Skype Traffic Identification full report
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Joined: Mar 2010
31-03-2010, 01:53 PM
SKYPE TRAFFIC IDENTIFICATION
A P2P VoIP network.
Instant messaging (chat) and file transfer is also possible within the Skype infrastructure.
Skype uses strong encryption to secure communications inside the whole Skype network.
The Skype P2P network consists of the following elements:
ordinary nodes (Skype clients, SCs)
Used for communication.
super nodes (SNs)
switching elements in the overlay network.
Stores the account information of users.
Used to update the software.
Contains the contact list of the user.
SC launched to establish a connection with SNs.
First run of client SNS are contacted and they are stored and used by the client during its next run.
SCs always connect to two servers:18.104.22.168 or 22.214.171.124.
When the user is on-line, there is a periodic message exchange between the host of the user and the selected SN.
SKYPE SIGNAL FLOW IDENTIFICATION
A SC maintains one permanent connection to a SN while the user is logged on to the Skype network.
At startup the client tries to establish several outbound connections to find an appropriate SN.
After few seconds transient connections are terminated and only one or two permanent TCP connections remain.
The timing of outgoing packets follows a well-defined pattern.
A TCP connection is regarded as SKYPE signaling if it obeys the rule:
1. Outbound and inbound data rate is not larger than 40
2. Number of packets per second (in outbound and inbound
direction separately) is not larger than 0.4 Packets/sec.
3. Every packet in outbound direction is smaller than 1000
bytes (including IP and TCP headers).
4. Periodicity of 1 minute is observable for outbound
packets of size between 70 byte and 250 byte. E.g. a
certain percentage of packets (between 70 B and 250 B in
size) arrive in a specific, periodic time slot.
SKYPE TRAFFIC SOURCES
Skype Message Framing Time
The time between two subsequent Skype Message
RF (Redundancy Factor)
The number of past blocks that Skype retransmits
SKYPE VOICE-TRAFFIC IDENTIFICATION
Skype prefers UDP as the primary transport protocol.
Switches to TCP when UDP is restricted.
Switches voice codec and transport protocol in middle of a call.
A loose upper bound of 400B for packet size and 128 kilobit/sec for flow bandwidth is defined.
A rate of 13 packets/sec is chosen as a lower bound and 53 packets/sec as an upper bound.
Inbound and outbound flows are paired to one another to create voice connections (sessions).
Flows not corresponding to the packet rate condition are discarded.
In call records about 1000 ADSL subscribersis multiplexed before entering the ATM access network.
Verification measurement carries traffic of 100 users.
In both measurements only IP and TCP/UDP headers were logged.
Flow level information is extracted from the traces including source addresses, ports, packet number, transmitted bytes, start time and end time of the flow.
For accurate identification both inbound and outbound traffic are required.
The purpose of validation is to verify the parameters of the identification method.
The signal flow identification method works well.
Login-Buddy-list and SN connection were rarely identified.
connection cannot be taken into account.
Only single event is detected i.e. login , update etc. and itâ„¢s unknown when user leaves skype network.
Calls are coming more frequently during daytime.
The bandwidth of Skype calls is usually between 18 and 70Kbps, typically around 40 Kbps.
Three peaks in the histogram of the packet rate of Skype speech flows, which correspond to the typical three inter-arrival times (20, 30 and 660 ms).
The typical packet size (including IP and TCP/UDP headers) is somewhere between 100 B and 200B, which is also confirmed by our test measurements.
Smaller packet size â€œ and bandwidth â€œ occurs in one direction when separate inbound and outbound TCP flows belong to the call.
Exponential distribution of skype calls duration.
Skype host are detected using traditional IP.
The algorithm uses only packet headers and the extracted flow-level information, but no packet payload is necessary.
It expects logged (offline) data as input.
Signal flow identification
Voice traffic identification
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