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An Experimental Study of theSkype Peer-to-Peer VoIP
System
Saikat Guha (Cornell)
Neil Daswani (Google)
Ravi Jain (Google)
IPTPS 2006
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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About Skype
I Voice over IP (VoIP)
I 50 million users
I Valued at $2.6 billion
I Proprietary
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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About Skype
“Internet Telephony that Just Works”
I Adaptive voice quality
modem, broadband, T1, . . .
I Works in any network topology
one or more NATs, Firewalls, . . .
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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Outrageous Opinion1 #1
I NAT Traversal in Skype:I Level 0: Initiator NAT’edI Level 1: Recipient NAT’edI Level 2: Both NAT’ed (well-behaved NATs)I Level 3: Both NAT’ed (broken NATs)
Outrageous Opinion
NAT Traversal is essential for P2P
I How to pick NAT Traversal complexity,
and make it scale?1does not, necessarily, reflect the views of all co-authors or employers
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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NAT Traversal in Skype
Skype SkypeNAT
Level 0: Initiator NAT’ed
Solution: Don’t embed IP address in payloadApps: Most old software, almost all new software
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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NAT Traversal in Skype
Rendezvous
Skype SkypeNAT
Level 1: Recipient NAT’ed
Solution: Use Rendezvous ServiceApps: Bittorrent, MSN, Yahoo, Skype, . . .
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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NAT Traversal in Skype
Rendezvous
Skype SkypeNAT NAT
Level 2: Both NAT’ed (well-behaved NATs)
Solution: Use STUN (UDP) or STUNT (TCP)Apps: MSN, Yahoo, Skype, . . .
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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NAT Traversal in Skype
Rendezvous
Skype SkypeNAT NAT
Level 3: Both NAT’ed (broken NATs)
Solution: Use TURNApps: MSN (limited), Yahoo (limited), Skype
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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NAT Traversal in Skype
Rendezvous
Skype SkypeNAT NAT
Level 3: Both NAT’ed (broken NATs)
Solution: Use TURN + P2PApps: Skype
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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Contributions
1. Properties of Skype’s P2PRendezvous-Relay Service
I load (low), structure (yes), churn (low) . . .
2. Skype user behaviorI call-length (long), file-transfer (small), online
activity (predictable) . . .
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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Skype’s P2P Network
I Inherent structure: [Baset et al. INFOCOM ’06]
I Supernodes (SN): no NAT, spare bandwidth
I Ordinary nodes (ON): associate with one (or
a few) supernodes. Don’t contribute to overlay.
I Skype network: FastTrack (likely)I Experiment: Capture traffic for a SN
I at Cornell, 4.5 months, ∼13GB
I Caveat: everything “encrypted”, only one SNSaikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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Skype’s P2P Network
00.10.20.30.40.50.60.70.80.9
1
30k10k3k1k3001003010
CD
F
Bandwidth (Bps)
Control trafficAll traffic
Relayed traffic
Supernode: Low Network Load
� Bandwidth: 51 Bps (median), 7.5 kBps (median relayed),34 kBps (peak)
� Control traffic (∼75%), Relayed traffic (∼10%)Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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Skype’s P2P Network
I Experiment: Estimate no. of active nodes
I Supernodes (SN):I Discover SN: crawl client SN cacheI App-level ping: replay “hello”-packetI 30%–40% of SNs active, 250k foundI Caveat: DHCP assigned SN address
I All Nodes (ON+SN):I Record number of active nodesI .4 million simultaneous usersI Caveat: reported by proprietary client
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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Outrageous Opinion1 #2
I Rough estimate: (just network, not CPU)
I ∼1–2 GBps median relay-trafficI .20 well-provisioned boxes at PoPsI ∼ $10 million per year
I Supernodes mostly at universities (EU, Asia, US)
Outrageous Opinion
Skype freeloads on university bandwidth
I Is there an ISP-friendly commercial
model for P2P?Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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Skype’s P2P Network
20%
40%
60%
80%
100%
Sun 10/2Sun 9/25Sun 9/18
Fra
ctio
n O
nlin
e
Date (noon UTC)
All NodesSupernodes
24h12h0h
UTC Thu 9/22
-5%
0%
5%
24h12h0h
Sup
erno
de C
hurn
UTC Thu 9/22
JoinsDeparts
Supernodes: Diurnal, Work-week Patterns
� Supernodes: low churn, .5% turnover (over 30-min)
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Outrageous Opinion1 #3
I Handling ChurnI a blood sportI Skype, perhaps intentionally, controls
churn (based on NAT, bandwidth, maybe session history, . . .)
Outrageous Opinion
Churn resistance is overrated. Control churn.
I Is there a price-performance tradeoff
between controlling, and handling
churn?Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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Skype Users
0.001
0.01
0.1
1
16d8d4d2d1d8h4h2h1h
P[s
essi
on ti
me
> x
]
Supernode Session Time
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
3h1h30m10m3m1m20s
CD
F
Relayed Conversation Duration
Skype users: differ from file-sharing, phone users
� Session Length: heavy-tailed, 5.5h median� File-Transfer: 346kB median� Call-Length: 12m 53s mean, 3h 26m max
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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Summary
1. Properties of Skype’s P2PRendezvous-Relay Service
I Low Load: NAT traversal, manysupernodes
I Stable: exploits heterogeneityI ISP-unfriendly, taxes universities
2. Skype usersI Different from file-sharing users
(longer session lengths, smaller files transferred)
I Different from phone users (longer calls)
3. Data available on requestSaikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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Discussion
For a peer-to-peer application:
I Complexity and scalability of NAT
traversal
I ISP-friendly commercial model
I Tradeoffs between controlling and
handling churn
www.cs.cornell.edu/∼saikat/skype/
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype
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Outrageous Opinions2
I NAT Traversal is essential for P2P
I Skype freeloads on university bandwidth
I Churn resistance is overrated. Control
churn.
www.cs.cornell.edu/∼saikat/skype/
2does not, necessarily, reflect the opinion of my co-authors, our employers, or
anyone else.
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Supernode Distribution
0%
20%
40%
60%
80%
100%
Sun 10/2Sun 9/25Sun 9/18
Dis
trib
utio
n of
Sup
erno
des
Date (noon UTC)
Europe
North America
Asia
Others
24h18h12h6h0h
UTC Thu 9/22
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Filesize Distribution
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
10M3M1M300k100k30k10k3k1k
CD
F
File Size (bytes)
Relayed File Size
Saikat Guha, Neil Daswani, Ravi Jain Experimental Study of Skype