Network interface

FIGURE 2.10 TCP/IP network architecture

FIGURE 2.10 TCP/IP network architecture

The application layer programs are intended to run directly over the transport layer. Two basic types of services are offered in the transport layer. The first service consists of reliable connection-oriented transfer of a byte stream, which is provided by the Transmission Control Protocol (TCP). The second service consists of best-effort connectionless transfer of individual messages, which is provided by the User Datagram Protocol (UDP). This service provides no mechanisms for error recovery or flow control. UDP is used for applications that require quick but necessarily reliable delivery.

The TCP/IP model does not require strict layering, as shown in Figure 2.10b. In other words, the application layer has the option of bypassing intermediate layers. For example, an application layer may run dirctly over the internet layer.

The internet layer handles the transfer of information across multiple networks through the use of gateways or routers, as shown in Figure 2.11. The internet layer corresponds to the part of the OSI network layer that is concerned with the transfer of packets between machines that are connected to different networks. It must therefore deal with the routing of packets across these networks as well as with the control of congestion. A key aspect of the internet layer is the definition of globally unique addresses for machines that are attached to the

Machine A

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