Protocols, Services, and Applications

Protocols, Services, and Applications As mentioned, TCP/IP provides a mechanism to allow systems to communicate with each other across a network.
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Protocols, Services, and ApplicationsProtocols, Services, and ApplicationsAs mentioned, TCP/IP provides a mechanism to allow systems to communicate with eachother across a network. If we refer back to our language analogy, most spoken languageshave certain rules that define how the communications occurs. By adhering to these rules,one is then able to understand and comprehend what is being communicated. TCP/IPfollows a similar process to define how the communications will occur through the use ofprotocols, services, and applications.You cannot just start throwing words together in any order that you feel like and expectpeople to understand what you are saying. You have to follow certain rules that areunderstood by all parties involved for them to understand what you are saying. Networkcommunications is no different. Although spoken languages have rules such as sentencestructure and noun and verb usage to define how the communications occurs, networkcommunication has protocols. The easiest way to think of a protocol is that it is merely aset of rules that defines how something occurs. So, much like how using a verb denotesan expression of existence, action, or occurrence, a network protocol defines how themethod of communication will occur, such as how TCP defines a mechanism forconnection-oriented communications. Protocols may be an open protocol (such as TCP,UDP, or IP), which means that the protocol is not owned by anyone in particular andcan be used by anyone that wants to use the protocol or they can be closed protocols(such as Cisco Discovery Protocol [CDP]), which means that the protocol can only beused by licensed or authorized entities. In general, open protocols are used to facilitatemost vendor-neutral communications processes; closed protocols are used by vendors toprovide vendor-specific communications processes.Whereas protocols define how something occurs, services typically define what is beingdone. The objective of a service is to produce some function or data of value andsubstance. This function or data can then be used by the systems to facilitatecommunications. In many cases, the function or data provided by a service is used byprotocols, such as how services like addressing services such as Domain Name System(DNS) might be used by IP to facilitate communication between hosts.Applications are nothing more than processes running on a host that take advantage of thenetwork services and protocols to provide data to the end user. Applications arefrequently known as end-user services because they exist to service end-user requests.The concept of protocols, services, and applications can be a difficult one to grasp. Afterall, how do they interact with each other? Which is responsible for what? Networkcommunications is a complex concept to master for many reasons, not the least of whichis that the concept is so large. I refer to this as the elephant problem.If you try to sit down and cook and eat an elephant all at once, you quickly realize that itis an insurmountable task. After all, there is a lot of elephant to chow down on. To besuccessful, one must take that elephant and break it down into smaller, easier-to-digeststeak-sized pieces. In doing so, what was once an insurmountable task just becamesomething easy to accomplish by virtue of the fact that you have taken a big thing andturned it into smaller, easier-to-manage pieces. To do this same thing with understandingnetwork communications, it is important to break the total task of communicationsbetween hosts into smaller, easier-to-understand and define layers. The benefits of alayered approach to network communications are as follows: • The complex process of network communications can be segmented into easier-to- understand components. • It provides a standard interface to allow for multivendor integration. Each layer merely needs to have a standard interface to the layer above and below, without concern for the details of what is done at other layers. • In conjunction with a standard interface, a layered approach allows the details of how something is done at a particular layer to be defined and changed without impacting the overall communications process at other layers.There are two predominant models for network communications, the Open SystemsInterconnection (OSI) model and the Department of Defense (DoD) model.The OSI ModelThe OSI model is a layered model that has been standardized for defining networkcommunications. The OSI model breaks the complex process of networkcommunications into seven distinct layers, each with it own distinct responsibilities. Asshown in Figure 3-1, the seven layers of the OSI model are as follows: • The application layer (Layer 7) Primarily responsible for interfacing with the end user • The presentation layer (Layer 6) Primarily responsible for translating the data from something the user understands into something the network understands and vice versa • The session layer (Layer 5) Primarily responsible for dialog and session control functions between systems • The transport layer (Layer 4) Primarily responsible for the formatting and handling of the transport of data between systems • The network layer (Layer 3) Primarily responsible for logical addressing • The data link layer (Layer 2) Primarily responsible for physical addressing • The physical layer (Layer 1) Primarily responsible for the physical transport of the data on the network Figure 3-1. Layers of the OSI ModelRather than focusing on detailing explicitly how communications occur, either in total orin each layer, the OSI model merely defines what needs to occur, and what each hostattempting to communicate should be able to expect in the communications process.Afte ...