Transcript Skype - dianyuan

VoIP Case Study: Skype
Dr. Danny Tsang
Department of Electrical & Electronic Engineering
Hong Kong University of Science and Technology
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Phone becomes “the Portal” ...
presence
p re s e n c e
audio
a u d io
Instant
In sta n t
messaging
m e ssa g in g
video
vid e o
XML
XM L
data
d a ta
HTTP
HTTP
… through which a whole new
world of services are delivered
VoIP Case Study
What Is Skype?
 It is a peer-to-peer (P2P) overlay network
for VoIP and other applications, developed
by the people who did KaZaA
 To a user, it is an Instant Messaging (IM)
system that supports P2P VoIP and other
applications
 The fee-based SkypeOut service allows
calls to regular phone numbers
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Skype - a raving success
 Skype - computer <-> computer telephony

You can talk to other computer based phones free
 Skype Out - computer -> PSTN telephony

You can call normal PSTN telephones - and pay for it
 Skype In - PSTN telephony -> computer telephony
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You can be called from PTSN telephones
You get a normal phone number
 Currently on-line: ~1.8 million users
 ~150 million skype software download (www.skype.com)
 Skype hardware phones available
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Overview of Skype P2P Network
Major Entities
 Login Server
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Skype login Server
Single central server
User authentication at login
(private/public keys)
Ensures uniqueness of Skype user
names
Message exchange with
The login server during login
 Super nodes
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Nodes with public IP address,
enough CPU, memory, and network
bandwidth
Registrar functionality
(distributed directory, presence
service)
Signaling/media proxy (reachability
of ordinary hosts)
 Ordinary nodes

Hosts behind FW/NAT
Ordinary Host
Super node
Neighbors relationships in the Skype network
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Highlights of Skype
P2P-based signaling
 to find and locate users, Skype uses "supernodes" that are running on
peer machines (In contrast, traditional VoIP systems use fixed central
servers)
 little management structure and overhead
 highly scalable
 Strong NAT/firewall traversal capability


e.g., use a peer relay to connect clients behind NATs, or use a TCP tunnel to a peer relay to
bypass a UDP blocking firewall
Better voice quality than the MSN and Yahoo IM applications
 Wideband voice codec: allow frequency between 50-8000Hz to pass
through
 Heavy software-based DSP operations at clients, including codecs,
mixer and fancy echo cancellation, e.g., Thinkpad X31, an active Skype
session consumes around 10-20% of CPU
 Security feature:
 encrypts calls end-to-end, and stores user information in a
decentralized fashion
 Integrated buddy list, presence information, chat, and audio

conferencing
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Reverse Engineering of Skype
 Based on “An Analysis of the Skype Peer-to- Peer Internet
Telephony Protocol” by S. Baset and H. Schulzrinne at
Columbia, September 15, 2004

“We observe, …, we think, …, we conjecture…, etc.”
 Terminologies:
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Skype client (SC)
Super node (SN)
Host cache (HC): list of IP addresses and port number of some
super nodes
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Some Observed Facts
 For NAT and firewall traversal, SC uses a
variation of typical schemes such as STUN to
determine the type of NAT and firewall it is
behind. SC refreshes this information periodically.
This information is stored, e.g., in the Windows
registry
 Use TCP to bypass UDP-restricted NAT/firewall
 Unlike its file sharing counter part KaZaa, a SC
CANNOT prevent itself from becoming a SC
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Skype Functions
 Startup
After installation, SC sends a HTTP 1.1 GET to
skype.com
Login
User search
Call establishment and tear down
Media transfer
Presence messages
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Skype Login
1. Advertises its presence to other peers and
its buddies.
2. Determines the type of NAT and firewall it
is behind.
3. Discover online Skype nodes with public IP
addresses.
•Stores Skype user names and passwords and ensures that Skype user
names are unique across the Skype name space.
•Login server: ns14.inet.telt.dk(Denmark)
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Skype Login algorithm
authentication with login server is not shown
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Skype Login in details
 A SC authenticates its user name and password with the login
server, advertises its presence to other peers and its buddies,
determines the type of NAT and firewall it is behind, and discovers
online Skype nodes with public IP addresses
 These newly discovered nodes were used to maintain connection
with the Skype network should the SN to which SC was connected
became unavailable
 SC must establish a TCP connection to a SN in order to connect to
the Skype network

