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11.1 | |
TCP/IP Transport Layer | |
| 11.1.6 | |
Acknowledgment |
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Reliable delivery guarantees that a stream
of data sent from one device is delivered through a data link to
another device without duplication or data loss. Positive
acknowledgment with retransmission is one technique that guarantees
reliable delivery of data. Positive acknowledgment requires a
recipient to communicate with the source and send back an
acknowledgment message when the data is received. The sender keeps a
record of each data packet (TCP segment), that it sends and expects an
acknowledgment. The sender also starts a timer when it sends a segment
and will retransmit a segment if the timer expires before an
acknowledgment arrives.
Figure
TCP provides sequencing of segments
with a forward reference acknowledgment. Each datagram is numbered
before transmission.
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shows the
sender transmitting data packets 1, 2, and 3. The receiver
acknowledges receipt of the packets by requesting packet 4. Upon
receiving the acknowledgment, the sender sends packets 4, 5, and 6. If
packet 5 does not arrive at the destination, the receiver acknowledges
with a request to resend packet 5. The sender resends packet 5 and
then receives an acknowledgment to continue with the transmission of
packet 7. 
At the
receiving station, TCP reassembles the segments into a complete
message. If a sequence number is missing in the series, that segment
is retransmitted. Segments that are not acknowledged within a given
time period will result in a retransmission.