Internet performance issues for safety-critical applications

(1)

Markus Fiedler: Internet Performance Issues for Safety-Critical Applications

Internet Performance Issues for Safety-Critical Applications

Markus Fiedler

Blekinge Institute of Technology School of Engineering

Dept. of Telecommunication Systems

Internet Issues (1)

• Its very nature

• A network of networks

• Connectivity – the only concern?

• “Can you ping?” (If yes, everything is OK.)

• Shared resources and the best-effort paradigm

– “Jantelagen”: “Du skall icke tro att du är bättre än vi.”

– No guarantees

• IP (layer 3) uses datagrams

– No explicit quality feedback between the layers

• TCP (layer 4) “times out”

(2)

Internet Issues (2)

• Few QoS standardization and handling efforts – IntServ does not scale

– Island-type solutions

• MPLS (DiffServ) in the core

• Industrial real-time Ethernet at the edge

• QoS handling – if any – is mainly left to the end systems/application

– RTCP (layer 7): monitors quality for real-time applications – TCP (layer 4): loss and sequence recovery at the cost of

delay

• Motto: Help yourself!

... And Some Consequences

• Packet streams

– Meet braking transport capacity limitations (bottlenecks)

– Interfere with (= brake) each other

• User-perceived Quality of Service (QoS) in terms of – Speed: Too low and varying delay and jitter – Accuracy: Too low losses

– Reliability: Not dependable time-/drop-outs

• Perceived speed < installed capacity

(3)

Perceived Speed

Low capacity

High cap.

Server

Switch Client

Ethernet

Transmission time Processing time ()

Queuing time Entity level Transmission

level

Legend:

time

Travel Length/capacity Client

Effective speed

Perceived Speed – Shared (1)

High cap.

Server

Switch Client

Ethernet

Queuing time

Entity level Transmission

level

Legend:

time

Effective speed

Low capacity Collision

(4)

Perceived Speed – Shared (2)

Low capacity

High cap.

Server

Switch Client

level

Legend:

time

Effective speed

Perceived Speed – Shaped

Low capacity

High cap.

Server

Switch Client

level

Legend:

time

Effective speed

Speed adaptation

(5)

Loss Due To Overload

Low capacity

High cap.

Server

Switch Client

Ethernet

level

Legend:

time

Dependability

Low capacity

Server

Switch Client

Ethernet

level

Legend:

time

(6)

Some QoS Degradation Measures (1)

• One-way delay

– Problematic due to time synchronization

• NTP useful in local environments

• GPS might help

• Round-trip delay

– Determines download times

– Approximately available through ping

• Active measurement: implies extra load; just probing – Upper bound for the one-way delay

• Asymmetrical delays are quite normal

Some QoS Degradation Measures (2)

• Delay jitter

– Comparably simple to obtain – Trend analysis required

• Loss

– To be observed on upper layers

• Reliability

– Real-time applications: Frequency of time-outs – General: Relative downtime

(7)

Safety-Critical Applications

• Needs:

– Minimal delay and (additional) jitter

• Keep timing relationships as good as possible – No loss

– Information = feedback in case of problems

• Characteristics:

– Streaming: amount ∆L within each ∆T

• Control; voice; video; ...

– Messaging: amount L to be received within T

• Alarms; notifications; ...

– Interactive: amounts L_C_S+ LSCto be received within T

Speed Considerations

• Close to limiting factor: lack of transport capacity

⇒ Queuing ⇒ delay, jitter

⇒ Loss

• Calculation of end-to-end-perceived speed – Time synchronization upon arrival of first packet

• As perceived by the receiver

– Bandwidth changes along the way through the network reflect jitter and loss

– Focus on streaming services

• Provides speed information even for messaging/interactive services

(8)

End-To-End Perceived Speed – Ideal Case

∆L

Delay of 1^stpacket

∆T

∆L/∆T

End-To-End Perceived Speed – Jitter (1)

∆T

∆L ∆L/∆T

Delayed packet speed change

(9)

