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
……
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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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D
DNS Server
Redirection dynamics sufficiently small for network control
En
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