ccna exploration1 chapter 3: application layer ......ccna exploration1 chapter 3: application layer...

CCNA Exploration1 Chapter 3: Application Layer Functionality and

ProtocolsLOCAL CISCO ACADEMY ELSYS TU

INSTRUCTOR: STELA STEFANOVA

Objectives

• Functions of the three upper OSI model layers, network services to end user applications.

• TCP/IP Application Layer protocols and services specified by the upper layers of the OSI model.

• Function of TCP/IP applications - World Wide Web and email, and their related services (HTTP, DNS, SMB, DHCP, STMP/POP, Telnet).

• File-sharing processes that use peer-to-peer applications and the Gnutella protocol.

Applications – The Interface Between Human and Data Networks

Applications provide the means for generating and receiving data that can be transported on the network

Application layer protocols:

• used to exchange data between programs running on the source and destination hosts.

Role of applications, services and protocols:• to convert communication to data that can be transferred across the data network.

Applications, services and protocols play different roles in transporting data through networks

OSI and TCP/IP Models

Presentation LayerPresentation layer primary functions:

• 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.• Compression of the data in a manner that can be decompressed by the destination device.• Encryption of the data for transmission and the decryption of data upon receipt by the destination.

Presentation layer• implementations are not typically associated with a particular protocol stack. Video standards:• Motion Picture Experts Group (MPEG) - standard for video compression and coding;• QuickTime - Apple Computer specification for video and audio. Graphic image formats:• Graphics Interchange Format (GIF) - compression and coding standards;• Joint Photographic Experts Group (JPEG) -compression and coding standards;• Tagged Image File Format (TIFF) - standard coding format for graphic images.

Session Layer

• creates and maintains dialogs between source and destination applications;• handles the exchange of information to initiate dialogs, keep them active;• restarts sessions that are disrupted or idle for a long period of time.

Most applications, like web browsers or e-mailclients, incorporate functionality of the OSIlayers 5, 6 and 7.

TCP/IP Application Layer Protocols

TCP/IP Application layer protocols:

• provide for the exchange of user information;• specify the format and control information necessary for many of the common Internet communication functions;• are generally defined by Requests forComments (RFCs);• Internet Engineering Task Force maintains the RFCs as the standards for the TCP/IPsuite.

TCP/IP Application Layer Protocols

Domain Name Service Protocol (DNS) used to resolve Internet names to IP addresses;• Hypertext Transfer Protocol (HTTP) used to transfer files that make up the Web pages of the World Wide Web;• Simple Mail Transfer Protocol (SMTP) used for the transfer of mail messages and attachments;• Terminal Emulation Protocol (Telnet) used to provide remote access to servers and networking devices;• File Transfer Protocol (FTP) - used for interactive file transfer between systems.

TCP/IP Application layer protocols

TCP/IP Application Layer SoftwareApplication layer software (processes):• provide access to the network: applications and services;• each application or network service uses protocols which define the standards and data formats to be used.

TCP/IP Application Layer Software

Applications:• are the software programs used to communicate over the network;

Network-Aware Applications:• end-user applications;• implement the application layer protocols;• are able to communicate directly with the lower layers of the protocol stack;• examples: e-mail clients and web browsers.

TCP/IP Application Layer Software

Application layer Services:• are programs that interface with thenetwork and prepare the data fortransfer;• file transfer or network print spooling -may need the assistance of Application layer services to use network resources;• different types of data (text, graphics, or video) - require different networkservices to ensure that it is properly prepared for processing by the functions occurring at the lower layers of OSI model.

Application Layer Protocols

Application layer protocols specify:

• what messages are exchanged between the source and destination hosts;

• syntax of the control commands;

• type and format of the data being transmitted;

• appropriate methods for error notification and recovery.

Client/Server ModelClient/Server Model• Client - device requesting the information;• Server - device responding to the request;• Client and server processes - in the Application

layer;• Application layer protocols describe the format

of the requests and responses between clients and servers;

• require control information:- user authentication;- identification of a data file to be transferred.

Client/Server Model

Client and server processes in data networks

Client/Server ModelServer• computer that contains information to beshared with many client systems;• may require authentication of user account information.

Server Daemon• is described as "listening"for a request from a client;• exchanges appropriate messages with the client; • proceeds to send therequested data to the client in the proper format.