End-To-End Perceived Speed – Jitter (2)

∆T

∆L ∆L/∆T

End-To-End Perceived Speed – Loss

∆T

∆L L/ ∆T

Lost packet speed change

(10)

”Kilroy was here”

• http://www.kilroywashere.org

• End-to-end-perceived speed measurements and histograms tell about

– Type of bottleneck – Severity of disturbance

• ”Kilroy indicator” = histogram@output – histogram@input – Observation window ∆W

– Averaging interval ∆T – Speed discretization ∆R

Shared Bottleneck – Example

In

Speed Out

Time interval

1

Speed

H

1

Speed

H

1

-1

∆H

Speed

(11)

Shaping Bottleneck – Example

In

Speed Out

Time interval

1

Speed

H

1

Speed

H

1

-1

∆H

Speed

Video Experience of an Artificial Bottleneck

Switch

Switch Internet

Karlskrona (SE) Generator

Consumer 10 Mbps half-duplex

Würzburg (D)

Distur- bing UDP traffic 010Mbps

Speed histogram

Karlskrona Würzburg ”Kilroy”

Difference

Measure- ment point Würzburg Measure- ment point Karlskrona

(12)

A Closer Look on Audio

• One packet à 492 B each 60 ms

• Inter-packet jitter

– Observation window = 1 min

– Jitter = Packet inter-arrival time – 60 ms – Discretization = 1 ms

• Speed

– Observation window = 1 min – Averaging interval = 60 ms – Speed discretization = 10 kbps

• Different levels of disturbance

Jitter – Disturbance

0 Mbps 6 Mbps

-0,15 -0,1 -0,05 0 0,05 0,1 0,15 0,2 0,25 0,3

-0,06 -0,04 -0,02 0 0,02 0,04

Jitter [s]

Rel. frequency

Jitter in Jitter out Difference

-0,15 -0,1 -0,05 0 0,05 0,1 0,15 0,2 0,25 0,3

-0,06 -0,04 -0,02 0 0,02 0,04 0,06

Jitter [s]

Rel. frequency

Jitter in Jitter out Difference

-0,15 -0,1 -0,05 0 0,05 0,1 0,15

-0,06 -0,04 -0,02 0 0,02 0,04

Jitter [s]

Rel. frequency

Difference

-0,15 -0,1 -0,05 0 0,05 0,1 0,15

-0,06 -0,04 -0,02 0 0,02 0,04 0,06

Jitter [s]

Rel. frequency

Difference

(13)

Speed – Disturbance 0 Mbps

-20%

0%

20%

40%

60%

80%

100%

0 20000 40000 60000 80000 100000 120000 140000 160000

Sender Receiver Difference Difference

-2,5%

-2,0%

-1,5%

-1,0%

-0,5%

0,0%

0,5%

1,0%

1,5%

0 20000 40000 60000 80000 100000 120000 140000 160000

bps→

Speed – Disturbance 6 Mbps

-20%

0%

20%

40%

60%

80%

100%

0 20000 40000 60000 80000 100000 120000 140000 160000

Sender Receiver Difference

Difference

-12,5%

-10,0%

-7,5%

-5,0%

-2,5%

0,0%

2,5%

5,0%

7,5%

0 20000 40000 60000 80000 100000 120000 140000 160000

bps→

(14)

Speed Comparisons

+ Reflects flow of packets

+ Takes packet lengths into account + Comparable to installed capacities + Comparable to expected behaviour

+ See whether packets come ”too late”

+ Comparable to behaviour at inlet

• Requires exchange of speed histograms + Feedback facility

− Dependency on first packet

− Short flows

− Granularity/discretization

Conclusion And Open Issues

• Quality of Service problems in terms of – Speed

– Accuracy – Reliability

are well reflected in speed changes

• Important: Time scale of interest

• Time synchronization issue and one-way delays via Internet

– Measurement – Guarantee

(15)

Internet Performance Issues for Safety-Critical Applications

Markus Fiedler Thanks for listening ☺☺☺☺

Any questions?