DNT 1013 DATA COMMUNICATIONS ------------------------------------------ CHAPTER 6:

DNT 1013DATA COMMUNICATIONS

------------------------------------------CHAPTER 6:

APPLICATION LAYER

Prepared By:Mdm Noor Suhana Bt SulaimanFKMT-NT, TATiUC

DOMAIN NAME SYSTEM

DNS: Domain Name SystemImagine a world without DNSYou would have to remember the IP addresses of

Every website you want to visitYour bookmarks will be a list of IP addressesYou will speak like

“I went to 167.33.24.10, and there was an awesome link to 153.11.35.81… “

DNS- HostnamesIP Addresses are great for computers

IP address includes information used for routing.

IP addresses are tough for humans to remember.

IP addresses are impossible to guess.ever guessed at the name of a WWW site?

The Domain Name SystemThe domain name system is usually used to

translate a host name into an IP address .

Domain names comprise a hierarchy so that names are unique, yet easy to remember.

DNS Hierarchy

edu com org jp

rpi albany

Host name structureEach host name is made up of a sequence of labels

separated by periods.Each label can be up to 63 charactersThe total name can be at most 255 characters.

Examples:whitehouse.govbarney.the.purple.dinosaur.commonica.cs.rpi.edu

Domain NameThe domain name for a host is the sequence

of labels that lead from the host (leaf node in the naming tree) to the top of the worldwide naming tree.

A domain is a subtree of the worldwide naming tree.

Top level domainsedu, gov, com, net, org, mil, …Countries each have a top level domain (2

letter domain name).New top level domains include:

.aero .biz .coop .info .name .pro

Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS servers

yahoo.comDNS servers

amazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com; 1st approx:Client queries a root server to find .com DNS serverClient queries com DNS server to get amazon.com

DNS serverClient queries amazon.com DNS server to get IP

address for www.amazon.com

Client wants IP for www.amazon.com; 1st approx:Client queries a root server to find .com DNS serverClient queries com DNS server to get amazon.com

DNS serverClient queries amazon.com DNS server to get IP

address for www.amazon.com

DNS: Root name serverscontacted by local name server that can not resolve nameroot name server:

contacts authoritative name server if name mapping not known

gets mappingreturns mapping to local name server

13 root name servers worldwideb USC-ISI Marina del Rey, CA

l ICANN Los Angeles, CA

e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 17 other locations)

i Autonomica, Stockholm (plus 3 other locations)

k RIPE London (also Amsterdam, Frankfurt)

m WIDE Tokyo

a Verisign, Dulles, VAc Cogent, Herndon, VA (also Los Angeles)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 11 locations)

REMOTE LOGGING, ELECTRONIC EMAIL &

FILE TRANSFER

Electronic MailThree major components: user agents mail servers simple mail transfer protocol:

SMTP

User Agenta.k.a. “mail reader”composing, editing, reading

mail messagese.g., Eudora, Outlook, elm,

Netscape Messengeroutgoing, incoming messages

stored on server

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

Electronic Mail: mail servers

Mail Servers mailbox contains incoming

messages for usermessage queue of outgoing

(to be sent) mail messagesSMTP protocol between

mail servers to send email messagesclient: sending mail

server“server”: receiving mail

server

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

Scenario: Alice Emails Bob1) Alice uses UA to

compose message and “to” [email protected]

2) Alice’s UA sends message to her mail server; message placed in message queue

3) Client side of SMTP opens TCP connection with Bob’s mail server

4) SMTP client sends Alice’s message over the TCP connection

5) Bob’s mail server places the message in Bob’s mailbox

6) Bob invokes his user agent to read message

useragent

mailserver

mailserver user

agent

1

2 3 4 56

SMTP Commands to send emailTelenet into port 25HELO hostnameMAIL FROMRCPT TORCPT TO …DATA… text ….QUIT

You can try doing this yourself

Try SMTP interaction for yourself:

telnet servername 25see 220 reply from serverenter HELO, MAIL FROM, RCPT TO, DATA, QUIT

commands above lets you send email without using email client

(reader)

SMTP: final wordsSMTP uses persistent

connectionsSMTP requires message

(header & body) to be in 7-bit ASCII

SMTP server uses CRLF.CRLF to determine end of message

Comparison with HTTP:

