Bandwidth Estimation in Broadband Access Networks

1
Bandwidth Estimation in Broadband Access Networks

• Venkat Padmanabhan
• Systems Networking Group
• Microsoft Research
• Joint work with
• Karthik Lakshminarayanan (Berkeley) Jitu Padhye
  (MSR)
• June 2004

2
Outline

• Bandwidth estimation
• Previous work
• Challenges in broadband access networks
• ProbeGap
• Experimental evaluation
• 802.11a testbed
• cable modem testbed
• Conclusion

3
Bandwidth Estimation

• Active area of networking research for 15 years
• Bandwidth refers to data rate
• CS bandwidth (bps), not EE bandwidth (Hz)
• Several notions of bandwidth
• bottleneck bandwidth, or capacity
• raw bandwidth of narrow link
• available bandwidth
• spare capacity of tight link
• other notions
• fair share bandwidth
• bulk transfer capacity

4
Bandwidth Estimation

• Of interest in several contexts
• congestion control (e.g., TCP)
• admission control (e.g., A/V streaming)
• background transfer (e.g., TCP Nice)
• server/peer selection (e.g., overlay multicast)
• Desirable attributes of an estimation scheme
• depends only on end hosts
• accurate
• fast
• lightweight non-intrusive

5
Previous Work on Capacity Estimation

• Packet-pair method
• Jacobson 88, Keshav 91
• cross-traffic ? underestimation/overestimation
• Refinement filtering to eliminate noise
• nettimer Lai 00, pathrate Dovrolis 01
• key observation capacity mode may not be
  dominant
• Single-packet techniques
• pathchar Jacobson 97, clink Downey 99
• dependence on ICMP msgs. limits applicability and
  accuracy

do
narrow link
6
Previous Work on Available Bandwidth Estimation

• Packet Rate Method (PRM)
• e.g., pathload Jain 02, PTR Hu 03
• probe at gradually increasing rates
• increasing trend in OWD indicates that pipe is
  full
• accurate but somewhat heavyweight
• Packet Gap Method (PGM)
• e.g., IGI Hu 03, Spruce Strauss 03
• send several carefully spaced probe pairs
• estimate cross-traffic based on the increase in
  spacing
• assumes that the tight link is also the narrow
  link
• relatively lightweight but susceptible to delays
  elsewhere
• RTT-based estimation Gunawardena 03
• derive analytical relationship between load and
  RTT
• perturb network by introducing known amount of
  additional load
• quite heavyweight, susceptible to delays
  elsewhere departure from the assumed traffic
  model

7
Packet Rate Method (PRM)
probes
cross-traffic
tight link
Probing rate lt available bandwidth ? no trend in
OWD
Probing rate gt available bandwidth ? increasing
OWD
8
Packet Gap Method (PGM)
probes
do
di
tight narrow link
di do ? no cross-traffic
cross-traffic
do
di
di lt do ? cross-traffic C(do-di)/di
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Traditional Link Model

• Assumptions made in previous work
• link has well-defined capacity
• point-to-point link with FIFO scheduling
• fluid cross-traffic (infinitesimal packet size)
• But these assumptions break down in broadband
  network settings

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Broadband Access Networks

• Various technologies
• cable modem, DSL, wireless (WiFi, WiMax)
• Why is broadband different?
• managed links (pricing flexibility)
• typically shared medium (lower cost)
• DSL is an exception
• conforms to the traditional link model
• Specific issues
• link may not have well-defined capacity
• contention and non-FIFO scheduling
• bursty cross-traffic

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Broadband Issues

• Link may not have well-defined capacity
• rate regulation (e.g., token bucket)
• dynamic multirate (e.g., 802.11)
• ? measured capacity may not be same as sustained
  capacity
• Non-FIFO scheduling due to frame-level contention
• fully distributed contention-based MAC (e.g.,
  802.11)
• centrally coordinated MAC (e.g., cable uplink)
• ? difficult for packet pairs to go through
  back-to-back
• ? probe packets may not see full impact of
  cross-traffic
• ? relative sizes of probe packets cross-traffic
  packets matter
• Bursty cross-traffic
• interference between links operating at different
  rates
• e.g., in 802.11a, a single packet CT packet on 6
  Mbps link would appear as a large burst on 54
  Mbps link
• ? makes it difficult to accurately sample the
  cross-traffic

