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Ebook Computer networks - A systems approach (4th edition): Part 2

(BQ) Part 2 book "Computer networks - A systems approach" has contents: Reliable byte stream, simple demultiplexer, performance, queuing disciplines, congestion avoidance mechanisms, data compression, cryptographic tools, secure systems,.and other contents. | End-to-End Protocols Victory is the beautiful, bright coloured flower. Transport is the stem without which it could never have blossomed. —Winston Churchill T he previous three chapters have described various technologies that can be used to connect together a collection of computers: direct links (including LAN technologies like Ethernet and token ring), packet-switched networks (including cell-based networks like ATM), and internetworks. The next problem is to turn this host-to-host packet delivery service into a process-to-process communication channel. This is the role played P R O B L E M by the transport level of the network architecture, which, because it supGetting Processes to ports communication between the Communicate end application programs, is sometimes called the end-to-end protocol. Two forces shape the end-to-end protocol. From above, the application-level processes that use its services have certain requirements. The following list itemizes some of the common properties that a transport protocol can be expected to provide: ■ ■ Delivers messages in the same order they are sent. ■ Delivers at most one copy of each message. ■ Supports arbitrarily large messages. ■ Supports synchronization between the sender and the receiver. ■ Allows the receiver to apply flow control to the sender. ■ 380 Guarantees message delivery. Supports multiple application processes on each host. Note that this list does not include all the functionality that application processes might want from the network. For example, it does not include security, which is typically provided by protocols that sit above the transport level. From below, the underlying network upon which the transport protocol operates has certain limitations in the level of service it can provide. Some of the more typical limitations of the network are that it may ■ Drop messages. ■ Reorder messages. ■ Deliver duplicate copies of a given message. ■ Limit messages to some finite .