Application Layers services and protocols

• servers typically have multiple clients requestinginformation at the sametime; • individual client requests must be handled simultaneously and separately for the network to succeed; • for each request, multiple processes may be executed.

Peer-to-Peer Model

Peer-to-Peer Model:• involves two distinct forms: peer-to-peer network

design and peer-to-peer applications (P2P);Peer-to-Peer Network• two or more computers 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;

• decentralize the resources on a network;• information can be located anywhere on any

connected device;• it is difficult to enforce security and access policies.

Peer-to-Peer Networking

One computer might assume the role of server for one transaction while simultaneously serving as a client for another.

Peer-to-Peer Applications (P2P)

• allows a device to act as both a client and a server within the samecommunication;

• every client is a server and every server a client;

• both can initiate a communication and are considered equal in the communication process;

• requires that each end device provide a user interface and run a background service;

• can be used on peer-to-peer networks, client/server networks, and across the Internet.

Peer-to-peer application (P2P)

DNS Services and Protocol

Domain Name System (DNS):• created for domain name to address resolution for

these networks;• uses a distributed set of servers to resolve the names

associated with these numbered addresses.DNS protocol• defines an automated service that matches resource

names with the required numeric network address;• includes the format for queries, responses, and data

formats;• uses a single format called a message;Message format:

- all types of client queries and server responses;- error messages;- transfer of resource record information between

servers.

Features of the DNS protocol and DNS support services

DNS Services and ProtocolDNS• is a client/server service;• differs from the other client/server services.DNS client• runs as a service itself (other services use a client that is an

application, such as web browser, e-mail client); • is called DNS resolver;• supports name resolution for other network applications

and other services that need it. DNS Server• when configuring a network device, we provide one or

more DNS Server addresses that the DNS client can use for name resolution;

• Internet service provider provides the addresses to use for the DNS servers.

DNS Services and ProtocolNslookup• computer operating system utility;• allows the user to manually query the name servers to resolve a

given host name;• can be used to troubleshoot name resolution issues;• to verify the current status of the name servers.• DNS server is dns-sj.cisco.com which has an address of

171.70.168.183.• can type the name of a host or domain for which to get the address;Example: query for www.cisco.com.• responding name server provides the address of 198.133.219.25.

DNS Services and ProtocolDNS server• provides the name resolution using the name daemon, which is often called named, (pronounced name-dee). • stores different types of resource records used to resolve names;• these records contain the name, address, and type of record.Record types:A - an end device address;NS - an authoritative name server;CNAME - the canonical name (or Fully Qualified Domain Name) for an alias; used when multiple services have the single network address but each service has its own entry in DNS;MX - mail exchange record; maps a domain name to a list of mail exchange servers for that domain.

Client makes a query:• the server's "named" process first looks at its own

records to see if it can resolve the name;• if it is unable to resolve the name using its stored

records, it contacts other servers in order to resolve the name;

• the request may be passed along to a number of servers, which can take extra time and consume bandwidth;

• once a match is found and returned to the original requesting server, the server temporarily storesthe numbered address that matches the name incache.

DNS Services and ProtocolDomain Name System• uses a hierarchical system to create a name database to provide name resolution;Top-level domains• the root servers maintain records about how to reach the top-level domain servers;• in turn have records that point to the secondary level domain servers and so on;• the different top-level domains represent:

- type of organization or- country or- origin;

Examples of top-level domains:.au - Australia .co - Colombia .com - a business or industry.jp - Japan.org - a non-profit organization

DNS Services and ProtocolSecond-level domain names, and below them are other lower level domains.• Each domain name is a path down this inverted tree starting from the root.

WWW Service and HTTP

Web address (or URL):• URL (or Uniform Resource Locator)• URI (Uniform Resource Identifier)• is typed into a web browser, the web browser establishes a connection to the web service running on the server using the HTTP protocol.Web browsers:• are the client applications to connect to the World Wide Web and access resources stored on a web server;• web clients make connections to the server and request the desired resources;• the server replies with the resources and, upon receipt, the browser interprets the data and presents it to the user;• web browsers can interpret and present many data types, such as plain text or Hypertext Markup Language (HTML)

HTTP protocol and support of the delivery of web pages to the client

Hypertext Transfer Protocol (HTTP)• one of the protocols in the TCP/IP suite;• was originally developed to publish and retrieve HTMLpages;• now used for distributed, collaborative information systems;• is used across the WWW for data transfer;• is one of the most used application protocols; • specifies a request/response protocol;• defines three common message types (when a client, typically a web browser, sends a request message to a server):

- message types the client uses to request the web page; - message types the server uses to respond;

• common message types are GET, POST, and PUT.

