Application Layer Protocols

What is Application Layer ?

An application layer is an abstraction layer that specifies the shared communications protocols and interface methods used by hosts in a communications network. The application layer abstraction is used in both of the standard models of computer networking: the Internet Protocol Suite and the OSI model.

The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to describe the functions of a networking system. The OSI model characterizes computing functions into a universal set of rules and requirements in order to support interoperability between different products and software.

An application layer protocol defines how application processes (clients and servers), running on different end systems, pass messages to each other. In particular, an application layer protocol defines:

• The types of messages, e.g., request messages and response messages.
• The syntax of the various message types, i.e., the fields in the message and how the fields are delineated.
• The semantics of the fields, i.e., the meaning of the information that the field is supposed to contain;
• Rules for determining when and how a process sends messages and responds to messages.

Sublayers

Originally the OSI model consisted of two kinds of application layer services with their related protocols. These two sublayers are the common application service element (CASE) and specific application service element (SASE). Generally, an application layer protocol is realized by the use of the functionality of a number of application service elements. Some application service elements invoke different procedures based on the version of the session service available.

Application Layer protocol:-

• TELNET
• FTP
• TFTP
• NFS
• SMTP
• LPD
• X window
• SNMP
• DNS
• DHCP

1. TELNET

• The main task of the internet is to provide services to users. For example, users want to run different application programs at the remote site and transfers a result to the local site. This requires a client-server program such as FTP, SMTP. But this would not allow us to create a specific program for each demand.
• The better solution is to provide a general client-server program that lets the user access any application program on a remote computer. Therefore, a program that allows a user to log on to a remote computer. A popular client-server program Telnet is used to meet such demands. Telnet is an abbreviation for Terminal Network.
• Telnet provides a connection to the remote computer in such a way that a local terminal appears to be at the remote side.

Working of TELNET

Advantages and Disadvantages of using TELNET

Advantages

Telnet client is versatile

Can be used cross-platform

Unlimited access to target resources

Disadvantages

Unencrypted data exchange

Full access makes it easier for hackers

Only few servers can be reached via Telnet

2. FTP

FTP stands for File transfer protocol. FTP is a standard internet protocol provided by TCP/IP used for transmitting the files from one host to another. It is mainly used for transferring the web page files from their creator to the computer that acts as a server for other computers on the internet. It is also used for downloading the files to computer from other servers.

Objectives of FTP

• It provides the sharing of files.
• It is used to encourage the use of remote computers.
• It transfers the data more reliably and efficiently.

Why FTP?

Although transferring files from one system to another is very simple and straightforward, but sometimes it can cause problems. For example, two systems may have different file conventions. Two systems may have different ways to represent text and data. Two systems may have different directory structures. FTP protocol overcomes these problems by establishing two connections between hosts. One connection is used for data transfer, and another connection is used for the control connection.

Working of FTP:

Advantages of FTP clients

• Allows you to transfer multiple files as well as directories
• The ability to resume a transfer if the connection is lost
• The ability to add items to a “queue” to be uploaded/downloaded
• Many FTP clients have the ability to schedule transfers

Disadvantages FTP clients

• Usernames, passwords and files are sent in clear text
• Filtering active FTP connections is difficult on your local machine (passive is preferred)
• Servers can be spoofed to send data to a random port on an unintended computer
• Hard to script jobs

3. TFTP

Trivial File Transfer Protocol (TFTP) is a simple lockstep file transfer protocol which allows a client to get a file from or put a file onto a remote host. One of its primary uses is in the early stages of nodes booting from a local area network. TFTP has been used for this application because it is very simple to implement.

Working of TFTP:

Message types in TFTP

4. NFS

NFS is an abbreviation of the Network File System. It is a protocol of a distributed file system. It is an architecture of the client/server, which contains a client program, server program, and a protocol that helps for communication between the client and server. It is that protocol which allows the users to access the data and files remotely over the network. Any user can easily implement the NFS protocol because it is an open standard.

Network File System is a protocol that works on all the networks of IP-based. It is implemented in that client/server application in which the server of NFS manages the authorization, authentication, and clients. This protocol is used with Apple Mac OS, Unix, and Unix-like operating systems such as Solaris, Linux, FreeBSD, AIX.

Working of NFS:

One major advantage that NFS provides is central management. Using a centrally managed server decreases the workload for the administrator in terms of back-ups, adding software that will be shared and computer repair. Another advantage is the granularity that can be used to give access to files.

