Presented by Sundar P Subramani

1. 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

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

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

4. 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

5. Why can’t 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

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

7. 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

8. 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

9. 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

10. Capacity of layer 3

11. Effect of packet size on capacity usage

12. 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

13. 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

14. 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

15. Average utilization • At any time • Link used fully  utilization =1 • Not used  utilization =0 • Avg utilization from time t-α to t is given by

16. Utilization of a link Link used 8 out of 20 time slots until T So the link utilization is 40%

17. 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

18. Pipe model

19. 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

20. 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

21. 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

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

23. Variable size packet probing • Measures capacity of each hop • Measure RTT • Limit packet propogation by TTL • Uses ICMP to measure RTT until that hop

24. 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

25. 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 = α • Based on packet size where

26. Variable size packet probing

27. packet size vs RTT

28. Packet pair dispersion probing • Measures end-to-end capacity

29. 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

30. 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

31. One way delay

32. 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

33. 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

34. 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

35. 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 train’s 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

36. 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

37. 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