Congestion Control and Traffic Management in High Speed Networks

1
Congestion Controland Traffic Management in
High Speed Networks

• Carey Williamson

University of Calgary
2
Introduction

• The goal of congestion control is to regulate
  traffic flow in the network in order to avoid
  saturating or overloading intermediate nodes in
  the network

3
Congestion Effects

• Congestion is undesirable because it can cause
• Increased delay, due to queueing within the
  network
• Packet loss, due to buffer overflow
• Reduced throughput, due to packet loss and
  retransmission
• Analogy rush hour traffic

4
Congestion Causes

• The basic cause of congestion is that the input
  traffic demands exceed the capacity of the
  network
• In typical packet switching networks, this can
  occur quite easily when
• - output links are slower than inputs
• - multiple traffic sources competing for same
  output link at the same time

5
Buffering A Solution?

• Buffering in switches can help alleviate short
  term or transient congestion problems, but...
• Under sustained overload, buffers will still fill
  up, and packets will be lost
• only defers the congestion problem
• More buffers means more queuing delay
• beyond a certain point, more buffering makes the
  congestion problem worse, because of increased
  delay and retransmission

6
Motivation

• The congestion control problem is even more acute
  in high speed networks
• Faster link speeds mean that congestion can
  happen faster than before
• e.g., 64 kilobyte buffer
• _at_ 64 kbps 8.2 seconds
• _at_ 10 Mbps 52 milliseconds
• _at_ 1 Gbps 0.52 milliseconds

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Motivation (Contd)

• Buffer requirements increase with link speeds
• e.g., to store 1 second worth of traffic
• _at_ 64 kbps 8 kilobytes
• _at_ 10 Mbps 1.25 Mbytes
• _at_ 1 Gbps 125 Mbytes

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Motivation (Contd)

• Heterogeneity of link speeds
• - just because you add new high speed links to
  a network doesnt mean that the old low speed
  links go away
• - interconnecting high speed and lower speed
  networks creates congestion problems at the point
  of interconnect

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Motivation (Contd)

• Traffic is bursty
• - high peak-to-mean ratio, peak rates
• - e.g., data traffic 10-to-1, 1-10 Mbps
• - e.g., video traffic 20-to-1, 5-100 Mbps
• - can statistically multiplex several
  channels, but if too many are active at the same
  time, congestion is inevitable

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Motivation (Contd)

• Reaction time is bounded by the propagation delay
• - in a high-speed wide-area network, the delay
  x bandwidth product is HUGE!!!
• - d x b tells you how many bits fit in the
  pipe between you and the receiver
• - by the time you realize that network is
  congested, you may have already sent another Mbit
  or more of data!!!

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Reactive versus Preventive

• There are two fundamental approaches to
  congestion control reactive approaches and
  preventive approaches
• Reactive feedback-based
• attempt to detect congestion, or the onset of
  congestion, and take action to resolve the
  problem before things get worse
• Preventive reservation-based
• prevent congestion from ever happening in the
  first place, by reserving resources

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Reactive versus Preventive (Contd)

• Most of the Internet approaches are reactive
  schemes
• TCP Slow Start
• Random-Early-Detection (RED) Gateways
• Source Quench
• The large d x b product means that many of these
  approaches are not applicable to high speed
  networks
• Most ATM congestion control strategies are
  preventive, reservation-based

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Congestion Control in ATM

• When people discuss congestion control in the
  context of high speed ATM networks, they usually
  distinguish between call-level controls and
  cell-level controls

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Call-Level Control

• An example of the call-level approach to
  congestion control is call admission control (to
  be discussed later this semester)
• Tries to prevent congestion by not allowing new
  calls or connections into the network unless the
  network has sufficient capacity to support them

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Call-Level Control (Contd)

• At time of call setup (connection establishment)
  you request the resources that you need for the
  duration of the call (e.g., bandwidth, buffers)
• If available, your call proceeds
• If not, your call is blocked
• E.g., telephone network, busy signal

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Call-Level Control (Contd)

• Tradeoff aggressive vs conservative
• Want to accept enough calls to have reasonably
  high network utilization, but dont want to
  accept so many calls that you have a high
  probability of network congestion (which might
  compromise the QOS requirements that you are
  trying to meet)

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Call-Level Control (Contd)

• Problems
• Can be unfair
• - denial of service, long access delay
• Hard to specify resource requirements and QOS
  parameters precisely
• - may not know, or may cheat
• - congestion can still occur

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Cell-Level Control

• Also called input rate control
• Control the input rate of traffic sources to
  prevent, reduce, or control the level of
  congestion
• Many possible mechanisms
• Traffic shaping, traffic policing, UPC
• Leaky bucket (token bucket)
• Cell tagging (colouring), cell discarding
• Cell scheduling disciplines

