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15-441 Computer Networking
Internet Video Delivery
Hui Zhang, Fall 2012
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Two Methodologies of Building
Networks and Applications

Methodology of building telephony network
- First, understand application (interactive voice) and specify the
detailed requirements of the application
- Then build a special network that optimize the application and
satisfy all the requirements

Methodology of building the Internet
- Build a general purpose protocol stack and network with very
minimal assumptions on applications
- Let diverse type of applications run top of the network
- When an application become popular and the performance
becomes unacceptable, start to optimize the performance of the
application
• deal with the disaster of application success later
• Have more constraints to fix the problem as an after-thought
Hui Zhang, Fall 2012
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Examples of Internet Applications:
Web

Web Initial design
- HTML + browser + stateless HTTP on top of TCP/IP
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Why the application performance is bad? What is the fix?
(homework question)
Why the server performance is bad? What is the fix?
(homework question)
- Has to do with TCP teardown process
- Server can only support limited # of simultaneously-open TCP
connections
• Each TCP connection consumes resources such as TCB data
structure, port # etc
- A server closes the TCP connection and reclaims all data
structures only after 2 MSL
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TCP Tearing Down Connection

Either side can initiate tear down
- Send FIN signal
- “I’m not going to send any more
data”

Other side can continue sending
data
- Half open connection
- Must continue to acknowledge

Acknowledging FIN
- Acknowledge last sequence
number + 1
Hui Zhang, Fall 2012
A
B
FIN, SeqA
ACK, SeqA+1
Data
ACK
FIN, SeqB
ACK, SeqB+1
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State Diagram: TCP Connection
Tear-down
CLOSE
send FIN
FIN
WAIT-1
ACK
FIN WAIT-2
Active Close
ESTAB
CLOSE
send FIN
10-31-2006
Hui Zhang, Fall 2012
CLOSE
WAIT
rcv FIN
snd ACK
CLOSE
snd FIN
rcv FIN+ACK
snd ACK CLOSING
LAST-ACK
rcv ACK of FIN
rcv FIN
snd ACK
Passive Close
rcv FIN
send ACK
rcv ACK of FIN
TIME WAIT
CLOSED
Timeout=2msl
delete TCB
Lecture 18: TCP Details
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Examples of Internet Applications:
Voice and Video

Telephony: bi-directional interactive voice (covered in last
lecture)
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Outline
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Multimedia requirements
Audio and Video Data
Streaming
Interactive Real-Time
Recovering from Jitter and Loss
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Internet Voice and Video
(Continuous Media)


Typically sensitive to delay, but can sometimes tolerate
packet loss (would cause glitches that can be concealed
somewhat)
Three classes of applications with different requirements
- Interactive Real-Time
• Two way communication: telephony (Skype)
• Multi-way conferencing: White house situation room, Google
hangout
- On-demand streaming
• Long form content: Netflix, Hulu
• Short form content: Youtube
- Broadcast: NFL, ESPN live
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Dimensions of Requirements to
Consider

How much signal distortion can be tolerated?
- Ear is less tolerant than eye

What is the range of bandwidth?
- Voice: 64 Kbps, can be even lower
- Video: 200Kbps, 1Mbps, 2 Mbps, 6 Mbps

What is the tolerance of maximum delay?
- Interactive Real-Time: 100 ms – 200 ms
- On-demand streaming:
• Long form content: 10 – 60 seconds
• Short form content: 1-3 seconds
- Broadcast: 5-10 seconds

Need to think of delay in terms of # of RTTs
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Audio Data

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Telephone system uses 8-bit samples at 8kHz:
64kbits/s.
Further compression may be pointless given packet
overhead.
Modern compression achieves equivalent perceptual
quality with about 1/10 to 1/5 of the bits.
Most audio compression is performed in "blocks" of
hundreds of original samples: adds latency.
Audio compression is lossy: it encodes something
perceptually similar but really different from the
original.
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Video Data

Unlike audio, video compression is essential:
- Too much data to begin with, but
- Compression ratios from 50 to 500


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Takes advantage of spatial, temporal, and perceptual
redundancy
Temporal redundancy: Each frame
can be used to predict the next ->
leads to data dependencies
To break dependencies, we
insert "I frames" or keyframes
that are independently encoded.
- Allows us to start playback from middle of a file

