ccna prep course

ccna prep course.2.2.3 Layer 5: Session LayerLayer 5 of the OSI reference model is the session layer, which is responsible for initiating the establishment and removal of connections. To perform these functions, the session layer must determine whether the data will remain local or be sent to a remote network component. If the latter is the case, the session layer will initiate the establishment of the connection. The session layer is also responsible for distinguishing between multiple network connections, ensuring that data is sent over the correct connection, and fetching data from the connection and forwarding it to the correct local application. However, the actual mechanism for this process is implemented at the transport layer. To establish or remove a connection, the session layer needs to communicate with the transport layer. Remote Procedure Call (RPC) is a TCP/IP session layer protocol; Network File System (NFS) uses RPC, which is also a session layer protocol. The session layer is also responsible for: reporting errors that occur at the application, representation, and session layers; and implementing Classes of Service (CoS) to prioritize certain types of traffic or connections.Exam Essentials The session layer is responsible for establishing, maintaining, and removing network connections; session layer protocols include RPC and NFS.2.2.4 Layer 4: Transport LayerLayer 4 of the OSI reference model is the transport layer, which is responsible for providing mechanisms for establishing, maintaining, and removing connections, where connections can be either reliable or unreliable. For reliable connections, the transport layer is responsible for detecting and repairing errors: when an error is detected, the transport layer will retransmit the data and thus repair the error; for unreliable connections, the transport layer only provides error detection, leaving the error repair to the higher layers (usually the application layer). In this sense, unreliable connections provide best-effort transfers: if the data reaches the target device, great; if it doesn't, leave it to fate! In the TCP/IP stack, the Transmission Control Protocol (TCP) is a reliable transport protocol, while the User Datagram Protocol (UDP) is a transport protocol that uses unreliable connections.For those who are new to the OSI reference model mechanism, it may be difficult to see the dividing line between the session and transport layers. The session layer handles timeouts, notifications, Hello packets to detect connection problems, and so on, while the transport layer provides the transport mechanism for moving data between network components.The transport layer (layer 4) provides both guaranteed and unguaranteed data transfers. Guaranteed transmission is achieved through sequence numbering, acknowledgement, and flow control. Traffic control methods include buffering, windowing techniques, and congestion avoidance. In the TCP/IP protocol stack, the transport layer protocols include TCP and UDP.There are 5 main functions of the transport layer. . Establish, maintain, and remove session connections between components. . Transmit data reliably or unreliably over connections. . Splitting data into manageable data segments. . Multiplexing connections to allow multiple applications on a network device to send and receive data simultaneously.. Use ready/not ready signaling or windowing techniques to control traffic and avoid one party sending more data over the connection than the other party can handlecapacity of the other party. Both of these traffic control methods typically use buffering and are used to avoid congestion. The next few subsections describe these functions in more depth.