Congestion%20Control

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Congestion Control

• Computer Networks

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Where are we?
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Recall

• Data Link Layer
• Link level specific transmission
• Network Layer
• End-to-End host addressing and routing
• Transport Layer
• End-to-End application multiplexing and message
  flow-control
• The expert Sally Floyd http//www.aciri.org/floyd
  /

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Note

• Flow control is a subset of congestion control.
  The former attempts to properly match the rate of
  the sender with that of the network and receiver.
  The later deals with the sustained overload of
  intermediate network elements such as
  internetwork routers.

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Congestion Collapse

• As the network load increases, packet drops and
  thus packet retransmissions increase
• Fragments dropped are especially annoying, the
  remaining fragments get sent, but cannot be used
• As retransmissions increase, less actual work
  gets done

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Some Congestion Fixes

• When congestion increases, slow down!
• Additive Increase, Multiplicative Decrease is
  used in TCP
• Setup reservations or service classes
• Packets failing to adhere to their class or
  reservation are simply discarded or put onto a
  low priority queue/link
• Discover end-to-end MTU if fragments are getting
  dropped

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Fairness

• Equal share bandwidth to end stations
• Fair share based on application
• Fair share based on timeliness of data
• Fair share based on value of data
• Fair share based on price paid
• ...and so on

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Active Congestion Control Mechanisms

• Eligible discard
• Queue management
• Network Signaling and Notification
• End station avoidance
• Class of service signaling
• Quality of service reservations

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Eligible Discard

• Frames, cells or packets are marked according to
  a drop priority
• Source or edge intermediate device may mark based
  on some policy
• watermark/threshold reached
• data type
• source
• destination
• cost
• Usually implemented at data link or network layer

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Eligible Discard Illustrated
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Queue Management

• First in, first dropped (FIFO)
• Tail drop (LIFO)
• Leaky bucket
• Token bucket
• Random early detection (RED)
• Weighted Fair Queueing
• Usually implemented in intermediate devices such
  as routers and switches

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First In, First Out Illustrated

• Queue pointers need to be updated
• Sender learns of drop sooner

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Last In, First Out Illustrated

• Simple - no queue pointers to update
• Source cannot react as quick

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Leaky Bucket Illustrated

• From Tanenbaum Figure 5-24, graphic will print to
  a Postscript printer

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Token Bucket Illustrated

• From Tanenbaum Figure 5-26, graphic will print to
  a Postscript printer

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RED Illustrated

• Probability marking applied to each packet based
  on queue length, packet being dropped

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Weighted Fair Queueing
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Network Signaling and Notification

• Also called choke packets
• In Frame Relay
• Forward Explicit Congestion Notification (FECN)
• Backward Explicit Congestion Notification (BECN)
• Bit in frame set
• Experimental Internet mechanism
• Explicit Congestion Notification (ECN)
• Bits set in packets to hosts

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End Station Avoidance

• Also called end-to-end control
• TCP
• Slow start
• Congestion avoidance
• Fast Retransmit
• Fast Recovery

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Class of Service Signaling

• Packets marked to a particular traffic class
• IEEE 802.1p
• Differentiated Services (DiffServ)
• Re-defines IP Type of Service (ToS) bit fields
• Asynchronous Transfer Mode

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Quality of Service Reservations

• Resource ReSerVation Protocol
• Reserve resources in routers
• Requires stateful path
• Asynchronous Transfer Protocol (ATM)