ccna exp1 - chapter03 - application layer functionality and protocols

8/8/2019 CCNA Exp1 - Chapter03 - Application Layer Functionality and Protocols

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Overview

Describe how the functions of the three upper OSI model layersprovide network services to end user applications.

Describe how the TCP/IP Application Layer protocols provide theservices specified by the upper layers of the OSI model.

Define how people use the Application Layer to communicate acrossthe information network.

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World Wide Web and email, and their related services (HTTP, DNS,SMB, DHCP, STMP/POP, and Telnet).

Describe file-sharing processes that use peer-to-peer applications andthe Gnutella protocol.

Explain how protocols ensure services running on one kind of devicecan send to and receive data from many different network devices.

Use network analysis tools to examine and explain how common userapplications work.


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Introduction

Explain that applications provide the means for generating andreceiving data that can be transported on the network.



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OSI model

To address the roblem of network incom atibilit , the International


Organization for Standardization (ISO) researched networking models likeDigital Equipment Corporation net (DECnet), Systems Network Architecture(SNA), and TCP/IP in order to find a generally applicable set of rules for allnetworks.

Using this research, the ISO created a network model that helps vendorscreate networks that are compatible with other networks.

The Open System Interconnection (OSI) reference model released in 1984was the descriptive network model that the ISO created.

It provided vendors with a set of standards that ensured greater compatibilityand interoperability among various network technologies produced bycompanies around the world.


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The OSI Model

Initially the OSI model was designed by the International Organization forStandardization (ISO) to provide a framework on which to build a suite of open

systems protocols.



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The OSI Model



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OSI model

Explain the role of applications, services and protocols in convertingcommunication to data that can be transferred across the data network.

Application layer: It is the layer that provides the interface between theapplications we use to communicate and the underlying network overwhich our messages are transmitted.



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OSI model



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OSI model



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OSI and TCP/IP Model

Functionality of the TCP/IP application layer protocols fit roughly into

the framework of the top three layers of the OSI model: Application,

Presentation and Session layers.



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The Presentation layer has three primary functions:

1. Coding and conversion of Application layer data to ensure that

data from the source device can be interpreted by the appropriate

application on the destination device.


.

by the destination device.

3. Encryption of the data for transmission and the decryption of data

upon receipt by the destination.

Example: QuickTime, Motion Picture Experts Group (MPEG),

Graphics Interchange Format (GIF), Joint Photographic Experts

Group (JPEG)


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The Session Layer

Functions at this layer create and maintain dialogs between source and

destination applications.

The Session layer handles the exchange of information to initiate


, ,

idle for a long period of time.


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Application Layer Protocols

These protocols specify the format and control information necessaryfor many of the common Internet communication functions. Amongthese TCP/IP protocols are:

Domain Name Service Protocol (DNS). Hypertext Transfer Protocol (HTTP).

Simple Mail Transfer Protocol (SMTP).


.

File Transfer Protocol (FTP).


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Application Layer Software

The functions associated with the Application layer protocols enableour human network to interface with the underlying data network.

Within the Application layer, there are two forms of software programs

or processes that provide access to the network: applications andservices.



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User applications, Services, and Application Layer Protocols

The Application layer uses protocols that are implemented withinapplications and services.

While applications provide people a way to create messages and

application layer services establish an interface to the network,protocols provide the rules and formats that govern how data is treated.



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Application Layer Protocol Functions

Application layer protocols are used by both the sourceand destination devices during a communication session.



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The Client-Server Model

In the client/server model, the device requesting the information iscalled a client and the device responding to the request is called aserver. Client and server processes are considered to be in the

Application layer.



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Servers

In a general networking context, any device that responds to requests fromclient applications is functioning as a server.

A server is usually a computer that contains information to be shared with

many client systems. Different types of server applications may have different requirements for

client access.

Some servers may


requ re au en ca on o

user account information

to verify if the user has

permission to access the

requested data or to use

a particular operation.


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Application Layer Services and Protocols

A single application may employ many different supporting Applicationlayer services.

Additionally, servers typically have multiple clients requesting

information at the same time.