HTTP: pullSMTP: push

both have ASCII command/response interaction, status codes

HTTP: each object encapsulated in its own response msg

SMTP: multiple objects sent in multipart msg

Mail message formatSMTP: protocol for

exchanging email msgsRFC 822: standard for text

message format:header lines, e.g.,

To: From: Subject:different from SMTP

commands!body

the “message”, ASCII characters only

header

body

blankline

Message format: multimedia extensionsMIME: multimedia mail extension, RFC 2045, 2056additional lines in msg header declare MIME

content typeThink of image attachments with your email

From: [email protected] To: [email protected] Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data

multimedia datatype, subtype,

parameter declaration

method usedto encode data

MIME version

encoded data

Mail access protocols

SMTP: delivery/storage to receiver’s serverMail access protocol: retrieval from server

POP: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download

IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server

HTTP: Hotmail , Yahoo! Mail, etc.

useragent

sender’s mail server

useragent

SMTP SMTP accessprotocol

receiver’s mail server

What’s theDifference?

What’s theDifference?

POP3 (more) and IMAPMore about POP3Previous example uses

“download and delete” mode.

Bob cannot re-read e-mail if he changes client

“Download-and-keep”: copies of messages on different clients

POP3 is stateless across sessions

IMAPKeep all messages in

one place: the serverAllows user to organize

messages in foldersIMAP keeps user state

across sessions:names of folders and

mappings between message IDs and folder name

FTPFile Transfer Protocol (FTP), a standard Internet

protocol, is the simplest way to exchange files between computers on the Internet.

Like the Hypertext Transfer Protocol (HTTP), which transfers displayable Web pages and related files, and the Simple Mail Transfer Protocol (SMTP), which transfers e-mail, FTP is an application protocol that uses the Internet's TCP/IP protocols.

FTP is commonly used to transfer Web page files from their creator to the computer that acts as their server for everyone on the Internet.

It's also commonly used to download programs and other files to computer from other servers.

Cont..User can use FTP with a simple command line

interface (for example, from the Windows MS-DOS Prompt window) or with a commercial program that offers a graphical user interface.

User Web browser can also make FTP requests to download programs you select from a Web page.

Using FTP, user can also update (delete, rename, move, and copy) files at a server.

User need to logon to an FTP server. However, publicly available files are easily accessed using anonymous FTP.

Cont..Basic FTP support is usually provided as part

of a suite of programs that come with TCP/IP. However, any FTP client program with a

graphical user interface usually must be downloaded from the company that makes it.

Web ServerA computer program that is responsible for accepting HTTP

requests from clients (user agents such as web browsers), and serving them HTTP responses along with optional data contents, which usually are web pages such as HTML documents and linked objects (images, etc.).

A computer that delivers (serves up) Web pages. Every Web server has an IP address and possibly a domain name

For example, if you enter the URL http://www.pcwebopedia.com/index.html in your browser, this sends a request to the server whose domain name is pcwebopedia.com.

The server then fetches the page named index.html and sends it to your browser.

Any computer can be turned into a Web server by installing server software and connecting the machine to the Internet.

There are many Web server software applications, including public domain software from NCSA and Apache, and commercial packages from Microsoft, Netscape and others.

Cont..Web server is a program that, using the client/server

model and the World Wide Web's Hypertext Transfer Protocol ( HTTP ), serves the files that form Web pages to Web users (whose computers contain HTTP clients that forward their requests).

Every computer on the Internet that contains a Web site must have a Web server program. Two leading Web servers are Apache , the most widely-installed Web server, and Microsoft's Internet Information Server ( IIS ).

Other Web servers include Novell's Web Server for users of its NetWare operating system and IBM's family of Lotus Domino servers, primarily for IBM's OS/390 and AS/400 customers.

Cont..Web servers often come as part of a larger

package of Internet- and intranet-related programs for serving e-mail, downloading requests for File Transfer Protocol ( FTP ) files, and building and publishing Web pages.

Considerations in choosing a Web server include how well it works with the operating system and other servers, its ability to handle server-side programming, security characteristics, and publishing, search engine, and site building tools that may come with it.

