Can you HEAR me now?

Transcription

1 Can you HEAR me now?

2 SEE Can you HEAR me now?

3 Video Telephony for End- consumers q Video Telephony: conceived in 1920 s business solueons q Recent AdopEons by End- consumers ichat Google+ Hangout Skype

4 Technical Challenges v RealEme voice/video transmission over best- effort Internet: ² high- rate (>500kbps) ² low- delay (<350ms) ² users highly sensieve to voice/video quality degradaeon

5 Reverse Engineering q Key QuesEons System Architecture Video GeneraEon and AdapEon User perceived Voice/Video Delay Performance Loss Recovery q DifficulEes all three systems use proprietary protocols, encrypt data and signaling, very li^le public informaeon on design choices

6 Methodology q Measure each system as black- box through controlled experiments probe the black- box under different network condieons record those systems behaviors analyze recorded traces to infer their designs q Agenda Skype Two- party video calls (Jan 2011 June 2011) MulE- party Video Conferencing (June 2011 May 2012) video telephony on mobile (June 2012 Present)

7 Testbed q Network emulaeon: BW capacity, propagaeon delay, packet loss q Video call emulaeon Repeatable video sequence Akiyo (experiment results comparable) Inject through virtual video camera q Data colleceon TCPDump for packet- level informaeon Store applicaeon display window for video- level informaeon

8 System Architecture v ichat: Central Peer- to- Peer v Google+: Server- centric ² each user connects to his local proxy server v Skype: Hybrid ² two- party call: voice and video using P2P ² mule- party: voice using central P2P, video relayed by servers ² video relay servers all located somewhere close to NJ/NY area

9 Video GeneraEon/AdaptaEon q If receivers download BWs are different, how sender generates and adapts his video? q ichat: one- version encoding heterogeneous receivers always receive same video version (determined by the weakest receiver) q Skype: source- side mule- version encoding source may generate muleple video versions and send them to relay server à large source upload bandwidth overhead receiver downloads from server a version matching his BW q Google+ employs layered video coding source video is encoded into muleple layers; high layers decodeable iff lower layers are received receivers get different subsets of layers, more layers à be^er quality

10 Delay Performance q End- to- end voice/video delay perceived by users video capturing and encoding delay at sender side network transmission delay + server/super node process?me (if exist) video decoding and playback delay at receiver side Internet Produced network packets Received network packets One- way Video Delay

11 Delay Performance over Internet q Round- trip Video Delay Measurement Running Current Clock Received Clock Clock From B Roundtrip Delay = s s = 1380ms Received Clock From A User A User B Internet

12 Delay Performance over Internet q Delay Experiments between Hong Kong and New York show that Skype mule- party s delay performance is much worse! q A larger measured delay could be due to: larger video packet delay - - propagaeon delay and queue delay video packet loss - > undecodeable frame - > long video delay

13 Behind the Scene architecture q Skype relay servers in one locaeon video flows traverse Internet all the way q Google+, Cross traffic Video packet proxy servers close to users worldwide transmission between servers over google backbone Internet Relay Server Internet Inter net Google Backbone Inter net Alice s Local Server Bob s Local Server

14 Behind the Scene loss recovery q How to recover from packet losses under Eght delay budget? q ConvenEonal Wisdom Retransmission would incur long retransmission delays Forward- Error- CorrecEon (FEC): add redundancies to do proteceon q ichat retransmits lost packet once or twice most vulnerable to losses q Skype uses FEC FEC raeo > 50% for source acceptable quality with <10% random loss q Google+ uses seleceve/persistent retransmissions reliably transfer important video frames acceptable quality with up to 40% random losses!

15 More details q Poster q Papers Profiling Skype Video Calls: Rate Control and Video Quality, Xinggong Zhang, Yang Xu, Hao Hu, Yong Liu, Zongming Guo, and Yao Wang, IEEE Conference on Computer and Communica>ons (INFOCOM 2012), March, 2012 Video Telephony for End- consumers: Measurement Study of Google+, ichat, and Skype, Yang Xu, Chenguang Yu, Jingjiang Li, and Yong Liu ACM Internet Measurement Conference (IMC 2012), November, 2012

16 Q & A Thanks!