WWW Service and HTTPGET• is a client request for data;• a web browser sends the GET message to request pages from a web server;• the server receives the GET request and responds with astatus line, such as HTTP/1.1 200 OK, • POST and PUT are used to send messages that upload data to the web server;POST• includes the data in the message sent to the server;• uploads information to the server in plain text that can be intercepted and read (HTTP is not a secure protocol)PUT• uploads resources or content to the web server.Secure HTTP (HTTPS) protocol• is used for accessing or posting web server information.• can use authentication and encryption to secure data as it travels between the client and server.

POP and SMTP protocols

POP and SMTP protocols and support of e-mail services

Post Office Protocol (POP)

Simple Mail Transfer Protocol (SMTP)

these protocols define client/server processes

Mail User Agent (MUA) or e-mail client• is an application used to compose e-mailmessages;• allows messages to be sent and places received messages into the client's mailbox, both of which are distinct processes; • can use POP in order to receive e-mailmessages from an e-mail server;• sending e-mail from either a client or a server uses message formats and command stringsdefined by the SMTP protocol.

E-mail server operates two separate processes:• Mail Transfer Agent (MTA)• Mail Delivery Agent (MDA)

Mail Transfer Agent (MTA) process:• is used to forward e-mail;• receives messages from the MUA or from another MTA on another e-mail server;• determines how a message has to beforwarded to reach its destination (based on the message header).

Mail Delivery Agent (MDA) process:

If the mail is addressed to a user whose mailbox is on the local server, the mail is passed to the MDA.

E-mail server operates two separate processes:• Mail Transfer Agent (MTA)• Mail Delivery Agent (MDA)

POP and SMTP protocolsMail Delivery Agent (MDA)• accepts a piece of e-mail from a Mail Transfer

Agent (MTA) and performs the actual delivery;• receives all the inbound mail from the MTA and

places it into the appropriate users' mailboxes;• can also resolve final delivery issues:

- virus scanning;- spam filtering;- return-receipt handling.

• most e-mail communications use the MUA, MTA, and MDA applications;

• there are other alternatives for e-mail delivery.

POP and SMTP protocolsPOP and POP3 deliver e-mail from the e-mail server to the client (MUA);SMTP transport of e-mail between e-mail servers

POP and SMTP protocolsPOP and POP3 (Post Office Protocol, version 3)• are inbound mail delivery protocols;• are typical client/server protocols;• 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. Simple Mail Transfer Protocol (SMTP)• governs the transfer of outbound e-mail from the sending

client to the e-mail server (MDA);• governs the transport of e-mail between e-mail servers

(MTA);• enables e-mail to be transported across data networks

between different types of server and client software and makes e-mail exchange over the Internet possible.

POP and SMTP protocols• SMTP protocol message format uses a rigid set of

commands and replies;• these commands support the procedures used in

SMTP, such as session initiation, mail transaction, forwarding mail, verifying mailbox names, expanding mailing lists, and the opening and closing exchanges.

Commands specified in the SMTP protocol:• HELO - identifies the SMTP client process to the SMTP

server process;• EHLO - is a newer version of HELO, which includes

services extensions;• MAIL FROM - identifies the sender;• RCPT TO - identifies the recipient;• DATA - identifies the body of the message.

FTP Application Layer Protocol

File Transfer Protocol (FTP)• is commonly used Application layer

protocol;• was developed to allow for file transfers

between a client and a server;• FTP client is an application that runs on a

computer that is used to push and pull filesfrom a server running the FTP daemon(FTPd);

• requires two connections between the client and the server:

- for commands and replies;- for the actual file transfer.

File Transfer Protocol (FTP)• the client establishes the first connection:

- to the server on TCP port 21;- is used for control traffic, consisting of client

commands and server replies. • the client establishes the second connection:

- to the server over TCP port 20;- is used for the actual file transfer;- is created every time there is a file transferred.

