student_compiled_cs6250_udacity_transcripts.pdf

Tcp congestion control in tcp congestion control the

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TCP Congestion Control In TCP congestion control, the senders continue to increase their rate until they see packet drops in the network. Packet drops occur because the senders are sending at a rate that is faster than the rate at which a particular router in the network might be able to drain its buffer. So you might imagine, for example, that if all three of these senders are sending at a rate that is equal to the rate at which the router is able to send traffic downstream, then eventually this buffer will fill up. TCP interprets packet loss as congestion. And when senders see packet loss, they slow down as a result of seeing the packet loss. This is an assumption. Packet drops are not a sign of congestion in all networks. For example, in wireless networks, there may be packet loss due to corrupted packets as a result of interference. But in many cases, packet drops do result because some router in the network has a buffer that has filled up and can no longer hold anymore packets and hence it drops the packets as they arrive. So senders increase rates until packets are dropped, periodically probing the network to check whether more bandwidth has become available; then they see packet loss, interpret that as congestion, and slow down. So, congestion control has two parts. One is an increase algorithm, and the other is a decrease algorithm. In the increase algorithm, the sender must test the network to determine whether the network can sustain a higher sending rate. In the decrease algorithm, the senders react to congestion to achieve optimal loss rates, delays in sending rates. Let's now talk about how senders can achieve these increase and decrease algorithms.
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Two Approaches to Adjusting Rate One approach is a window based algorithm. In this approach, a sender can only have a certain number of packets outstanding, or quote, in flight. And the sender uses acknowledgements from the receiver to clock the retransmission of new data. So let's suppose that the sender's window was four packets. At this point, there are four packets outstanding in the network. And the sender cannot send additional packets until it has received an acknowledgement from the receiver. When it receives an acknowledgment, or an ACK from the receiver, the sender can then send another packet. So at this point there are still four outstanding or four unacknowledged packets in flight. In this case if a sender wants to increase the rate at which it's sending, it simply needs to increase the window size. So, for example, if the sender wants to send at a faster rate, it can increase the window size from four, to five. A sender might increase its rate anytime it sees an acknowledgement from the receiver. In TCP, every time a sender receives an acknowledgement, it increases the window size.
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  • Fall '08
  • Staff
  • IP address, Transmission Control Protocol

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