CCNA Exploration Ch 3

8/8/2019 CCNA Exploration Ch 3

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KC KHOR, Multimedia Univ. Cyberjaya 2

OSI and TCP/IP Models – Application Layer

The Application layer, Layer seven, is the top

layer of both the OSI and TCP/IP models. It provides the interface between the

applications we use to communicate withnetwork over which our messages are

transmitted. Application layer protocols are used to exchange

data between programs running on the sourceand destination hosts.

There are many Application layer protocols andnew protocols are always being developed



The functionality of the TCP/IP applicationlayer protocols fit roughly into the framework of

the top three layers of the OSI model:Application, Presentation and Session layers




The Presentation layer (of OSI model) has 3primary functions:

- Coding and conversion of Applicationlayer data to ensure that data from the sourcedevice can be interpreted by the appropriateapplication on the destination device.

- Compression of the data in a manner thatcan be decompressed by the destinationdevice.

- Encryption of the data for transmissionand the decryption of data upon receipt bythe destination.




The Session Layer

- It creates and maintains dialogs between

source and destination applications.- It handles the exchange of information toinitiate dialogs, keep them active, and to

restart sessions that are disrupted or idle fora long period of time

Most applications, like web browsers or e-mail clients, incorporate functionality of theOSI layers 5, 6 and 7.




The most widely-known TCP/IP Applicationlayer protocols are those that provide for the

exchange of user information.- Domain Name Service Protocol (DNS)

- Hypertext Transfer Protocol (HTTP)

- Telnet, a terminal emulation protocol

- File Transfer Protocol (FTP)




Application Layer Software Within the Application layer, there are two

forms of software programs or processes thatprovide access to the network.

Network-Aware Applications: Some end-

user applications are network-aware,meaning that they implement the applicationlayer protocols and are able to communicatedirectly with the lower layers of the protocolstack. Ex: Email client, web browser




Application layer Services: Other programsmay need the assistance of Application layer

services to use network resources, like filetransfer or network print spooling.

Each application or network service uses

protocols which define the standards anddata formats to be used. Without protocols,the data network would not have a commonway to format and direct data




Application Layer Protocol Functions

In order for the communications to be successful, the

application layer protocols implemented on the sourceand destination host must match.

Protocols establish consistent rules for exchangingdata between applications and services loaded on the

participating devices Many different types of applications communicate

across data networks. Therefore, Application layerservices must implement multiple protocols to provide

the desired range of communication experiences




The Client-Server Model In the client/server model, the device requesting the information is

called a client and the device responding to the request is called aserver.

Client and server processes are considered to be in the Applicationlayer.

Data transfer from a client to a server is referred to as an upload and

data from a server to a client as a download.




Servers In a client/server network, the server runs a service,

or process, sometimes called a server daemon. Like most services, daemons typically run in the

background and are not under an end user's directcontrol.

When a daemon "hears" a request from a client, itexchanges appropriate messages with the client, asrequired by its protocol, and proceeds to send therequested data to the client in the proper format

httpd, telnetd, ftpd and etc




Peer-to-Peer Model

Peer-to-peer networking involves two distinctforms.

In a peer-to-peer network, two or morecomputers are connected via a network and can

share resources (such as printers and files)without having a dedicated server.

Every connected end device (known as a peer)

can function as either a server or a client..




A peer-to-peer application (P2P), unlike apeer-to-peer network, allows a device to act

as both a client and a server within thesame communication.




Some P2P applications use a hybrid systemwhere resource sharing is decentralized but

the indexes that point to resource locationsare stored in a centralized directory

Peer-to-peer applications can be used on

peer-to-peer networks, client/servernetworks, and across the Internet.




Some Specific Used Protocols The Transport layer uses an addressing scheme

called a port number. Port numbers identifyapplications and Application layer services that are thesource and destination of data

Domain Name System (DNS) - TCP/UDP Port 53

Hypertext Transfer Protocol (HTTP) - TCP Port 80

Simple Mail Transfer Protocol (SMTP) - TCP Port 25

Post Office Protocol (POP) - UDP Port 110

Telnet - TCP Port 23 Dynamic Host Configuration Protocol - UDP Port 67

File Transfer Protocol (FTP) - TCP Ports 20 and 21






A DNS server provides the name resolution using thename daemon, which is often called named,(pronounced name-dee).

