Internet Measurement Methods

1. Internet Measurement Methods Workshop on QoS Hanoch Levy Feb 2004 Hanoch Levy, CS, TAU

2. B A D C The objective: • A talks to B. • A wants to know how well it goes. • How well it will go? Hanoch Levy, CS, TAU

3. Performance Measures • Delay: • The time it takes a packet to go from A to B. • Loss: • Will the packet arrive or not? • What fraction of packets will get lost. • Jitter: • What is the variability of the delay? • Bandwidth: • At what rate can I transfer my bits to the destination? Hanoch Levy, CS, TAU

4. What are typical performance measures • Look at: http://special.matrixnetsystems.com/ratings/index.asp • Round trip delay: 40ms – 700 ms • Loss: 0% - 12% • Goes down on terrestrial links, yet high on wireless • Jitter: 10ms, 30 ms 100 ms. • Bandwidth: depends. Hanoch Levy, CS, TAU

5. The causes for problems Hanoch Levy, CS, TAU

6. The causes for problems • Sources for delay: Packets have to: • Traverse links: Propagation delay • Function of distance – roughly speed of light, 0.1-0.2 sec around the globe • Be transmitted: Transmission delay • Line rate / packet size • Wait on line: Queueing delay • Number of packets * line rate / packet size • Sources for loss: • Queue is full • Noisy line (quite uncommon today except for wireless) Hanoch Levy, CS, TAU

7. Who is Who + references • IETF: the standardization body of Internet. • IPPM = IP Performance Metrics: a working group of measurements under IETF. • http://www.ietf.org/html.charters/ippm-charter.html • In there: find drafts for measuring: • Delay • Loss • Delay variation (jitter). Hanoch Levy, CS, TAU

8. References (2) • Conferences: • INFOCOM • SIGCOMM • SIGMETRICS Hanoch Levy, CS, TAU

9. Performance Measures and applications (1) • Applications differ in the quality measures they require • Voice: • Send a packet every 30 ms. • Packets must arrive at “real time” (<200 msec) • If don’t arrive makes no sense! • If arrive irregularly – confuse recipient! • Lost packets are OK if not often. •  major quality factors: • Delay • Loss • Jitter Hanoch Levy, CS, TAU

10. Applications (2): FTP • FTP: File transfer protocol • Want the file to transfer as soon as possible • Packet delay: not important: • Packet Loss: Directly – not important • Packet jitter: not important • Bandwidth: Important! (loss indirectly). Hanoch Levy, CS, TAU

11. Applications (3) • Web: • Want your page to make it in a few seconds • SEMI real time. • Network delay – is not major factor (since it is anyhow less than a second). • Losses -- a factor  can slow down the transfer • “Bandwidth” (how much bandwidth is available) Hanoch Levy, CS, TAU

12. Bandwidth Measurement • The route to destination consists of routers and links. • The delay incurred on the path: • Where q_i = queueing delay • l_i = latency • S/b_i = transmission delay Hanoch Levy, CS, TAU

13. Bandwidth Measurement • Path Capacity bandwidth (PCB): • Min_i (b_i) • Interpretation: If the pipes were in my hands – how much could I push? • Path Available Bandwidth(PAB): • Min (b_i-c_i) where c_i is the cross traffic. • Interpretation: how much BW can (will) I get. Hanoch Levy, CS, TAU

14. Bandwidth Measurement (3) : Packet pair • Send a packet pair (two packets back to back) on the route • Property: the arrival time difference is equal to the propagation delay over the bottleneck (S. Keshav 91 ): Hanoch Levy, CS, TAU

15. Bandwidth Measurement (4) : Packet pair: Explanation Hanoch Levy, CS, TAU

16. Bandwidth Measurement (5) : Packet pair: Conditions • The routers on the path are store and forward • The two packets are sent sufficiently close to one another • Both packets take the same route to the destination. • There are no multi-channel links. • There is no cross traffic! Hanoch Levy, CS, TAU

17. Bandwidth Measurement (5) : Packet pair • If no cross traffic: Measures PCB • If there is cross traffic and routers use FairQueueing measures PAB • Fair Queueing: • Will explain in the sequel • More or less divides the channel to “pieces” where each application gets it share of the channel. Hanoch Levy, CS, TAU

18. Bandwidth Measurement : What we want on the project • Want to get the route where one can push the data as much as possible. • Want the route with the highest Available BW: • Better a pipe of 100MByte with 30Mbyte avail than 200MB with 20 avail Hanoch Levy, CS, TAU

19. Bandwidth Measurement • Carter and Mark E. Crovella 96 • Bprobe: pairs of packets sent for a roundtrip • Use echo packets (ICMP) by the sender • Send several pairs • Use histograms to clean those affected by cross traffic • Measures PCB • Measures round-trip BW  • Does not need receiver cooperation  Hanoch Levy, CS, TAU

