1. 1 S. Sen, J. Rexford and D. Towsley {sen,towsley}@cs.umass.edu, UMass Amherst jrex@research.att.com, AT&T Labs Presented by : Shubho Sen Proxy Prefix Caching for Multimedia Streams

2. 2 Outline Outline Motivation Proxy Prefix Caching Prefix Caching in conjunction with VBR video smoothing Performance evaluation using MPEG traces Conclusions

3. 3 Streaming Video across Network Problem Setting Streaming prerecorded multimedia High bandwidth, multi-timescale burstiness Poor Internet delay, loss, and throughput Protocol Delay : 3-4 end-end roundtrips to start receiving video Goal: Efficient transmission of low latency, high quality streaming video Video Server Client

4. 4 Proxy Prefix Caching Central Idea Proxy caches initial part of popular streams  starts satisfying client request more quickly  requests remainder of stream from server Path from proxy to client has low latency, and better QoS control Benefits  reduces client delay - masks server-proxy delay  aids bandwidth smoothing  better QoS at client Video Server Client Proxy

5. 5 Smoothing at Proxy (Review) Goal: Reduce peak and variability on path from proxy to client  Controlled transmission to client - do workahead transmission proxy bsbs bc bc DiDi AiAi SiSi client streaming video A i bits arrive by time i b s bits staging buffer at proxy S i bits sent by proxy by time i b c bits playout buffer at client D i bits playout by time i

6. 6 Smoothing Reduces Variability Delay client playback for smoothing  w frames smoothing window (playout delay)  Server delay d, client startup latency L = w + d Smoothing reduces peak rate and rate variability  Larger w greater smoothing gains dw+d A shifted D Cum. bytes S Smoothing, No prefix cachingNo smoothing or prefix caching d A D Cum. bytes

7. 7 Prefix Caching and Smoothing Use prefix caching to mask smoothing delay b p : Prefix cache at proxy Total proxy buffer becomes B = b s + b p Step1: Transmit content of b p Step2: Transmit from b s Smoothing gains determined by: b s, b p, b c D i-L new bpbp bcbc AiAi SiSi bsbs proxy client Step 1 Step 2

8. 8 Prefix Caching hides delay d Cache first d frames  At time d, frame (d + 1) arrives at proxy  Can shift D to left client startup delay L reduces to smoothing window w new A A D Cum. bytes dw+d2d Cum. bytes d w new D w+d No prefix cached-frame prefix First d frames

9. 9 Prefix Caching to hide delay w+d Cache first (d + w) frames  At time d, frame (d + w + 1) arrives at proxy  Can shift D further left - no client delay (d+w)-frame prefix Cum. bytes First (d + w) frames A D w w+d d Cum. bytes w w+d new A new D d d-frame prefix Decouple client delay L from window w For delay L, prefix = d + w - L frames

10. 10 Performance Evaluation Pro: can have large window and small start-up delay Pro: performance is virtually indistinguishable from model without prefix Con: storing prefix nearly doubles buffer requirement Con: may be difficult to smooth at beginning of video Trace based evaluations:  17 min MPEG-2 Blues Brothers - peak 44.5 Mbps, mean 1.48 Mbps  23 min MPEG-1 Wizard of Oz - peak 10.9 Mbps, mean 1.25 Mbps

11. 11 Effect of Proxy Buffer Size B A few MB of proxy buffer yields most benefits Blues Brothers, 8 MB client buffer, client startup L = 1 sec B = b s + b p

12. 12 Allocation of prefix and staging buffers Small prefix buffer limits w, large prefix buffer limits staging buffer Symmetric allocation works best Convex, cup-shaped curve of peak rate vs. buffer  Utilize for resource allocation among multiple streams Wizard of Oz, 32 MB client buffer, L = 1 sec, d = 5 sec

13. 13 Prefix Caching Issues Caching initial frames of the video  Server replication of the prefix  Proxy prefetching of the prefix  Proxy caching of prefix after first request Transparent retrieval of remaining frames  Range request operation in HTTP 1.1  Absolute positioning in RTSP

14. 14 Conclusions Prefix caching at a proxy Prefix caching in conjunction with video smoothing Hides playback startup delay for smoothing With moderate proxy buffer, substantial smoothing gains Ongoing Work  Effective resource allocation schemes at the proxy  Multiplexing proxy staging buffer between different clients  Potential for masking poor QOS between server and proxy Retransmission of lost packets TCP-like congestion control