5. SMTP

SMTP stands for Simple Mail Transfer Protocol. SMTP is a set of communication guidelines that allow software to transmit an electronic mail over the internet is called Simple Mail Transfer Protocol. It is a program used for sending messages to other computer users based on e-mail addresses. It provides a mail exchange between users on the same or different computers, and it also supports:

• It can send a single message to one or more recipients.
• Sending message can include text, voice, video or graphics.
• It can also send the messages on networks outside the internet.

The main purpose of SMTP is used to set up communication rules between servers. The servers have a way of identifying themselves and announcing what kind of communication they are trying to perform. They also have a way of handling the errors such as incorrect email address. For example, if the recipient address is wrong, then receiving server reply with an error message of some kind.

Working of SMTP:

• The main drawback of sending through an SMTP server is that it is insecure, it can be easily hacked.
• Another disadvantage is the server limitation.
• Regarding the advantages, we have recently launched a new feature to get statistics from messages sent by SMTP.

6. LPD

The Line Printer Daemon protocol/Line Printer Remote protocol is a network printing protocol for submitting print jobs to a remote printer.

Working of LPD:

7. X window

It defines a protocol for the writing of graphical user interface–based client/server applications. The idea is to allow a program, called a client, to run on one computer. It is primarily used in networks of interconnected mainframes.

The X Window System core protocol is the base protocol of the X Window System, which is a networked windowing system for bitmap displays used to build graphical user interfaces on Unix, Unix-like, and other operating systems. The X Window System is based on a client–server model: a single server controls the input/output hardware, such as the screen, the keyboard, and the mouse; all application programs act as clients, interacting with the user and with the other clients via the server. This interaction is regulated by the X Window System core protocol. Other protocols related to the X Window System exist, both built at the top of the X Window System core protocol or as separate protocols.

Working of X- Windows:

8. SNMP

SNMP stands for Simple Network Management Protocol. SNMP is a framework used for managing devices on the internet. It provides a set of operations for monitoring and managing the internet. SNMP has two components Manager and agent.

The manager is a host that controls and monitors a set of agents such as routers. It is an application layer protocol in which a few manager stations can handle a set of agents. The protocol designed at the application level can monitor the devices made by different manufacturers and installed on different physical networks. It is used in a heterogeneous network made of different LANs and WANs connected by routers or gateways.

Working of SNMP:

9. DNS

An application layer protocol defines how the application processes running on different systems, pass the messages to each other.

DNS stands for Domain Name System. DNS is a directory service that provides a mapping between the name of a host on the network and its numerical address DNS is required for the functioning of the internet. Each node in a tree has a domain name, and a full domain name is a sequence of symbols specified by dots. DNS is a service that translates the domain name into IP addresses. This allows the users of networks to utilize user-friendly names when looking for other hosts instead of remembering the IP addresses.

For example, suppose the FTP site at EduSoft had an IP address of 132.147.165.50, most people would reach this site by specifying ftp.EduSoft.com. Therefore, the domain name is more reliable than IP address.

Working of DNS:

10. DHCP

Dynamic Host Configuration Protocol (DHCP) is a network management protocol used to dynamically assign an IP address to nay device, or node, on a network so they can communicate using IP (Internet Protocol). DHCP automates and centrally manages these configurations. There is no need to manually assign IP addresses to new devices. Therefore, there is no requirement for any user configuration to connect to a DHCP based network.

DHCP can be implemented on local networks as well as large enterprise networks. DHCP is the default protocol used by the most routers and networking equipment. DHCP is also called RFC (Request for comments) 2131.

What does the DNCP do ? :

• DHCP manages the provision of all the nodes or devices added or dropped from the network.
• DHCP maintains the unique IP address of the host using a DHCP server.
• It sends a request to the DHCP server whenever a client/node/device, which is configured to work with DHCP, connects to a network. The server acknowledges by providing an IP address to the client/node/device.

Working of DHCP:

Conclusion:

As we know all An application layer protocol defines how application processes (clients and servers), running on different end systems, pass messages to each other. So, by understanding all these protocols we will be able to implement these protocols during working with the computer system.

Contributors

Vidhisha Kamat
Rohan Katkar
Rohan Katta
Kushal Kela
Aayush Khandekar