qr code c# free Layers of the OSI Reference Model in Objective-C

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Layers of the OSI Reference Model
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Window sizes affect efficiency
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As you can see, with a window size of 1, the flow control process is not very quick or efficient Let s look at an example with a window size of 3, as illustrated at the bottom of Figure 2-2 With a window size of 3, the source can send three segments at once before waiting for an ACK Once segments are sent (each with its own unique sequence number: 1, 2, and 3), the source must wait for an acknowledgment In this instance, the destination sends an ACK back with the number 4 in it, indicating that the fourth segment is expected next The source can then proceed to send segments 4, 5, and 6, and then wait for the destination s acknowledgment In this case, having a larger window size is more efficient: only one acknowledgment is required for every three segments that are sent Therefore, the larger the window size, the more efficient the transfer of information becomes However, this is not always the case For example, let s assume that one segment gets lost on its way to the destination, as is shown in Figure 2-3 In this example, the window size negotiated is 3 PC-A sends its first three segments, which are successfully received by PC-B PC-B acknowledges the next segment it expects, which is 4 When PC-A receives this acknowledgment, it sends segments 4, 5, and 6 For some reason, segment 4 becomes lost and never reaches the destination, but segments 5 and 6 do arrive Remember that the destination is keeping track of what was received: 1, 2, 3, 5, and 6 In this example, the destination sends back an ACK of 4, indicating that segment 4 is expected next
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Lost segments and retransmissions
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At this point, how PC-A reacts depends on the transport layer protocol that is used Here are some possible options:
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PC-A understands that only segment 4 was lost and therefore resends
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segment 4 It then sends segments 7 and 8, filling up the window size
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PC-A doesn t understand what was or wasn t received, so it sends three
segments starting at segment 4, indicated by PC-B Of course, if two segments are lost, the first option listed won t work unless the destination can send a list of lost segments Therefore, most protocol stacks that use windowing will implement the second option Given this behavior, the size of the window can affect your performance You would normally think that a window size of 100 would be very efficient; however, if the very first segment is lost, some protocols will have all 100 segments resent! As mentioned earlier, most protocol stacks use a window size that is negotiated up front and can be renegotiated at any time Therefore, if a connection is experiencing a high number of errors, the window size can be dropped to a smaller value to increase efficiency And once these errors disappear or drop down to a lower rate, the window size can be increased to maximize the connection s throughput What makes this situation even more complicated is that the window sizes on the source and destination devices can be different For instance, PC-A might have
Layers of the OSI Reference Model
a window size of 3, while PC-B has a window size of 10 In this example, PC-A is allowed to send 10 segments to PC-B before waiting for an acknowledgment, while PC-B is allowed to send only 3 segments to PC-A Flow control through the use of sequence numbers and acknowledgments is covered in more depth in 9, where TCP is discussed
Ready/not ready signals and windowing are used to implement ow control Ready/not ready signals are not ef cient, causing drops of unnecessary traf c and delays in the transmission of
traf c Windowing addresses these issues With windowing, a window size is established, which de nes the number of segments that can be transferred before waiting for an acknowledgment from the destination
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