Curs PC si internet cap 13 The Session Layer

Overview After data packets provided by the four lower layers travel through the transport layer, they are turned into sessions by the layer 5 protocol or OSI session layer. This is done by implementing various control mechanisms. In this chapter, you will learn about these mechanisms including accounting, conversation control, that is, determining who can talk when, and session parameter negotiation. This chapter also describes how the session layer coordinates service requests and responses that occur when applications communicate between different hosts. You will learn about the processes that occur as data travels through the session layer, such as dialogue control and dialogue separation that enable applications to communicate between the source and destination. 13.1 The Session Layer 13.1.1 The session layer overview Instructor Note The purpose of this target indicator is to start to justify the need for layer 5. Layer 5 has traditionally been the "least filled" layer. But the students need to flesh out their understanding of the OSI Model and they should have something tangible to relate to Layer 5. Networking processes often occur in less than a second, making them difficult to "see". By using analogies you can understand more clearly what happens during these processes. The following analogy helps explain the session layer:You have just had an argument with a friend. You are now communicating (referred to, here, as a "rap session" or "session") with him/her, to discuss the state of your friendship. You are using the Instant Mail feature on America On Line (AOL) or an Internet Relay Chat (IRC). However, there are two problems that may interfere with your session. The first problem is that your messages may cross during your conversation. You may both type messages at exactly the same time, thus interrupting each other. The second problem is that you need to pause (to save your current conversation as a file) or to check each others previous conversation (for clues to the cause of the argument), or re-synchronize your communication after an interruption. To solve the first problem, you should establish a protocol, or set of protocols, that dictate rules for communicating with each other. This means that each of you would agree to a set of guidelines to use during the conversation (e.g. taking turns sending messages to avoid interrupting each other). This is referred to as two-way alternate communication. Another solution is that each person would type whenever he/she wishes, regardless of who is transmitting, and you would assume that more information is always on the way. This is referred to as two-way simultaneous communication. To solve the second problem, you should send a checkpoint to each other, which means that each person should save the conversation as a file. Then, each person should re-read the last part of his/her conversation and check the time on the clock. This is referred to as synchronization. Two very important checkpoints are how the conversation starts and how it ends. This is referred to as orderly initiation and termination of the conversation. For example, when you use Instant Mail or Internet Relay Chat, usually you exchange good-byes with the other person, before you terminate the session, to make sure the other person understands that you are ending the session. Another analogy to help understand what the session layer does is to re-use this same analogy, but this time imagine that you are communicating with a pen pal via the postal service. The same problems might occur. Messages could pass each other because you haven't agreed to use two-way simultaneous communication rather than two-way alternate control; or you could experience poor communication because you haven't synchronized the subjects of your conversations. 13.1 The Session Layer 13.1.2 Session layer analogies Instructor Note The purpose of this target indicator is to provide two analogies for dialogue control. The first analogy is an online analogy, familiar to Internet Service Provider "chat" session. The second analogy is a postal analogy. The session layer establishes, manages, and terminates sessions between applications. This includes starting, stopping, and re-synchronizing two computers that are having a "rap session". The session layer coordinates applications as they interact on two communicating hosts. Data communications travel on packet-switched networks, not like telephone calls that travel on circuit-switched networks. Communication between two computers involves many mini-conversations, thus ensuring that the two computers can communicate effectively. One requirement of these mini-conversations is that each host plays dual roles: requesting service - like a client; and, replying with service - like a server. Determining which role they are playing at any given moment is called dialogue control. 13.1 The Session Layer 13.1.3 Dialogue control Instructor Note The purpose of this target indicator is that the student develop a clear understanding of the two major forms of dialog control -- two-way alternate and two-way simultaneous. As an activity, first toss a paper (representing data) back and forth to one of the students. This represents two-way alternate communication. Then have each person throwing a piece of paper simultaneously (it may require some concentration to keep this going!) -- this represents two way alternate communication. In reference to layer 2 communications, the terms half-duplex and full-duplex are used instead of two-way-alternate and two-way-simultaneous. The session layer decides whether to use two-way simultaneous conversation or two-way alternate communication. This decision is referred to as dialogue control. If two-way simultaneous communication is allowed, then the session layer does little in the way of managing the conversation, and allows the other layers of the communicating computers to manage the conversation. It is possible to have session layer collisions, although these are very different than the media-collisions that occur in Layer 1. At this level, collisions can only occur as two messages pass each other, and cause confusion in either, or both, communicating hosts. If these session layer collisions are intolerable, then dialogue control has another option: - two-way alternate communication. Two-way alternate communication involves the use of a session layer data token that allows each host to take turns. This is similar to the way a Layer 2 Token Ring handles Layer 1 collisions. 13.1 The Session Layer 13.1.4 Dialogue separation Instructor Note The purpose of this target indicator is to convey the somewhat abstract idea of minor (one way) and major (two way) synchronizations. Point out that the vertical axis is time. Emphasize that there should be no horizontal lines -- this would imply a message traveled in zero time from one host to another. We recommend a kinesthetic activity where two students with watches sending messages to each other and periodically sharing their watch readings as checkpoints within the communication. Checkpointing is similar to the way a word processor on a stand-alone computer pauses for a second when it performs an AutoSave of the current document. However, these checkpoints are used, instead, to separate parts of a session previously referred to as dialogues. Dialogue separation is the orderly initiation, termination, and managing of communication. The main graphic illustrates a minor synchronization. At the "Time Axis, t = checkpoint", the host A session layer sends a synchronization message to host B, at which time both hosts perform the following routine: back up the particular files save the network settings save the clock settings make note of the end point in the conversation A major synchronization would involve more back-and-forth steps and conversation than is shown in this diagram. Summary In this chapter, you learned about the functions of the session layer and the different processes that occur as data packets travel through this layer. More specifically, you learned that: The session layer establishes, manages, and terminates sessions between applications Communication sessions consist of mini-conversations that occur between applications located in different network devices Requests and responses are coordinated by protocols implemented at the session layer The session layer decides whether to use two-way simultaneous conversation or two-way alternate communication by using dialogue control The session layer uses dialogue separation to orderly initiate, terminate, and manage of communication Now that you have completed this chapter, you should have a firm understanding of how the session layer provides transport services from the host to the destination. In the next chapter, you will examine what happens to data packets as they travel through the presentation layer of the OSI Model.