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The OSI Reference Model
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called positive acknowledgment with retransmission--a technique that
requires a receiving machine to communicate with the transmitting source by
sending an acknowledgment message back to the sender when it receives
data. The sender documents each segment it sends and waits for this
acknowledgment before sending the next segment. When it sends a segment,
the transmitting machine starts a timer and retransmits if it expires before an
acknowledgment is returned from the receiving end.
In Figure 1.8, the sending machine transmits segments 1, 2, and 3. The
receiving node acknowledges it has received them by requesting segment 4.
When it receives the acknowledgment, the sender then transmits segments 4,
5, and 6. If segment 5 doesn't make it to the destination, the receiving node
acknowledges that event with a request for the segment to be resent. The send-
ing machine will then resend the lost segment and wait for an acknowledgment,
which it must receive in order to move on to the transmission of segment 7.
F I G U R E 1 . 8
Transport layer reliable delivery
The Network Layer
The Network layer (also called layer 3) manages devices addressing, tracks
the location of devices on the network, and determines the best way to move
1
2
3
4
5
6
1
2
3
4
5
6
Send 1
Send 2
Send 3
Ack 4
Send 5
Connection lost!
Send 5
Send 4
Send 6
Ack 5
Ack 7
Sender
Receiver
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