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Application Layer in OSI Model: Architecture, Client-Server & Peer-to-Peer, Schemes and Mind Maps of Computer Networks

Institute of Business and Medical CareersComputer Networks

A comprehensive explanation of the application layer in the osi model, covering its functions, services, and architecture. It delves into the client-server and peer-to-peer models, highlighting their characteristics and advantages. The document also explores the domain name system (dns), its role in network communication, and its working principles. This information is valuable for understanding the fundamental concepts of network communication and the role of the application layer in enabling user interactions.

Typology: Schemes and Mind Maps

2024/2025

Available from 04/13/2025

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Application Layer
The application layer in the OSI model is the closest layer to the end user which means that the application layer
and end user can interact directly with the software application. The application layer programs are based on
client and servers.
The Application layer includes the following functions:
o Identifying communication partners: The application layer identifies the availability of communication
partners for an application with data to transmit.
o Determining resource availability: The application layer determines whether sufficient network
resources are available for the requested communication.
o Synchronizing communication: All the communications occur between the applications requires
cooperation which is managed by an application layer.
Services of Application Layers
o Network Virtual terminal: An application layer allows a user to log on to a remote host. To do so, the
application creates a software emulation of a terminal at the remote host. The user's computer talks to
the software terminal, which in turn, talks to the host. The remote host thinks that it is communicating
with one of its own terminals, so it allows the user to log on.
o File Transfer, Access, and Management (FTAM): An application allows a user to access files in a remote
computer, to retrieve files from a computer and to manage files in a remote computer. FTAM defines a
hierarchical virtual file in terms of file structure, file attributes and the kind of operations performed on
the files and their attributes.
o Addressing: To obtain communication between client and server, there is a need for addressing. When a
client made a request to the server, the request contains the server address and its own address. The
server response to the client request, the request contains the destination address, i.e., client address. To
achieve this kind of addressing, DNS is used.
o Mail Services: An application layer provides Email forwarding and storage.
o Directory Services: An application contains a distributed database that provides access for global
information about various objects and services.
Network Application Architecture
Application architecture is different from the network architecture. The network architecture is fixed and provides
a set of services to applications. The application architecture, on the other hand, is designed by the application
developer and defines how the application should be structured over the various end systems.
Application architecture is of two types:
o Client-server architecture: An application program running on the local machine sends a request to
another application program is known as a client, and a program that serves a request is known as a
server. For example, when a web server receives a request from the client host, it responds to the request
to the client host.
Characteristics Of Client-server architecture:
o In Client-server architecture, clients do not directly communicate with each other. For example, in a web
application, two browsers do not directly communicate with each other.
o A server is fixed, well-known address known as IP address because the server is always on while the client
can always contact the server by sending a packet to the sender's IP address.
Disadvantage Of Client-server architecture:
It is a single-server based architecture which is incapable of holding all the requests from the clients. For example,
a social networking site can become overwhelmed when there is only one server exists.
o P2P (peer-to-peer) architecture: It has no dedicated server in a data center. The peers are the computers
which are not owned by the service provider. Most of the peers reside in the homes, offices, schools, and
universities. The peers communicate with each other without passing the information through a
dedicated server, this architecture is known as peer-to-peer architecture. The applications based on P2P
architecture includes file sharing and internet telephony.
Features of P2P architecture
o Self scalability: In a file sharing system, although each peer generates a workload by requesting the files,
each peer also adds a service capacity by distributing the files to the peer.
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Application Layer

The application layer in the OSI model is the closest layer to the end user which means that the application layer and end user can interact directly with the software application. The application layer programs are based on client and servers. The Application layer includes the following functions: o Identifying communication partners: The application layer identifies the availability of communication partners for an application with data to transmit. o Determining resource availability: The application layer determines whether sufficient network resources are available for the requested communication. o Synchronizing communication: All the communications occur between the applications requires cooperation which is managed by an application layer.

Services of Application Layers

o Network Virtual terminal: An application layer allows a user to log on to a remote host. To do so, the application creates a software emulation of a terminal at the remote host. The user's computer talks to the software terminal, which in turn, talks to the host. The remote host thinks that it is communicating with one of its own terminals, so it allows the user to log on. o File Transfer, Access, and Management (FTAM): An application allows a user to access files in a remote computer, to retrieve files from a computer and to manage files in a remote computer. FTAM defines a hierarchical virtual file in terms of file structure, file attributes and the kind of operations performed on the files and their attributes. o Addressing: To obtain communication between client and server, there is a need for addressing. When a client made a request to the server, the request contains the server address and its own address. The server response to the client request, the request contains the destination address, i.e., client address. To achieve this kind of addressing, DNS is used. o Mail Services: An application layer provides Email forwarding and storage. o Directory Services: An application contains a distributed database that provides access for global information about various objects and services.

