All language subtitles for 6. TCP Acknowledgement and Windowing

In its basic form, TCP may have a window size of 1
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which means that for every segment transmitted by the sender
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the receiver sends an acknowledgement for that individual segment.
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This however slows down the throughput dramatically
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because the sender cannot transmit anymore data
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until it to receives acknowledgement of that single segment transmitted.
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The throughput would be very low
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depending on the round trip timer
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between sending data and receiving the acknowledgement.
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TCP however thus allow for greater window sizes
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to allow for more segments
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to be transmitted before receiving an acknowledgement.
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The window is the number of data segments
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the sender is allowed to send
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without getting an acknowledgement from the receiver.
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In this case we’ve set it to 1
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that means when host A wants to send traffic to host B
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it can send 1 segment because the window size is set to 1
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host B, once it receives that segment, sends an acknowledgement.
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In this example, let’s assume that host A, send the segment with the sequence number of 1
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host B would acknowledge for segment 2.
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Host A will then send segment 2
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and host B, once successfully receiving that segment
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will acknowledge or ACK for segment 3.
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Host A will then send segment 3 to host B.
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This process will then continue for the duration of the session
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this is obviously very reliable however, the throughput is very low.
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A would need to buffer outgoing segments
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until it receive an acknowledgement for the segment transmitted.
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now for argument sake
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let’s assume that it takes 1 second for traffic
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with the start with the small window size
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and then exponentially increase the window size to gauge
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the amount of data that the receiver can receive and what the network can handle.
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You’ve probably notice this, when downloading a file from the internet
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initially the download speed is slow
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but then increases to a certain point over time.
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This is because the window size initially are small
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but then increases exponentially until a packet is dropped
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or the receiving host cannot handle the amount of data it's receiving.
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So you’ll notice initially, that the download speed is very slow
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increases very quickly and then gets to a point
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and then stays around that speed.
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So once again, let’s assume
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that the hosts in this example have a fixed window size of 3.
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That means that A can send 3 segments before receiving an acknowledgement.
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So in this case host A sends segment 1,2 and 3
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host B acknowledges for segment 4
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thus letting A know that it successfully received segment 1,2 and 3
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host A then sends segment 4,5 and 6 because it has a fix window size of 3
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and host B acknowledges for segment 7
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thus letting A know, that it had received segments 4,5 and 6.
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Remember with acknowledgements, acknowledge the next segment that you want to received
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not the segment that you have already received.
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So host B does not acknowledge for segment 6
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but acknowledges segment 7.
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As I've mentioned, a sliding window allows hosts to gauge
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the amount of data the receiver can receive and what the network can handle.
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So in this example let’s assume that the host are using a sliding window
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the way that they determine what the network can handle
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is when a packet is dropped by the network, the host will slow down.
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This information is covered in a lot more detail
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in courses that contain quality of service.
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With this course just assume that when a packet gets drop
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they reduced their window size dramatically
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in brief, the window size is either the window granted to the sender by the receiver
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or a calculated window called the congestion window or CWND
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the congestion window or CWND is initially set to very low value
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at connection establishment and then increases at an exponential rate.
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For every lost segment, the congestion window is half
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after lost segment has been successfully re-transmitted
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the congestion window grows again until it reaches
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a value half of the original congestion window
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it then slows down its growth, using an algorithm called congestion avoidance.
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It does exponentially grows up to half the original congestion window size
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and then increases slowly at the linear rate.
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In quality of service Weighted Random Early Detection or WRED
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can be used improve efficiency of TCP transmissions across the link
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as packets are randomly dropped from various flows or various sessions
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going across an individual interface
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rather than packets from multiple senders being drop at the same time.
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this avoid an issue called global synchronization
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where packets from multiple TCP sessions are drop at the same time
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and therefore multiple host reduce their window size and slowdown at the same time
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and then gradually increase their window size
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and therefore their throughput at the same time.
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So you have a many hosts slowing down and speeding up at the same time
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with WRED some hosts will be slowing down
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and other hosts will be increasing their throughput because or random drops.
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Please refer to quality of service documentation for more detail about WRED
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In this example let’s assume that we start with the initial window size of 3.
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So A transmits 3 segments to B, however only segment 1 and 2 arrive at host B
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segment 3 goes missing.
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Host B may reduce its window size in this example to 2 and acknowledge segment 3.
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Remember the acknowledgement is for the next packet that that hosts expects to receive
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and because host B did not receive segment 3
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it’s acknowledging successful receipt of segment 1 and 2.
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Host A will re-transmit segment 3 and in this example send segment 4
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but also request a window size of 3.
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Both those segment are successfully received by host B
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so host B will acknowledge for segment 5
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but in this example, still wants a window size of 2.
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Host A will only send 2 segments because the negotiated window size is 2
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but A may still request that the window size be increase to 3.
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With the sliding window, there is dynamic negotiation of a window size
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and that window size may change dramatically during a session
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depending on what the receiver can process and what the network can handle.