[Iccrg] Congestion control definition and requirements of a new protocol

[Saverio Mascolo]

hi,

you say: " 1. In a high bandwidth-delay product environment, high throughputs can only
be achieved if the packet loss rate is unrealistically low."

does anyone know what it is a realistic packet loss rate  not due to congestion in gigabit nets?

Saverio



On 2/24/06, S. Keshav <keshav at uwaterloo.ca> wrote: 
  Folks,
     Its been quiet on this list for a while. Can I take it as consensus on
  the following:

  A. Definition of Congestion:

  Network congestion is a state of degraded performance from the perspective
  of a particular user. A network is said to be congested from the perspective
  of a user if that user's utility has decreased due to an increase in network
  load.

  B. Problems with TCP congestion control:

  1. In a high bandwidth-delay product environment, high throughputs can only
  be achieved if the packet loss rate is unrealistically low.



  2. TCP has low throughput under lossy environments because it uses loss as
  an indication of congestion.

  3. Due to additive increase, it takes a long time for a flow to ramp up to
  transient increases in available capacity which results in unnecessarily
  long flow-completion times.

  4. Even when the flow is capable of completing within a round-trip time,
  slow-start makes flows last multiple round-trip times just to find their
  fair share rate. Many flows complete before they exit slow-start phase. 

  5. TCP fills up all available buffers at the bottleneck links, which results
  in long latency.

  6. TCP  shares bandwidth inversely proportional to flow RTTs

  7. TCP builds a standing queue at the point of congestion, which increases
  the delay.
  -------

  If we agree on this, (and if you do not, this is the time to speak up) I
  would like to propose that we discuss the requirements of a new congestion
  control protocol, both theoretically and practically.

  To start off this debate, I would like to state the following top level
  requirements.

  First, given the definition of congestion, I argue that the proposed
  protocol should allow two things: decoupling and observability.

     0 Decoupling means that the traffic from one user should not affect (or
  minimally affect) other users.
     0 Observability means that users should be able to observe the network
  state in some fashion, so that they can control their input so as to not
  cause overload and a consequent decrease in utility.

  Second, in addition to these, the new mechanism should also not suffer from
  the seven problems with TCP.

  Comments?

  thanks

  keshav



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