Cont..The origin of the content sent by server is

called:static if it comes from an existing file lying on

a filesystem; dynamic if it is dynamically generated by

some other program or script or application programming interface (API) called by the web server.

Serving static content is usually much faster (from 2 to 100 times) than serving dynamic content, especially if the latter involves data pulled from a database.

Web Server featuresAlthough web server programs differ in detail, they all

share some basic common features.HTTP: every web server program operates by accepting

HTTP requests from the client, and providing an HTTP response to the client. The HTTP response usually consists of an HTML document, but can also be a raw file, an image, or some other type of document (defined by MIME-types). If some error is found in client request or while trying to serve it, a web server has to send an error response which may include some custom HTML or text messages to better explain the problem to end users.

Logging: usually web servers have also the capability of logging some detailed information, about client requests and server responses, to log files; this allows the webmaster to collect statistics by running log analyzers on these files.

HTTP overviewHTTP: hypertext transfer

protocolWeb’s application layer

protocolclient/server model

client: browser that requests, receives, “displays” Web objects

server: Web server sends objects in response to requests

HTTP 1.0: RFC 1945HTTP 1.1: RFC 2068

PC runningExplorer

Server running

Apache Webserver

Mac runningNavigator

HTTP request

HTTP request

HTTP response

HTTP response

HTTP overview (continued)Uses TCP:client initiates TCP

connection (creates socket) to server, port 80

server accepts TCP connection from client

HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)

TCP connection closed

HTTP is “stateless”server maintains no

information about past client requests

Protocols that maintain “state” are complex!

past history (state) must be maintained

if server/client crashes, their views of “state” may be inconsistent, must be reconciled

aside

HTTP connectionsNonpersistent HTTPAt most one object is

sent over a TCP connection.

HTTP/1.0 uses nonpersistent HTTP

Persistent HTTPMultiple objects can be

sent over single TCP connection between client and server.

HTTP/1.1 uses persistent connections in default mode

HTTP Protocol HTTP (Hypertext Transfer Protocol) is the set of rules for

transferring files (text, graphic images, sound, video, and other multimedia files) on the World Wide Web.

As soon as a Web user opens their Web browser, the user is indirectly making use of HTTP.

HTTP is an application protocol that runs on top of the TCP/IP suite of protocols (the foundation protocols for the Internet).

In order to fetch a web page for you, your web browser must "talk" to a web server somewhere else.

When web browsers talk to web servers, they speak a language known as HTTP, which stands for HyperText Transfer Protocol.

The latest version of HTTP is HTTP 1.1.

Cont..HTTP concepts include (as the Hypertext part of the name

implies) the idea that files can contain references to other files whose selection will elicit additional transfer requests.

Any Web server machine contains, in addition to the Web page files it can serve, an HTTP daemon, a program that is designed to wait for HTTP requests and handle them when they arrive.

Your Web browser is an HTTP client, sending requests to server machines.

When the browser user enters file requests by either "opening" a Web file (typing in a Uniform Resource Locator or URL) or clicking on a hypertext link, the browser builds an HTTP request and sends it to the Internet Protocol address (IP address) indicated by the URL.

The HTTP daemon in the destination server machine receives the request and sends back the requested file or files associated with the request. (A Web page often consists of more than one file.)

Cont..Hypertext Transfer Protocol (HTTP) is an application-

level protocol for distributed, collaborative, hypermedia information systems.

Its use for retrieving inter-linked resources led to the establishment of the World Wide Web.

HTTP development was coordinated by the World Wide Web Consortium and the Internet Engineering Task Force (IETF), culminating in the publication of a series of Requests for Comments (RFCs), most notably RFC 2616 (June 1999), which defines HTTP/1.1, the version of HTTP in common use.

Cont..HTTP is a request/response standard between a client and

a server. A client is the end-user, the server is the web site. The

client making a HTTP request—using a web browser, spider, or other end-user tool—is referred to as the user agent.

The responding server—which stores or creates resources such as HTML files and images—is called the origin server.

In between the user agent and origin server may be several intermediaries, such as proxies, gateways, and tunnels.

HTTP is not constrained to using TCP/IP and its supporting layers, although this is its most popular application on the Internet. Indeed HTTP can be "implemented on top of any other protocol on the Internet, or on other networks.

HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used

Cont..HTTP predominantly uses TCP and not UDP because

much data must be sent for a webpage, and TCP provides transmission control, presents the data in order, and provides error correction.

Resources to be accessed by HTTP are identified using Uniform Resource Identifiers (URIs) (or, more specifically, Uniform Resource Locators (URLs)) using the http: or https URL schemes.

HTTP Transaction HTTP uses only one TCP connection on Well-Known port 80 Ex) protocol://hostname[:port]/path/filename

Cont..Step 1: Connection Establishment

TCP/IP connection set up uses a port number as application reference usually port 80 ports ≤ 1024 are privileged (>1024 are open)

Step 2: Client’s request HTTP message sent with a request line

Step 3: Server Response server sends HTTP message and optionally requested

dataStep 4: Server Disconnection

usually the server sometimes the client “stops” it

Cont..http is “stateless”

server maintains no information about past client requests

each operation or transaction makes a new connection

each operation is unaware of any other connection

each click is a new connection

MessageMessage categories

Request message : Client ServerResponse message : Server Client

Request message format

Request Line Request line

Request type : methods (GET,HEAD,POST…) Uniform Resource Locator (URL) : Protocol (FTP,HTTP,TELNET…) HTTP version : HTTP 1.1(1.0, 0.9 are still in use)

Cont..Resources

A resource is a chunk of information that can be identified by a URL(Universal Resource Locator)

A resource can be A file A dynamically created page

What we see on the browser can be a combination of some resources

Cont.. URL(Uniform Resource Locator)

Method is the protocol used to retrieve the document among them are FTP, HTTP, News and TELNET

MethodsMethods

Methods is actual command or request that a client issues to the server

Methods kinds GET

Client wants to retrieve a document from the server Responds with the contents of the document in the

body of the response message HEAD

Client wants some information about a document Response from the server does net contain a body

(different from GET) POST

Client provides some information for the server ex) send input to a server

Cont.. Methods kinds

PUT Client provides a new or replacement document to be stored on the

server Included in the body of the request and will be stored in the

location defined by the URL PATCH

Only a list of differences that should be implemented in the existing file

COPY Used to copy a file to another location Location of the source file is given in the request line (URL),

destination is given in the entity header MOVE

Used to move a file to another location Location of the source file is given in the request line (URL),

destination is given in the entity header

Cont..Methods kinds

DELETE Used to remove a document on the server

LINK Used to create a link or links from a document to another

location Location of the source file is given in the request line

(URL), destination is given in the entity header UNLINK

Used to delete links created by the LINK methods OPTION

Used by the client to ask the server about available options

CHECKIN, CHECKOUT, SHOWMETHOD, SPACEJUMP etc.

Response Message

Status Line Status Line

Defines the status of the response message consists : HTTP version + a space + a status code +

a space + a status phrase HTTP version : HTTP 1.1 (1.0, 0.9 are also in use)

Status Code

There is no content in the bodyNo content204

The request is accepted, but it is not immediately acted upon

Accepted202

A new URL is createdCreated201

The request is successfulOK200

Success

The server is complying with a client request to switch protocols defined in the upgrade header

Switching101

The initial part of the request has been received and the client may continue with is request

Continue100

Informational

DescriptionPhraseCode

Cont..

The format requested is not acceptableNot acceptable406

The method is not supported in this URLMethod not allowed405

Server Error

There is an error, such as a crash, in the server siteInternal server error

500

The requested URL is no longer used by the serverMoved permanently302

The request lacks proper authorizationUnauthorized401

Service is deniedForbidden403

The service is temporarily unavailable, but may be requested in the future

Service unavailable503

The action requested act not be performedNot implement501

The document is not foundNot found404

There is a syntax error in the requestBad request400

Client Error

The requested URL has moved temporarilyMoved temporarily304

The requested URL refers to more than one resource

Multiple choices301

Redirection

DescriptionPhraseCode

And one truly last word…

ThAnKs

DNT 1013 DATA COMMUNICATIONS ------------------------------------------ CHAPTER 6:

Documents

Transcript of DNT 1013 DATA COMMUNICATIONS ------------------------------------------ CHAPTER 6: