Drafting Behind Akamai (Travelocity-Based Detouring)

Dr. Yingwu Zhu

Motivation

• Overlay networks– Build the “view” of the underlying network– Passive and active measurements• To determine overlay paths according to some metric,

e.g., latency?• Redundant, non-scalable, & overhead (expensive!)

• What common service we may need in place?– An Internet weather-report service

Proposed Approach• Fact– CDNs (e.g., Akamai) perform extensive network and

server measurements• Publish the results through DNS over short time scales

• Can overlay networks recycle measurements collected by production CDNs?– Significantly reduce the amount of measurements (a

complementary service) – No new infrastructure need to be deployed– Inherit the robustness of DNS– Easy integration with existing systems

CDN-Driven One-Hop Source Routing

S

A1

An

A2

……

..

D

DNS Server

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E2

E1

Key Questions

• How does Akamai work?– DNS translation– How many web replicas does a client see?– Impact of different sites (e.g., Yahoo vs. NY Times)?– DNS redirection dynamics?– Network or server latency?

• An example application: one-hop source routing– Mapping CDN servers to overlay nodes– Low-overhead protocols for exploiting CDN

redirections

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DNS “Black Magic”

WebClient

Akamai High-LevelDNS Server

Akamai Low-LevelDNS Server

Local DNS ServerPCWorld AuthoritativeDNS Server

PCWorld Web Server

AkamaiEdge Server

Root DNS Server

Top-Level Domain DNS Server

www.pcworld.com

www.pcworld.comwww.pcworld.com

www.pcworld.com

http request/response

images.pcworld.com

CNAME: a1694.g.akamai.netimages.pcworld.com

a1694.g.akmai.net

a1694.g.akmai.net2 ip addresses of Akamai Edge Servers

2 ip addresses of Akamai Edge Servers

fetch image files

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Measuring Akamai

• 2-months long measurement• 140 PlanetLab (PL) nodes

– 50 US and Canada, 35 Europe, 18 Asia, 8 South America, the rest randomly scattered

• Every 20 sec, each PL node queries an appropriate CNAME for– Yahoo, CNN, Fox News, NY Times, etc.

PL Node

AkamaiEdge Server 2

AkamaiEdge Server 3

Akamai Low-LevelDNS Server

.……

AkamaiEdge Server 1

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Initial Results

day

night

Berkeley Purdue

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Server Diversity for Yahoo

Good overlay-to-CDN mapping candidates

Majority of PL nodessee between 10 and 50 Akamai edge-servers

Nodes far away from Akamaihot-spots

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Multiple Akamai Customers

Amazon is not hosted on UMass and LBNL

Yahoo and NYTimes are not hosted in U. Oregon

FEMA is poor!

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Redirection Dynamics

Berkeley Brazil

Korea

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Key Questions

• How does Akamai work?– DNS translation– How many web replicas does a client see?– Impact of different sites (e.g., Yahoo vs. NY Times)?– DNS redirection dynamics?– Network or server latency?

• Potentials for one-hop source routing?– Mapping CDN servers to overlay nodes– Low-overhead protocols for exploiting CDN

redirections

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Methodology

Planet Lab Node

Akamai Low-LevelDNS Server

10 Best Akamai Edge Servers

………

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Do CDN redirections correlate to network latencies? (1)

• Rank = r1+r2-1– 16 means perfect correlation

– 0 means poor correlationBrazil is poor

MIT and Amsterdam are excellent

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Do CDN redirections correlate to network latencies? (2)

CNN is poor!

70% paths are among best 10%

97% better than average

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Akamai-Driven One-Hop Source Routing

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A1

An

A2

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DNS Server

Redirection dynamics sufficiently small for network control

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E2

E1

Redirections driven by network conditions

Potential for CDN-to-overlay mapping

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Methodology

Source

Akamai Low-LevelDNS Server

Destination

Direct PathAkamai path 3

Akamai path 1

……

....

.

Akamai path 10

Akamai path 2

Akamai path 1

Akamai path 2

Akamai path 3

Akamai path 10

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Akamai-driven source routing (1)

80% Taiwan15% Japan5 % U.S.

75% U.K.25% U.S.

Taiwan-UK UK-Taiwan

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Akamai-driven source routing (2)

Direct paths betterthan Akamai paths

25% of Akamaipaths better thandirect pathsAkamai and direct

paths have similar performance

Experiment: US (6), Europe (3), S. America (2), Asia (3)

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Path pruning

• Fact– Not always is Akamai-driven path better than the

direct one

• Practical issues– How frequently to make a decision whether to use

the direct or the Akamai path?– Should one use • the first (of the 2 paths) returned by Akamai (FAS)• the better (of the 2 paths) returned by Akamai (BTAS)

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Path Pruning Result

Direct path accounts for 78% of the gain

2 hours update frequency before the performance declines

BTAS better than FAS

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Conclusions

• Reuse measurements performed by CDNs• Reverse-engineering Akamai– DNS redirections sufficiently small– Strong correlation to network conditions– All clients see a large number of paths

• CDN-driven one-hop source routing– 25% of Akamai paths outperform direct paths– 50% of nodes “discovered” by Akamai outperform

direct paths– Low-overhead pruning algorithms

• Global Internet “weather-report” service for little to no cost