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TCP Sliding Window Acknowledgment System For Data Transport, Reliability and Flow Control
(Page 3 of 9)
Providing Basic Reliability Using Positive Acknowledgment with Retransmission (PAR)
Basic reliability in a protocol running
over an unreliable protocol like IP can be implemented by closing
the loop so the recipient provides feedback to the sender. This
is most easily done with a simple acknowledgment system. Device A
sends a piece of data to Device B. Device B, receiving
the data, sends back an acknowledgment saying, “Device A,
I received your message”. Device A then knows its transmission
was successful.
Of course, since IP is unreliable,
that message may in fact never get to where it is going. Device A
will sit waiting for the acknowledgment and never receive it. Conversely,
it is also possible that Device B gets the message from Device
A, but the acknowledgment itself vanishes somehow.
In either case, we don't want Device A to sit forever waiting
for an acknowledgment that is never going to ever arrive.
To prevent this from happening, Device
A starts a timer when it first sends the message to Device
B, which allows sufficient time for the message to get to B
and the acknowledgment to travel back, plus some reasonable time to
allow for possible delays. If the timer expires before the acknowledgment
is received, A assumes there was a problem and retransmits
its original message. Since this method involves positive acknowledgments
(“yes, I got your message”) and a facility for retransmission
when needed, it is commonly called (ta-da!) positive acknowledgment
with retransmission (PAR), as shown in Figure 204.
Figure 204: Basic Reliability: Positive Acknowledgment With Retransmission (PAR) This diagram shows one of the most common simple techniques for ensuring reliability. Each time a message is sent by Device A, it starts a timer. Device B sends an acknowledgment back to A when it receives a message so A know it was successfully transmitted. If a message is lost, the timer goes off and A retransmits the data. Note that only one message can be outstanding at any time, making this system rather slow.
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 Key Concept: A basic technique for ensuring reliability in communications uses a rule that requires a device to send back an acknowledgment each time it successfully receives a transmission. If a transmission is not acknowledged after a period of time, it is retransmitted by its sender. This system is called positive acknowledgment with retransmission (PAR). One drawback with this basic scheme is that the transmitter cannot send a second message until the first has been acknowledged.
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