In the previous section we reviewed several aspects of the Transport Layer. We learned a great deal of information; covering sockets, ports, TCP, UDP, segments, and datagrams. Now we will take a look at the fourth and final layer of the TCP/IP stack: the Application Layer.
By breaking the TCP/IP Application Layer into three separate layers, we can better understand what responsibilities the Application Layer actually has.
- 1. Application Layer - The seventh OSI model layer (which shouldn’t be confused with the TCP/IP stack’s Application Layer). It supports network access, as well as provides services for user applications.
- 2. Presentation Layer - The Sixth OSI model layer is the Presentation Layer. It translates data into a format that can be read by many platforms. With all the different operating systems, programs, and protocols floating around, this is a good feature to have. It also has support for security encryption and data compression.
- 3. Session Layer - The fifth layer of the OSI model is the Session Layer. It manages communication between applications on a network, and is usually used particularly for streaming media or using web conferencing.
Application Layer APIs
There are many types of APIs to delve into. You may have heard of NetBIOS, Winsock, or WinAPI among others. The world of APIs has also extended to web services. You may have heard of a Google API, for instance. In this case Google allows developers to use its internal functions, yet also keeps Google’s internal code safe from prying eyes. (Otherwise, there would be a few security concerns on Google’s part.)
A redirector, otherwise known as a requester, is a service that is largely taken for granted. It is a handy little service that looks at requests a user may make: if it can be fulfilled locally, it is done so. If the request requires a redirection to another computer, then the request is forwarded onto another machine. This enables users to access network resources just like they were an integral part of the local system. A user could browse files on another computer just like they were located on the local computer- obviously redirector services are fairly powerful.
Lastly we have file and print services. If a computer needs to access a file server or a printer, these services will allow the computer to do so. It is fairly self-explanatory, but worth reviewing nonetheless.
- 1. ARP - Arp stands for Address Resolution Protocol. It is used to map an IP address to a physical address found on your NIC card. Using this command can tell us what physical address belongs to which IP address.
- 2. Netstat - Netstat is a handy tool that displays local and remote connections to the computer. It displays IP addresses, ports, protocol being used, and the status of the connection.
- 3. Ping - Ping is a simple diagnostic tool that can check for connectivity between two points on a network. It is one of the most used TCP/IP utilities when setting up a network or changing network settings.
- 4. TraceRT - Tracert, or traceroute, is a command that will show the path that packets of data take while being sent. It’s handy for checking to see where a possible network failure lies, or even for ensuring that data packets are taking the fastest route possible on a network.
- 5. FTP / TFTP - FTP and TFTP are both used for transferring files. It is important to note that FTP is a TCP utility, while TFTP is a UDP utility. TFTP tends to be less secure than FTP, and is generally only used for transferring non-confidential files over a network when speed is concerned.
- 6. Hostname - Hostname is a simple command that displays the hostname of the current computer. Simple, yet effective.
- 7. Whois - Whois information is just like an online phonebook. It shows the contact information for owners of a particular domain. By using a Whois search, you will find that Google is based in California.
The Application Layer isn’t as exciting as the others. We don’t really have much physical interaction with the Application layer, and most of the fun applies to developers and geeks only.
There is still much to learn- TCP/IP is just the very beginning of the networking world. But with this lesson on the final TCP/IP layer complete, you can now say that you have a much better understanding of the TCP/IP model. (And networking in general.)