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Signals used to control congestion in networks

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• Motivation
  • Goal:

    Transmit a (large) file from point A to point B as FAST as possible

  • It is a mind-boggling unintuitive fact that the quickest way to to transmit data is not always transmitting the data as fast as you can....

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  • It may help to understand this unintuitive fact by observing traffic on the highway:
    • traffic on the highway will actually move faster if the traffic load is high enough but not too high.

    • When the traffic load is higher than what the highway can handle, a congestion will build at some bottleneck point (usually an on or off ramp) and causes traffic on the whole highway to slow down to a trickle....

    Key: It is beneficial to a flow to restrict its own transmission rate if the current transmission rate is more than the network can handle.

    This "sacrificial act" of restricting one's own transmission rate is known as congestion control

  • On the other hand, if the network's capability can handle more traffic, then it is "natural" than one would increase one's transmission rate.

    Congestion control mechanism also include the ability to increase one's transmission rate (although it's kinda unintuitive to call this action "congestion control"... since it would initiate congestion :-))

  • Congestion control is one of a few application of communism that will actually work.... because it benefit oneself if one will do it (as long as everyone else will do it also).

    (Communistic principles fail in general because it did not take into account the "self-beneficial" motives of humanoids... ordinary humans would only do something if it benefits oneself... Marx assumed that that everyone is a saint :-)...)

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• Congestion control
  • The basic principles of congestion control is simple:

    Decrease your transmission rate if you detect congestion Increase your transmission rate if you detect no congestion

  • One (actually two) question remains:

    How fast should you decrease your transmission rate if you detect congestion How fast should you increase your transmission rate if you detect no congestion

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• Designing congestion control methods
  • You need to solve 2 problems when you are designing a congestion avoidance or control methods:

    How to detect an onset or presence of congestion How fast do you: decrease transmission rate when you do detect congestion... and increase transmission rate when no congestion is detected

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• Detecting Congestion

  • Network congestion is detected by using one or more "signals" from the network...

  • Type of signals used to detect congestion:
    1. Explicit signals:

       An explicit signal is a value that is sent inside a packet Example: In the DECBit method, the packet header contains a congestion bit When the packet is sent by the sender, this bit is reset When the packet passes through a congested router, (the queue length exceeds a certain threshold) the router will set the congestion bit NOTE: Explicit signals requires the assistence of routers and therefore, it is NOT useful practically because present day Internet routers do not flip bits in packets...

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    2. Implicit signals:

       An implicit signal is a value that is derived (concluded) from events and it is not convey through packets Examples: Packet loss Congestion is detected when the sender detects that some packet was lossed Packet loss can be detected through timeout: Or by duplicate ACKs: Packet delay The sender measures the time for the ACK of packets to return back. When the time exceeds a certain threshold, the source assumes congestion.

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  • Due to the fact that routers in the Internet do NOT provide explicit signals, we will only consider congestion control using implicit signals

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• Reliability of the Packet Loss Signal

  • When a packet is lossed, it's a sure sign that the network path is congested

  • However, packet loss is the worst possible feedback signal that you can use !!!

  • The purpose of congestion avoidance is to avoid congestion

  • The loss of a packet is in fact an indication that the network is already congested - some buffer was full at some router

    What we want to achieve is to avoid the buffer being filled up...

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  • It is somewhat akin to a medical situation where the patient is having a heart attack:
    • What we really want is to prevent the heart attack

    • What we are really doing is: fix the patient when the heart attack is happening...

  • NOTE:
    Because "it was too late", the congestion control measure taken by the sender when packet loss is detected will have to be VERY DRASTIC (discussed later)

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• Reliability of the End-to-end Delay Signal

  • The End-to-end delay is a far better indication than packet loss:

    By measuring the end-to-end delay, the source can detect an impeding collapse and take measure to reduce the traffic before the collapse happens:

  • But the End-to-end delay signal is also harder to use
    • It is much more difficult to determine if the buffer of a router is about to overflow by observing the end-to-end delay...

      Specifically:

      you cannot accurately tell from the increase in the delay how bad the situation is.

      Example:

      1. When the drain rate is high (1 Gbps), a small increase in delay can correspond to a buffer overflow:

         • The additional buffer space occupied is 5 Mbits

         • The additional delay inncured is only 0.005 sec

      2. Now consider a lower drain rate:

         • The additional buffer space occupied is ONLY 2 Mbits

         • But the additional delay inncured is 0.02 sec

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    Conclusion:

    • Absolute increase in delay is not a good indicator

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  • A study on delayed based congestion control methods can be found in:
    • Raj Jain (ofcourse - he is the foremost expert in congestion research) paper: click here

  • TCP Vegas uses delay as congestion indication