monitoring persistently congested internet links

Monitoring Persistently CongestedInternet Links

Leiwen (Karl) Deng

Aleksandar Kuzmanovic

Northwestern University

http://networks.cs.northwestern.edu

2Karl Deng Monitoring Persistently Congested Internet Links

Objective

New probing methods that can improve measurement observability for core congestionPong – a tool specialized in measuring a subset of non-edge links exhibiting repetitive congestion– Can reveal systematic problems such as routing

pathologies, poorly-engineered network policies, or non-cooperative inter-AS relationships

Lightweight: monitoring in addition to on-demand measuring– A building block of a large scale triggered

monitoring system for Internet congestion


Repetitive Congestion

We focus on locating and monitoring non-edge links that exhibit repetitive congestion– Queuing delay as congestion indicator

– Queue building-up repetitively happens on time scales of one or more minutes.


Methodology Highlights

Coordinated probing– Probe from both endpoints of a path– Combine end-to-end probes with (TTL limited)

probes to intermediate routers

Infer underlying path topology conditions– Implicit inference– Based on measured queuing delays on different

probing paths

Use statistics over longer time scales

Quantify measurement accuracy– Link measurability score


S D

Probe

fs

db

f (“forward”) probe b (“backward”) probe

s (“source”) probe d (“destination”) probe

, ,

,

A Simplified Case – Symmetric Path

Coordinated Probing


S D

fs

db

Δfs

Δfd

Half-path queuing delay

Locating

Congested

Links

Tracing

Congestion

Status

Probe

Δd

Δb

Δf

Δs

Coordinated Probing


Infer Underlying Path Topology Conditions

S D

fs

db

Probe

Δd

Δb

Δf

Δs

Condition: Δf +Δb ≈Δs +Δd

Path Pattern: 4-p probing scenario


S D

fs

bd

Observed by b probe only Paired d probe

CongestionPair up

Fsd probing

S D

fs

b

No suitable d probes to pair up with this s probe

Congestion

Fsb probing

Probing technique Condition

4-p probing Δf +Δb ≈Δs +Δd

Δf ≈Δs +ΔdFsd probing

Δs ≈Δf +ΔbFsb probing

unconditional2-p probing

S D

fs

bd

Paired d probe

Congestion

4-p probing

Pair up

Δfs

Δfd

Estimates of half-path queuing delay

Probe Probe

Probe

Infer Underlying Path Topology Conditions


Probing technique Condition

4-p probing

Fsd probing

Fsb probing

2-p probing

Δf +Δb ≈Δs +Δd

Δf ≈Δs +Δd

Δs ≈Δf +Δb

unconditional

max(Δf +Δb, Δs +Δd)QoM4p = 1 −|(Δf +Δb) − (Δs +Δd)|

max(Δf, Δs +Δd)QoMfsd = 1 −|(Δf − (Δs +Δd)|

QoMfsb = 1 −|(Δs − (Δf +Δb)|max(Δs, Δf +Δb)

Dem

ote

Pro

mo

te

(Last resort)

Definition of QoM

Select Probing Techniques

Adjust probing technique online based on quality of measurability (QoM)


S D

Probe Probe Probe Probe Probe

Δfs ΔfdΔfs ΔfdΔfs ΔfdΔfs Δfs Δfd Δfd

Probe all nodes simultaneously

Locating Congested Links

Perform coordinated probing for all intermediate nodes


Correlate probes to neighboring nodes

S D

Probe Probe

Switch Point Approach

Congested link is identified

Congestion

Locating Congested Links


S DLink C (Identified congested link)

Link C

Congestion Status

Time

Congestion

Use fast rate end-to-end probing

Tracing Congestion Status

fffffffffffffff


1 2 3 4 5 6 7 81 3 4 6 72 5

0.37s on/off 0.71s on/off 0.53s on/off

0.47s on/off 0.83s on/off

12111098 10 119

Topology: 12 nodes (PCs), 11 links

Link: 100Mbps, 2ms

Cross traffic: Each consists of 3 parallel TCP flows, 50% time on and 50% time off.

Build multiple bottlenecks: Cross traffics are added to corresponding links concurrently.

Emulab Experiment Example


1 2 3 4 5 6 7 81 3 4 6 72 5


12111098 10 119

At the Beginning



1 2 3 4 5 6 7 81 3 4 6 72 5



12111098 10 119

After adding backward bottlenecks



1 2 3 4 5 6 7 81 3 4 6 72 5


12111098 10 119


After adding two more forward bottlenecks



Optimizing Pong in the Internet

Set queuing delay threshold– Based on distribution of queuing delay samples

Tune other parameters based on experiments on the PlanetLabMinimize measurement errors– Detect and react to anomalies (clock skews, router alterations, ICMP

queuing, etc)– Use instantaneous quality of measurement value as sample weight

Quantify measurement quality– Help select vantage points


Quantify Measurement Quality

Help select vantage points

Link measurability score1. Probing technique and quality of measurability

2. Queuing delay threshold quality

3. Observability score• Congestion observed on a less frequently congested link

can be blurred by a much more frequently congested link on the same path.


Conclusion

Pong – a tool specialized in measuring a subset of non-edge links exhibiting repetitive congestion

Coordinated probing

Infer underlying path topology conditions

Select probing techniques online– Quality of measurability

Quantify measurement quality– Link measurability score


Thank you!

Questions?

monitoring persistently congested internet links

Documents

congested internet linksat

congested internet linksafter

edukarl deng monitoring

subset of nonedge links

f b s dpath pattern

pathcombine endtoend

corresponding links

ttl limited probes