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Peer-to-Peer Networking and Applications (p2p)

The Peer-to-Peer Model

Peer-to-peer networking involves two distinct forms: peer-to-peer networkdesign and peer-to-peer applications (P2P). Both forms have similar features

but in practice work very differently.Peer-to-Peer Networks

In a peer-to-peer network, two or more computers are connected via a networkand can share resources (such as printers and files) without having a


e ca e server.


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Peer-to-Peer Networking and Applications (p2p)

Peer-to-Peer Applications

A peer-to-peer application (P2P), unlike a peer-to-peer network, allows adevice to act as both a client and a server within the same communication.

In this model, every client is a server and every server a client. Both can initiatea communication and are considered equal in the communication process. Peer-to-peer applications can be used on peer-to-peer networks, client/server

networks, and across the Internet.



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Application Layer Protocols and Services Examples

Now that we have a better understanding of how applications providean interface for the user and provide access to the network, we willtake a look at some specific commonly used protocols.

Some of these services are: Domain Name System (DNS) - TCP/UDP Port 53

Hypertext Transfer Protocol (HTTP) - TCP Port 80


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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


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DNS Services and Protocol


A domain naming system was developed in order to associate thecontents of the site with the address of that site. The Domain NameSystem (DNS) is a system used on the Internet for translating namesof domains and their publicly advertised network nodes into IP

addresses.


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DNS Services and protocol

The Domain Name System (DNS) was created for domain name toaddress resolution for these networks. DNS uses a distributed set ofservers to resolve the names associated with these numbered

addresses. The DNS protocol defines an automated service that matches resource

names with the required numeric network address.

DNS rotocol communications use a single format called a message.


DNS is used for all types of client queries and server responses, errormessages, and the transfer of resource record information betweenservers.


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DNS - Example



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DNS - Example



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DNS - Example



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DNS - Example



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DNS - Example



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DNS Services and Protocol

The DNS server stores different types of resource recordsused to resolve names. These records contain the name,address, and type of record.



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DNS Services and protocol

When configuring a network device, we generally provide one or moreDNS Server addresses that the DNS client can use for nameresolution.

Computer operating systems also have a utility called nslookup thatallows the user to manually query the name servers to resolve a givenhost name. This utility can also be used to troubleshoot nameresolution issues and to verify the current status of the name servers.



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WWW Service and HTTP

When a web address (or URL) is typed into a web browser, the web browserestablishes a connection to the web service running on the server using theHTTP protocol.

URLs (or Uniform Resource Locator) and URIs (Uniform Resource Identifier)are the names most people associate with web addresses.



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In order to access the content, web clients make connections to theserver and request the desired resources. The server replies with theresources and, upon receipt, the browser interprets the data and

presents it to the user.



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Browsers can interpret and present many data types, such as plain textor Hypertext Markup Language (HTML, the language in which webpages are constructed). Other types of data, however, may require

another service or program, typically referred to as plug-ins or add-ons.



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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 and

also the message types the server uses to respond. The three commonmessage types are GET, POST and PUT.



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E-mail Services and SMTP/POP Protocols

E-mail, the most popular network service and run on a computer or other enddevice, e-mail requires several applications and services.

POP/SMTP define client/server processes.

Mail User Agent (MUA): allows messages to be sent and places receivedmessages into the client's mailbox, both of which are distinct processes. MUAinclude: POP and SMTP.

POP: used to receive e-mail messages from an e-mail server.


: use to sen e-ma rom e t er a c ent or a server uses messageformats and command strings.


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E-mail Server Processes - MTA and MDA

The e-mail server operates two separate processes: Mail Transfer Agent (MTA).

Mail Delivery Agent (MDA).



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We see that the Mail Delivery Agent (MDA) accepts a piece of e-mail from aMail Transfer Agent (MTA) and performs the actual delivery.

The MDA receives all the inbound mail from the MTA and places it into theappropriate users' mailboxes.

The MDA can also resolve final delivery issues, such as virus scanning, spamfiltering, and return-receipt handling.



