CONGESTION CONTROL

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CONGESTION CONTROL
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Flow Control, Congestion control, Quality of
Service

• Flow control
• adjusting the speed between sender and receiver
• Congestion control
• penalty for dynamically shared resources
• Similarities with the highway system
• The influence of routing on the congestion
• Quality of service

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Quality of Service

• Internet
• a heterogeneous network that uses different lower
  layer technologies
• the best effort service
• Service quality - not precisely defined
• Parameters used
• delay, jitter, bandwidth, reliability

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Consequences in the Case of Overload
DELAY
OFFERED LOAD
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Controlled Environment
Ideal
Controlled
EFFECTIVE THROUGHPUT
Uncontrolled
OFFERED LOAD
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PROBLEMS CAUSED BY OVERLOAD

• Loss of efficiency
• Unfair allocation of resources among competing
  users
• Various types of loops and deadlocks

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LOSS OF EFFICIENCY

• The main cause - wastage of resources
• The most commonly wasted are
• communications capacity
• storage capacity
• Some users unnecessary monopolize the buffers at
  some congested node

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EXAMPLE FOR WASTAGE OF BUFFERS
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UNFAIRNESS

• A natural byproduct of uncontrolled competition
• Some users can obtain preferential treatment due
  to
• relative position in the network
• particular selection of network and traffic
  parameters

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EXAMPLE
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Another example
r14 1
1
1
1
1
2
3
4
r34 1
r23 1
r12 1

• When 1, 2, 3, and 4 are fully using the capacity
  of the links 1 to 4 traffic is stopped
• If 1/2 of traffic is allowed to everyone, the
  total traffic is smaller

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Loops and Deadlocks

• Routing loops
• caused by inconsistency in routing tables
• Deadlocks
• Caused by lack of buffers in the nodes
• reassemble deadlocks
• store-and-forward deadlocks

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Performance Goals

• Minimum delay and maximum throughput
• conflict between them
• Power - often used to reflect tradeoff between
  the delay and throughput
• Power throughput /delay
• a gt 1 if throughput is more important (bulk
  traffic)
• a lt1 if delay is more important (interactive
  traffic)
• a 1 if they are equally important

a
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Maximum Power

• Obtained at the knee of the curve
• Below the knee - underutilized
• Over the knee - overutilized

DELAY
Max. power
OFFERED LOAD
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Handling Congestion

• Two categories of handling congestion
• congestion recovery
• reactive in nature
• without congestion recovery, the network may come
  to a zero throughput
• congestion avoidance
• preventive in nature
• even when it is implemented, congestion recovery
  is still required

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Quality of Service in Internet

• Most exploited expression in the last years
• Hard to implement in the best effort
  environment
• Usual response is
• using technology that implements QoS
• ATM, Frame relay
• increase of bandwidth
• alternate physical paths
• Congestion avoidance mechanisms

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Methods proposed

• Source quench
• Drop of packets using various policies
• Queuing discipline
• Slow start
• Slow Start and Search
• DUAL
• TCP Vegas

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Source quench

• The oldest method in Internet used for congestion
  recovery
• ICMP message
• when a router drops the packets from the queue,
  it sends the ICMP message to the source
• the message is sent with a limited frequency to
  conserve CPU resources during the time of heavy
  load

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Source Quench Drawbacks

• End system is not informed about the reasons of
  ICMP message
• the last packet in the queue is drooped
• not always indication that that particular system
  is cause of congestion

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Random Drop

• Proposed as an improvement to packet dropping
• Packet to be dropped is selected randomly
• users packet with demand is with low
  probability of drop
• users with high demand should be asked to throttle

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Other Proposals for Packet Drop

• Type of service bit should be considered
• the higher the precedence, the lower the
  probability of drop
• has to be implemented on every router in the path
  - sometimes impossible, having in mind different
  paths packet can take

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Queue Discipline

• FIFO - usual discipline
• Priority queues
• form several queues with different priorities,
  the one with highest priority is first
• Weighted round-robin
• form different queues, but serve all of them with
  weighted preference
• Weighted fair queuing

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Weighted Fair Queuing

• Calculates time of each queued packet
• Packets with smallest finish time is transmitted
  first
• Pretty fair to everyone
• Complicated implementation

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TCPs Slow Start Mechanism

• A window flow-control mechanism
• At initial startup phase - one segment is
  transmitted
• Upon receiving an ACK two segments are
  transmitted for every ACK received
• Upon packet loss, the window is drooped to one
  half and after that it is increased one segment
  after every ACK

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TCP Flow Behavior
Packet drop level
Network buffer capacity
Available bandwidth
TCP flow rate
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Effectiveness of Slow Start

• For four TCP conversations going between eight
  computers on 10Mbps LAN and 230Kbps link over
  Internet 200 increase increase in effective
  bandwidth was noticed when Slow start was
  implemented

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Slow Start and Search (Three-S)

• Uses the change in throughput as an indication of
  congestion
• At first the window size is set to one basic
  adjustment unit (BAU) and increased by one packet
  after after each ACK
• When a packet times out window is set to one BAU
  and increased by one BAU

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DUAL

• Similar to Slow start
• It uses the round trip time (RTT) in addition to
  packet loss to detect detection
• Every two RTT DUAL checks if the current RTT is
  greater than the previous and reduces the
  congestion window by 7/8

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TCP Vegas

• Proposes a new implementation of TCP
• Uses a time stamp on each packet sent to
  calculate the RTT for each ACK received
• Can predict with certain probability if packet
  will be lost
• Retransmit the packets before they time out

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Conclusion

• Congestion avoidance and congestion recovery
  should receive attention
• For the quality of service in the Internet there
  are no perfect solutions, at least not for the
  foreseeable future
• It is possible to deliver differentiated levels
  of best effort traffic