Video streaming

Basic background
- sequence of frame sizes: f_i
- variable frame sizes due to detail, motion, etc.
- playout buffer: buffer space b at client
- playout delay: wait d seconds to start playing
- rebuffering: empty playout buffer
- video quality: high frame quality, low quality variation, limited rebuffering

Problems with TCP
- retransmission of lost packets not useful after playout time
- slowing down after a packet loss may starve the client
- overhead of TCP header and ACK packets, particularly for audio streams
- cannot easily cancel an ongoing download to switch to a lower level
- cannot easily count data in receive buffer, or retract data from send buffer

Adaptive bit rate selection
- multiple encoding rates (from 235 Kbps to 5 Mbps)
- client requests each chunk at a particular level
- balance tradeoff between video quality and likelihood of rebuffering
- estimate future bandwidth from past observations, but be more conservative when buffer is low

Netflix video streaming paper
- capacity estimation is too hard, due to variability
- sources of variability: WiFi interference, network congestion, overloaded server
- better to use the buffer occupancy (once the buffer has built up)
- cannot cancel transfer, so keep buffer above a reservoir (at least one chunk, but perhaps
       more because of variable chunk sizes)
- video rate vs. buffer occupancy: flat for reservoir, linear increase, flat for upper

Dynamic Adaptive Streaming over HTTP (DASH)
- break content into small HTTP file segments
- content made available at many bit rates
- client requests a chunk at the appropriate rate
- media presentation description gives segment info, URL, media characteristics
      -- could be a segment list, a template with time or chunk number, or one URL with byte range

Netflix measurement paper
- infrastructure
     *  Netflix data centers (www.netflix.com)
           - redirect users to movies.netflix.com or to registration (payment info)
           - not involved in video streaming
     * Cloud (e.g., Amazon)
           - content ingestion
           - log recording/analysis
           - DRM
           - CDN routing 
           - user sign-in
           - mobile device support
     * CDNs
           - Akamai, Limelight, and Level3
     * Players
           - desktop: Silverlight
           - mobile phones: various apps
- Dynamic adaptive streaming over HTTP (DASH)
- Manifest file 
      * list of CDNs
      * chunk URLs for multiple quality levels
- Trickplay
      * thumbnail images for periodic (10-second) snapshots

Sprout paper
- highly variable throughput and RTT in cellular networks
- bad for interactive apps
- existing schemes are reactive (to packet loss and RTT increase)
- infer link speed from interarrivel times, and predict future speed