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POP and POP3 (Post Office Protocol, version 3) are inbound mail deliveryprotocols and are typical client/server protocols. They deliver e-mail from the e-mail server to the client (MUA).

The MDA listens for when a client connects to a server. Once a connection is

established, the server can deliver the e-mail to the client. Some of the commands specified in the SMTP protocol are: HELO, EHLO,

MAIL FROM, RCPT TO, DATA.



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FTP

The File Transfer Protocol (FTP) is another commonly used Application layerprotocol.

FTP was developed to allow for file transfers between a client and a server. AnFTP client is an application that runs on a computer that is used to push and

pull files from a server running the FTP daemon (FTPd).

The client establishes the first connection to the server on TCP port 21. Theclient establishes the second connection to the server over TCP port 20.


.

DHCP


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DHCP

The Dynamic Host Confirmation Protocol (DHCP) service enables devices on anetwork to obtain IP addresses and other information from a DHCP server.

DHCP allows a host to obtain an IP address dynamically when it connects tothe network.


Wh U DHCP?


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Why Use DHCP?

DHCP reduces the complexity and amount of administrative work by

using automatic TCP/IP configuration

Manual TCP/IP Configuration

IP addresses are enteredmanually on each client computer

Automatic TCP/IP Configuration

IP addresses are suppliedautomatically to client computers


Possibility of entering incorrect orinvalid IP address

Incorrect configuration can leadto communication and networkissues

Administrative overload onnetworks where computers arefrequently moved

Ensures that clients always usecorrect configuration information

Client configuration is updatedautomatically to reflect changesin network structure

Eliminates a common source ofnetwork problems

Wh t A DHCP S ?


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What Are DHCP Scopes?

A scope is a range of IP addresses that are available to be leased

DHCP Server


Scope Properties

Network ID Lease duration Scope name Subnet mask Router Exclusion range

Network IPaddress range

Scope BScope A

DHCP


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DHCP

The DHCP server maintains a pool of IP addresses and leases an address toany DHCP-enabled client when the client is powered on.

DHCP Packet: DHCP DISCOVER.

DHCP OFFER.

DHCP REQUEST.

DHCP ACK.


How the DHCP Operation


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How the DHCP Operation

DHCP

DHCP

Server2


DHCP client broadcasts a DHCPDISCOVER packet1

DHCP servers broadcast a DHCPOFFER packet2

DHCP client broadcasts a DHCPREQUEST packet3

DHCP Server1 broadcasts a DHCPACK packet4

Client

Server1

File Sharing Services and SMB Protocol


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The Server Message Block (SMB) is a client/server file sharing protocol. Unlike the file sharing supported by FTP, clients establish a long term

connection to servers. Once the connection is established, the user of theclient can access the resources on the server as if the resource is local to the

client host.

SMB file-sharing and print services have become the mainstay of Microsoftnetworking.




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The SMB protocol describes file system access and how clients can makerequests for files. All SMB messages share a common format. This format usesa fixed-sized header followed by a variable-sized parameter and data

component.

SMB messages can: Start, authenticate, and terminate sessions.

Control file and printer access.


ow an app ca on o sen or rece ve messages o or rom ano er ev ce.

P2P Services and Gnutella Protocol


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P2P Services and Gnutella Protocol

Many P2P applications do not use a central database to record all the filesavailable on the peers.

When a user is connected to a Gnutella service, the client applications willsearch for other Gnutella nodes to connect to. These nodes handle queries for

resource locations and replies to those requests.

The Gnutella protocol defines five different packet types: ping, pong, query,query hit, push


Telnet Services and Protocol


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Telnet provides a standard method of emulating text-based terminal devicesover the data network. Both the protocol itself and the client software thatimplements the protocol are commonly referred to as Telnet.




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Telnet is a client/server protocol and it specifies how a VTY session isestablished and terminated.

Each Telnet command consists of at least two bytes. The first byte is a specialcharacter called the Interpret as Command (IAC) character.

If security is a concern, the Secure Shell (SSH) protocol offers an alternate andsecure method for server access.


Labs


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Labs


Summary


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Summary



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ccna exp1 - chapter03 - application layer functionality and protocols

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