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March/7/2005 Hanoch Levy, CS, TAU 1
Internet Measurement Methods
Workshop on QoSHanoch Levy
March 2005
To Start 23/3/05To Start 23/3/05
March/7/2005 Hanoch Levy, CS, TAU 2
• A talks to B. • A wants to know how
well it goes. • How well it will go?
A
B
DC
The objective:
March/7/2005 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 (rate): [ “bandwidth” taken from EE]– At what rate can I transfer my bits to the destination?
March/7/2005 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.
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The causes for problems
March/7/2005 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• Packet size / line rate
– Wait on line: Queueing delay• Number of packets * packet size/line rate
• Sources for loss: – Queue is full– Noisy line (quite uncommon today except for wireless)
March/7/2005 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.ht
ml• In there: find drafts for measuring:
– Delay– Loss– Delay variation (jitter).
March/7/2005 Hanoch Levy, CS, TAU 8
References (2)
• Conferences: – INFOCOM – SIGCOMM – SIGMETRICS
March/7/2005 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
March/7/2005 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).
March/7/2005 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)
March/7/2005 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 (S=pckt size)
ni
1ii
qi
li
b
ST
March/7/2005 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.
March/7/2005 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 transmission delay over the bottleneck (S. Keshav 91 ):
)/(max01iinn bStt
)/()(min 01nnii ttSb
March/7/2005 Hanoch Levy, CS, TAU 15
Bandwidth Measurement (4) : Packet pair: Explanation
Bandwidth
Time
Bottleneck Link
March/7/2005 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!
March/7/2005 Hanoch Levy, CS, TAU 17
Bandwidth Measurement (5) : Packet pair
• If no cross traffic: Measures PCB
• If there is cross traffic and routers use Fair Queueing 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.
March/7/2005 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
March/7/2005 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
March/7/2005 Hanoch Levy, CS, TAU 20
A histogram C&C
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Another histogram
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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)
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Bandwidth Measurement: Train accuracy
• Examine the train value against the available BW of a real stream
March/7/2005 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!)
March/7/2005 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
March/7/2005 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 (mix of control and measurement)
March/7/2005 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?
March/7/2005 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.
•
March/7/2005 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.
March/7/2005 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/
• Kapoor, Chen, Lao, Gerla and Sanadid, “CAProbe: A simple and accurate capacity estimation technique,” SIGCOMM’ 04, August 2004