Network Application Architecture

Application architecture is different from the network architecture. The network architecture is fixed and provides a set of services to applications. The application architecture, on the other hand, is designed by the application developer and defines how the application should be structured over the various end systems. Application architecture is of two types: o Client-server architecture: An application program running on the local machine sends a request to another application program is known as a client, and a program that serves a request is known as a server. For example, when a web server receives a request from the client host, it responds to the request to the client host. Characteristics Of Client-server architecture: o In Client-server architecture, clients do not directly communicate with each other. For example, in a web application, two browsers do not directly communicate with each other. o A server is fixed, well-known address known as IP address because the server is always on while the client can always contact the server by sending a packet to the sender's IP address. Disadvantage Of Client-server architecture: It is a single-server based architecture which is incapable of holding all the requests from the clients. For example, a social networking site can become overwhelmed when there is only one server exists. o P2P (peer-to-peer) architecture: It has no dedicated server in a data center. The peers are the computers which are not owned by the service provider. Most of the peers reside in the homes, offices, schools, and universities. The peers communicate with each other without passing the information through a dedicated server, this architecture is known as peer-to-peer architecture. The applications based on P2P architecture includes file sharing and internet telephony.

Features of P2P architecture

o Self scalability: In a file sharing system, although each peer generates a workload by requesting the files, each peer also adds a service capacity by distributing the files to the peer.

o Cost-effective: It is cost-effective as it does not require significant server infrastructure and server bandwidth. Client and Server processes o A network application consists of a pair of processes that send the messages to each other over a network. o In P2P file-sharing system, a file is transferred from a process in one peer to a process in another peer. We label one of the two processes as the client and another process as the server. o With P2P file sharing, the peer which is downloading the file is known as a client, and the peer which is uploading the file is known as a server. However, we have observed in some applications such as P2P file sharing; a process can be both as a client and server. Therefore, we can say that a process can both download and upload the files. Client and Server model o A client and server networking model is a model in which computers such as servers provide the network services to the other computers such as clients to perform a user based tasks. This model is known as client-server networking model. o The application programs using the client-server model should follow the given below strategies: o An application program is known as a client program, running on the local machine that requests for a service from an application program known as a server program, running on the remote machine. o A client program runs only when it requests for a service from the server while the server program runs all time as it does not know when its service is required. o A server provides a service for many clients not just for a single client. Therefore, we can say that client- server follows the many-to-one relationship. Many clients can use the service of one server. o Services are required frequently, and many users have a specific client-server application program. For example, the client-server application program allows the user to access the files, send e-mail, and so on. If the services are more customized, then we should have one generic application program that allows the user to access the services available on the remote computer. Client A client is a program that runs on the local machine requesting service from the server. A client program is a finite program means that the service started by the user and terminates when the service is completed. Server A server is a program that runs on the remote machine providing services to the clients. When the client requests for a service, then the server opens the door for the incoming requests, but it never initiates the service. A server program is an infinite program means that when it starts, it runs infinitely unless the problem arises. The server waits for the incoming requests from the clients. When the request arrives at the server, then it responds to the request. Advantages of Client-server networks: o Centralized: Centralized back-up is possible in client-server networks, i.e., all the data is stored in a server. o Security: These networks are more secure as all the shared resources are centrally administered. o Performance: The use of the dedicated server increases the speed of sharing resources. This increases the performance of the overall system. o Scalability: We can increase the number of clients and servers separately, i.e., the new element can be added, or we can add a new node in a network at any time. Disadvantages of Client-Server network: o Traffic Congestion is a big problem in Client/Server networks. When a large number of clients send requests to the same server may cause the problem of Traffic congestion. o It does not have a robustness of a network, i.e., when the server is down, then the client requests cannot be met.

The inverse domain is used for mapping an address to a name. When the server has received a request from the client, and the server contains the files of only authorized clients. To determine whether the client is on the authorized list or not, it sends a query to the DNS server and ask for mapping an address to the name. Working of DNS o DNS is a client/server network communication protocol. DNS clients send requests to the. server while DNS servers send responses to the client. o Client requests contain a name which is converted into an IP address known as a forward DNS lookups while requests containing an IP address which is converted into a name known as reverse DNS lookups. o DNS implements a distributed database to store the name of all the hosts available on the internet. o If a client like a web browser sends a request containing a hostname, then a piece of software such as DNS resolver sends a request to the DNS server to obtain the IP address of a hostname. If DNS server does not contain the IP address associated with a hostname, then it forwards the request to another DNS server. If IP address has arrived at the resolver, which in turn completes the request over the internet protocol.