Analyzing Cross-layer Interaction

in Overlay Networks

Srinivasan SeetharamanSeptember 2007

22

Overlay Networks

Overlay networking helpsovercome functionalitylimitations of the Internetby forming a virtual networkover the native IP networkthat is:

Independent Customizable

33

Service Overlay Networks

Offer enhanced or new services by deploying intelligent routing schemes.

Relaying

Overlay link

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Service Overlay Networks (contd.)

Characteristics: Nodes and links are persistent Perform overlay routing independent of native layer

routing Each Overlay path comprises one or more Overlay

links, based on a certain selfish objective

Many types of services can be offered Multicast (e.g. ESM, Overcast) QoS (e.g. OverQoS, SON) Security (e.g. DynaBone, SOS) Better routes (e.g. RON, Detour, X-Bone)… and much more

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Cross-Layer Interaction

Performing dynamic routing at both overlay and native IP layers leads to:

Conflict due to mismatch or misalignment of routing objectives

Contention for limited physical resources

Functionality overlap (Both overlay layer and IP layer perform similar set of functions)

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Cross-Layer Interaction (contd.)

These issues are amplified in the presence of

Selfish motives and aggressive behavior

Lack of information about other layer

Increasing impact ( #overlays |Traffic| )

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Context

Native network topology Intra-domain Inter-domain

Attitude of native network Restrictive Oblivious Cooperative

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Thesis Organization

. Oblivious Restrictive Cooperative

Intra-domain

Interaction with failure recovery

[Chapter III]

OR vs Load balancing

[Chapter V]

Overlay-friendly native

network[Chapter VII]

Inter-domain

OR vs Policy constraints[Chapter IV]

BT vs Load balancing

[Chapter VI]

INTERACTION BETWEEN FAILURE RECOVERY IN THE NATIVE

AND OVERLAY LAYERS

Chapter III

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Dual Rerouting

A

OVERLAY1

LAYER

NATIVE IP

LAYER

X

A

D

E F H

FH

GA

EC

C

BD

B

G

Each layer performs rerouting, with no knowledge of which layer leads to optimal restoration

Overlay reroutin

g

Native rerouting

Failure

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1. Overlap of functionality between layers causing Unnecessary route changes (esp when connectivity in

native network is very dynamic) Increased probing overhead

2. Unawareness of other layer’s decisions leading to Multiple simultaneous failures

3. Lack of flexibility and control

Downside to Dual Rerouting

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Tuning Dual Rerouting

Intra-domain (keepAlive-time = 1 sec, hold-time = 3 secs)

Dual Rerouting

Suppress overlay rerouting at 0.5 prob.

Defer overlay rerouting by 0.375 secs

Native-only rerouting

Average route changes 125.08% 101.59% 109.85% 1.567

Stabilized inflation 100% 108.32% 100% 1.202

Time when stable 113.7% 100.48% 107.33% 2.481

Peak inflation 114.22% 109.98% 110.73% 1.202

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Further Improving Recovery

Adjust the functioning of native layer:

Tuning the native layer keepAlive-time:

This produces the best tradeoff

between # of route changes,

stabilization time and recovery time

keep

Alive

-tim

e

keep

Alive

-tim

e

Tuning

INTERACTION BETWEEN OVERLAY ROUTING AND TRAFFIC ENGINEERING

Chapter V

2121

Repeated Non-Cooperative Game

Player1: Overlay Routing - Latency-optimized paths between nodes

Player2: Traffic Engineering - Optimal load-balanced routes

OverlayRoutingOverlay Link

Latencies

Overlay layertraffic

Overlay routes

TrafficEngineerin

g

Traffic on each overlay

link

Background traffic

Nativeroutes

Native linkdelays

TM

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Simulation Results

TEobjective

Overlayobjective

Overallstability

Round

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Our goal

.. is to propose strategies that

obtain the best possible performance for a particular layerwhile steering the system towards a stable state.

