Application Layer in Networking

Application Layer Introduction

• Application layer is the top layer in the OSI and TCP/IP models
• It is the closest layer to the end user, enabling direct interaction with software applications
• Application layer programs operate on a client and server
• This layer exists in different models because of its significance in user and application interaction
• The application initiates and shows actual communication
• It relies on the Transport layer and layers below for communication/data transfer to remote host
• Application layer hands over data or information to the Transport layer for transmission

Application Layer Functions

• Identifies availability of communication partners for an application with data
• Determines if adequate network resources exist for requested communication
• Manages cooperation necessary for all communications between applications

Services of Application Layers

• Network Virtual Terminal: Lets users log onto remote hosts by creating a software terminal emulation
• File Transfer, Access, and Management (FTAM): Allows file access, retrieval, and management on remote computers and defines a hierarchical virtual file structure
• Addressing: Uses DNS to provide addresses to allow communication between clients and servers
• Mail Services: Provides email forwarding and storage
• Directory Services: Provides global information access via a distributed database
• Authentication: Authenticates the sender/receiver's message, or both

Network Application Architecture

• Application architecture differs from network architecture
• Network architecture offers fixed set of service for application
• Application architecture is designed by the developer, and defines application structure
• There are two main application architectures: client-server and peer-to-peer (P2P)

Client-Server Architecture

• A client sends requests to the server, which provides resources
• Clients do not directly communicate to each other
• Server's IP address is fixed and well-known for client contact
• A single server is in charge of holding the requests from multiple clients
• Social sites may become overwhelmed if they only have one server

P2P Architecture

• Does not use a dedicated server
• Peers run on computers not owned by a service provider
• Peers reside in homes, offices, schools, and universities
• Peers communicate directly, making it the peer-to-peer architecture
• P2P is used for file sharing and internet telephony

Client-Server Model Details

• Clients run on local machines and request services from servers
• Client-server interactions are finite programs that terminates when the service is completed
• Servers operate on remote machines, providing services to clients
• Servers only open the doors for incoming requests from clients
• Any process can act as a server or client, depending on its ability to serve requests
• A system can act as both server and client, even on the same machine

Centralized Networking

• A model where servers provide network services to clients for user-based tasks
• Centralized backups are possible with all data stored on a server
• Enhanced security through central administration of shared resources
• Better performance through use of dedicated servers
• Scalability because clients/servers can be added independently

Client-Server Network Disadvantages

• Traffic congestion can occur when many clients request from the same server
• Lacks robustness if the server goes down, requests cannot be fulfilled
• Regular hardware cannot serve a high number of clients
• Clients may not have access to everything that is available at the server

Application Protocols

• Operate in the Application Layer for user needs
• Protocols are categorized as those used by users (e.g., email/eMail) and those supporting user protocols (e.g., DNS)
• Examples include DNS, SMTP, FTP, WWW, and SNMP

DNS (Domain Name System) Explained

• An application layer protocol that dictates communications across processing systems
• Provides a mapping between host names and numerical addresses
• Is crucial for internet's functionality
• Hierarchy of domains specified by dots
• Translates domain names into IP addresses for user-friendly access
• A domain name is more reliable than an IP address

DNS as a TCP/IP Protocol

• Used across different platforms
• Divides domain name space into generic domains, country domains, and inverse domains

Generic Domains

• Defines registered hosts based on their behavior (e.g., .com, .org, .net)
• Defines the domain name of registered hosts
• Uses three-character labels defining organization type

Country Domains

• Uses two-character country abbreviations, such as ".us" for the United States, instead of generic organizational abbreviations
• Same format as generic domains

Inverse Domain

• Is employed to map an address to a name
• Used when the server contains files for authorized clients only
• To verify client authorization, it queries the DNS server to map the IP address to a name

DNS Operation Explained

• A client/server network communication protocol
• DNS clients send requests, DNS servers send responses
• Requests either convert name to IP address (forward DNS lookup) or IP address to name (reverse DNS lookup)
• DNS uses a distributed database for all available internet hosts
• A DNS resolver forwards hostname requests to a DNS server to get its address
• If the first DNS server doesn't have the IP address, it forwards the hostname request to another

SMTP (Simple Mail Transfer Protocol)

• Communication guides for software to transmit electronic mail over the internet
• Facilitates sending messages between computer users via email
• Supports mail exchange among users including: single message to multiple recipients, includes voice, video, and graphics, or sending messages on networks outside the internet

SMTP Communication

• Sets up communication rules between servers
• Servers announce their identities and intended actions
• Servers handle errors, like incorrect email addresses
• Example: If address is wrong, the recipient server will reply with en error message

SMTP Components

• User Agent (UA): Prepares the message (body and envelope)
• Mail Transfer Agent (MTA): Transfers mail across the internet

Complex SMTP Systems

• Complex systems use relaying
• In a relaying system, multiple MTAs can be added, acting either as a client or sever to relay the email

Relaying Systems

• Relaying systems can use TCP/IP protocol to send emails to users via mail gateways
• Mail gateway: A relay MTA to receive emails