For a given entry in HC, SC tries to make the connection in the following
order: TCP with the IP address and the port listed, the IP address and
HTTP port (80), the IP address and SHTTP port (443)
 After SC is connected to a SN, the SC gets the IP address of the
Skype login server, and authenticates the username and password
with the login server
 SC sends UDP packets to 22 distinct nodes at the end of login
process
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Advertise its arrival
Build an “alternative node tables” that provide a list of available nodes
SC can use should its current SN becomes unavailable
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User Search
 Perhaps use Chord-like hash-based
methods, combined with traditional blind
searching techniques
 SN acts like a search proxy for CN and
caches searched results
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Use of An Immediate Node
 Two reasons
Passing NAT and firewall
 Acting as a mixer for conferencing traffic

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Conferencing
A
A+C
C
B
B
A+B
C
 CPU and bandwidth usages at the bridging point (e.g., the A
node above) increase with # conference participants
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Conferencing
A
A+C
B
B
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A+B
C
C
A call was established between A (the most powerful one) and B. Then B
decided to include C in the conference.
It mixed its own packets with those of B and sent them to C over UDP and
vice versa
Even if user B or C started a conference, A, which was the most powerful
amongst the three, always got elected as conference host and mixer.
If iLBC codec is used, the total call 36 KB/s for a two-way call. For threeuser conference, it jumps to 54 kb/s for the machine hosting the conference.
For a three party conference, Skype does not do full mesh conferencing.
To host a conference with 5 parties you need a big PC, a Pentium 4 or
thereabouts. With a PIII CPU of 450 MHz you will be limited to hosting 3
parties.
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Reflection of Skype
Pros:
Cons:
 “scales as it grows”
 Proprietary protocols
 Ease of use (self
 Closed world, no SIP/H.323 GWs


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
configuration)
 No bandwidth control at super
Intelligence at the edge
nodes
Transparent FW/NAT
 Reliability? Are there enough super
traversal
nodes?
Integration of IM,
 Not suitable for business usage,
presence, voice
privacy?
Encrypted communication
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Broad Impact
 Great application for WLAN-enabled handsets


A new opportunity for fixed-line operators
New opportunity and challenge to mobile phone operators and
manufacturers (think when WLAN infrastructure and handsets
become widespread)
 New challenges to SIP/ENUM-based approaches
 Since naming on overlay networks could work well, DNS may
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not need to expand its scope
VoIP is a new must-to-have feature for IM systems
A new view on P2P networks, which previously focused on
content rather than communications
A large number of online Skype VoIP clients distributed
throughout the world can provide valuable network
performance measurements and other information
Other issues, including anonymity vs. law enforcement
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Profound Impact
 Skype has shown, or at least has suggested, the following:
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Signaling, the most unique property of traditional phone
systems, can now be accomplished effortlessly with selforganizing P2P networks
P2P overlay networks can scale up to handle large-scale
connection-oriented real-time services such as voice
 For Skype, the Internet is just a dumb "bit pipes“

We should focus on applications and services that run on layers
above pipes, since pipes or roads always only have relatively
small business value themselves
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Reference
 B. Goode, “Voice over Internet protocol (VoIP),”
IEEE, vol. 90, no. 9, pp. 1495 – 1517, Sep. 2002.
Proceedings of the
 M. Maresca, N. Zingirian, and P. Baglietto, “Internet protocol
support for telephony,” Proceedings of the IEEE, vol. 92, no. 9, pp.
1463 – 1477, Sep. 2002.
 S. A. Baset and H. Schulzrinne, “An analysis of the Skype peer-topeer internet telephony protocol,” Computer Science Department,
Columbia University, Tech. Rep. CUCS-039-04, 2004. [Online].
Available: http://arxiv.org/pdf/cs.NI/0412017
 C. Schlatter, “Why SIP?” 2005, Swiss Education and Research
Network, SWITCH. [Online]. Available:
http://vconf.switch.ch/econf_docs/sip_h323_skype.pdf
 H. T. Kung, “P2P VoIP,” 2004, lecture notes on CS-143/CSCI-E135:
Computer Networks. [Online]. Available:
http://www.eecs.harvard.edu/cs143/
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