A proxy smoothing service for variable bit rate streaming video
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A Proxy Smoothing Service for Variable-Bit-Rate Streaming Video. Jennifer Rexford AT&T Labs - Research Florham Park NJ. http://www.research.att.com/~jrex. Joint work with Subhabrata Sen, Don Towsley, and Andrea Basso. Outline. Background and motivation

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A proxy smoothing service for variable bit rate streaming video l.jpg

A Proxy Smoothing Service for Variable-Bit-Rate Streaming Video

Jennifer Rexford

AT&T Labs - Research

Florham Park NJ


Joint work with Subhabrata Sen, Don Towsley, and Andrea Basso

Outline l.jpg
Outline Video

  • Background and motivation

    • Burstiness of compressed video streams

    • Smoothing techniques for stored video

  • Online smoothing of variable-bit-rate video

    • Sliding-window smoothing algorithm

    • Performance evaluation on MPEG traces

  • Integration of smoothing with prefix caching

    • Caching initial frames of popular video streams

    • Resource allocation across multiple streams

  • Prototype proxy smoothing service

    • Software design of proxy service in Windows NT

    • MPEG-2 PC-based video streaming testbed

  • Conclusions and ongoing work

Video streaming applications l.jpg
Video Streaming Applications Video

  • Live, interactive video

    • Video teleconferencing, video phones, etc.

    • Tight delay constraints to support interactivity

  • Stored, non-interactive video

    • Movies, distance learning, Web videos, etc.

    • Video recorded in advance; loose delay constraints

  • Live, non-interactive video

    • Course lectures, news, sporting events, conferences

    • Video not recorded in advance; loose delay constraints

Challenges of video streaming l.jpg
Challenges of Video Streaming Video

  • High bandwidth requirements of compressed video

    • 4-6 Megabits/second for high quality MPEG2 streams

  • Burstiness of frame sizes on several time scales

    • MPEG group-of-pictures structure (I, P, B frames)

    • Differences in action and detail within/across scenes

  • Bandwidth limitations on clients and links

    • 10 or 100 Mbps shared local area network

    • 27 Mbps cable channel, 1.5 Mbps ADSL

  • Lack of end-to-end control of path from source

  • Poor delay, throughput, and loss in the Internet

Approaches to handling variability l.jpg
Approaches to Handling Variability Video

  • Constant-bit-rate encoding of each stream

    • Adjust quality of encoding to stay at constant rate

    • Quality degradation during scenes with action & detail

  • Statistical multiplexing of variable rate streams

    • Rely on mixing to reduce the aggregate peak rate

    • Limited effectiveness on access links

  • Selective discard of packets/frames in stream

    • Discard packets/frames during transient congestion

    • Noticeable degradation in video quality

  • Transcoding or layered encoding to reduce bit rate

    • Re-encode the video stream at different quality at proxy

    • Quality degradation; hard to transcode at link speeds

Smoothing stored video l.jpg
Smoothing Stored Video Video

For prerecorded video streams:

  • All video frames stored in advance at server

  • Prior knowledge of all frame sizes (fi, i=1,2,..,n)

  • Prior knowledge of client buffer size (b)

  • workahead transmission into client buffer



b bytes




Smoothing constraints l.jpg
Smoothing Constraints Video

Given frame sizes {fi} and buffer size b

  • Buffer underflow constraint (Lk = f1 + f2 + … + fk)

  • Buffer overflow constraint (Uk = min(Lk + b, Ln))

  • Find a schedule Sk between the constraints

  • O(n) algorithm minimizes peak and variability

  • U

    number of bytes

    rate changes



    time (in frames)

    Limitations of smoothing model l.jpg
    Limitations of Smoothing Model Video

    • Assumes prerecorded stored video

      • but… need to support live and precorded video

    • Assumes smoothing is performed by server

      • but… server is in the domain of another provider

    • Assumes end-to-end control of the network

      • but… the Internet is decentralized

    • Assumes server knows the client buffer size

      • but… the client may be in a different domain

    Online smoothing l.jpg
    Online Smoothing Video

    Source or proxy can delay the stream by w time units:

