Bandwidth Estimation: Metrics Mesurement Techniques and Tools By Ravi Prasad, Constantinos Dovrolis,

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Bandwidth Estimation Metrics Mesurement
Techniques and ToolsBy Ravi Prasad,
Constantinos Dovrolis, Margaret Murray and Kc
ClaffyIEEE Network, Nov/Dec 2003

• Presented by
• Sundar P Subramani

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Outline

• Introduction
• Metrics
• Capacity
• Available bandwidth
• TCP throughput and Bulk transfer capacity
• Bandwidth estimation techniques
• Taxonomy of tools
• Conclusion

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Introduction

• Bandwidth
• Physical layer Spectral width of
  electromagnetic signals
• Data networks Data rate
• In this paper they discuss about the data networks

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Why b/w estimation needed?

• P2P applications form user-level networks based
  on b/w between them
• Overlay n/w s configure routing tables based on
  b/w of the links
• Service agreements between n/w provider and
  consumer done based on b/w availability at
  crucial points of the n/w

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Why cant SNMP be used?

• Network administrators can read router/switch
  information using SNMP protocol
• End-to-end bandwidth estimation cannot be done in
  the above way

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Outline

• Introduction
• Metrics
• Capacity
• Available bandwidth
• TCP throughput and Bulk transfer capacity
• Bandwidth estimation techniques
• Taxonomy of tools
• Conclusion

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Hops and segments

• Segment
• Links at layer 2
• Physical point-to-point link
• Virtual circuit
• Shared access LAN (ETHERNET, FDDI)
• Hop
• Links at layer 3
• Sequence of segments connected by switches,
  bridges and other layer 2 devices
• Path p from s to v
• Sequence of hops from s to v

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Capacity

• Transmission rate limited by
• Capacity of the physical link
• Speed of the transmitter/receiver hardware
• Overhead in Layer 2 in terms of encapsulation and
  framing produces lower rate as far as layer 3 is
  concerned

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Capacity

• Tx time of IP packet of size LL3 in a link of
  capacity CL2 is
• Where,
• HL2 is the length of the layer 2 header

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Capacity of layer 3
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Effect of packet size on capacity usage
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Capacity

• Capacity of a hop
• Maximum possible IP layer transfer rate at that
  hop
• Maximum layer 2 transfer can occur only with MTU
  sized packets
• Bit rate mesured at IP layer transferring MTU
  sized packets

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Capacity of a path

• Minimum link capacity determines capacity of the
  path
• Where
• H is the number of hops
• Ci Is the capacity of the ith hop

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Problems

• Traffic shapers
• Rate limiters
• Wireless networks like 802.11
• Operate at different rates
• 11, 5.5, 2 or 1 Mbps
• Definition holds during time at which the
  capacity remains constant

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Average utilization

• At any time
• Link used fully ? utilization 1
• Not used ? utilization 0
• Avg utilization from time t-a to t is given by

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Utilization of a link
Link used 8 out of 20 time slots until T So the
link utilization is 40
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Available bandwidth

• Let ui be the average utiliztion of the link i
  over a period of time
• Let Ci be the capacity of the hop i
• Then the available bandwidth during that period
• Ai (1 ui) Ci
• Available bandwidth along the path

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Pipe model
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Assumptions

• Link utilization remains constant over the
  duration of mesurement
• Reasonable for short intervals
• Load variations impact the measurement over a
  long period
• So available b/w mesurements should be done
  quickly
• Since capacity remains constant those
  measurements need not be made quickly

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TCP throughput and Bulk transfer capacity

• TCP throughput depends on various parameters
• Congestion window
• RTT
• Slow start mechanism
• Capacity and load along the path
• BTC
• Maximum capacity obtainable by a TCP connection

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Difference between BTC and available b/w

• BTC is TCP specific
• Available b/w is transport protocol independent
• BTC depends on the how a TCP connection
  throughput is affected by other flows
• Available b/w assumes average load remains
  constant and estimates additional bandwidth

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Outline

• Introduction
• Metrics
• Capacity
• Available bandwidth
• TCP throughput and Bulk transfer capacity
• Bandwidth estimation techniques
• Taxonomy of tools
• Conclusion

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Variable size packet probing

• Measures capacity of each hop
• Measure RTT
• Limit packet propogation by TTL
• Uses ICMP to measure RTT until that hop

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Variable size packet probing

• RTT includes
• Serialization delay
• Delay to send packet of length L across channel
  of capacity C L/C
• Propogation delay
• Time taken to traverse the link
• Queuing delay
• Delay in routers/Switches

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Variable size packet probing

• Send multiple packets and calculate minimum RTT
• Assumption for minimum RTT no queuing delay
• RTT has two terms
• Delay independent of packet size a
• Based on packet size

where
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Variable size packet probing
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packet size vs RTT
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Packet pair dispersion probing

• Measures end-to-end capacity

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Problems

• Assumption that no other traffic exists is not
  real
• Existing traffic can increase/decrease the
  estimate
• Solution?
• Send multiple pairs and get a statistical
  estimate
• Does not always yield a correct estimate

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Self-loading periodic streams

• Measures end-to-end available bandwidth
• Sends k packets at different rates
• Receiver notifies the one way delay trends
• If stream rate greater than available b/w
• One way delay will grow large
• Else
• Packets will not make the one way delay large

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One way delay
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Train dispersion probing

• Similar to packet pair dispersion probing
• Instead of sending just two packets send a train
  of packets
• Calculate the average dispersion rate

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Taxonomy of estimation tools Per-hop capacity
estimation tools

• Pathchar
• First tool to implement
• Clink
• On routing instability collects data along all
  paths
• Until one path provides statistically significant
  estimate
• Pchar
• Uses linear regression algorithms

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Taxonomy of estimation tools end-to-end
capacity estimation tools

• BProbe
• Uses packet pair dispersion
• Uses variable sized packets to improve efficiency
  
• Access needed only in sender side, uses ICMP
  messages
• Nettimer
• Uses sophisticated kernel density algorithm to
  provide better accuracy
• Pathrate
• Sprobe

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Available bandwidth estimation tools

• CProbe
• Measures dispersion of a train of eight maximum
  sized packets
• It measures dispersion rate and not available
  bandwidth
• Dispersion rate depends on all links of the path
  and the trains initial rate
• Available b/w depends only on tight link of the
  path
• Pathload
• Implements SLoPS
• Used UDP and requires access at both ends
• Reports range
• Center represents the average
• Range represents values during mesurement period

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TCP throughput and BTC measurement tools

• Treno
• emulates TCP sends UDP packets to receiver
• Replies with ICMP port unreachable
• Does not require access to remote end
• ICMP rate limited
• Accuracy of Treno affected
• Cap
• More accurate than Treno
• Uses UDP for TCP data and ACK
• Requires access at both ends

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Intrusiveness

• If probe packets comparable to available b/w
• VPS are non intrusive
• One packet per RTT
• PPTD tools create bursts which last only for a
  very short duration
• Only a small fraction of available b/w used
• BTC tools are intrusive
• They capture all b/w for a specific duration