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Congestion Control in ATM

• There is actually a complete spectrum of traffic
  control functions, ranging from the very
  short-term (e.g., traffic shaping, cell
  discarding) to the very long-term (e.g., network
  provisioning)
• See Gilbert et al 1991

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ATM Traffic Control Schemes
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ATM Traffic Control Schemes
Short Term
usec
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ATM Traffic Control Schemes
Long Term
Months, years
Short Term
usec
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ATM Traffic Control Schemes
Long Term
Resource Provisioning
Admission Control Routing, Load Balancing
Call Duration
Explicit Congestion Notification Fast Reservation
Protocol Node to Node Flow Control
Propagation Delay Time
Usage Parameter Control Priority Control Traffic
Shaping Cell Discarding
Cell Time
Time Scale
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ATM Traffic Control Schemes
Usage Parameter Control Priority Control Traffic
Shaping Cell Discarding
Cell Time
Time Scale
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ATM Traffic Control Schemes
Explicit Congestion Notification Fast Reservation
Protocol Node to Node Flow Control
Propagation Delay Time
Time Scale
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ATM Traffic Control Schemes
Admission Control Routing, Load Balancing
Call Duration
Time Scale
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ATM Traffic Control Schemes
Long Term
Resource Provisioning
Time Scale
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ATM Traffic Control Schemes
Long Term
Resource Provisioning
Admission Control Routing, Load Balancing
Call Duration
Explicit Congestion Notification Fast Reservation
Protocol Node to Node Flow Control
Propagation Delay Time
Usage Parameter Control Priority Control Traffic
Shaping Cell Discarding
Cell Time
Time Scale
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ATM Traffic Control Schemes

• Preventive controls
• Resource provisioning
• Connection admission control
• Call routing and load balancing
• Usage parameter control
• Priority control
• Traffic shaping
• Fast reservation protocol

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ATM Traffic Control Schemes

• Reactive controls
• Adaptive admission control
• Call routing and load balancing
• Adaptive usage parameter control
• Explicit congestion notification
• (forward or backward)
• Node to node flow control
• Selective cell discarding

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Leaky Bucket

• One of the cell-level control mechanisms that has
  been proposed is the leaky bucket (a.k.a. token
  bucket)
• Has been proposed as a traffic policing mechanism
  for Usage Parameter Control (UPC), to check
  conformance of a source to its traffic descriptor
• Can also be used as a traffic shaper

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Leaky Bucket (Contd)

• Think of a bucket (pail) with a small hole in the
  bottom
• You fill the bucket with water
• Water drips out the bottom at a nice constant
  rate drip, drip, drip...

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Leaky Bucket (Contd)
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Leaky Bucket (Contd)
Bucket
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Leaky Bucket (Contd)
Empty
Bucket
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Leaky Bucket (Contd)
Bucket
Hole
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Leaky Bucket (Contd)
Water
Bucket
Hole
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Leaky Bucket (Contd)
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Leaky Bucket (Contd)
Drip
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Leaky Bucket (Contd)
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Leaky Bucket (Contd)
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Leaky Bucket (Contd)
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Leaky Bucket (Contd)
Constant rate stream of drips, all nicely spaced,
periodic
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Leaky Bucket (Contd)
Storage area for drips waiting to go
Constant rate stream of drips, all nicely spaced,
periodic
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Leaky Bucket (Contd)

• A leaky bucket flow control mechanism is then a
  software realization of this very simple idea
• Packets (cells) waiting for transmission arrive
  according to some (perhaps unknown) arrival
  distribution
• Tokens arrive periodically (deterministically)
• Cell must have a token to enter network

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Leaky Bucket (Contd)
Incoming Tokens at rate r tokens/sec
Incoming Cells (generated by traffic source with
rate X)
To Network

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Leaky Bucket (Contd)
Incoming Tokens at rate r tokens/sec
Incoming Cells
To Network

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3
4
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Leaky Bucket (Contd)
Incoming Tokens
Incoming Cells
To Network

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1
2
3
4
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Leaky Bucket (Contd)
Incoming Tokens
Incoming Cells
To Network

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2
3
4
5
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Leaky Bucket (Contd)
Incoming Tokens
Incoming Cells
To Network

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1
3
4
5
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Leaky Bucket (Contd)
Incoming Tokens
Incoming Cells
To Network

1
2
3
4
5
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Leaky Bucket (Contd)
Incoming Tokens
Incoming Cells
X
To Network

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3
1
4
5
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Leaky Bucket (Contd)
Incoming Tokens
Incoming Cells
X
X
To Network

