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The Session Layer of the Osi Model

By roguespwnu Aug 27, 2010 1514 Words
A Glance at the Session Layer of the OSI Model

Throughout the history of networking, one model has stood the test of time for providing not only insight, but also a foundation by which all networking systems utilize in some form or fashion. The seven layers of the OSI model, which stands for Open System Interconnect, are: Application, Presentation, Session, Transport, Network, Data link, and Physical. These layers are the framework for networking, because they represent the roadmap, rules, and process that every message must adhere to when it is sent across networks. Just as a package is sent from UPS or Fed-Ex with a routing number, a sender, a receiver, an address, and many other things that are involved with the delivery of the package, the same principles can be applied to networking messages or packets by using the OSI model. The Session layer of the OSI Model, “Provides the control functions necessary to establish, manage, and terminate the connections as required to satisfy the user requests,” (Beasley 123). This layer is in direct contact with the user through the different interfaces and the connections can be managed and manipulated in many different ways. For instance if I wanted to terminate my connection because of a severe thunderstorm there are a bunch of ways I could do that. I could pull the plugs; I could disable the router then power it down and power down my switch and then power down the rest of my network. If I operated ITT-Tech’s network and in order to control the employees’ use of the internet and to keep them working, I could put a filter using barracuda on all websites that wouldn’t pertain to work related subjects. It would manage their use of the network and keep them focused on their work. These are just several examples of the Session layer of the OSI Model in action. The OSI model interacts with protocols that correspond with each layer and help regulate and manage the network communications. There are many protocols which work directly with or around the session layer of the OSI Model. One set of protocols is TCP/IP which stands for Transmission Control Protocol / Internet Protocol. Near the last pages of my report I have attached a copy of Figure 6-5 from page 102 of Irwin Handbook of Telecommunications. This figure illustrates how the OSI Model works in directly in correlation to TCP/IP. This suite of protocols is used for internetworks such as the internet. The protocols within this suite that are used to process requests and work in and around the session layer are: Telnet, FTP, SMTP, HTTP, DNS, and SNMP. Telnet is a terminal emulation application that acts as a virtual terminal connection and allows certain users or administrators to log in remotely across a TCP/IP. FTP is File transfer protocol which helps regulate file transfers across TCP/IP networks. SMTP stands for Simple Mail Transfer Protocol this pertains to directly to e-mail as it is considered simple mail. HTTP hypertext transport protocol is used in website addressing it also helps the web browser run like a client and the computer hosting the website to function like a server. It focuses more on the application layer but portions of it pertain to the session layer such as when users send requests to different servers that utilize HTTP. DNS is domain name system this can be used in requests across TCP/IP networks by working with the DHCP to store info about the host name to the IP address. SNMP or “simple network management protocol is a connectionless protocol using the user datagram protocol for the transmission of data to and from UDP port 161,” (Beasley 264). Other things involved with the session layer are SIP, SQL, NetBIOS, and NFS. SIP which stands for session initiation protocol is used as a signaling protocol to help manage communication implemented with multimedia. In other words it helps with voice over IP, video conferencing, chat, file transfer, or games over the network. So some programs that utilize this that you would be familiar with are Skype, AOL instant messenger, Ventrilo, Lime Wire, and many others including certain video games which incorporate network communications. SQL or structured query language is used when queries are sent between hosts; it helps with managing data around databases. NetBIOS aka the network basic input and output system services the users by allowing separate computers to communicate across a LAN. Network file system protocol or NFS is responsible for helping to regulate and control file transfers across the network. There are a number of networking standards that are involved in the Session layer of the OSI model as well. ISO 8306 and 8037, ITU-T X.215 and X.225 are all depicted in the table 6-1 on page 95 of Irwin Handbook of Telecommunications. Also the ISO 9548 standard is a connectionless session protocol (Green 95). The most common functions of the session layer of the OSI model is to establish, manage and terminate sessions (Beasley 123). In order to do these three functions one must understand that there are a lot of things included that are important to the session layer and help it function properly. Such relevant things include: programs, buttons, power cords and connections, commands, protocols, standards, applications, users. All of these control and manage the sessions in different ways and manipulate the communications in each session. The Session layer interacts directly with the presentation and the transport layers of the OSI model. It also corresponds to the session layer at the receiver or receivers’ host machines. The presentation layer, which follows the application layer, is where the user presents his requests with the message intact. Then during the session layer the user manipulates the rest of the details regarding the transmission of the information and the termination of the communication if necessary. After that the transport layer acts like the security guard with the x-ray machine who double checks your luggage. He “ensures error-free packets” and sends the luggage along (Beasley 123). Then your message is marked and routed for transport and follows the correct protocols and utilizes the standards necessary to be delivered. It goes on through the rest of the network layer then to the data link layer and then on to the physical layer and then up the model in reverse order. The communications between the presentation session and transport are controlled by a combination of the applications, standards, protocols, and the user. Even though the session layer is farther down the chain it also works in correlation with the application layer. Since the application layer interacts with programs that utilize some form of a communication component, it is very dependent on the session layer’s management of the connections. One can’t make a Skype call to someone who can’t establish a session. A breakdown in communications can be detrimental to anyone anywhere in the world. Also one can’t continue communicating in a Skype call, be it video and voice over IP or just voice over IP, if the session gets terminated. Also reoccurring problems with connectivity during the session usually aren’t caused by faulty session protocols applications or standards, but they are usually caused by a bad connection. The standards protocols and applications in place help manipulate these multimedia forms of communication but if they weren’t so carefully adhered to and coded then we could be subject to even more problems than just a bad connection. Since the application layer directly logs messages in and interprets requests its success completely depends on the management of the sessions or it cannot send the messages across the network. This relates right into the data link layer as well because the error recovery and flow control within the data link layer would inform the user or the application of any problems and either fix them or inform the user to fix them. If the messages are terminated or the call is dropped, like I mentioned earlier with the Skype example, this could be a problem with the physical layer. Thus one would be correct in stating that the session layer is very dependent on the physical layer to help distribute the sessions and provide a connection. If a tree falls on the telephone line or more commonly a builder digs up and breaks a telephone or internet cable, then the session layer is terminated and not at the users request nor at the request of the protocols or standards. In conclusion one could say the OSI Model’s layers are dependent on one another. The session layer is both dependant on the physical layer and the constant watching security guard on which the application layer depends.

Green, James Harry. Irwin Handbook of Telecommunications. 5th ed. New York, New York: McGraw-Hill Professional, 2006. 93-110. Print. Beasley, Jeffrey, and Cram101 Reviews. Networking - With CD by Jeffrey S. Beasley, ISBN. 2nd ed. Ann Arbor, Michigan: Edward Brothers, 2009. Print.

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