    Larger window w reduces burstiness, but…

    • Larger buffer at the source/proxy

    • Larger processing load to compute schedule

    • Larger playback delay at the client

    stream with

    delay w



    b bytes



    Slide13 l.jpg

    proxy Video







    Online Smoothing Model

    • Arrival of Aibits to proxy by time i in frames

    • Smoothing buffer of B bits at proxy

    • Smoothing window (playout delay) of w frames

    • Playout of Di-w bits by client by time i

    • Playout buffer of b bits at client

    • transmission of Si bits by proxy by time i

    Online smoothing14 l.jpg
    Online Smoothing Video

    • Must send enough to avoid underflow at client

      • Si must be at least Di-w

    • Cannot send more than the client can store

      • Si must be at most Di-w + b

    • Cannot send more than the data that has arrived

      • Si must be at most Ai

    • Must send enough to avoid overflow at proxy

      • Si must be at least Ai - B

    max{Di-w, Ai - B} <= Si <= min{Di-w + b, Ai}

    Online smoothing constraints l.jpg
    Online Smoothing Constraints Video

    Source/proxy has w frames ahead of current time t:

    don’t know

    the future

    number of bytes




    time (in frames)



    Modified smoothing constraints as more frames arrive...

    Smoothing star wars l.jpg
    Smoothing VideoStar Wars

    GOP averages

    30-second window

    2-second window

    • MPEG-1 Star Wars,12-frame group-of-pictures

    • Max frame 23160 bytes, mean frame 1950 bytes

    • Client buffer b=512 kbytes

    Reducing computational complexity l.jpg
    Reducing Computational Complexity Video

    • No need to compute schedule at every time unit

      • Limited information from new frame arrivals

      • Limited impact on trajectory of the schedule

    • Execute online algorithm every a time units

      • Perform O(w) work every a time units

      • Limit number of rate changes

    • Performance implications

      • Very small increases in peak and variance of rates

      • Setting a = w/2 performs almost as well as a = 1

    Parameters in smoothing model l.jpg
    Parameters in Smoothing Model Video

    • Algorithm parameters

      • Window w (in number of frame slots)

      • Client buffer size b (in bytes)

      • Source/proxy buffer size B (in bytes)

      • Computation interval a (in frames)

      • Frame-size prediction interval p (in frames)

    • Performance metrics

      • Peak rate of the smoothed stream

      • Coefficient of variation (standard-deviation/mean)

      • Effective bandwidth (given buffer and loss rate)

    Peak rate vs window size varying client buffer size for mpeg 1 wizard of oz l.jpg
    Peak Rate vs. Window Size Video(varying client buffer size for MPEG-1 Wizard of Oz)

    • Dramatic decrease in bandwidth variability

    • Online algorithm approaches offline scheme

    • Ten-second window gives most of the gain

    Peak rate vs client buffer varying window size for mpeg 1 wizard of oz l.jpg
    Peak Rate vs. Client Buffer Video(varying window size for MPEG-1 Wizard of Oz)

    • Significant reductions with a few Mbytes of buffer

    • Diminishing returns for larger client buffer sizes

    • Window size w should scale with buffer size b

    Proxy vs client buffer varying prediction under 512 kbyte total buffer 30 frame window l.jpg
    Proxy vs. Client Buffer Video(varying prediction under 512-kbyte total buffer & 30-frame window)

    • Need buffer at each end for good performance

    • Even buffer for large P, more at proxy for small P

    • Simple prediction schemes are very effective

    Prefix caching to avoid start up delay l.jpg
    Prefix Caching to Avoid Start-Up Delay Video

    • Avoid start-up delay for prerecorded streams

      • Proxy caches initial part of popular video streams

      • Proxy starts satisfying client request more quickly

      • Proxy requests remainder of the stream from server

    • smooth over large window without large delay

    • Use prefix caching to hide other Internet delays

      • TCP connection from browser to server

      • TCP connection from player to server

      • Dejitter buffer at the client to tolerate jitter

      • Retransmission of lost packets

    • apply to “point-and-click” Web video streams

    New questions l.jpg
    New Questions Video

    • Video streaming protocol

      • How to get the proxy in the path?