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Assume: Each layer has a general notion of the other layer’s selfish objective

Designate leader / follower

Operate leader such thata. Follower has no desire to change Friendlyb. Follower has no alternative to pick Hostile

Use history to learn desired action gradually.

Resolving Conflict – Our Approach

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Performance of Preemptive Strategies

We proposed four strategies that improve performance for one layer and achieve a stable operating point

Inflation factor= Steady state obj value with strategy

Best obj value achievedLeader Strategy Overlay TE

Overlay Friendly: Load-constrained LPHostile: Dummy traffic injection

1.0821.023

1.1221.992

Native Friendly: Hopcount-constrained LPHostile: Load-based Latency tuning

1.0271.938

1.1841.072

Inflation

Chapter VI

CROSS-LAYER INTERACTION OF PERFORMANCE-AWARE OVERLAY

APPLICATIONS

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BitTorrent File-Sharing

Popular file-sharing application that generates a large volume of Internet traffic

Characteristics: Service capacity increases with demand Centralized tracker regulating neighborhood Dynamically change active peers by

choke/unchoke protocol

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Comparison to Overlay Routing

AX

Data1

BY

Data2

A2

A1

B2B1A3

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BitTorrent Protocol

Tit-for-tat based incentive for uploading decisions Leecher: Unchoke the fastest uploaders Seed: Unchoke the fastest downloaders

Popular strategy to improve performance Optimistic unchoke: periodically look for faster

peers

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UnchokeRequest

BitTorrent Dynamics

B

X

A

L1

L2

ED

C

When bottlenecked on link L1

ChokeChoke

Load distribution across links is balanced

BitTorrent apps use all available b/wPeer TFT

Upload stats Download stats

Status Opt? Interested Pieces? Status My interest

Unchoke

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BitTorrent Dynamics

B

X

A

L1

L2

ED

C

When NOT bottlenecked on link L1

ChokeChoke

Load distribution across links is unbalanced

Unchoke

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Cross-Layer Interaction

Operating BitTorrent disrupts load balance and can result in high max util: This can be a problem for background traffic

Objective of native layer: Minimize ( Max Util.)

Objective of BitTorrent: Minimize (Overall finish time)

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Simulation Setup

Pick 100 ASes with 60% of them being non-stub ASes

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Simulation Setup

A

B

C

D

E

F

Generate 1-50 peers. Each associating with 1-3 torrents

F

L1

L2

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Simulation Performance Metrics

Max util across access links

= MaxaE ( Xa/Ca ), E is set of all links

X is the load, C is the capacity

Average finish time inflation of leechers

= 1/Nl ( ’i / i ) Nl is # of leechers

’ is finish time after strategy

Nl

i=1

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Reducing Impact – Traffic Engg

TE can be performed across inter-domain access links, in order to minimize (Max util)

Two flavors: Ingress / Egress

Determines which access link to pick for a certain destination or source IP address

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Reducing Impact – Traffic Engg (contd.)

Applying TE does not make much difference

Performance of a random AS (Focus AS)

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Alter certain BitTorrent protocol components or tune the associated parameters Minimal reduction of the max util Significant inflation of finish time

Specifically, we tried each of the following: Make peer selection random Make piece selection random Reduce duration of optimistic unchoking Freeze list of unchoked peers after 10 mins Tune the unchoking timers

Reducing Impact – Tuning BitTorrent

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Reducing Impact – Locality-awareness

Locality-based traffic management Give priority to peers within AS No change to BitTorrent clients Also try caching of requests sent outside AS

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Reducing Impact – Bandwidth Throttling

Limit bandwidth consumed by BT traffic Popular strategy among most ASes Involves lesser infrastructure cost

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Cross-layer Conflict

Native layer and BitTorrent layer constantly retaliate to other layer’s disruptive behavior

Peers deploy BitTorrent Protocol Encryption to avoid detection by native layer

We develop two “friendly” BitTorrent strategies that achieve a mutually agreeable point by reducing peak load

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A. Limit # of parallel downloads

The unchoking protocol and their timeline is uncoordinated across neighborsA

vera

ge

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A. Limit # of parallel downloads (contd.)

