1. ICDCS’07, Toronto, Canada1 SCAP: Smart Caching in Wireless Access Points to Improve P2P Streaming Enhua Tan 1, Lei Guo 1, Songqing Chen 2, Xiaodong Zhang 1 1 The Ohio State University 2 George Mason University

2. 2 Background Wireless access to Internet is pervasive: On campus, in offices, at home, and public utilities Most are supported by Wireless LANs Peer-to-Peer applications are widely used: Streaming: PPLive, Joost, etc … VoIP: Skype, etc … Large file distribution: BitTorrent, etc … Our Focus: Interaction between wireless users and P2P streaming applications

3. 3 Wired/wireless Communications WLAN Internet Access Point (AP) Wireless users Wired users

4. 4 P2P Streaming for Wired/wireless Users: Workflow WLAN Internet Source Peer Wireless Peer Viewing Peer Access Point

5. 5 P2P Streaming for Wired/wireless Users: Problems WLAN Internet Downstream traffic for other wireless users AFFECTED Generating upstream traffic Streaming quality degraded Wireless Peer (Relay/Viewing) Viewing Peer Streaming content Other packets Source Peer

6. 6 Problem Summary Peers in WLAN may relay streaming content by uploading a lot of traffic: Congest the WLAN due to channel competitions Provide low quality of service to the Internet peers Downstreams have lower priority due to upstreams Extra upstream traffic: further increase the number of transmission errors increase the cost of contention window back-off upstream relay traffic Major problem source: upstream relay traffic  Can we minimize upstream traffic with low overhead? to improve WLAN throughput to improve service quality for Internet peers

7. 7 WLAN Internet Wireless Peer Viewing Peer Access Point The same content is transferred twice in the WLAN!  Duplicated traffic P2P Streaming for Wired/wireless Users: Workflow Source Peer

8. 8 Contributions Our measurements show that > 75% upstream traffic is duplicated with the downstream traffic for three representative applications SCAP: Smart Caching in the Access Point for minimizing upstream traffic: design & prototype implementation Evaluation results show SCAP can improve the throughput of the WLAN by up to 88%: SCAP also reduces the delay to Internet peers

9. 9 Outline Problem Summary and Contributions Measurement & Analysis of P2P Streaming Traffic SCAP Design & Implementation Evaluation Summary

10. 10 Measurement & Analysis of P2P Streaming Traffic Aim to answer two questions: How much duplicated traffic in practice? How much overhead in identifying such duplications? Measurement: Collect traces of three representative P2P live streaming applications: PPLive, ESM, and TVAnts In LAN (100Mbps) and WLAN (802.11b)

11. 11 Workload Statistics Downstream throughput is typically 300~400Kbps Upstream traffic to downstream traffic: Can be as large as 10 times for PPLive due to its popularity Between 2 to 4 times for TVAnts Not too much for ESM PPLive and ESM: most in TCP TVAnts: 74% in UDP for WLAN

12. 12 Downstream packet FIFO buffer Duplication Detection Methods: Fixed Hashing Offline workload analysis: Fixed Hashing (FH) Compute only 1 fingerprint (hash value) for a downstream packet; s tore this fingerprint in a hash table, and cached the packet in FIFO buffer For each upstream packet, also compute the fingerprint, and look it up in the hash table to locate the duplicated downstream packet; If found the same fingerprint, do further byte-to-byte comparison Downstream packet fingerprint hash table Upstream packet Upstream packet fingerprint Lookup

13. 13 Duplication Detection Methods: Rabin Fingerprinting Rabin Fingerprinting (RF) A unique hash function: produce fingerprints for a continuous data stream quickly (NSDI’07 BitTyrant) We scan the whole packet and only store fingerprints ending with 8 zeros over 64 bytes content averagely 5 fingerprints for a 1400 bytes packet (1/2 8 ) FIFO Buffer: stores latest 50,000 downstream packets Buffer + hash table: need about 75MB memory totally

14. 14 RF can detect more duplications than FH All the duplication ratios are larger than 75% Offline analysis processing throughput of RF is less than FH: Still large enough (> 90Mbps) for process P2P streaming (400 Kbps) Dup Ratio & Tput

15. 15 Duplication Beginning Offset FH can only detect the duplication when the offsets for up/downstream packets are the same (no re-packetizing) ESM does not have any offset differences  FH performs well TVAnts has a lot of re- packetizing  FH performs the worst