      • How to receive an initial copy of the prefix?

      • How to retrieve the remaining frames of the video?

    • Smoothing model

      • What changes in the smoothing constraints?

      • What changes in the basic performance properties?

    • Proxy resource allocation

      • How much prefix is needed to hide Internet delays?

      • How to allocate between caching and smoothing?

      • How to allocate resources across multiple streams?

    Protocol issues l.jpg
    Protocol Issues Video

    • Ensuring that requests go through the proxy

      • Configuration of proxy in client browser or player

      • Placement of transparent proxy in the path

    • Caching of the 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

    Changes to smoothing model l.jpg
    Changes to Smoothing Model Video

    • Separate parameter s for client start-up delay

    • Prefix cache stores the first w-s frames

    • Arrival vector Ai includes cached frames

    • Prefix buffer does not empty after transmission

    • Send entire prefix before overflow of bs

    • Frame sizes may be known in advance (cached)







    Performance evaluation l.jpg
    Performance Evaluation Video

    • Comparison to original online smoothing model

      • Pro: can have large window and small start-up delay

      • Pro: performance is virtually indistinguishable

      • Con: storing prefix nearly doubles buffer requirement

      • Con: may be difficult to smooth at beginning of video

    • Allocation of prefix and smoothing buffers

      • Small prefix buffer limits size of smoothing window

        • small window w restricts workahead smoothing

      • Large prefix buffer limits size of smoothing buffer

        • small bs requires aggressive transmission schedule

    Peak rate vs window size varying total proxy buffer size for mpeg 1 wizard of oz l.jpg
    Peak Rate vs. Window Size Video(varying total proxy buffer size for MPEG-1 Wizard of Oz)

    • Convex, cup-shaped curve of peak rate vs. buffer

    • Simple binary search for optimal allocation

    • Heuristic: pick largest w that does not constrain bs

    Peak rate vs prefix buffer size varying total proxy buffer size for mpeg 1 wizard of oz l.jpg
    Peak Rate vs. Prefix Buffer Size Video(varying total proxy buffer size for MPEG-1 Wizard of Oz)

    Allocating resources across streams l.jpg
    Allocating Resources Across Streams Video

    • Performance issues

      • Limited buffer (M) and/or bandwidth (B) at proxy

      • Collection of V videos with different popularity

      • Videos with different sequences of frame sizes

    • Optimization problem

      • Allocate prefix buffer bp for each video v =1,…, V

      • Allocate smoothing buffer bs for each of nv requests

      • Obey constraint on buffer (M) or bandwidth (B)

      • Minimize the usage of the other resource (M or B)

    Simplifying the problem l.jpg
    Simplifying the Problem Video

    • Complex resource allocation problem

      • Assign bp, bs, and w for each video v

      • Buffer requirement: sumv{bp(v) + nv * bs(v)}

      • Bandwidth requirement:sumv{nv * peak(v)}

    • Reduce problem to selecting w for each video

      • Select same bs and w across all requests for v

      • Select prefix buffer bp as first w-s frames

      • Select bs as max smoothing buffer for window w

    Greedy algorithm l.jpg
    Greedy Algorithm Video

    • Further simplifying the problem

      • Selecting w determines bp(v), bs(v), and peak(v)

