Pathload A measurement tool for end-to-end available bandwidth

1
Pathload A measurement tool for end-to-end
available bandwidth

• Manish Jain, Univ-Delaware
• Constantinos Dovrolis, Univ-Delaware
• Sigcomm 02

2
Overview

• Capacity and available bandwidth metrics
• Self-Loading Periodic Streams (SLoPS) methodology
• Description of pathload
• Verification experiments

3
Part I

• Background

4
Definition of capacity

• Capacity maximum possible end-to-end throughput
• End-to-end capacity C is limited by narrow link
  n
• Pathrate measurement tool based on packet
  pairs/trains (Infocom01)
• See www.pathrate.org

DS3/ATM
Fast Ethernet
Fast Ethernet
S
45
100
100
R
36(IP) 9 (ATM)
5
Definition of available bandwidth

• ui utilization of link i in time interval T
  ( 0 ? ui ? 1 )
• Available bandwidth of link i
• Available bandwidth is limited by tight link t

C2
C3
C1
A
Sink
Source
t
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Measuring per-hop available bandwidth

• Network managers are very interested in available
  bandwidth
• Can be measured at each link from router
  utilization statistics (via SNMP) - Multi Router
  Traffic Grapher
• MRTG graphs 5-minute averages (each graph one
  direction)
• BUT, users do not normally see this data and it
  is not end-to-end

7
Previous work on available bandwidth estimation

• Blast the path with UDP traffic (bad and wrong!)
• Measure throughput of large TCP transfer
• TCP does not get available bandwidth in
  under-buffered paths
• TCP gets more than available bandwidth in
  over-buffered paths
• TCP saturates the path (intrusive measurements)
• Carter Crovella Dispersion of packet trains
  (see cprobe)
• Does not measure available bandwidth (see
  Infocom01)
• Melander et. al. (Global Internet00) and Ribeiro
  et.al.(ITC00)
• Correct estimation when queuing only at single
  link in path

8
Part II

• Self-Loading Periodic Streams (SLoPS) Methodology

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Major Idea

• SLoPS analyzes One-Way Delays (OWDs) of packets
  from sender S to receiver R
• OWD Di TRarrive-TSsend Tarrive - Tsend
  Clock_Offset(S,R)
• Relative OWDs between successive packets Di
  Di1
• S and R do not have synchronized clocks.

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Basic Idea

• Periodic Stream K packets, size L bytes, rate R
  L/T
• If RgtA, OWDs gradually increase due to
  self-loading of stream

T L/R
1
2
3
4
K4
4
1
2
3
D1
D2
D3
D4
1
2
3
4
D1
D2
D3
D4
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Experimental result R gt A case

• K 100 packets, A 74Mbps, R96Mbps, T100?s

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Experimental result R lt A case

• K 100 packets, A 74Mbps, R37Mbps, T100?s

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Experimental result R A case

• K 100 packets, A 74Mbps, R82Mbps, T100?s

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Iterative algorithm in SLoPS

• At source Send periodic stream n with rate
  R(n)
• At receiver Measure OWDs Di for i1K
• At receiver Check for increasing trend in OWDs
  and notify source
• At source If trend is -
• increasing (i.e. R(n)gtA ), ?repeat with
  R(n1) lt R(n)
• non-increasing (i.e. R(n)ltA ),? repeat
  with R(n1)gtR(n)
• Terminate if R(n1) R(n) lt ?
• ? resolution of final estimate

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Part III

• Description of Pathload

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Selection of L, T and K

• L can not be less than certain number of bytes
• L should not be greater than path MTU, to avoid
  fragmentation
• T should be small to complete transmission of
  stream before context switch
• Large K may overflow the queue of the tight link
  when R gt A
• Small K does not give enough samples to infer
  trend robustly

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Use of Several Streams

• N streams allow us to examine for N consecutive
  times whether R gt A or not
• Disjoint streams, separated by silence period
  allow queues in network to drain measurement
  traffic
• Duration of a fleet of N streams
• Effects of stream duration and fleet duration
  described in a technical report, End-to-end
  available bandwidth measurement methodology,
  dynamics, and relation with TCP throughput

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How do we detect an increasing trend?

• Pairwise Comparison Test (PCT)
• EPCT0.5 for independent OWDs,
  when increasing trend
• Pairwise Difference Test (PDT)
• EPDT0 for independent OWDs,
  when increasing trend

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Illustration of PCT and PDT metrics

• Infer increasing trend when PCT or PDT trend ? 1.0

20
PCT variation for 3 fleets
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PDT variation for 3 fleets
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Rate adjustment algorithm

• Increasing trend
• Rmax R(n)
• R(n1) (Gmax Rmax)/2
• Non-increasing trend
• Rmin R(n)
• R(n1) (Gmax Rmin)/2
• Grey region R(n) gt Gmax
• Gmax R(n)
• R(n1) (Gmax Rmax )/2
• Grey region R(n) lt Gmin
• Gmin R(n)
• R(n1) (Gmin Rmin )/2

Grey region
Terminate if Rmax Rmin lt ? or Gmax
Gmin lt ?
23
Detecting sender side context switch

• Context Switches on sender machine can distort
  the inter-packet spacing in a stream.
• Receiver detects the sender side context switches
  in a stream by testing t(i1) - t(i) gt CS T

24
Detecting receiver side context switch

• Receiver side context switch results in packets
  being accumulated in the buffer in the kernel
• Receiver detects the receiver side context by
  checking if a(i1) - a(i) ? 0

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Other pathload features

• Clock skew between sender and receiver can
  distort the relative OWD.
• Clock skew not an issue in pathload due to small
  stream duration.
• Pathload aborts the fleet if
• stream encounters excessive loss ( gt10 )
• a fraction of streams encounter moderate loss
• For default tool parameters, and avail-bw ? 10
  Mbps, pathload takes 12 seconds

26
Part IV

• Experimental Results

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Verification Approach

• Use paths from U-Delaware to Greek universities
  and U-Oregon.
• Routes through UDel, Abilene, Dante, GRnet
• MRTG graphs for all links in path report 5-min
  averages for avail-bw
• In 5-min interval, pathload runs W times, each
  for qi secs
• 5-min average avail-bw R reported by pathload

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Verification I

• Tight link U-Ioannina to AUTH(C8.2Mbps),
  resolution w1Mbps

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Verification II

• Tight link U-Oregon gigapop-Abilene(C155Mbps),
• Resolution w1 Mbps

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Summary

• Avail-bw estimation has numerous application
• SLoPS fast, accurate and non-intrusive
  measurement
• First release of pathload in Spring02
• Examined avail-bw variability using pathload, and
  results published in a technical report,
  End-to-end available bandwidth measurement
  methodology, dynamics, and relation with TCP
  throughput
• Future work incorporate avail-bw estimation in
  transport,QOS and routing