20. A histogram C&C Hanoch Levy, CS, TAU

21. Another histogram Hanoch Levy, CS, TAU

22. Bandwidth Measurement • Carter and Mark E. Crovella 96 • Cprobe: Send a train of packets • Measures PAB (avail BW) • Again: packets are ICMP •  Does not require target cooperation  •  Measures only the round trip BW  • Train length: used several trains of 8 packets • Do ICMP represent real traffic?  • (low priority at routers) Hanoch Levy, CS, TAU

23. Bandwidth Measurement: Train accuracy • Examine the train value against the BW of a real stream Hanoch Levy, CS, TAU

24. Bandwidth Measurement : Issues (1) • How large should the train be? • Too small train may not represent reality • Too large: • Waist “ammunition” • May affect the BW on the route (if very large!) • How frequently should we send it? • Too low frequency: What you see now is not what will happen later (traffic on network changes) • Too high frequency: • Waist “ammunition” • May affect the BW on the route (if very large!) Hanoch Levy, CS, TAU

25. Bandwidth Measurement : Issues (2) • Should we use ICMP or regular packets? • ICMP: A special IP packet that when sent to destination, destination returns it right away. • Used to implement ping. • Advantage: Can be used for measurement without cooperation of other side. • Disadvantages: • Round-trip and not one-way. • Does not reflect the REAL TRAFFIC • Routers on the route may treat ICMP differently • Give low priority • Drop Hanoch Levy, CS, TAU

26. Bandwidth Measurement : Issues (3) • Should we measure the real transaction? • Reflects the real traffic best • If data is not used at other end – then we wasting data. • If we measure the real transaction (FTP, say): • What kind of window should we use to estimate the BW. Hanoch Levy, CS, TAU

27. In case you have Questions • Ippm@advanced.org • hi, i'm writing from italy, my name is david • i saw you wrote: • ******************************************************* • > By the way, I have a question ... • > What's the difference between a bottleneck b/w and an available b/w? • > Which of b/w's can "pathchar" give me? • The available bandwidth along a path between two Internet hosts is • equal to the bandwidth at the bottleneck. In other words, in the • context of pathchar, they're the same thing. • ******************************************************** • yes, but pathchar doen't work with crosstraffic. the vps(variable packet • size) works • with a empty link, don't it? Hanoch Levy, CS, TAU

28. References (1) • Internetworking with TCP/IP By Douglas E. Comer, Prentice Hall 1995 • Van Jacobson. Congestion avoidance and control. In SIGCOMM ’88 Conference Proceedings, pages 314-329, Stanford, CA, USA, August, 1998. • Van Jacobson. Pathchar - a tool to infer characteristics of Internet paths. Presented at the Mathematical Sciences Research Institute (MSRI); Slides available from ftp://ftp.ee.lbl.gov/pathchar/, April 1997. • S. Keshav - "Congestion Control in Computer Networks". Ph.D Dissertation.Department of EECS at UC Berkeley, August 1991. • Vern Paxson. Measurements and Analysis of End-to-End Internet Dynamics. Ph.D. dissertation, University of California, Berkeley, April 1997. Hanoch Levy, CS, TAU

29. References (2) • Robert L. Carter and Mark E. Crovella. Measuring bottleneck link speed in packet-switched networks. Technical Report TR-96-006, Boston University Computer Science Department, Boston, MA, USA, March 1996. • Constantinos Dovrolis, Parameswaran Ramanathan, David Moore What Do Packet Dispersion Techniques Measure In Proceedings of IEEE INFOCOM 2001 http://www.cs.utk.edu/~dunigan/pktprb.ps. • Kevin Lai, Mary Baker, Nettimer: A Tool for Measuring Bottleneck Link Bandwidth, In Proceedings of the USENIX Symposium on Internet Technologies and Systems March 2001 • Kevin Lai and Mary Baker. Measuring link bandwidth using a deterministic model of packet delay. In SIGCOMM 2000 Conference Proceedings, Stockholm, Sweden, August 28-September 01, 2000. Hanoch Levy, CS, TAU

30. References (3) • Allen B. Downey. Using pathchar to estimate Internet link characteristics. In SIGCOMM ’99 Conference Proceedings, pages 241–250, Cambridge, MA, USA, August 31–September 3, 1999. ACM SIGCOMM Computer Communication Review, 29(4). • W. Jiang, T. F. Williams, Detecting and measuring Asymmetric Links in IP Network, Tech Rep, CUCS-009-99, Columbia University, 1999 http://www.cs.columbia.edu/~wenyu/papers/asym-gi99-ea.ps. • J. Postel. Internet control message protocol. Request for Comments (Standard) 792, Internet Engineering Task Force, September 1981. • http://bandwidthplace.com/speedtest/ Hanoch Levy, CS, TAU