Chapter 6 Congestion Control and Resource Allocation

1
Chapter 6 Congestion Control and Resource
Allocation

• Packets contend at a router for the use of a
  link, they are placed in a waiting queue
• Too many packets in a queue and some are dropped
• The network is then said to be congested
• Resources are bandwidth and buffer space

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Factoids

• Because of the bursty nature of Internet traffic,
  queues can become full very quickly, and then
  become empty again. (page 477)
• The complexity of routing in the Internet is such
  that simply obtaining a reasonably direct,
  loop-free route is about the best that you can
  hope for. Routing around congestion is icing on
  the cake. (page 449)

3
More Factoids

• The essential strategy of TCP is to send packets
  into the network without a reservation and then
  to react to observable events that occur
  (congestion). (page 464)
• The main reason that packets are not delivered
  and dropped is due to congestion not errors
  during transmission. (page 465)

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Congestion Control a Definition

• We use the term congestion control to describe
  the efforts made by network nodes to prevent or
  respond to overload conditions. (page 448)

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Source
1
10-Mbps Ethernet
Router
Destination
1.5-Mbps T1 link
100-Mbps FDDI
Source
2
6
Source
1
Router
Destination
1
Router
Source
2
Router
Destination
2
Source
3
7
Router-Centric vs. Host-Centric

• Throughout chapter 6 there will be this
  particular choice should the router or host be
  responsible (more) for deciding which packets to
  drop and how to adjust the network behavior.

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Throughput/delay
Optimal
Load
load
9
Queuing

• FIFO (with tail drop) is the most widely used
  approach in Internet routers
• This approach actually has the effect of turning
  over the responsibility for most congestion
  control in the Internet to the edges of the
  network TCP is used here.

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Arriving
Next free
Next to
packet
buffer
transmit
Free buffers
Queued packets
(a)
Arriving
Next to
packet
transmit
(b)
Drop
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Flow 1
Flow 2
Round-robin
service
Flow 3
Flow 4
12
Flow 1
Flow 2
Flow 1
Flow 2
Output
Output
(arriving)
(transmitting)
F 10
F 10
F 8
F 5
F 2
(a)
(b)
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TCP Congestion Control

• Section 6.3 is a large and important topic
• Extends the chapter 2 discussion of sliding
  windows
• ACKs are used to pace the transmission of packets
• TCP is said to be self-clocking
• Available bandwidth changes over time, so a
  source must adjust the number of transmitted
  packets
• Sliding windows are used by TCP for congestion
  control

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TCP Congestion Control in Action

• The next few slides show TCP in action
• First additive increase in packets sent
• Second, a typical TCP transmission pattern
• Slow start
• A graph of typical behavior of TCP congestion
  control

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Source
Destination

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Source
Destination

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Congestion Avoidance

• In general predict when congestion is about to
  happen and then reduce the rate at which hosts
  send data (page 475)
• Not widely adopted yet
• Three different methods are discussed
• First two depend on adding a small amount of
  additional logic to the routers

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Queue Length/Buffer of the Router

• Buffer space is one of the two main scarce
  resources of the Internet (bandwidth is the
  other)
• The router passes information about its queue
  length (equivalent to remaining buffer space)
  onward to eventually end up at the sending host
• The sending host can change it packet sending
  rate based on this information

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Measuring Congestion/Buffers/Queues

• Following are several slides that show various
  graphs
• Some are measurements
• Some represent thresholds and probability

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Queue length
Current
time
T
ime
Current
Previous
cycle
cycle
A
veraging
interval
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MaxThreshold
MinThreshold
A
vgLen
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Quality of Service

• Real-time applications need some assurance
  (guarantee) from the network that the data is
  likely (certainly) going to arrive on time.
  (page 488)
• First we start with application requirements

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Sampler
,
Microphone
Buffer
,
A D
D A
converter
Speaker
30
Playback Buffer

• Study figure 6.21
• Packet generation is at a constant rate
• Packet playback is at a constant rate
• In-between generation and playback is the network
• Packet arrival will have some random components
  and appears wavy because of intermittent network
  delay

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Packet
arrival
Packet
generation
Sequence number
Playback
Buffer
Network
delay
T
ime
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Overview of Real-Time Applications (Taxonomy)

• Figure 6.23 tells it best
• Study over the text on pages 492-494

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Applications
Real time
Elastic
Interactive
Asynchronous
Interactive
T
olerant
Intolerant
bulk
Adaptive
Nonadaptive
Rate-adaptive
Nonadaptive
Delay-
Rate-
adaptive
adaptive
35
QOS Mechanisms

• We will only cover this briefly
• The textbook is also brief about this rather
  extensive topic
• Many different ideas are being tried out

36
3
Flow B
2
Bandwidth (MBps)
Flow A
1
1
2
3
4
T
ime (seconds)
37
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