Streaming Media

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Streaming Media
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• Unicast
• Redundant traffic
• Multicast
• One to many

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Video Multicast

• Multicast Support
• multicast enabled network
• Real-time Requirements Support

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Supporting Real-time Requirements

• QoS and Resource reservation
• Resource Reservation to bound data delivery
  delay, loss, jitter etc.
• Adaptive Rate Control
• Adjust video traffic characteristics to suit the
  Internet.

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Multicast

• Group Addressing
• Distribution Tree
• Join(graft)
• Leave(prune)

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Video Traffic

• Periodic generation of frames at regular
  intervals.
• Variable bit rate.
• Frame periodicity must be maintained for the
  video to appear smooth
• Data unavailable at playout is useless.
• Jitter (variability in interarrival times)

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• Buffering and Start-up Latency
• Congestion leads to Data Loss
• Decreasing Data Rate
• Error Control
• Summary
• Delay Sensitive, Loss Insensitive

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Multicast and Heterogeneity

• The Internet is Heterogeneous
• Infra-structural (Spatial)
• Traffic density (Temporal)
• Administrative
• Fairness Goal
• Every receiver should receive video that is
  commensurate to the resources available.
• Is this fair to other traffic?

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Fairness

• Intra-session fairness
• Inter-session fairness

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Rate Adaptive Digital Video

• Compression, Scene Complexity, Motion

Feedback
Smoothing Buffer
Video Encoder
Network
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• Raw video stream is fed to encoder
• Encoder sends encoded data to buffer
• Buffer level provides feedback
• Feedback is used to control data output rate at
  encoder.
• Quantization, frame rate, pixel resolution etc.
  are controlled.

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• Network feedback can also be used.
• Queueing information (internal to the network)
• End-system information

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Adaptive Bit-Rate Video

• Single Stream Adaptive Approach
• Replicated Streams Adaptive Approach
• Layered Video Streams Approach

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RTP

• Real-Time Transport Protocol
• End-to-end Protocol
• NO notion of Connection. (hence unreliable)
• Application level
• Requires framing and segmentation be taken care
  of by lower layers.

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RTP (continued)

• Divided into two parts (consecutive ports for
  UDP)
• Data (audio video packets, even-numbered port)
• Control
• Can use single PDU in case UDP is not used.

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Real-time Transport Protocol
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RTP Data Packets

• 12 byte header
• data (video/audio)
• can be encapsulated in encoding-specific layer.

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RTP Data Packet Header

• Payload Type (1 byte)
• eg JPEG etc.
• Timestamp (32 bits)
• generation instant of the data
• Sequence marker (16 bits)
• packet seq. number to help loss detection
• Marker bit
• end of frame for video
• beginning of talk-spurt for audio

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RTP Data Packet Header (contd)

• Synchronization Source Identifier (32 bits)
• randomly generated identifying session source.

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RTP Control Channel

• Control protocol called RTCP
• QoS monitoring and Congestion Control
• multicast
• all other receivers know how others are doing
• sender-report, helps receivers compute data-rate
• Intermedia Synchronization
• wall-clock time RTP timestamp
• allows synch of audio and video
• Identification
• Detailed identification of participant instead of
  just a 32 bit identifier.
• Session size estimation and scaling
• scaled to 5 of data rate

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RTCP Packets

• Several types to carry a variety of information
• Source description (SDES)
• CNAME, email, location, name, ...
• Sender report (SR)
• Bytes sent -gt estimated rate
• Timestamp -gt synchronization
• Receiver report (RR)
• Loss rate, interarrival jitter, roundtrip delay
• Explicit leave (BYE)
• Compound packets (SDES CNAME RR)

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RTCP traffic Control

• RTP session scale two to thousands of
  participants
• RTCP traffic increases with session size
• Want to keep to small fraction of data bandwidth
  (5)
• Randomized response with rate decreasing as
  number of participants increases
• Give active senders more bandwidth
• But limited by tolerable age of status

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Single Stream Video Multicast

• Adjust video output rate
• Three parameters
• refresh rate (?)
• quantizer (color scheme 422, 411.)
• movement detection threshold (what defines
  motion)
• Application can specify which of these to control

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Single Stream Video Multicast

• RTCP is used for feedback
• Feedback implosion
• probabilistic probing
• Fair? (No..)

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Replicated Streams

• Destination Set Grouping
• Multiple replicated streams on different
  multicast addresses.
• Different quality and data rates.
• Receivers can switch streams

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Switching Streams

• Congestion due to presence of two streams
  simultaneously on the same link
• Bandwidth Control Protocol
• Congestion History Checking before stream switch.
• Local Area Bandwidth Limit restricts the number
  of streams received in local area.

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Layered Video Multicast

• Disjoint layers on different addresses
• Cumulative subscription
• Many protocols making different assumptions

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RLM

• Receiver based
• Sender does not participate
• Receivers share loss information
• Receivers join and drop groups based on these
  shared loss reports.
• Receivers back off when they or other receivers
  see congestion.
• The higher the layer, the longer the back-off
  duration.

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Problems with RLM

• Receiver Consensus
• Fast Leaves and Joins
• Impact of failed experiments on topologically
  unrelated receivers.
• UNFAIR
• Arguably the most cited and most maligned
  protocol!!

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TCP rate-based Congestion Control

• Analyze TCP to come with a magic formula to
  describe
• Bandwidth 1.3 MTU / (RTT sqrt(Loss))
• Adapt sender rate to match such a formula.
• But what is RTT?
• Let receivers make this decision.
• Define loss thresholds based on this formula,
  for each layer.
• If loss exceeds this threshold, drop a layer

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• http//www.psc.edu/networking/papers/tcp_friendly.
  html

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Summary

• Multicast
• RTP/RTCP
• Rate Adaptation
• Issues
• Fairness
• Intra-session
• Inter-session
• Stability
• Deployability
• Administrative Issues