• the file transfer can happen in the following direction:- the client can download (pull) a file from the

server;- the client can upload (push) a file to the server.

DHCP Application Layer ProtocolDynamic Host Configuration Protocol (DHCP) service• enables devices on a network to obtain IP addresses and

other information from a DHCP server;• automates the assignment of IP addresses, subnet masks,

gateway and other IP networking parameters.DHCP• allows a host to obtain an IP address dynamically when it

connects to the network;• DHCP server is contacted and an address requested;• DHCP server chooses an address from a configured range

of addresses called a pool and assigns ("leases") it to the host for a set period.

• is preferred on larger networks, local networks, or where the user population changes frequently;

• is used for general purpose hosts such as end user devices;• fixed addresses are used for network devices such as

gateways, switches, servers and printers.

DHCP Application Layer Protocol

DHCP processDHCP-configured device boots up or connects to the network:• client broadcasts a DHCP DISCOVER packet to identify any

available DHCP servers on the network;• DHCP server replies with a DHCP OFFER, which is a lease offer

message with an assigned IP address, subnet mask, DNS server, and default gateway information as well as the duration of the lease;

• client may receive multiple DHCP OFFER packets if there is more than one DHCP server on the local network, so it must choose between them;

• client broadcasts a DHCP REQUEST packet that identifies theexplicit server and lease offer that the client is accepting. A client may choose to request an address that it had previously been allocated by the server.

• assuming that the IP address requested by the client, or offered by the server, is still valid, the server would return a DHCP ACK message that acknowledges to the client the lease is finalized.

File Sharing Services and SMB Protocol

Server Message Block (SMB)• is a client/server file sharing protocol;• IBM developed SMB in the late 1980s to describe the structure of shared network resources, such as directories, files, printers, and serial ports;• is a request-response protocol;• SMB file-sharing and print services have become the mainstay of Microsoft networking:

- Microsoft changed the underlying structure for using SMB (with the introduction of the Windows 2000 series of software);

- all subsequent Microsoft products use DNS naming;- this allows TCP/IP protocols to directly support

SMB resource sharing.

SMB protocol and its support in file sharing in Microsoft-based networks

SMB protocol• describes file system access and how clients can make requests for files;• describes the SMB protocol inter-processcommunication;• All SMB messages share a common format that uses:

- fixed-sized header;- variable-sized parameter;- data component.

SMB messages can:• Start, authenticate, and terminate sessions;• Control file and printer access;• allow an application to send or receive messages to or from another device.

Gnutella protocol and P2P servicesP2P applications based on the Gnutella protocol• used to make files on hard disks available to others for downloading;• Gnutella-compatible client software allows users to connect to Gnutella services over the Internet and to locate and access resources shared by other Gnutellapeers;• user is connected to a Gnutella service, the client applications will search for other Gnutella nodes to connect to;• the nodes handle queries for resource locations and replies to those requests;• the nodes govern control messages, which help the service discover other nodes;• the actual file transfers usually rely on HTTP services.

Gnutella protocol and P2P services

Gnutella protocol

Gnutella protocol and P2P services

Gnutella protocolDifferent packet types:

ping - for device discovery pong - as a reply to a pingquery - for file locationquery hit - as reply to a query push - as a download request

Gnutella protocol

TELNET Services and Protocol

Telnet• was developed to the early 1970s;• provides a standard method of emulating text-based

terminal devices over the data network;• refers both the protocol itself and the client software that

implements the protocol;• Virtual Terminal (VTY) session (or connection) - a

connection using Telnet;• uses software to create a virtual device that provides the

same features of a terminal session with access to the server command line interface (CLI);

• Telnet daemon – is a service that the server runs to support Telnet client connections;

• on a Microsoft Windows PC, Telnet can be run from the command prompt;

TELNET Services and Protocol

Telnet• a virtual terminal connection is established from an end

device using a Telnet client application;• most operating systems include an Application layer Telnet

client;• common terminal applications that run as Telnet clients:

- HyperTerminal;- Minicom;- TeraTerm.

• Telnet connection is established:- users can perform any authorized function on the server

(just as if they were using a command line session on the server itself);- users can start and stop processes, configure the

device, and even shut down the system.

TELNET Services and ProtocolTelnet - Services and Protocol

Summary

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