When a client makes a query, the server's "named"process first looks at its own records to see if it canresolve the name. If it is unable to resolve the nameusing its stored records, it contacts other servers inorder to resolve the name

Once a match is found and returned to the originalrequesting server, the server temporarily stores thenumbered address that matches the name in cache.

ipconfig /displaydns command displays all of thecached DNS entries on a Windows XP or 2000computer system.




The Domain Name System uses a hierarchical system to create aname database to provide name resolution.

At the top of the hierarchy, the root servers maintain records about

how to reach the top-level domain servers.




WWW Service & HTTP When a web address (or URL) is typed into a web browser, the web

browser establishes a connection to the web service running on theserver using the HTTP protocol.

The browser interprets the 3 parts of the URL:

1. http (the protocol or scheme)

2. www.cisco.com(the server name)

3. web-server.htm (the specific file name requested).

HTTP specifies a request/response protocol. When a client, typically aweb browser, sends a request message to a server, the HTTP protocoldefines the message types the client uses to request the web page andalso the message types the server uses to respond. The three commonmessage types are GET, POST, and PUT.

GET is a client request for data. A web browser sends the GETmessage to request pages from a web server.

POST and PUT are used to send messages that upload data to the webserver




Email Services & SMTP/POP Protocols

Post Office Protocol (POP) and Simple Mail

Transfer Protocol (SMTP) are involved in emailservices.

Users use an application called a Mail User

Agent (MUA), or e-mail client to allow messages

to be sent and places received messages intothe client's mailbox.

In order to receive e-mail messages from an e-mail server, the e-mail client can use POP.

Sending e-mail from either a client or a serveruses message formats and command stringsdefined by the SMTP protocol.




The e-mail server operates two separate processes:

Mail Transfer Agent (MTA)

Mail Delivery Agent (MDA)


SMTP

POP



FTP FTP was developed to allow for file transfers between a

client and a server. An FTP client is an application that runson a computer that is used to push and pull files from aserver running the FTP daemon (FTPd).

The client establishes the first connection to the serveron TCP port 21. This connection is used for control traffic,

consisting of client commands and server replies.

The client establishes the second connection to theserver over TCP port 20. This connection is for the actualfile transfer and is created every time there is a file

transferred. The file transfer can happen in either direction. The client

can download (pull) a file from the server or, the client canupload (push) a file to the server.




DHCP The Dynamic Host

Confirmation Protocol

(DHCP) serviceenables devices on anetwork to obtain IPaddresses and other

information from aDHCP server. Thisservice automatesthe assignment of IP

addresses, subnetmasks, gateway andother IP networkingparameters.




The DHCP server maintains a pool of IP addresses andleases an address to any DHCP-enabled client when theclient is powered on.

DHCP DISCOVER packet to identify any available DHCPservers on the network

A DHCP server replies with a DHCP OFFER. The client may receive multiple DHCP OFFER packets if

there is more than one DHCP server on the local network, soit must choose between them, and broadcast a DHCPREQUEST packet that identifies the explicit server and leaseoffer that the client is accepting.




File Sharing Service & SMB The Server Message Block (SMB) is a client/server file

sharing protocol.

Unlike the file sharing supported by FTP, clientsestablish a long term connection to servers.

Once the connection is established, the user of theclient can access the resources on the server as if theresource is local to the client host.

SMB file-sharing and print services have become themainstay of Microsoft networking.

The LINUX and UNIX operating systems also providea method of sharing resources with Microsoft networksusing a version of SMB called SAMBA.




P2P Service & Gnutella Protocol

With P2P applications based on the Gnutella protocol,people can make files on their hard disks available toothers for downloading.

Many client applications are available for accessingthe Gnutella network, including: BearShare, Gnucleus,LimeWire, Morpheus, WinMX and XoloX. (samplesfrom materials)




Telnet Telnet provides a standard method of emulating text-based

terminal devices over the data network.

A connection using Telnet is called a Virtual Terminal(VTY) session, or connection.

Rather than using a physical device to connect to theserver, Telnet uses software to create a virtual device that

provides the same features of a terminal session withaccess to the server command line interface (CLI).

To support Telnet client connections, the server runs aservice called the Telnet daemon. A virtual terminalconnection is established from an end device using aTelnet client application.

If security is a concern, the Secure Shell (SSH) protocoloffers an alternate and secure method for server access.





The end…

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CCNA Exploration Ch 3