1. Bandwidth Estimation: Metrics Measurement Techniques and Tools
By Ravi Prasad, Constantinos Dovrolis, Margaret Murray and Kc Claffy IEEE Network, Nov/Dec by Phuong Nguyen, Josiah McClurg

2. Internet as a “black box”
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
Several network properties are important for applications and transport protocols:
Delay, loss rate, capacity, congestion, load, etc
But routers and switches do not provide direct feedback to end-hosts (except ICMP, also of limited use)
Mostly due to scalability, policy, and simplicity reasons

3. Can we guess what is in the black box?
probe
packets
End-systems can infer network state through end-to-end (e2e) measurements
Without any feedback from routers
Objectives: accuracy, speed, non-intrusiveness

4. Outline Metrics Bandwidth estimation techniques Taxonomy of tools
Capacity
Available bandwidth
TCP throughput and Bulk transfer capacity
Bandwidth estimation techniques
Taxonomy of tools

5. Hops, segments, and paths
Segment: a link at layer 2
Physical point-to-point link
Virtual circuit
Shared access LAN (ETHERNET, FDDI)
Hop: a link 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

6. Capacity At layer 2 link (segment): At IP layer:
Transmission rate limited by: i) capacity of the physical link, ii) speed of the transmitter/receiver hardware
At IP layer:
Lower rate due to overheads in layer 2 by encapsulation and framing

7. Effect of packet size on capacity usage

8. Capacity (cont.) Capacity of a hop Capacity of a path:
Maximum possible IP layer transfer rate at that hop
Maximum layer 2 transfer can occur only with Maximum Transmission Unit sized packets
Bit rate measured at IP layer transferring MTU sized packets
Capacity of a path:
Minimum link capacity determines capacity of the path

9. Problems with estimating capacity
Traffic shapers
Rate limiters
Wireless networks like
Operate at different rates
11, 5.5, 2 or 1 Mbps
Definition holds during time at which the capacity remains constant

10. Available bandwidth
The available bandwidth of a link relates to the unused, or “spare”, capacity of the link during a certain time period
At any time
Link used fully  utilization =1
Not used  utilization =0
Average utilization from time t-α to t is given by
Then the available bandwidth during that period: Ai = (1 – ui) Ci

11. Available bandwidth (cont.)
Available bandwidth along the path

12. Assumptions
Link utilization remains constant over the duration of measurement
Reasonable for short intervals
Load variations impact the measurement over a long period
So available b/w measurements should be done quickly
Since capacity remains constant those measurements need not be made quickly

13. 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

14. Differences 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

15. Outline Metrics Bandwidth estimation techniques Taxonomy of tools
Capacity
Available bandwidth
TCP throughput and Bulk transfer capacity
Bandwidth estimation techniques
Taxonomy of tools

16. Bandwidth estimation techniques
Variable Packet Size (VPS) probing
Packet Pair/Train Dispersion (PPTD)
Self-Loading Periodic Streams (SLoPS)
Trains of Packet Pairs (TOPP)

17. Variable size packet probing
Measures capacity of each hop along a path
Based on measuring RTT
Limit packet propagation by using TTL in IP header
Uses ICMP to measure RTT until that hop
RTT includes delays:
Serialization delay
Delay to send packet of length L across channel of capacity C = L/C
Propagation delay
Time taken to traverse the link
Queuing delay at routers/switches

18. Variable size packet probing
Send multiple packets and calculate minimum RTT
Assumption: for minimum RTT no queuing delay
RTT has two terms
A term ~ delay independent of packet size = α
Another term proportional to packet size (i.e., serialization delay)

19. Packet pair dispersion probing
Measures end-to-end capacity
Dispersion after a link
Dispersion at the receiver

20. 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 (Packet train probing)
Does not always yield a correct estimate but helps reduce the variance among the measurements

21. Self-loading periodic streams
Measures end-to-end available bandwidth
Method:
Sends K packets at a certain rate R
Receiver notifies the one way delay trends:
If the rate is greater than available b/w: one way delay will keep increasing
Otherwise: one way delay will not increase

22. Self-loading periodic streams
Sender attempts to bring the stream rate close to available bandwidth, following an iterative algorithm similar to binary search

23. Trains of packet pairs
Estimate the available bandwidth of a network path
Method:
Sends many packet pairs at gradually increasing rates from the source to the receiver
Notify the difference between offered rate & measured rate
TOPP increases the offered rate linearly, while SLoPS uses a binary search to adjust the offered rate

24. Trains of packet pairs

25. Intrusiveness of bandwidth estimation tools
All active measurement tools inject probing traffic in the network and thus are all intrusive
How to quantify the intrusiveness?
An active measurement tool is intrusive when its average probing traffic rate during the measurement process is significant compared to the available bandwidth in the path.

26. Intrusiveness of bandwidth estimation tools
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

27. Outline Metrics Bandwidth estimation techniques Taxonomy of tools
Capacity
Available bandwidth
TCP throughput and Bulk transfer capacity
Bandwidth estimation techniques
Taxonomy of tools

28. Taxonomy of estimation tools

29. Discussions Limited accuracy in estimating bandwidth
Applications of using bandwidth estimates to support applications
Routers with multiple queues
Simplified assumptions:
Link utilization remains constant over the duration of measurement
Assumption that no other traffic exists

30. Thank you!

31. 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

32. 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

33. 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

34. 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