      • Consider the nv*peak(v) vs. bp(v)+nv*bs(v)curve

      • Curve is piecewise-linear, convex, non-increasing

    • Greedy algorithm for buffer constraint M

      • Select the videowith steepest initial slope

      • Assign buffer space to this video for max gain

      • Repeat until reaching the buffer constraint M

    • Greedy algorithm for bandwidth constraint B

      • Repeat until not exceeding bandwidth constraint B

    Illustration of greedy algorithm l.jpg
    Illustration of Greedy Algorithm Video




    bandwidth for video 2

    bandwidth for video 1




    buffer for video 1

    buffer for video 2

    Building a smoothing proxy l.jpg
    Building a Smoothing Proxy Video

    • Performance results

      • Memory: a few megabytes of RAM is sufficient

      • CPU: 1-2 msec to smooth 30 sec (300 MHz PC)

      • Bandwidth: 2-4 Mbps feasible on personal computer

    • Solution with off-the-shelf components

      • 300 MHz Pentium Pro with 192 megabytes of RAM

      • Input and output on 10 megabit/second Ethernet

      • Windows NT operating system with WinSock 2.0

    Reality sets in l.jpg
    Reality Sets In Video

    • Video stream is packetized, not a fluid

      • Smoothing constraints must be applied to packets

      • Proxy cannot transmit the stream at arbitrary rates

    • System does not have support for traffic shaping

      • Cannot control the inter-packet spacing at fine scale

      • E.g., 2 msec spacing for 15-packets frames (30 fps)

    • Interrupt latency, timer jitter, and data copying

      • Limited control over time expiration times

      • Latency in processing I/O and timer operations

      • Need to avoid extra copying of video frames

    Time sharing the processor l.jpg
    Time-Sharing the Processor Video

    • Reception of incoming packets

      • Smooth over more frames by receiving often

      • Avoid double-copy from kernel to user space

      • Avoid the worst-case scenario of overflow

    • Computation of smooth schedule

      • Must run often enough to maximize smoothing

      • Fortunately, does not need to read or write data

    • Transmission of packets according to schedule

      • Must run often enough to control packet spacing

      • Avoid the bad case of sending a large burst

      • Avoid the worst case of client underflow

    Key design decisions l.jpg
    Key Design Decisions Video

    • Single thread of control

      • No operating system control over fine-grain sharing

    • High-performance counter for timing operations

      • Timers are too inaccurate (tens of milliseconds)

      • How often should the counter be checked?

    • Overlapped I/O to avoid double copying

      • Receive and send directly to/from the user-space buffer

      • How many outstanding sends and receives?

    • Explicit pacing of packet transmissions

      • How often should the send routine be invoked?

    Livenet mpeg 2 testbed developed by andrea basso glenn cash and reha civanlar l.jpg
    LiveNet MPEG-2 Testbed Video(developed by Andrea Basso, Glenn Cash, and Reha Civanlar)

    • MPEG-2 encoder

      • MPEG-2 encoder board (MPEGXpress)

      • Software to read into buffers and stream into network

    • Real-time packetizer

      • Parses MPEG-2 stream and divides frames into slices

      • Packing slices into Real-Time Protocol (RTP) packets

    • MPEG-2 decoder

      • Software for packet reception and error concealment

      • MPEG-2 decoder board (DarimVision)

    Slide39 l.jpg

    Conclusions Video

    • Online smoothing model

      • Applicable to many non-interactive applications

      • Significantly lowers burstiness of compressed video

      • Enables high-quality video across access networks

    • Prefix caching

      • Hides start-up delay for smoothing and other operations

      • Effective resource allocation schemes at the proxy

    • Practical application

      • Transparent to the origin video source/server

      • Implementation with commercial off-the-shelf parts

      • Integration with MPEG-2 and Real-Time Protocol

    Slide40 l.jpg

    Ongoing Work Video

    • Prototyping the proxy smoothing service

      • Completion of implementation of proxy service

      • Performance evaluation of parameterized system

    • Combining smoothing with other mechanisms

      • Discard, transcoding, feedback, and retransmission

      • Exploiting prefix cache to hide additional latency

    • Measurement of Web-initiated video streaming

      • Collection of video packet traces in AT&T WorldNet

      • Study of potential for (partial) caching at the proxy