The sender mails several envelopes on the same day. The postal service again delivers each
envelope by any truck using any route. The sender returns to the post of?¬?ce each day
sending several envelopes each requiring a return receipt. The receiver signs a separate
receipt for each envelope in the batch as they are received. If one envelope is lost in transit,
the sender would not receive a certi?¬?cate of delivery for that numbered envelope. The
sender might have already sent the pages that follow the missing one, but would still be able
to resend the missing page. After receiving all the envelopes, the receiver puts the pages in
the right order and pastes them back together to make the book. TCP provides these levels
of services.
UDP is another transport layer protocol that was added to the TCP/IP protocol suite. This
transport layer protocol uses a smaller header and does not provide the reliability available
with TCP.
The early IP suite consisted only of TCP and IP, although IP was not differentiated as a
separate service. However, some end user applications needed timeliness rather than
accuracy. In other words, speed was more important than packet recovery. In real-time
voice or video transfers, a few lost packets are tolerable.
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