Congestion Control Algorithms

1
Congestion Control Algorithms

• General Principles of Congestion Control
• Congestion Prevention Policies
• Congestion Control in Virtual-Circuit Subnets
• Congestion Control in Datagram Subnets
• Load Shedding
• Jitter Control

2
Congestion

• When too much traffic is offered, congestion sets
  in and performance degrades sharply.
• Factors
• Insufficient memory
• Slow processors
• Low bandwidth.
• Congestion Control vs Flow Control
• Congestion control has to do with making sure the
  subnet is able to carry the offered traffic.
• Flow control relates to the point-to-point
  traffic between a given sender and a given
  receiver.

3
General Principles of Congestion Control

• Open loop solution
• Attempt to solve the problem by good design. It
  makes decisions without regard to the current
  state of network.
• Closed loop solution based on the concept of a
  feedback loop.
• Monitor the system .
• detect when and where congestion occurs.
• Pass information to where action can be taken.
• Adjust system operation to correct the problem.

4
Congestion Control in Virtual-Circuit Subnets

• Admission control technique once congestion has
  been signalled, no more virtual circuits are
  setup until the problem has gone away.
• Allow new virtual circuits but carefully route
  all new virtual circuits around problem areas.
• Negotiating an agreement between the host and
  subnet when a virtual circuit is set up. The
  agreement specifies the volume the shape of the
  traffic, Qos etc. To keep part of the agreement,
  the subnet, the subnet will typically reserve
  resources along the path when the circuit is set
  up.

5
Congestion Prevention Policies

• Policies that affect congestion.

6
Congestion Control in Virtual-Circuit Subnets

• (a) A congested subnet. (b) A redrawn subnet,
  eliminates congestion and a virtual circuit from
  A to B.

7
Congestion control in Datagram Subnets

• Theory
• Each line has a real variable, u, whose value
  between 0.0 and 1.0 reflects the recent
  utilization of that time.
• To maintain a good estimate of u, a sample of the
  instantaneous line utilization, f (0, or 1), can
  be made periodically and u updated according to
• u2au1(1-a)f
• When u moves above the threshold the output line
  enters a warning state

8
Congestion control in Datagram Subnets

• The Warning Bit
• The warning state is set as a special bit in the
  packets header.
• As long as the router was in the warning state,
  it continued to set the warning bit, which meant
  that the source continued to get acknowledgements
  with it set.
• As long as the warning bits continued to flow in,
  the source continued to decrease its transmission
  rate.
• Choke Packets
• In the warning state, the router sends a choke
  packet back to the source host.
• When the source host gets the choke packet. It
  reduce the traffic by x. It ignore choke packets
  referring to the destination for a fixed time
  interval, and check again if there is still the
  choke packets sent back.

9
Hop-by-Hop Choke Packets

• A choke packet that affects only the source.
• A choke packet that affects each hop it passes
  through. More buffer request for router F at this
  moment.
• The net effect of this hop-by hop scheme is to
  provide quick relief at the point of congestion
  at the price of using up more buffers upstream.

10
Jitter Control

• High jitter. (b) Low jitter.
• The variation (i.e., standard deviation) in the
  packet arrival times is called jitter.
• What a packet arrives at a router, the router
  checks to see how much the packet is behind or
  ahead of its schedule. If it is behind schedule,
  the router tries to get it out the door quickly
  and if it is ahead of schedule, it is held just
  long enough to get it back on schedule.