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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 stationsFair share based on applicationFair share based on timeliness of dataFair share based on value of dataFair share based on price paid...and so on
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Active Congestion Control Mechanisms
Eligible discardQueue managementNetwork Signaling and NotificationEnd station avoidanceClass of service signalingQuality 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 updatedSender learns of drop sooner
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Last In, First Out Illustrated
Simple - no queue pointers to updateSource cannot react as quick
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Title:(05-W.eps)Creator:Adobe Illustrator(TM) 5.0Preview:This EPS picture was not savedwith a preview included in it.Comment:This EPS picture will print to aPostScript printer, but not toother types of printers.
Leaky Bucket Illustrated
From Tanenbaum Figure 5-24, graphic will print to a Postscript printer
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Title:(05-X.eps)Creator:Adobe Illustrator(TM) 5.0Preview:This EPS picture was not savedwith a preview included in it.Comment:This EPS picture will print to aPostScript printer, but not toother types of printers.
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 packetsIn 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 controlTCP
Slow start Congestion avoidance Fast Retransmit Fast Recovery
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Class of Service Signaling
Packets marked to a particular traffic class
IEEE 802.1pDifferentiated Services (DiffServ)Re-defines IP Type of Service (ToS)
bit fieldsAsynchronous Transfer Mode
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Quality of Service Reservations
Resource ReSerVation Protocol Reserve resources in routers Requires stateful path
Asynchronous Transfer Protocol (ATM)