Skype Relay Calls
Measurements and Experiments
Wookyun Kho
Salman Abdul Baset
Henning Schulzrinne
Contents
• Background
• Experimental Setup
• Results and Discussion
– Factors impacting Skype relay calls
– Characterizations of Skype relay nodes
• Conclusion and Future Work
Background
SN B
SN A
Media
SC 1
SC 2
Host cache (HC)
NAT1. A
2. B
3. C
4. D
5. E
6. …
Skype Network
NAT
SN C
SN E
Relay Node (RN)
SN D
SC = Skype Client
SN = Super Node
Experimental Setup
Internet
Unrestricted
SC 1
SC 2
Internet
NATed
SC 1
NAT
NAT
SC 2
Internet
Direct-blocked
SC 1
Block SC 2
SC 2
Results and Discussion
• Factors Impacting Skype Relay Calls
– Internet connectivity, host cache.
• Characterizations of Skype Relay Nodes
– Relay distribution (geographical, ISP, AS)
– Round trip time (RTT)
– Call distribution over relay nodes
– Uptime of relay nodes
Factors Impacting Skype Relay Calls
Experimental Setups
Unrestricted
NAT
Total
Direct-blocked
HC delete Not delete HC delete
Not delete
Call trials
867
4,649
5,658
15,774
11,680
38,628
Succ calls
867
339
5,567
2,586
8,597
17,962
Succ rate
100%
100%
7.3%
7.3%
98.4%
98.4%
16.4%
16.4%
73.6%
73.6%
46.5%
# Possible Failure Reasons of Skype Relay Calls
a) The stale information about the IP address and port number of the callee
and its SN in the cache of other Skype nodes.
b) The inability of Skype to find a relay at the time of call establishment.
Relay Distribution (1/3)
Geographical distribution
Relay Distribution (2/3)
Domain name suffix
vs. % RNs
Domain name suffix
vs. % RCs
Relay Distribution (3/3)
Organization
AS #
% of RNs
% Succ Calls Median RTT
(ms)
6128
9.2
6.1
15.3
12271
7.3
5.9
16.4
812
4.5
2.5
52.1
SBC
7132
4.4
2.5
46.6
Comcast
7015
4.4
3.6
16.2
AOL
1668
3.9
3.4
95.6
33287
3.7
2.8
30.1
14
3.1
17.4
0.28
Cox
22773
2.9
1.7
65.6
Comcast
33657
2.6
2.0
34.5
Cable Vision
RR-NYC
Rogers
Comcast
Columbia Univ.
Top Ten ASes
Round Trip Time (RTT)
Total
Average : 52.2ms
Median : 43.6ms
Partial Median
NAT : 95.6ms
Direct-blocked : 13.3ms
Call Distribution Over Relays (1/2)
Through RNs in US
- NAT : 81.2%
- Direct-blocked : 92.4%
Uptime of Relays
Uptime
Maximum : 42.5 days
Median : 3.5 hrs
Downtime
Maximum : 63.5 days
Median : 2 hrs
Median lifetime cycle
3.5 + 2 = 5.5 hrs
Median uptime
probability
63.6%
Conclusions and Future Work
• The success rate of Skype relay calls depends on:
– The network conditions
– The presence of a host cache
– Caching of callee reachable address by super nodes
• Skype’s relay selection mechanism can further be
improved.
– The median one-way network latency for relay calls in a
NAT setup is 95.7ms.
Q&A