1. TRIO: Measuring Asymmetric Capacity with Three Minimum Round-trip Times Edmond Chan, Ang Chen §, Xiapu Luo §, Ricky Mok §, Weichao Li §, and Rocky Chang § Corporate Research Department, Huawei Research, China § The Hong Kong Polytechnic University

2. Network path capacity Network path capacity: smallest transmission rate of a set of links 20 Mbits/s 8 Mbits/s 100 Mbits/s 50 Mbits/s 15 Mbits/s 10 Mbits/s Measuring node Remote node Forward path Reverse path C f (or C b ) C r (or C B ) 2 Our goal: Measuring any asymmetric capacity without remote nodes control Forward capacityC f = 8 Mbits/s Reverse capacityC r = 10 Mbits/s Slower-path (Round-trip) capacityC b = min{C f, C r } = 8 Mbits/s Faster-path capacityC B = max{C f, C r } = 10 Mbits/s

3. Measuring asymmetric capacity Asymmetric capacity is common due to, e.g., – xDSL/cable broadband – Multi-homing and load balancing Available tools: AsymProbe [ 05networking ], DSLprobe [ 08conext ], MPI tool [ 07imc ], SProbe [ 02infocom ] – Packet-dispersion: AsymProbe, DSLprobe, SProbe – Buffer saturation: MPI tool 3 89% of broadband subscriptions (OECD, Sept. 2010) Average capacity (Mbps) UpstreamDownstream Cable2.6729.73 DSL2.4814.18 All of them rely on packet-size asymmetry!

4. Contributions Generalize packet-dispersion methods into round-trip probes (RTPs) and two-way probes (TWPs) with detailed analysis Propose TRIO which exploits RTPs and TWPs for asymmetric capacity measurement without requiring packet-size asymmetry Implement TRIO to support the measurement with remote web servers or residential users 4

5. Packet-size asymmetry k-Round-Trip Probe (k-RTP) – Packet-dispersion method: Capacity estimate = kS max /δ. S max = max{S f, S r }. Measuring C f : AsymProbe and SProbe (k=1), DSLProbe (k1) Measuring C r : AsymProbe (k=1), DSLProbe (k1) 5 Measuring Remote CrCr CfCf δ = kS f /C f kS f /C f pjpj p j-k pjpj δ = kS r /C r Measuring C f with S f > S r Measuring C r with S f < S r kS f /C f

6. Problem: Probe/response interference The packet-dispersion method (kS max /δ) underestimates the faster- path capacity! Solutions for C f measurement 1)Increase probe packet size, but unaware of IP fragmentation! 2)Increase number of probe packets, but number of response packets also increases (e.g., SProbe, DSLprobe)! 6 Measuring Remote CrCr CfCf δ = kS min /C r > kS max /C f kS max /C f pjpj p j-k pjpj δ = kS min /C f > kS max /C r Measuring C f Measuring C r kS min /C f Response interference Probe interference C f >C r C f <C r

7. Example C f /C r = 24M/1M, k=1 (packet pair), 20% cross traffic – Measuring C r (slower-path capacity) with S f /S r =240B/1500B δ = 1500B/1M > 240B/24M OK! – Measuring C f (faster-path capacity) with S f /S r = 1500B/240B δ = 240B/1M > 1500B/24M Response interference! C r = S r /δ C f = S f /δ 7

8. Measuring only reverse capacity (v,k)-Two-Way Probe ((v,k)-TWP) – Based on the packet-dispersion method: C r = kS r /δ. Avoid probe interference! Independent of v and S f – Existing tool SProbes C r measurement: (0,1)-TWP 8 Measuring Remote CrCr CfCf p u-v pupu δ = kS r /C r rjrj r j-k

9. Exploits three minRTTs from 1-RTP and (1,1)-TWP r0r0 p1p1 d T0 p1p1 r1r1 d T1 Measuring Remote CrCr CfCf p1p1 p0p0 p0p0 p1p1 r1r1 1-RTP (1,1)-TWP r0r0 p1p1 Our solution: TRIO 9 C f = S f /(d T0 -d R0 ). C r = S r /(d T1 -d T0 ). Avoid response interference! r0r0 p1p1 d T0 p0p0 d R0 Reuse d T0 Avoid probe interference! S f /C f d T1 -d T0 =S r /C r Independent of S r d T0 -d R0 =S f /C f

