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2.1 PROTOCOLS > 2.1.1 Transport and Routing Protocols - Pg. 17

2.1 Protocols 17 Implementors familiar with operating systems should skip Section 2.4, except for Example 8 (receiver livelock as an example of how operating system structure influences protocol implementations). Even those unfamiliar with an area such as operating systems may wish to consult these sections if needed after reading the specific chapters that follow. 2.1 PROTOCOLS Section 2.1.1 describes the transport protocol TCP and the IP routing protocol. These two examples are used to provide an abstract model of a protocol and its functions in Section 2.1.2. Section 2.1.3 ends with common network performance assumptions. Readers familiar with TCP/IP may wish to skip to Section 2.1.2. 2.1.1 Transport and Routing Protocols Applications subcontract the job of reliable delivery to a transport protocol such as the Trans- mission Control Protocol (TCP). TCP's job is to provide the sending and receiving applications with the illusion of two shared data queues in each direction -- despite the fact that the sender and receiver machines are separated by a lossy network. Thus whatever the sender application writes to its local TCP send queue should magically appear in the same order at the local TCP receive queue at the receiver, and vice versa. TCP implements this mechanism by breaking the queued application data into segments and retransmitting each segment until an acknowl- edgment (ack) has been received. A more detailed description of TCP operation can be found in Section A.1.1. If the application is (say) a videoconferencing application that does not want reliability guarantees, it can choose to use a protocol called UDP (User Datagram Protocol) instead of