1. Accuracy of Link Capacity Estimates using Passive and Active Approaches with CapProbe
Rohit Kapoor, Ling-Jyh Chen, M. Y. Sanadidi, Mario Gerla
Dept. of Computer Science, University of California at Los Angeles

2. Packet pair Techniques
Ideal Case:
June 2004
ISCC 2004

3. Packet Pair and Train Dispersion
June 2004
ISCC 2004

4. Packet Pairs Bandwidth Histogram
Packet-pair estimates: multimodality with cross traffic:
Light load conditions(20%) (b) heavy load conditions(80%)
SCDR is caused by dispersion expansion
PNCM is caused by dispersion compression
June 2004
ISCC 2004

5. Packet Train Bandwidth Histogram
As trains get longer, get “Asymptotic Dispersion Rate” or ADR
ADR is not equal to Residual (available) Capacity
We found and proved a physical interpretation (to be published): ADR is the flow share, when merges are proportional to arrival rates at each link
Dovrolis’ results obtained for non-responsive cross traffic flows
June 2004
ISCC 2004

6. CapProbe: The Main Idea
Observation: Both expansion and compression of dispersion involve queuing due to cross traffic:
Dispersion expansion => second packet queued more
Dispersion compression => first packet queued more
Packet pair with minimal end-to-end delay sum, is likely to be dispersed corresponding to narrow link capacity
Looking for packet pair with minimal delay sum is inexpensive
CapProbe appears accurate in most of our experiments, simulations and measurements
CapProbe fails under heavy (~>75%) utilization by non-responsive (UDP) traffic
June 2004
ISCC 2004

7. CapProbe Ideal Case: no cross traffic
Real Case: dispersion may be compressed or expanded by the cross traffic
Under-estimation due to expansion
Over-estimation due to compression
June 2004
ISCC 2004

8. CapProbe Both expansion and compression are due to queuing
A Packet-Pair sample with Minimal Delay Sum can be used for Capacity Estimation
4 mbps narrow link, TCP cross traffic,
June 2004
ISCC 2004

9. Wireless Measurements
Bad channel retransmission larger dispersions lower estimated capacity
Experiment No.
Capacity Estimated by CapProbe (kbps)
Capacity Estimated by Strongest Mode (kbps) 1 2
462.8 3 4 5
449.73
Results for Bluetooth-interfered b, TCP cross-traffic
June 2004
ISCC 2004

10. Comparison to Earlier Tools
UCLA-2
UCLA-3 UA
NTNU
Time
Capacity
Cap
Probe
0’03
5.5
0’01 96
0’02 98
0’07 97
5.6
0’04 79
0’17 83
0’22
0’09 99 95
Pathrate
6’10
0’16
5’19 86
0’29
6’14
5.4
5’20 88
0’25
6’5
5.7
5’18
133
6.8
0’26
6’20
5.8
132
Pathchar
21’12
4.0
22’49 18
3 hr 34
21’21
22’53 31 35
21’45
22’48 32
20’43
3.9
27’41
21.18
29’47 30
June 2004
ISCC 2004

11. Implementation Issues
User vs. Kernel Mode generation of probes and measurements
End systems processing speed
Probe packet size
June 2004
ISCC 2004

12. Testbed User mode and kernel mode implementations
Slow system: Pentium II 500MHz CPU; Fast system: Pentium IV 1.8 GHz CPU
Probe packet sizes varied from 500 Bytes to 5K Bytes
June 2004
ISCC 2004

13. Measurement Experiments on Internet
Unit: Mbps
PKSize
(bytes)
YAHOO
NTNU
WLSH 1 2 3
User mode 1
(slow machine)
500
571.4
285.7
452.8
133.3
67.8
1.45
1.41
1000
421.1
61.5
160.0
381.0
216.2
1.51
1.47
1.5
3000
338.0
237.6
79.2
827.6
444.4
1.48
5000
231.2
156.2
239.5
209.4
210.5
243.9
User mode 2
(fast machine)
64.5
148.1
400
54.1
56.3
40.8
1.26
1.28
156.9
150.9
160
82.5
95.2
2.17
1.50
103.4
112.1
117.0
111.6
123.7
93.1
1.49
1.46
99.5
100.8
102.6
89.9
102.3
107.8
Kernel mode
9.2
10.2
89.4
90.9
74.0
1.39
1.42
69.6
79.3
78.6
93.0
85.1
91.6
1.44
92.0
86.3
90.0
96.2
98.4
95.6
95.0
86.8
91.3
96.4
Narrow Link Capacity
100
NTNU: National Taiwan Normal University; WLSH: Wuling Senior High School, Taiwan, Ling Jyh taught computer use in that school !!
NTNU connects to UCLA with 100Mbps bottleneck link; WLSH connects to UCLA with 1.5Mbps bottleneck link (T1)
User Mode 1 user level process, no kernel implementation, runs ICMP packets (modified Ping program)
User Mode 1 produces inaccurate time stamps and thus wrong dispersion measurements, except for low speed link (1.5Mbps)
User Mode 2 similar to above, except fast machine
Kernel mode: in Linux Kernel , uses ICMP still like above
June 2004
ISCC 2004

14. Discussion
For high speed networks, either a high time resolution machine or a large probing packet size is needed for accuracy
Fine resolution may not be possible in user mode
A large packet size increases the chances expansion of dispersion
Required time resolution T = pksize / C:
Packet Size
Narrow Link Capacity
100 Mbps
10 Mbps
1 Mbps
500 bytes
0.04 ms
0.4 ms
4 ms
1000 bytes
0.08 ms
0.8 ms
8 ms
3000 bytes
0.24 ms
2.4 ms
24 ms
5000 bytes
0.40 ms
40 ms
.04 instead of .05 because 500 bytes!!!
Required time resolution for accurate estimation
June 2004
ISCC 2004

15. Passive CapProbe
CapProbe is an active approach and using ICMP packets.
Passive approach is less intrusive, thus more scalable
Passive CapProbing within TCP requires back to back TCP packet transmission
Simulation => 15~20% of TCP data packets are sent back-to-back
June 2004
ISCC 2004

16. Simulation
The network topology used in our simulations consists of a six-hop path with capacities {10, 7.5, 5.5, 4, 6 and 8} Mbps.
DelACK is disabled in the simulation.
Different cross traffic are used, with packet size 1000 bytes and 200 bytes.
June 2004
ISCC 2004

17. Passive CapProbe Active CapProbe June 2004 ISCC 2004
Comparing Passive and Active
Cross traffic indicated in boxes
Capacity is 4 Mbps
As cross traffic load increase, more inaccuracy in passive approach
CBR cross traffic is also a problem in active approach at high load
June 2004
ISCC 2004

18. Conclusion
Either a high time resolution machine or a large probing packet size is necessary for accurate capacity estimation
Passive CapProbing within TCP is feasible, minor TCP sender modification helps a lot (future work)
Other Future work:
Experiments at speeds higher than 100 Mbps
Passive CapProbing in TFRC, other applications
Use of capacity estimates in TCPW and overlays construction
June 2004
ISCC 2004

19. T h a n k s Improving Wireless Link Throughput via Interleaved FEC
June 2004
ISCC 2004