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2
3
4
5
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Leaky Bucket (Contd)
Incoming Tokens
Incoming Cells
X
X
To Network

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2
3
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Leaky Bucket (Contd)

• Cells must obtain tokens in order to proceed into
  the network
• If no token available, then cell is discarded
• Constrains the rate at which cells enter the
  network to be the rate negotiated at the time of
  call setup
• Shapes traffic, reduces burstiness

56
Buffered Leaky Bucket

• Arriving cells that find a token waiting can
  proceed directly into the network
• Arriving cells that find no token ready must wait
  in queue for a token
• Cells that arrive to a full queue are lost
• Tokens that arrive to a full token pool are
  simply discarded

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Buffered Leaky Bucket
Incoming Tokens at rate r tokens/sec
Pool of at most M waiting tokens
Incoming Cells
To Network

Queue of at most B waiting cells
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Buffered Leaky Bucket (Contd)

• Incoming cell rate X
• Token rate r
• If X gt r, then cells wait in buffer until tokens
  are available
• Output traffic is r cells/sec, nicely paced
• If X lt r, then tokens always ready
• Output traffic is X (lt r)
• Use for traffic shaping or UPC

59
Buffered Leaky Bucket (Contd)

• A station can save up at most M tokens
• Limits the maximum burst size in the network
• Can send at most M cells back to back
• B can be set to balance the tradeoff between cell
  loss and cell delay

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Leaky Bucket UPC

• The token rate r is set based on the rate
  declared at the time of call setup
• Makes sure that each source obeys rate that was
  used when the call admission decision was made
  (i.e., descriptor)
• Can use single leaky bucket to police just the
  peak cell rate (PCR)
• Can use dual leaky bucket to police both PCR
  and SCR

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Variations

• There are several different variations of the
  basic leaky bucket concept described in the
  literature, such as the virtual leaky bucket,
  spacer, others
• Basic idea rather than strictly enforcing rates,
  allow senders to occasionally exceed their
  prescribed rate, as long as they mark or tag
  their extra cells

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Cell Marking Scheme

• Uses leaky bucket to regulate cell transmissions
  as before, but rather than having cells wait for
  tokens when there are no tokens ready, the
  station can transmit the cell and mark it as a
  violation cell (i.e., cell colouring)
• Green (CLP 0) for cells that obey rate
• Red (CLP 1) for cells that dont

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Cell Colouring (Contd)

• If the network detects congestion at any point,
  then it does not hesitate to throw away red cells
  (CLP 1), but always gives preference to green
  cells
• Green cells must get through
• Red cells get through only if there is spare
  capacity in the network
• No harm in trying! principle

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Selective Cell Discard (SCD)

• A cell-level control mechanism in ATM switches
  called selective cell discard can be implemented
  quite easily using a CLP threshold on each
  queue/buffer
• Below the threshold, can accept both green and
  red cells
• Beyond the threshold, can only accept green cells

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Selective Cell Discard (Contd)
Buffer in an ATM switch
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Selective Cell Discard (Contd)
Buffer in an ATM switch
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Selective Cell Discard (Contd)
Some cells waiting to go
Buffer in an ATM switch
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Selective Cell Discard (Contd)
CLP Threshold
Buffer in an ATM switch
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Selective Cell Discard (Contd)
CLP Threshold
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Explicit Congestion Notification

• There are some proposals to use reactive
  congestion control approaches for end-to-end flow
  control in ATM
• One of the mechanisms proposed is called Explicit
  Forward Congestion Notification (EFCN) (or EFCI,
  for Explicit Forward Congestion Indication)

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EFCI Basic Operation

• Switches can detect the onset of congestion
  (e.g., buffers filling up)
• Switches set a control bit in cell headers to
  indicate this congestion condition
• Sources react by reducing the volume of traffic
  that they are sending through that switch
• Suitable for VBR or ABR traffic

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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
Buffer
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
Occupied
Unoccupied
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
Data Cell
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
!!!
EFCI Threshold
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EFCI Basic Operation (Contd)
Ack Cell
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
!!!
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
112
EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Basic Operation (Contd)
Traffic Sink
Traffic Source
Switch
Switch
Switch
EFCI Threshold
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EFCI Issues

• How to set EFCI threshold
• What should sources do when EFCI signal is seen
• What should sources do when no EFCI signal is
  seen
• Forward versus backward notification
• Effect of feedback delay
• Delay x bandwidth product

115
Summary

• Congestion control in high speed ATM networks is
  a difficult problem
• Lots of good ideas of how to do it, but no real
  standard (yet?)
• Will likely require a combination of schemes at
  different time scales and for different classes
  of traffic
• Lots more remains to be done