[Iccrg] Congestion control definition and requirements of a new
mascolo at poliba.it
Mon Feb 27 18:52:48 GMT 2006
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?
On 2/24/06, S. Keshav <keshav at uwaterloo.ca> wrote:
Its been quiet on this list for a while. Can I take it as consensus on
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
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
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
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.
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