(Please note the differences between the OSI & the TCP/IP Reference Models. will try to maintain certain understanding, as well as reference.)
As mentioned before the Application Layer is where the user has a direct connection to the computer by inputting data, or making requests. This layer is also responsible for resolving the availability of communication and sufficiency of resources for data input. The protocols associated with this layer are HTTP, FTP, and SMTP.
HTTP began as an extremely basic protocol, which permitted a client to send a simple request and to receive the hypertext file from the server. As the web has grown so has the complexity of the request, but the simplistic job of http has truly stayed the same.
In this segment we will discuss the Application layer protocols, which are: Telnet, FTP, TFTP, NFS, SMTP, LPD, X-Window, SNMP, DNS, and DHCP/BootP. Each protocol has a different function and is used in different ways. So, let us start…
Telnet: This is represented as the chameleon of protocols, because its specialty is terminal emulation. It allows a user on a remote client computer, etc., also called the Telnet client, to access the resources of another machine, the Telnet server. It does so by creating the illusion that the Telnet server is connected to a valid Telnet client machine, but is virtual in nature. It is able to execute and determine system statuses as well as being the causation of procedural execution.
FTP: File Transfer Protocol is the protocol responsible for allowing us to transfer files…really big surprise, eh? FTP is both a protocol and a program. As a protocol it is used by applications; as a program it is used by operators to perform file tasks manually. It teams up with Telnet to permit logging in to the FTP server and then provides for file transferring.
TFTP: Trivial File Transfer Protocol is a stripped down, no bells, and no whistles version of FTP. If you know exactly what you want, where it is, this is what you want. It is fast and because it is stripped down it does not have an abundance of functions to bog it down.
NFS: Network File System is a protocol that specializes in file sharing, allowing two different types of files to interoperate. It permits and allocates RAM on the server to transparently store another operating system based application so that it may run along side of the operating system of the server itself. (i.e., Server runs NT, Win2003, etc, will allow a portion of the RAM to store and run an application which is Unix or Linux based.)
SMTP: Simple Mail Transfer Protocol is another tough one to figure out…it aids us in our desire to stay in touch with others through email by using a spooled or queued method of mail delivery. SMTP is used to send email and POP3 is used for receiving email.
LPD: Line Printer Daemon protocol is used for…printer sharing. LPD along with the Line Printer (LPR) program allows jobs to be spooled and sent to the network’s printers using TCP/IP.
X-Window: This is designed for client/server operations; X Window defines a protocol for writing client/server applications based on a graphical user interface (GUI). The purpose is to run a program (a client) run on one computer and permit it to be displayed through a window server on another computer.
SNMP: Simple Network Management Protocol collects and manipulates data. The data manipulated is valuable network information. Data is gathered by polling devices on the network from a management station at designated intervals, which requires those devices to disclose certain information. SNMP receives what is called a “baseline” which is a report that delimits the operations of a healthy or unhealthy network. This protocol can be a watchdog over the network by advising of any sudden events.
DNS: Domain Name Service resolves “hostnames”, such as www.google.com, www.yahoo.com, etc., to an internet provider (IP) address, such as 192.168.100.1. If you type in an IP address DNS is not being used, because the software knows what to do with it and how to use it. DNS simply makes our lives easier as users so we are not required to type in IP addresses for any specific thing we desire.
DHCP/BootP: Dynamic Host Configuration Protocol assigns IP addresses to hosts. It creates an easier environment for both small and very large scale networks administratively. All types of hardware can be used as a DHCP server; most home networks use their router as a DHCP server. The difference between DHCP and BootP is that with BootP the addresses must be manually keyed in to the BootP table. The DHCP server can provide this information:
- IP address
- Subnet mask
- Domain Name
- Default gateway routes
- WINS information.
See also: Part 1, Part 2, Part 3
Lammle, T. (2007). CCNA Cisco Certified Network Associate Study Guide. Indianapolis: Wiley Publishing, Inc.
Odom, W. (2012). Official Cert Guide ICND1. Indianapolis: Cisco Press.