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Congestion Control
Computer Networks
Spring 2000
John Kristoff
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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
Title:
(05-W.eps)
Creator:
Adobe Illus trator(TM) 5.0
Prev iew :
This EPS picture w as not s av ed
w ith a preview inc luded in it.
Comment:
This EPS picture w ill print to a
Pos tSc ript printer, but not to
other ty pes of printers.
 From Tanenbaum Figure 5-24, graphic will print to a Postscript printer
Spring 2000
John Kristoff
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Token Bucket Illustrated
Title:
(05-X.eps)
Creator:
Adobe Illus trator(TM) 5.0
Prev iew :
This EPS picture w as not s av ed
w ith a preview inc luded in it.
Comment:
This EPS picture w ill print to a
Pos tSc ript printer, but not to
other ty pes of printers.
 From Tanenbaum Figure 5-26, graphic will print to a Postscript printer
Spring 2000
John Kristoff
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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)
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