SMTP Workflow

• Mail composition: A user composes an email using MUA
• Submission stage: Email client submits the completed e-mail to a SMTP server
• Delivery: Composed of a username and domain name
• Receipt and processing: The exchange server delivers the the email to the incoming server
• Access and retrieval: Using login and password

FTP (File Transfer Protocol)

• Standard internet protocol via TCP/IP
• Transmits files from one host to another
• Transfers webpage from creator to a server for other computers
• Used for downloading files to a remote computer from other servers
• Aims to reliably and efficiently share

Additional Points on Transferring

• Although transferring files is straightforward, it can potentially cause problems
• Problems can be: differing ways to express data, different file conventions different directory structures
• FTP establishes two network connections between hosts to solve transfer problems
• One network connection is used for data transfer and another network connection is used for transfer control

FTP Mechanics

• FTP client consist of a user interface, transfer control and data transfer process
• FTP server consist of a transfer control and data transfer process

FTP Connection Types

• Control Connection: Simple communication rules, transmits commands/responses and stays connected
• Data Connection: Complex, variable, made for data transfer and opens/closes as commands are being transferred

FTP Clients

• A program, which implements a file transfer protocol to transfer files between hosts
• Allows users to connect a remote host to upload/download files with a set of commands

FTP Speed and Security

• One of the fastest ways to transfer files between computers
• Transfers efficiently, eliminating the need to complete all operation to get the file
• Requires logging in with a username and password to access the FTP server
• Transferring files is a back and forth movement

FTP Disadvantages

• Standard requirement is that all FTP transfers should be encrypted by FTP Provides to offer extra encryption
• The size limit is 2GB that can be sent but it doesn't allow users to run a file transfer multiple different connections
• Passwords and file contents are sent in clear text for eavesdropping
• Attackers carry out brute force attack to guess passwords

WWW (World Wide Web) Defined

• Also called the Web
• Invented by Tim Berners-Lee in 1989
• A collection of website, web pages and other resources known as URLs
• Linked via hypertext and searchable through the internet
• Users can access data like text, images, audios, videos, etc., using devices
• Enables the retrieval and display of text and media

Further WWW Information

• Organized via the HTML protocol, building blocks connected by hyperlinks which redirects to other sources containing additional information
• WebPages of a site are stored on a single server while other websites/organizations host their pages by using different servers
• The WWW can direct a person to the closest server to their location

WWW Architecture

• The client requests information from the server via a browser
• The server finds the document, then sends it to the client

WWW's Components

• Consists of the Uniform Resource Locator (URL), HyperText Transfer Protocol (HTTP) and Hyper Text Markup Language (HTML)
• The URL is used for accessing resources
• The URL consists of four parts: protocol, host computer, port, path, and serves as a network on the web
• WebPages have online addresses that belong to that sites URL, like Facebook, or google

HyperText Transfer Protocol Defined

• Specifies communication between browser and server
• HTTP is an application layer protocol used to transmit hypermedia documents (such as HTML)
• It was designed for communications which can be used for different purposes
• Follows a client or server protocol making the client request
• Stateless protocol, meaning server does note keep data

Hyper Text Markup Language

• A HTML protocol that tells the structure, organization, and web page content.
• Invented by Berners-Lee in the year of 1991
• Is the building blocks of content meaning it is the most basic code in of web
• tags surrounds HTML elements

HTML Facts

• Hypertext is used to link web pages together single website or across different websites
• Is used for web pages development and web navigations
• Supports Responsive and offline UIs

WWW Operation

• Employs a client-server approach including several steps:
• Users enters the URL into an address bar
• Browser requests Domain Name Server for the IP address corresponding to the URL
• The server interprets, then presents it to the browser

Internet vs WWW

• Internet and Web are not the same
• The internet uses technology to transmit data

SNMP (Simple Network Management Protocol)

• Is an application layer protocol that follows standards of the Internet engineering task force that is used to manage tools and technology
• Uses a framework for managing internet devices with a TCP/IP protocol
• Delivers a set of operations used to maintain the internet
• Used to detect faults in the network/ internet along with monitoring the remote devices and networks

SNMP Concept

• The SNMP protocol makes the use of the network through the manager an agent, the manager normally oversees the agents, but they are still individual
• SNMP can assist the device that has been made to detect errors
• The protocols is used in heterogeneous networks, which connect the lans and wams

SNMP

• Is standard for use in the Internet protocols and language and allows management through distributed management

SNMP Components

• Has three systems of protocol architecture made to manage networks. SNMP uses systems to control network and control agents likes: PC, routers, switches, etc
• Information Management Base: Contains various hierarchs of to be managed

SNMP and The Exchange

• Main role it to format a packet from agents/managers
• The data is interpret and made into statistics and objects
• Packets between agents/managers contains objects is values and sends/recieves them

Benefits of SNMP

• Is a standard with different OS, tools and is easy to design
• Universal, support device of all kinds and is easy to use

Disadvantages of SNMP

• Can lead to a reduction of network bandwith
• Does not have good security or information