Data dependency
Video data is highly structured
Credit: http://www.icsi.berkeley.edu/PET/GIFS/MPEG_gop.gif
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Challenges

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TCP/UDP/IP suite provides best-effort, no guarantees on
expectation or variance of packet delay
Streaming applications delay of 5 to 10 seconds is typical
and has been acceptable, but performance deteriorate if
links are congested
Real-Time Interactive requirements on delay and its jitter
have been satisfied by over-provisioning (providing plenty
of bandwidth), what will happen when the load
increases?...
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On-Deman Streaming



Important and growing application due to
reduction of storage costs, increase in high
speed net access from homes, enhancements
to caching
Interactive control by user
(but often with long response time)
Ubiquitous on the web:
-
YouTube, Netflix, Vimeo
Television networks, Hollywood, etc.
Most local radio & TV stations
Virtually everywhere on websites
15-441 Fall 2011
Hui Zhang, Fall 2012
Multimedia
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Helper Application

Displays content, which is typically requested via a Web
browser; typical functions:
- Decompression
- Jitter removal
- Error correction: use redundant packets to be used for
reconstruction of original stream
- GUI for user control

Examples:
-
RealPlayer
Adobe Flash Player
Windows Media Player
QuickTime
DivX Web Player
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First Generation: HTTP Download

A simple architecture is to have the Browser request the
object(s) and after their reception pass them to the player
for display
- No pipelining
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First Gen: HTTP Progressive
Download (2)
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Alternative: set up connection between server and
player; player takes over
Web browser requests and receives a Meta File
(a file describing the object) instead of receiving
the file itself;
Browser launches the appropriate Player and
passes it the Meta File;
Player sets up a TCP connection with Web Server
and downloads or streams the file
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Meta file requests
15-441 Fall 2011
Hui Zhang, Fall 2012
Multimedia
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Buffering Continuous Media

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Jitter = variation from ideal timing
Media delivery must have very low jitter
- Video frames every 30ms or so
- Audio: ultimately samples need <1ns jitter

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But network packets have much more jitter that
that!
Solution: buffers
- Fill them with best effort
- Drain them via low-latency, local access
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HTTP Progressive Download


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With helper application doing the download, playback can start
immediately...
Or after sufficient bytes are buffered
Sender sends at maximum possible rate under TCP; retransmit when
error is encountered; Player uses a much larger buffer to smooth
delivery rate of TCP
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Max Buffer Duration
"Bad": Buffer
overrflows
Max Buffer Size
File Position
Streaming, Buffers and Timing
Buffer
Duration
"Good" Region:
smooth playback
Buffer
Size
= allowable jitter
"Bad": Buffer
underflows and
playback stops
Buffer almost empty
Time
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Drawbacks of HTTP Progressive
Download (2)

HTTP connection keeps data flowing as fast as possible
to user's local buffer
- May download lots of extra data if you do not watch the video
- TCP file transfer can use more bandwidth than necessary

Mismatch between whole file transfer and stop/start/seek
playback controls.
- However: use file range requests to seek to video position

Cannot change video quality (bit rate) to adapt to network
congestion
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2nd Generation:
Real-Time Streaming