16. 16 Forwarding Delay PPLive and TVAnts: most upstream packets forwarded in 200 seconds <20 seconds for 70% ESM: within 10 ms Implies the downstream buffer can be quite small 200 seconds 200 seconds 10 ms 10 seconds 20 seconds

17. 17 Outline Problem Summary and Contributions Measurement & Analysis of P2P Streaming Traffic SCAP Design & Implementation Evaluation Summary

18. 18 SCAP (Smart Caching in Access Points) Overview WLAN Internet Downstreams buffer Metadata upstream packet (If duplications found in downstream buffer) Relay/Viewing Peer Access Point Original upstream packet Downstream buffer

19. 19 Design Issues Buffer size: Need 7.5MB for storing recent 200 seconds traffic (in 300Kbps rate), which is affordable for a wireless station But AP will need to buffer for multiple stations: AP should dynamically adjust the buffer space for each station according to its duplication ratios in order to achieve highest traffic reduction with limited buffer space Buffer synchronization between AP and station: If a metadata upstream packet cannot be reassembled on AP due to a cache miss, TCP flow will be stalled Wireless station caches several copies of recent sent upstream packets and resends the uncompressed packet when needed

20. 20 Prototype Implementation Modified HostAP driver in Linux kernel 2.6.16 for the AP and stations Wireless card is based on Intersil Prism 2.5 chipset (802.11b) Identification of the downstream packet For AP to locate the packet in decompressing the upstream packet Cannot use Sequence Control field (2 bytes) because it is filled by the firmware Have to use the first fingerprint value (8 bytes)

21. 21 Outline Problem Summary and Contributions Measurement & Analysis of P2P Streaming Traffic SCAP Overview Design & Implementation Evaluation Summary

22. 22 Performance Evaluation: LAN Experiment Station first receives a file from a server, then sends it back RF: little overhead for the downstream throughput (1.5% decrease), and 88% improvement for the upstream throughput FH: cannot have any improvement due to constant TCP re- packetizing 4.50 4.43 Mbps 4.7 8.9 Mbps

23. 23 Performance Evaluation: Internet Experiment Evaluate PPLive, TVAnts, and ESM Run the applications in a VMWare-based Windows XP guest OS for HostAP driver to work Measurement methods: Because P2P Streaming is a Constant Bit Rate stream:  Upstream throughput will not change even if we reduces its traffic Running iperf on another wireless station to observe the impact to WLAN TCP throughput Running Ping to observe the impact to response time Run multiple trials to get comparable P2P downstream throughput for comparison Each trial runs for 600 seconds

24. 24 Internet Experiment: Evaluation Results RF/FH performs best for TVAnts since it has the largest volume of upstream traffic: Increases TCP throughput by 0.95 Mbps (54% of upstream traffic) Decrease Ping round-trip time by 83 ms (-26%) Also performs well for PPLive/ESM

25. 25 Summary With the increasing popularity of P2P streaming applications and pervasive deployment of 802.11 WLANs, more peers will be connected by wireless We study the impact of wireless peers to the performance of wireless and Internet users Without a proper control of P2P traffic, the performance of both parties can be significantly affected We designed and implemented SCAP (Smart Caching in Access Points) in order to reduce the upstream traffic for P2P live streaming applications Our prototype based evaluation shows the effectiveness of SCAP: SCAP improves the throughput of the WLAN by up to 88% SCAP reduces the response delay to Internet peers as well

26. 26 Thank you! Enhua Tan: etan@cse.ohio-state.edu http://www.cse.ohio-state.edu/hpcs/

27. 27 SCAP (Smart Caching in Access Points) – Basic Idea AP stores downstream data in buffer (1) Station stores downstream data in buffer (2) Compare upstream packet (3) with (2), upload difference (4) AP will assemble upstream packet with data in (1) to the Internet

28. 28 Workflow of SCAP

29. 29 Rabin Fingerprinting Rabin Fingerprinting (RF) can produce fingerprints for a continuous data stream quickly: Advance the fingerprint only requires an addition, a multiplication, and a mask Lack of this property for other hash functions like MD5/SHA (and they are also more complex)

30. 30 Some Related Work XORs in the Air: Practical Wireless Network Coding (Sigcomm’06) Utilizing the broadcasting nature of wireless networks to improve throughput of multi-hop network (instead of application characteristics) Our scheme is utilizing the traffic pattern of P2P applications A Protocol-Independent Technique for Eliminating Redundant Network Traffic (Sigcomm’00) reduces redundant traffic using Rabin Fingerprinting A Low-bandwidth Network File System (SOSP’01) Exploits similarities between different versions of a file to reduce update traffic