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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Transport Layer Protocols
           9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
                9  TCP/IP Transmission Control Protocol (TCP)
                     9  TCP Fundamentals and General Operation

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TCP Data Handling and Processing: Streams, Segments and Sequence Numbers
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TCP Sliding Window Acknowledgment System For Data Transport, Reliability and Flow Control
(Page 2 of 9)

The Problem With Unreliable Protocols: Lack of Feedback

A simple “send and forget” protocol like IP is unreliable and includes no flow control for one main reason: it is an open loop system where the transmitter receives no feedback from the recipient. (I'm ignoring error reports using ICMP and the like for the purpose of this discussion.) A datagram is sent and it may or may not get there, but the transmitter will never have any way of knowing because no mechanism for feedback exists. This is shown conceptually in Figure 203.


Figure 203: Operation Of An Unreliable Protocol

In a system such as that used by IP, if it gets there, great; otherwise, nobody will have a clue. J Some external mechanism is needed to take care of the lost message, unless the protocol doesn’t really care whether a few bits and pieces are missing from its message stream.

 


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