This gets us around HTTP, allows a choice of UDP vs.
TCP and the application layer protocol can be better
tailored to Streaming; many enhancements options are
possible
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Multimedia
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Example: Real Time Streaming
Protocol (RTSP)
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For user to control display: rewind, fast forward,
pause, resume, etc…
Out-of-band protocol (uses two connections, one
for control messages (Port 554) and one for
media stream)
RFC 2326 permits use of either TCP or UDP for
the control messages connection, sometimes
called the RTSP Channel
As before, meta file is communicated to web
browser which then launches the Player; Player
sets up an RTSP connection for control
messages in addition to the connection for the
streaming media
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RTSP Operation
15-441 Fall 2011
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Multimedia
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RTSP Exchange Example
C: SETUP rtsp://audio.example.com/xena/audio RTSP/1.0
Transport: rtp/udp; compression; port=3056; mode=PLAY
S: RTSP/1.0 200 1 OK
Session 4231
C: PLAY rtsp://audio.example.com/xena/audio.en/lofi RTSP/1.0
Session: 4231
Range: npt=0
(npt = normal play time)
C: PAUSE rtsp://audio.example.com/xena/audio.en/lofi RTSP/1.0
Session: 4231
Range: npt=37
C: TEARDOWN rtsp://audio.example.com/xena/audio.en/lofi RTSP/1.0
Session: 4231
S: 200 3 OK
15-441 Fall 2011
Hui Zhang, Fall 2012
Multimedia
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RTSP Media Stream
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Stateful Server keeps track of client's
state
Client issues Play, Pause, ..., Close
Steady stream of packets
- UDP - lower latency
- TCP - may get through more firewalls, reliable
Credit: some content adapted from Alex Zambelli
15-441 Fall 2011
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Multimedia
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Example 2: RTMP - Real-Time
Messaging Protocol
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Proprietary Adobe protocol
Runs over TCP
Manages audio, video, and other
Multiplex multiple streams over TCP connection
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Hui Zhang, Fall 2012
Multimedia
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Drawbacks of RTSP, RTMP
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Web downloads are typically cheaper than streaming
services offered by CDNs and hosting providers
Streaming often blocked by routers
UDP itself often blocked by firewalls
HTTP delivery can use ordinary proxies and caches
Conclusion: rather than adapt Internet to streaming,
adapt media delivery to the Internet
15-441 Fall 2011
Hui Zhang, Fall 2012
Multimedia
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3rd Generation: HTTP Streaming

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Other terms for similar concepts: Adaptive Streaming, Smooth
Streaming, HTTP Chunking
Client-centric architecture with stateful client and stateless server
- Standard server: Web servers
- Standard Protocol: HTTP
- Session state and logic maintained at server
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Video is broken into multiple chunks
Chunks begin with keyframe so independent of other chunks
A series of HTTP progressive downloads of chunks
Playing chunks in sequence gives seamless video
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Adaptive Bit Rate with HTTP Streaming
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Encode video at different levels of quality/bandwidth
Client can adapt by requesting different sized chunks
Chunks of different bit rates must be synchronized
- All encodings have the same chunk boundaries and all chunks start
with keyframes, so you can make smooth splices to chunks of
higher or lower bit rates
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Adaptive HTTP Streaming System
(Protocol)

Server
- Can be standard web
server
- Media segment can be
prepared in-line or offline
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Client
- Sends series of HTTP GET
segment requests and
receives segments
- Performs rate adaptation
before sending a new GET
segment request
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Advantages of HTTP Streaming
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Easy to deploy: it's just HTTP, work with exsiting
caches/proxies/CDN/Firewall
Fast startup by downloading lowest quality/smallest chunk
Bitrate switching is seamless
Many small files
- Small with respect to the movie size
- Large with respect to TCP
• 5-10 seconds of 1Mbps – 3Mbps  0.5MB – 4MB per chunk
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Hui Zhang, Fall 2012
Multimedia
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Example of HTTP Streaming Protocols
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Apple HLS: HTTP Live Streaming
Microsoft IIS Smooth Streaming: part of Silverlight
Adobe: Flash Dynamic Streaming
DASH: Dynamic Adaptive Streaming over HTTP
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Hui Zhang, Fall 2012
Multimedia
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Terms and Definitions of Adaptive
HTTP Streaming

Need
- Media Presentation Description (MDP) which provides
metadata
• For requesting (GET request) media segments
• For rate adaptation purpose
- Segment which may include media data or metadata to decode

Use DASH an example in the few slides
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Example: DASH Client
Thomas Stockhammer, Qualcomm, “DASH – Design Principles and Standards ,
Presentation at MMSys 2011
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Meta Data


DASH uses MPD (Media Presentation Descriptor) and
Index Information as metadata for DASH Access Client
Initialization and Media Segments for Media Engine
- Reuse of existing container format
Source: Stockhammer, Qualcomm, “DASH – Design Principles and
Standards , Presentation at MMSys 2011
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Media Presentation Data Model
MDP - description of accessible segments and corresponding timing
Source: Stockhammer, Qualcomm, “DASH – Design Principles and
, Presentation at MMSys 2011
Hui Zhang,Standards
Fall 2012
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Example of HLS Meta Data
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