10. minRTTs of 1-RTP and (1,1)-TWP When S=S f =S r, – D is the total transmission and propagation delays. Hence, 10 Measuring Remote CrCr CfCf p1p1 p0p0 p0p0 p1p1 r1r1 1-RTP (1,1)-TWP r0r0 p1p1 d T0 d T1 d R1 d R0 d T0 = D + S/C f. d T1 = D + S/C f + S/C r. d R0 = D. d R1 = D + max{S/C f,S/C r }. C f = S/(d T0 - d R0 ). C r = S/(d T1 - d T0 ). C B = S/(d T1 - d R1 ). C b = S/(d R1 - d R0 ). For self-diagnosis The MDDIF method [ 09conext ] The MDDIF method [ 09conext ]

11. Implementation of TRIO Based on OneProbes probing technique [ 09usenix ] Client-side measurement – As a web client to measure with any remote web server – Probe packet: HTTP GET request – Response packet: HTTP data response Server-side measurement – As a web server to accept measurement requests from any remote web client – A client-side Flash object opens a TCP connection with TRIO for measurement. 11

12. Self-diagnosis tests Packet loss/reordering filtering – Ignore unexpected TCP response packets minRTT estimate validation: d R0 < d T0 d R1 < d T1. Capacity estimate validation: min{C f, C r } = C b. max{C f, C r } = C B. 12

13. Testbed evaluation Testbed configuration – n = 16 hops – Pareto cross traffic 13 Ran Click v1.8 to emulate different C f and C r Launch AsymProbe, DSLprobe, and SProbe with S max /S min =1518B/64B; and TRIO with S=1518B

14. Testbed results 14 Highlight results with relative error > 0.1 AsymProbe, DSLprobe, and SProbe underestimate faster-path capacity for 20/0.512 and 0.512/20. DSLprobes sanity check withholds C f estimations for highly asymmetric path and Fast-Reverse paths.

15. Example: A client with C f /C r = 250K/5M (emulated using residential broadband network model [07imc]) 50 public Debian mirror sites – Implement PDProbe to perform RTP and TWP measurements used by AsymProbe, DSLprobe, and SProbe TRIO is also accurate for (i) Fedora/Gentoo/openSUSE mirror sites and (ii) other configurations for C f /C r. Internet evaluation 15 Due to probe interference

16. Conclusion TRIO – Three minRTTs for asymmetric capacity measurement – Avoid probe and response interference – Self-diagnosis tests – More accurate than existing packet-size asymmetry techniques Future work – Packet size impact – Measurement with multichannel bottlenecks – Large-scale measurement of the bottleneck location and degree of capacity asymmetry of Internet paths 16

17. THANKS Our project website: www.oneprobe.org 17

18. Measurement capabilities of k-RTPs Valid C f measurement: δ = kS max /C f Valid C r measurement: δ = kS max /C r Response interference (measuring C f ): δ = kS min /C r > kS max /C f Probe interference (measuring C r ): δ = kS min /C f > kS max /C r 18 Capacity-asymmetric path Fast-Forward (FF) path (C f /C r >1)Fast-Reverse (FR) path (C f /C r <1) Measure C f : (S f /S r =S max /S min1) Measure C r : (S f /S r =S min /S max 1) (I) C f /C r > S max /S min 1 (II) C f /C r < S min /S max 1 (1) S max /S min C f /C r > 1 (2) S max /S min 1 > C f /C r (3) S min /S max 1 < C f /C r (4) S min /S max C f /C r < 1 Capacity estimate = kS max /δ Correct! Capacity estimate = kS max / δ Underestimated!

19. Evaluation of probe interference C f /C r = 1M/24M, k=1 (packet pair), 20% cross traffic – S f /S r = 1500B/240B for measuring C f (slower-path capacity) δ = 1500B/1M > 240B/24M OK! – S f /S r = 240B/1500B for measuring C r (faster-path capacity) δ = 240B/1M > 1500B/24M Probe interference! 19 C f = S f /δ C r = S r /δ

20. C f /C r = 1M/24M, 20% cross traffic Example: (0,1)-TWP vs (1,1)-TWP Consistent results for v={0,1} and S f ={240,1500}B – Due to this property, TRIO can use the same TWP to measure both C f and C r ! 20