An Analysis of the Skype Peer-to-Peer internet telephony Protocol[Skype的对等网络网络电话协议分析](PPT-48).pptVIP

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2018-03-02 发布于江西
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An Analysis of the Skype Peer-to-Peer internet telephony Protocol[Skype的对等网络网络电话协议分析](PPT-48).ppt

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An Analysis of the Skype Peer-to-Peer internet telephony Protocol[Skype的对等网络网络电话协议分析](PPT-48)

User Search (cont’d):UDP-Restricted Firewall SuperNode performs search TCP TCP 16B 52B 406B 1104B … * Intermediate Node Search Caching Local caches cleared on User B client User A User B User A User B Search “User B” Search “User B” (6-8 secs) (3-4 secs) Search Results Search Results ? * Call Signaling TCP Challenge-Response Mechanism If calling a non-buddy, search function is first performed At the end of the Call Signaling phase, the Callee’s Skype client will “ring” * Call Signaling (cont’d)Public IP * Call Signaling (cont’d)Caller Behind NAT Another online node relays TCP signaling packets NAT Caller Callee Online Node * Media Transfer Internet Speech Audio Codec (iSAC) Frequency range: 50-8000Hz Public IPs communicate directly NAT users use a media proxy Uses UDP Transport if possible 67 byte UDP voice packets 5 kilobytes/sec UDP-restricting firewall users communicate over TCP No Silence Suppression * Why No Silence Suppression? Voice packets continue flowing during periods of silence For UDP connections, this allows Skype to maintain NAT bindings For TCP connections, it is ideal to avoid drops in congestion window * On the use of proxy nodes… Enables users behind NAT and Firewall to talk Natural solution for conferencing However, creates lots of traffic on the proxy Remember, these are regular (mostly unsuspecting) Skype users! * Conferencing A: 2GHz P4 w/ 512MB RAM B, C: 300MHz P2 w/ 128MB RAM “Mixer” elections – A always wins * Additional Findings If a Skype call is put on “hold,” a packet is sent every 3 seconds (think lack of silence suppression) 2-byte SN Keep-Alive messages every minute To maintain reasonable call quality, Skype needs roughly 4 KB/s of available bandwidth The same user can log in from multiple machines simultaneously Buddy lists stored locally Cannot select your own SuperNode by manually populating the Host Cache with a Skype Client’s IP * Outline Skype Overview/Network and NAT Refresher Experimental Setup Skype Components INFOCO