congestion control
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Autumn 2000 John Kristoff 3
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
An expert: Sally Floyd <http://www.aciri.org/floyd/>An expert: Van Jacobson
Autumn 2000 John Kristoff 4
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.
Autumn 2000 John Kristoff 5
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
Autumn 2000 John Kristoff 6
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
Autumn 2000 John Kristoff 7
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
Autumn 2000 John Kristoff 8
Active Congestion Control Mechanisms
Eligible discard Queue management Network Signaling and Notification End station avoidance Class of service signaling Quality of service reservations
Autumn 2000 John Kristoff 9
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
Autumn 2000 John Kristoff 11
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
Autumn 2000 John Kristoff 12
First In, First Out Illustrated
Queue pointers need to be updated Sender learns of drop sooner
Autumn 2000 John Kristoff 13
Last In, First Out Illustrated
Simple - no queue pointers to update Source cannot react as quick
Autumn 2000 John Kristoff 14
Leaky Bucket Illustrated
From Tanenbaum Figure 5-24, graphic will print to a Postscript printer
Autumn 2000 John Kristoff 15
Token Bucket Illustrated
From Tanenbaum Figure 5-26, graphic will print to a Postscript printer
Autumn 2000 John Kristoff 16
RED Illustrated
Probability marking applied to each packet based on queue length, packet being dropped
Autumn 2000 John Kristoff 18
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
Autumn 2000 John Kristoff 19
End Station Avoidance
Also called end-to-end control TCP
Slow start Congestion avoidance Fast Retransmit Fast Recovery
Autumn 2000 John Kristoff 20
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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