Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

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76 CHAPTER 5 The OSI Model and Networking Protocols stacks are referred to as routable because they include addressing schemes that identify both the network or subnet and the particular client on that network or subnet. Each subnet must be unique, and each local area network (LAN) will need to know how to get to the other LANs. Routing refers to forwarding packets from one network or subnet to another. Logical addressing is important since it defines how and where the packets are sent. Its characteristics and functions are as follows: ■ ■ ■ ■ ■ A TCP/IP called Address Resolution Protocol (ARP) helps map an IP address to a physical machine address. This is the first layer responsible entirely for the logical connection between two hosts, not the physical one. It defines the mechanisms used to route packets between networks. Without these Layer 3 functions, only local communications would be able to take place. It provides additional levels of flow control and error control. The devices that operate at this layer include, most prominently, routers and Layer 3 switches. Layer 4: Transport The transport layer is responsible for transporting the data from one node to another, and it provides transparent data transfer between nodes and manages the end-to-end flow control, error detection, and error recovery. The transport layer protocols initiate contact between host computers and set up a virtual circuit. The transport protocols on each host computer verify that the application sending the data is authorized to access the network and that both ends are ready to initiate the data transfer. When this synchronization is complete, the data can be sent. As the data is being transmitted, the transport protocol on each host monitors the data flow and watches for transport errors. If transport errors are detected, the transport protocol can provide error recovery. Its characteristics and functions are as follows: ■ It is responsible for providing reliability and connection-oriented or connectionless communications. ■ The two protocols most commonly associated with the transport layer are the TCP, which is connection-oriented, and the User Datagram Protocol (UDP), which is connectionless. ■ It handles ports, which are another aspect of logical addressing. Ports are used to determine which incoming data belongs to each application running on a particular host. ■ It is responsible for name resolution.
The OSI Model 77 Layer 5: Session The session layer is responsible for establishing, monitoring, and terminating sessions, using the virtual circuits established by the transport layer. Its characteristics and functions are as follows: ■ ■ ■ ■ ■ ■ It is responsible for putting header information into data packets to indicate where the message begins and ends. It performs synchronization between the sender’s session layer and the receiver’s session layer. The use of acknowledgement messages (ACKs) helps coordinate transfer of data at the session layer. It controls whether the communications within a session are sent as full duplex or half duplex messages. It establishes a connection between two processes. A process is a defined task related to an application. The session layer is responsible for setting up the connection between an application process on one computer and an application process on another computer, after the transport layer has established the connection between the two machines. Protocols that operate at the session layer include Windows Sockets (the Winsock interface) and the Network Basic Input/Output interface (NetBIOS). Layer 6: Presentation In the presentation layer, data translation is the primary activity performed. The sender’s application passes data down to the presentation layer, where it is put into a common format. When the data is received on the other end, the presentation layer changes the data from the common format back into a format that is useable by the application. Protocol translation, the conversion of data from one protocol to another so that it can be exchanged between computers that use different platforms or operating systems, takes place here. Its characteristics and functions are as follows: ■ ■ ■ Gateway services operate at this layer. Gateways are connection points between networks that use different platforms or applications. It is responsible for data compression, which is used to minimize the actual number of bits that must be transmitted on the network media to the receiver. It is responsible for data encryption and decryption. Layer 7: Application The application layer is the point at which the user application program interacts with the network. This layer of the OSI model should not be confused with the application itself. Application processes are initiated within a user application and then the data created by that process are handed to the application layer of the
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