Reduces peak load from 0.94 to 0.852Finish time inflation is 1.1501

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B. Avoiding common neighbors

Problem is that two peers in same AS often contact same peer outside AS

Algorithm Perform bilateral info exchange where each

peer A finds out if its neighbor B has a neighbor C inside its own AS

If yes, toss a coin to determine if we can download from this peer B (Randomization acts as a load balancing strategy)

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B. Avoiding common neighbors (contd.)

Reduces max util from 0.94 to 0.85Finish time inflation is 1.187

ANALYZING INTER-DOMAIN POLICY VIOLATIONS IN OVERLAY

ROUTES

Chapter IV

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Inter-Domain Policy Violations

Two types of violations exist

Provider 1

Provider 1

Client 1Client 1

A

Client 2

Client 2

BClient

3Client

3C

Provider 2

Provider 2Peer

Legitimate native route

Overlay route

Exit violation

$

Transitviolation

$$

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Each transit violation has a corresponding exit violation upstream

Extent of exit policy violations in multihop paths

Measurement Results

Violation Type % paths

Next hop AS violated 72.05

Exit point violated 15.63

Total 87.68

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Policy Enforcement by Native Layer

As ISPs become aware of the negative impact of overlays and commence filtering, this leads to

drastic deterioration in overlay route performance commensurate with the number of ASes enforcing policy

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Overlay Service Provider (OSP) adopts a combination of the following strategies for achieving good legitimate paths:

1. Obtain transit permit from certain AS for $T

2. Add new node to certain provider AS for $N

3. Obtain exit permit from certain AS for $E

Resolving Conflict

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With no filtering,

Illustration of Mitigation Strategy

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21

32

22

11 13

23

33

Cust-Prov relation

Peering relation

Transitviolation

AS hosting overlay node

Tier-1 provider

Tier-2 provider

Stub customer

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With filtering, we have no multi-hop paths

Illustration of Mitigation Strategy (contd.)

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21

32

22

11 13

23

33

Cust-Prov relation

Peering relation

AS hosting overlay node

Tier-1 provider

Tier-2 provider

Stub customer

8080

Option 1: Add new overlay node to provider AS 22

Option 2: Obtain transit permit from stub AS 32

Illustration of Mitigation Strategy (contd.)

31

21

32

22

11 13

23

33

Cust-Prov relation

Peering relation

AS hosting overlay node

Tier-1 provider

Tier-2 provider

Stub customer

22

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Objective of Mitigation Strategy

For a certain budget, determine which ASes to obtain transit permit from to add new node to to obtain exit permit from

… so as to achieve the best possible gainGain = Native route latency – Overlay path

latency Native route latency

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Mitigation Results

When all permit fee = P, new node fee = N

Add new node

Permit

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Summary of Cross-Layer Interaction

Overlays offer valuable services needed by end-systems. But, lead to complex cross-layer interaction with potentially detrimental effects

Layer awareness is essential to reduce negative effects and to improve performance of both layers. We propose simple strategies that achieve this goal in an effective manner.

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Contributions of Thesis

Knobs for better control over the cross-layer interaction

Analysis and mitigation of the conflict in objective between native and overlay layers:

inter-domain: OR vs Policy enforcement intra-domain: OR vs Traffic engg

Ways to improve coexistence between BitTorrent file-sharing and native layer

Framework for network layer support of overlay services

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Future of Overlays

Overlays are essential as… Means for end-systems to collaborate Environment for testing future innovations (GENI) Architecture for Future Internet in the form of Network

Virtualization

Cross-layer interaction will affect performance. How best to design protocols and services in the future?

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Future Work

Need to address the network impasse. How to tune the network for

.. the new breed of Internet applications? (e.g., file sharing) …and new paradigms of communication? (e.g., wireless)

Which layer to implement a service at? For example, a service like multicast can be performed at both native layer and overlay layer!

Which layer to use for a particular scenario? How can the other layer support this service?