12
Is AvlbBw Still Interesting?

• With a fair MAC it may be feasible to estimate
  the fair share bandwidth
• e.g., Keshavs original packet-pair work
• However, available bandwidth remains interesting
• TCP ramp-up
• safe option is to quickly ramp-up to available
  bandwidth and then probe gradually for fair share
• admission control for A/V streams
• letting new stream exercise its fair share might
  cause disruption of existing streams

13
ProbeGap

• New technique for estimating available bandwidth
• designed to address some of these issues
• non-FIFO scheduling, bursty cross-traffic
• Key idea probe for idle gaps in the link
• gather OWD samples
• knee in CDF identifies idle fraction
• multiply by capacity to obtain available
  bandwidth estimate
• Issues
• very lightweight
• 200 probes of 20-bytes each
• clock drift is a concern
• can estimate and neutralize
• susceptible to delays at other links
• like PGM and RTT-based method

14
Experimental Evaluation

• We focus on the broadband network in isolation
• Testbeds
• 802.11a
• cable modem
• controlled testbed
• commercial connections
• Tools evaluated
• capacity pathrate
• available bandwidth pathload, spruce, probegap
• Validation
• capacity measured using intrusive packet train
  probes
• available bandwidth determined by observing
  impact on cross-traffic

15
802.11a Evaluation

• Experimental setup
• 6 nodes in ad hoc configuration
• one pair used for bandwidth estimation
• other two pairs used to generate cross-traffic
• cross-traffic
• link rate 6 Mbps
• traffic rate 0-4 Mbps, packet size 300 or
  1472 B
• estimation link
• single-rate case link rate 6 Mbps
• multi-rate case link rate 54 Mbps

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Impact of Packet Size (802.11a)
Significant per-packet overhead, especially at 54
Mbps
17
Capacity Estimation (802.11a)

• Pathrate uses 1472-byte probe packets
• Single-rate case
• capacity mode identified consistently in the
  5.1-5.5 Mbps range, even with cross-traffic
• enough packet pairs go through back-to-back,
  despite non-FIFO fair MAC
• situation might be different with a larger number
  of contending stations
• Multi-rate case
• capacity mode identified in the 23-30 Mbps range
  in most cases
• exception with heavy cross-traffic (4 Mbps, 300
  B)
• capacity mode identified was 10-11 Mbps

Packet-pair sampling with suitable filtering
mostly works
18
AvlbBw Estimation (802.11a single-rate)
Overestimation due to tendency towards fair share
(Pathload) and differential packet size
(Spruce). Probegap only overestimates slightly
under high load.
19
ProbeGap (802.11a single-rate)
Overestimation at high loads. Possible fix send
probes in bunches and pick max OWD.
20
AvlbBw Estimation (802.11a multi-rate)
The single-rate issues persist. But new anomalies
with both Pathload and Spruce due to the
burstiness of cross-traffic.
21
Pathload (802.11a multi-rate)
Pathload fails to detect consistent increasing
trend even though probing rate (9.79 Mbps)
exceeds avlb bw (6.5 Mbps).
22
Impact of Token Bucket (Cable Modem)

• Experimental setup
• raw bandwidth of downlink 27 Mbps
• token bucket rate 6 Mbps, depth 9600 bytes
• cross-traffic rate 0-6 Mbps
• Capacity estimation
• pathrate consistently estimates 26 Mbps
  regardless of cross-traffic
• Available bandwidth estimation
• pathload overestimates slightly
• token bucket can accommodate large train of
  300-byte probes
• spruce overestimates significantly
• a pair of probes is less likely to be regulated
  than a train
• unclear what right capacity to assume is

23
Pathload (cable modem)
Slight overestimation because of token bucket
24
Spruce (cable modem)
Significant overestimation because of token
bucket. Unclear what the right capacity to assume
is.
25
Conclusion

• Broadband access networks present new challenges
  to bandwidth estimation
• performance experienced by probes may not be
  indicative of true performance
• tendency to estimate fair share rather than
  available bandwidth
• ProbeGap looks promising
• More info www.research.microsoft.com/padmanab/pr
  ojects/PeerMetric/
• MSR tech report (MSR-TR-2004-44)
• IMC 2003 paper (macroscopic properties of
  broadband networks)