lect-15 - Transport Protocols Congestion Control ECE/CS 438...

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ECE/CS 438 1 Transport Protocols – Congestion Control ECE/CS 438 Fall 2011 Slides mostly from Peterson & Davies
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TCP State Diagram
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Congestion Control and Resource Allocation n When too many packets are contending for the same link ¡ The queue overflows ¡ Packets get dropped n Network is congested! n Network should provide a congestion control mechanism to deal with such a situation
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Congestion Control and Resource Allocation n Congestion control and Resource Allocation ¡ Two sides of the same coin n If the network takes active role in allocating resources ¡ The congestion may be avoided ¡ No need for congestion control
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Issues in Resource Allocation n Network Model ¡ Packet Switched Network A potential bottleneck router.
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TCP Congestion Control n TCP congestion control was introduced into the Internet in the late 1980s by Van Jacobson, roughly eight years after the TCP/IP protocol stack had become operational. n Immediately preceding this time, the Internet was suffering from congestion collapse— ¡ hosts would send their packets into the Internet as fast as the advertised window would allow, congestion would occur at some router (causing packets to be dropped), and the hosts would time out and retransmit their packets, resulting in even more congestion
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TCP Congestion Control n The idea of TCP congestion control is for each source to determine how much capacity is available in the network, so that it knows how many packets it can safely have in transit. ¡ Once a given source has this many packets in transit, it uses the arrival of an ACK as a signal that one of its packets has left the network, and that it is therefore safe to insert a new packet into the network without adding to the level of congestion. ¡ By using ACKs to pace the transmission of packets, TCP is said to be self-clocking .
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TCP Congestion Control n Additive Increase - Multiplicative Decrease ¡ TCP maintains a new state variable for each connection, called CongestionWindow , which is used by the source to limit how much data it is allowed to have in transit at a given time. ¡ The congestion window is congestion control s counterpart to flow control s advertised window. ¡ TCP is modified such that the maximum number of bytes of unacknowledged data allowed is now: min ( CongestionWindow , AdvertizedWindow )
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TCP Congestion Control n Additive Increase - Multiplicative Decrease ¡ TCP s effective window is revised as follows: n MaxWindow = MIN(CongestionWindow, AdvertisedWindow) n EffectiveWindow = MaxWindow (LastByteSent LastByteAcked). ¡ That is, MaxWindow replaces AdvertisedWindow in the calculation of EffectiveWindow. ¡ Thus, a TCP source is allowed to send no faster than the slowest component —the network or the destination host— can accommodate .
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TCP Congestion Control n Additive Increase Multiplicative Decrease ¡ The problem, of course, is how TCP comes to learn an appropriate value for CongestionWindow. ¡ Unlike the AdvertisedWindow, which is sent by the receiving side of
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This note was uploaded on 02/08/2012 for the course ECE 438 taught by Professor Luo during the Fall '08 term at University of Illinois, Urbana Champaign.

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lect-15 - Transport Protocols Congestion Control ECE/CS 438...

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