HTML) were designed to meet that goal.
URL
They provide information that allows objects on the Web to be located, and they look like the following:
If you opened that particular URL, your Web browser would open a TCP connection to the Web server at a machine called and immediately retrieve and display the file called index.html.
CSS432: Applications

33. 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 information (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
By design, the POST request method requests that a web server accept the data enclosed in the body of the request message, most likely for storing it.[1] It is often used when uploading a file or when submitting a completed web form.
In contrast, the HTTP GET request method retrieves information from the server. As part of a GET request, some data can be passed within the URL's query string, specifying (for example) search terms, date ranges, or other information that defines the query.
As part of a POST request, an arbitrary amount of data of any type can be sent to the server in the body of the request message. A header field in the POST request usually indicates the message body's Internet media type.
The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request-Line
The PUT method requests that the enclosed entity be stored under the supplied Request-URI. 
The DELETE method requests that the origin server delete the resource identified by the Request-URI.
The TRACE method is used to invoke a remote, application-layer loop- back of the request message.  TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information.
The OPTIONS method represents a request for information about the communication options available on the request/response chain identified by the Request-URI. On the server
CSS432: Applications

34. HTTP Example Web [css432@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

35. The University of Adelaide, School of Computer Science
11 February 2018
HTTP 1.0
TCP Connections
The original version of HTTP (1.0) established a separate TCP connection for each data item retrieved from the server
HTTP 1.0 behavior
Chapter 2 — Instructions: Language of the Computer

36. The University of Adelaide, School of Computer Science
11 February 2018
Persistent Connection
the client and server can exchange multiple request/response messages over the same TCP connection.
Reduces latency, can avoid slow start each time due to TCP’s congestion window mechanism
HTTP 1.1 behavior with persistent connections
Chapter 2 — Instructions: Language of the Computer

37. 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
Server assigns an expiration date when it sends back a page to the client after which it can no longer be cached
CSS432: Applications

38. Exercises: try the following example by yourself.
Reviews
RFC
Naming service: DNS
SMTP, MIME, and IMAP
File transfer: FTP
WWW: HTTP
Exercises: try the following example by yourself.
FTP
CSS432: Applications

39. HW4
NAT: Network Address Translation
a network device, usually a firewall, assigns a public address to a computer (or group of computers) inside a private network when connecting to an external network such as internet
Dynamic Host Configuration Protocol (DHCP) is a network protocol that enables aserver to automatically assign an IP address to a computer from a defined range of numbers (i.e., a scope) configured for a given network
The main use of NAT is to limit the number of public IP addresses an organization or company must use, for both economy and security purposes
CSS432: Applications

40. Similar to linux FTP client
Project: FTP client
Similar to linux FTP client
ftp ftp.tripod.com
ls, open, close, quit commands
Map to the functions specified by RFC 959
E.g., ls  PASV + LIST
Establish TCP connection with server
Use read and write commands
User issues ls, client should send PASV, LIST command to server and read back the response from server
File Transfer issues
CSS432: Applications