ppt of computer netwokrs.cse engineering.part 5

Editor's Notes

• #4: When computers, terminals, and/or other data processing devices exchange data, the procedures involved can be quite complex. eg. file transfer. There must be a data path between the two computers. But also need: Source to activate communications Path or inform network of destination Source must check destination is prepared to receive File transfer application on source must check destination file management system will accept and store file for its user May need file format translation Instead of implementing the complex logic for this as a single module, the task is broken up into subtasks, implemented separately. In a protocol architecture, the modules are arranged in a vertical stack, each layer in the stack performs a related subset of the functions. It relies on the next lower layer to perform more primitive functions. It provides services to the next higher layer. The peer layers communicate using a set of rules or conventions known as a protocol.
• #5: The TCP/IP protocol architecture is a result of protocol research and development conducted on the experimental packet-switched network, ARPANET, funded by the Defense Advanced Research Projects Agency (DARPA), and is generally referred to as the TCP/IP protocol suite. This protocol suite consists of a large collection of protocols that have been issued as Internet standards by the Internet Activities Board (IAB).
• #7: TCP/IP doesn’t have an “official” layer model (& it predates the OSI Reference Model we’ll introduce later), but it does have a “working” layer model, as shown.
• #9: The physical layer covers the physical interface between a data transmission device (e.g., workstation, computer) and a transmission medium or network. This layer is concerned with specifying the characteristics of the transmission medium, the nature of the signals, the data rate, and related matters.
• #10: The network access layer is concerned with the exchange of data between an end system (server, workstation, etc.) and the network to which it is attached. The sending computer must provide the network with the address of the destination computer, so that the network may route the data to the appropriate destination. The sending computer may wish to invoke certain services, such as priority, that might be provided by the network. The specific software used at this layer depends on the type of network to be used; different standards have been developed for circuit switching, packet switching (e.g., frame relay), LANs (e.g., Ethernet), and others. Thus it makes sense to separate those functions having to do with network access into a separate layer.
• #11: The internet layer provides procedures used to allow data to traverse multiple interconnected networks, to provide communications between devices are attached to different networks. The Internet Protocol (IP) is used at this layer to provide the routing function across multiple networks. This protocol is implemented not only in the end systems but also in routers. A router is a processor that connects two networks and whose primary function is to relay data from one network to the other on its route from the source to the destination end system.
• #12: The host-to-host layer, or transport layer, collects mechanisms in a common layer shared by all applications to provide reliable delivery of data. Regardless of the nature of the applications, there is usually a requirement that data be exchanged reliably, ensuring that all of the data arrives at the destination application and that the data arrives in the same order in which they were sent. These mechanisms for providing reliability are essentially independent of the nature of the applications. The Transmission Control Protocol (TCP) is the most commonly used protocol to provide this functionality.
• #14: Finally, the application layer contains the logic needed to support the various user applications. For each different type of application, such as file transfer, a separate module is needed that is peculiar to that application.
• #15: For successful communication, every entity in the overall system must have a unique address. Actually, two levels of addressing are needed. Each host on a subnetwork must have a unique global internet address; this allows the data to be delivered to the proper host. Each process with a host must have an address that is unique within the host; this allows the host-to-host protocol (TCP) to deliver data to the proper process. These latter addresses are known as ports.
• #18: Stallings DCC8e Figure 2.1 indicates how these protocols are configured for communications. To make clear that the total communications facility may consist of multiple networks, the constituent networks are usually referred to as subnetworks. Some sort of network access protocol, such as the Ethernet logic, is used to connect a computer to a subnetwork. This protocol enables the host to send data across the subnetwork to another host or, if the target host is on another subnetwork, to a router that will forward the data. IP is implemented in all of the end systems and the routers. It acts as a relay to move a block of data from one host, through one or more routers, to another host. TCP is implemented only in the end systems; it keeps track of the blocks of data to assure that all are delivered reliably to the appropriate application.
• #20: Consider a simple operation where a process on host A, wishes to send a message to another process on host B. The process at A hands the message down to TCP with instructions to send it to host B. TCP hands the message down to IP with instructions to send it to host B. Note that IP need not be told the identity of the destination port. Next, IP hands the message down to the network access layer (e.g., Ethernet logic) with instructions to send it to router J (the first hop on the way to B). To control this operation, control information as well as user data must be transmitted, as suggested in Stallings DCC8e Figure 2.2. The sending process generates a block of data and passes this to TCP. TCP may break this block into smaller pieces to make it more manageable. To each of these pieces, TCP appends control information known as the TCP header, forming a TCP segment. Next, TCP hands each segment over to IP, with instructions to transmit it to B. These segments must be transmitted across one or more subnetworks and relayed through one or more intermediate routers. This operation, too, requires the use of control information. Thus IP appends a header of control information to each segment to form an IP datagram. Finally, each IP datagram is presented to the network access layer for transmission across the first subnetwork in its journey to the destination. The network access layer appends its own header, creating a packet, or frame. The packet is transmitted across the subnetwork to router J.