two samples are enough: opportunistic flow-level latency estimation using netflow

Two Samples are Enough:Opportunistic Flow-level Latency Es-

timation using NetFlow

Myungjin Lee†, Nick Duffield‡, Ramana Rao Kompella†

†Purdue University, ‡AT&T Labs–Research

2

Per-hop Measurements are important

R3R2

R1IPTV/VoIP/VoD

Server

Which router causes the prob-lem??

100 ms

AS 2

AS 1

Why 100 ms?!

3

Aggregate vs. Per-Flow

R3R2

R1IPTV/VoIP/VoD

Server5 ms10 ms

AS 2

AS 1

Why 100 ms?!

flow-level latency measurements on a per-hop basis

Aggregate la-tencies look all right. Why?

4

Existing Approaches Active probes and tomography

Chen et al. [SIGCOMM’04], Duffield et al. [IMC’03] Problems

Problem formulation is under-constrained No per-flow latency measurements

Lossy Difference Aggregator (LDA) Kompella et al. [SIGCOMM’09] Problems

Require hardware modification No per-flow latency measurements

5

Basic Framework: NetFlow A measurement framework widely deployed in

routers Maintains per-flow state in the form of a flow record

Packet and byte counts Flow duration (flow start and end timestamps)

Usage Normally used for accounting, traffic matrix estimation,

etc. Does not support per-flow latency measurements

Goal: Enable per-flow latency estimation Harness flow start and end timestamps in NetFlow

framework

6

Obtaining Two Delay SamplesFlow

IDStart

TSEnd TS

1 1

B

A

Flow ID

Start TS

End TS

2 2

12

Flow ID

Start TS

End TS

2 4−

−

=

= Delay2

Delay1

Two delay samples / flowEnd TS

4Start

TS2

R2R1

Flow ID

Start TS

End TS

1 2

End TS2

Start TS1

R3R2

R1IPTV/VoIP/VoD

Server

AS 2

AS 1

7

Problem 1: Independent Packet Sam-pling

Flow ID

Start TS

End TS

1 1

B

A

Flow ID

Start TS

End TS

4 4

12

Only up-date

1st packet

Only up-date2nd

packet

R2R1No coordination between NetFlow instances

8

Solution: Hash-based SamplingFlow

IDStart

TSEnd TS

1 1

B

A

Flow ID

Start TS

End TS

2 2

12R2R1

Hash Space

Sampling Space

Hash-based sampling achieves coordination

Sampled at both NetFlow

Instances

Not sampled at both Net-

Flow In-stances

1 2

9

Problem 2: Packet LossFlow

IDStart

TSEnd TS

1 1

B

A

Flow ID

Start TS

End TS

2 2

12

Flow ID

Start TS

End TS

1 2

R2R1

X

Updateboth

packets

Only up-date

1st packet

Packet losses may cause inconsistencies

10

Solution: Packet DigestsFlow

IDStart

TSEnd TS

1 1

B

A

Flow ID

Start TS

End TS

2 2

Flow ID

Start TS

End TS

1 2

Start PD

End PD

0x01 0x01

Start PD

End PD

0x01 0x01

Start PD

End PD

0x01 0x02

12

X Detect un-usable

timestamp

Detect unus-able

timestampR1 R2Packet digest achieves packet association

11

Consistent NetFlow (CNF) Architecture Issue I: No coordination between NetFlow instances

Different packets are sampled on different NetFlow instances Solution: Hash-based sampling (filtering) in RFC 5475

Same packets are selected on different NetFlow instances IETF PSAMP working group

Issue II: Packet losses Discrepancy in selected packets due to packet losses

Solution: Maintaining packet digests Hash of the invariant packet contents Use timestamps iff packet digests match at the two routers

12

Trivial Estimator: Endpoint Use two delay samples belonging to the same

flow Obtain accurate latency estimates for small flows

Problem: Accuracy penalty for large flows Solution: Multiflow estimator

Time

Endpoint estimator Avg ( , )=

Flow ID

13

Better Estimator: Multiflow Key insight: Packets experiencing same queu-

ing busy periods will experience similar delays Use background delay samples from other flows Use only delay samples between the start and end

of a flow

Time

Multiflow estimator Avg ( , , , )=

Flow ID

14

Evaluation Simulation Setting

Endpoint vs. Multiflow estimators

Comparison with Trajectory sampling

15

Simulation Setting Modified YAF

Simulate a queuing model with RED active queue management policy

Dataset CHIC trace

1 min. trace collected from an OC-192 backbone link About 13M packets and 1M flows

16

Multiflow vs. Endpoint Estimators

1 10 100 1000 100000

0.10.20.30.40.50.60.70.80.9

1MultiflowEndpoint

Flow size

Med

ian

rela

tive

err

or o

fde

lay

mea

n es

tim

ates

Endpoint obtainsgood accuracy

Multiflow performsbetter than Endpoint

17

Trajectory Sampling Shares some similarity with CNF architecture

Routers use consistent hash function to sample packets

Facilitates direct observation of packet trajectories

Requires flow ID and timestamps for per-flow latency estimation Aggregate all sampled packets with same flow key Compute their average latency

18

Comparison with Trajectory Sampling

0.001 0.01 0.1 1 100

0.10.20.30.40.50.60.70.80.9

1MultiflowTrajectory

Relative error of delay mean estimates

CDF

Packet sampling rate = 0.01

Multiflow is 2-3xbetter than Trajectory

19

Summary Our approach retrofits per-flow latency esti-

mates in the NetFlow framework

Two main ideas Consistent NetFlow architecture ensures that dif -

ferent routers record the same set of flows

Multiflow estimator achieves significantly accurate estimates of per-flow latencies compared to prior approach

20

Questions?