Failed acknowledgments alert the sender to slow
down or stop sending. The receiver can also lower the ?¬‚ow to slow the sender down.
?– Packet recovery services: The receiver can request retransmission of a packet. If
packet receipt is not acknowledged, the sender resends the packets.
TCP is a reliable transport layer protocol. Reliable data delivery services are critical for
applications such as ?¬?le transfers, database services, transaction processing, and other
mission-critical applications in which delivery of every packet must be guaranteed.
An analogy to TCP protocol services would be sending certi?¬?ed mail through the postal
service. For example, someone who lives in Lexington, Kentucky, wants to send this book
to a friend in New York City, New York, but for some reason, the postal service handles only
letters. The sender could rip the pages out and put each one in a separate envelope. To
ensure the receiver reassembles the book correctly, the sender numbers each envelope.
Then, the sender addresses the envelopes and sends the ?¬?rst envelope certi?¬?ed mail. The
66 Chapter 1: Building a Simple Network
postal service delivers the ?¬?rst envelope by any truck and any route. Upon delivery of that
envelope, the carrier must get a signature from the receiver and return that certi?¬?cate of
delivery to the sender.
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