Curs PC si internet cap 12.1 The Transport Layer

Overview As you know, a router has the ability to make intelligent decisions regarding the best path for delivery of data over a network. This is based on a Layer 3 or network layer addressing scheme. The router uses this information to make forwarding decisions. Once data packets go through the network layer, the transport layer, Layer 4, assumes that it can use the network as a "cloud" to send data packets from the source to the destination. The cloud resolves issues such as "Which of several paths is best for a given route?" In this chapter, you will start to see the role that routers perform in this process. In addition, you will learn how the transport layer regulates the flow of information from source to destination reliably and accurately. This chapter explains the primary functions that occur at the transport layer. This includes end-to-end control provided by sliding windows and the reliability in sequencing numbers and acknowledgements. In addition, this chapter describes how the transport-layer data stream is a logical connection between the endpoints of a network. Keeping this in mind, you will learn how the transport-layer data stream provides transport services from the host to the destination, often referred to as end-to-end services. In addition, you will learn about TCP and UDP and how they use port numbers to keep track of different conversations that cross the network at the same time, to pass information to the upper layers. 12.1 The Transport Layer 12.1.1 Purpose of the transport layer Instructor Note The purpose of this target indicator is to start to justify the need for layer 4. Layer 1 allows bit streams to be created and to travel; layer 2 packages those data packets into frames to be converted to bit streams and makes LAN delivery possible; layer three packages data from upper layers in packets and makes routing and WAN delivery possible. But we have made no provision for assuring our data reliably travels end-to-end across the often vast network path. Layer 4 performs multiple functions to provide this "quality of service." Another purpose of this target indicator to is assist the student in visualizing one of the somewhat abstract but absolutely crucial functions of Layer 4 -- flow control. You can have the students act this out, with one student speaking very quickly and the other student trying to keep up, using their native or second languages. The phrase "quality of service" is often used to describe the purpose of Layer 4 - the transport layer. Its primary duties are to transport and regulate the flow of information from source to destination, reliably and accurately. The end-to-end control, provided by sliding windows, and reliability in sequencing numbers and acknowledgements are primary duties of Layer 4. To understand reliability and flow control, think of a student who studies a foreign language for one year. Now imagine he/she visits the country where the language is used. In conversation he/she must ask everyone to repeat their words (for reliability) and to speak slowly, so he/she can catch the words (flow control). 12.1 The Transport Layer 12.1.2 Layer 4 protocols Instructor Note The Layer 4 protocol data unit (PDU), the segment, is introduced. Two particularly important Layer 4 protocols -- Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are introduced and briefly described. The emphasis of this curriculum is on TCP/IP Ethernet networks. The TCP/IP protocol of the OSI model Layer 4 (transport layer) has two protocols - TCP and UDP.TCP supplies a virtual circuit between end-user applications. These are its characteristics: connection-oriented reliable divides outgoing messages into segments reassembles messages at the destination station re-sends anything not received reassembles messages from incoming segments. UDP transports data unreliably between hosts. Following are the characteristics of UDP: connectionless unreliable transmit messages (called user datagrams) provides no software checking for message delivery (unreliable) does not reassemble incoming messages uses no acknowledgements provides no flow control 12.1 The Transport Layer 12.1.3 Comparing TCP and IP Instructor Note The curriculum explicitly refers to and students may have heard from others about TCP/IP. So in many students' minds they are correctly related. But they are not the same -- most obviously TCP is a layer 4 protocol and IP is a layer 3 protocol. Less obvious is that TCP is connection-oriented and ensures reliability and IP is connection-less with best effort attempts at delivery. TCP/IP is a combination of two individual protocols - TCP and IP. IP is a Layer 3 protocol - a connectionless service that provides best-effort delivery across a network. TCP is a Layer 4 protocol - a connection-oriented service that provides flow control as well as reliability. Pairing the protocols enables them to provide a wider range of services. Together, they represent the entire suite. TCP/IP is the Layer 3 and Layer 4 protocol on which the Internet is based. Lab Activity In this lab, you will use Protocol Inspector (or equivalent) software to view dynamic TCP operations.