Streaming Stored Video- Computer Networking

Presented To
Sajeeb Saha
Assistant Professor
Dept of CSE, Jagannath University
Presented By
Anisuzzaman Khan
ID: B150305028
CSE, JnU
Md. Ashik Jiddney
ID: B150305030
CSE, JnU
Mahbubur Rahman
ID: B150305042
CSE, JnU

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Streaming Stored Video
Streaming: Can begin playout before downloading entire file
Stored (at server): Can transmit faster than audio/video will be rendered (implies
storing/buffering at client) e.g., YouTube, Netflix, Hulu
Video: Can begin playout before downloading entire file
For streaming video applications, pre recorded videos are placed on servers, and users
send requests to these servers to view the videos on demand.

Streaming Stored
Video
Streaming video systems can be classified into three categories:
I. UDP streaming
II. HTTP streaming
III. Adaptive HTTP streaming
Although all three types of systems are used in practice, the majority of today’s systems
employ HTTP streaming and adaptive HTTP streaming.
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Streaming Stored Video : Challenges
Continuous playout constraint: once client playout begins, playback must match original
timing but network delays are variable (jitter), so will need client-side buffer to match
playout requirements
Other Challenges:
➔ client interactivity: pause, fast-forward, rewind, jump through video
➔ video packets may be lost, retransmitted
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Streaming Stored Video : Client Side Buffer
All the three types of video streaming client side buffering is used because
❏ Mitigate the effects of end to end delay.
❏ Varying amounts of available bandwidth between server and client.
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Streaming Stored Video : Client Side Buffer
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Streaming Stored Video: UDP Streaming
★ Server sends at rate appropriate for client
• often: send rate = encoding rate = constant rate
• transmission rate can be oblivious to congestion levels
★ Short playout delay (2-5 seconds) to remove network jitter
★ Error recovery: application-level, time permitting
★ RTP [RFC 2326]: multimedia payload types
★ UDP may not go through firewalls

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In HTTP streaming, the video is simply stored in an HTTP server as an ordinary file with
a specific URL. When a user wants to see the video, the client establishes a TCP
connection with the server and issues an HTTP GET request for that URL. The server
then sends the video file, within an HTTP response message, as quickly as possible, that
is, as quickly as TCP congestion control and flow control will allow.
Streaming Stored Video: HTTP Streaming

● The use of HTTP over TCP also allows the video to traverse firewalls and NAT
(Network address translation) more easily (which are often configured to block most
UDP traffic but to allow most HTTP traffic).
● Streaming over HTTP also obviates the need for a media control server, such as an
RTSP( Real Time Streaming Protocol) server, reducing the cost of a large-scale
deployment over the Internet.
● Due to all of these advantages, most video streaming applications today—including
YouTube and Netflix—use HTTP streaming (over TCP) as its underlying streaming
protocol.
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● Multimedia File Retrieved Via HTTP Get.
● Send At Maximum Possible Rate Under TCP.
● Fill Rate Fluctuates Due To TCP Congestion Control,
Retransmissions (In-order Delivery).
● Larger Playout Delay: Smooth Tcp Delivery Rate.
● HTTP/TCP Passes More Easily Through Firewalls.

Functionalities in HTTP Streaming
1. Prefetching Video.
2. Client Application Buffer And TCP Buffer.
3. Analysis of Video Streaming.
4. Early Termination And Repositioning The Video.
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❖ Prefetching Video
● The client can attempt to download the video at a rate higher than the consumption rate.
● This prefetched video is naturally stored in the client application buffer.
● TCP’s congestion avoidance mechanism will attempt to use all of the available bandwidth
between server and client.
● when the average TCP throughput is roughly twice the media bit rate, streaming over TCP
results in minimal starvation and low buffering delays. [Wang 2008]
Example: When video consumption rate is 1 Mbps but the network is
capable of delivering the video from server to client at a constant rate of 1.5
Mbps. Then the client will not only be able to play out the video, but will
also be able to increase the amount of buffered video data by 500 Kbits
every second.
In this manner, if in the future the client receives data at a rate of less than
1Mbps for a brief period of time, the client will be able to continue to
provide continuous playback due to the reserve in its buffer.

15❖ Client Application Buffer And TCP Buffer
Figure 7.2 illustrates the interaction between client
and server for HTTP streaming.
At the server side, the portion of
the video file in white has already
been sent into the server’s socket,
while the darkened portion is what
remains to be sent. After “passing
through the socket door,” the
bytes are placed in the TCP send
buffer before being transmitted
into the Internet.

16what happens when the user pauses the video during the streaming process ?
● Bits are not removed from the client application buffer, even though bits continue to
enter the buffer from the server.
● If the client application buffer is finite, it may eventually become full, which will cause
“back pressure” all the way back to the server.
● Once the client application buffer becomes full, bytes can no longer be removed from the
client TCP receive buffer, so it too becomes full.
● Once the client receive TCP buffer becomes full, bytes can no longer be removed from
the client TCP send buffer, so it also becomes full.
● Once the TCP send buffer becomes full, the server cannot send any more bytes into the
socket. Thus, if the user pauses the video, the server may be forced to stop transmitting,
in which case the server will be blocked until the user resumes the video.

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what happens when client application buffer becomes full during the regular
streaming process ?
Back pressure will cause the TCP buffers to become full, which will force the
server to reduce its rate.
when the client application removes f bits, it creates room for f bits in the client
application buffer, which in turn allows the server to send f additional bits.
Thus, the server send rate can be no higher than the video consumption rate at
the client. Therefore, a full client application buffer indirectly imposes a limit
on the rate that video can be sent from server to client when streaming over
HTTP.

18❖ Analysis of video streaming
B denote the size (in bits) of the
client’s application buffer,
Q denote the number of bits that
must be buffered before the client
application begins playout.
r denote the video consumption
rate

19❖ When x < r (that is, if the server send rate is less than the video consumption
rate), then the client buffer will never become full! Indeed, starting at time, the
buffer will be depleted at rate r and will only be filled at rate x < r. Eventually the
client buffer will empty out entirely, at which time the video will freeze on the
screen while the client buffer waits another seconds to build up Q bits of video.
Thus, when the available rate in the network is less than the video rate, playout
will alternate between periods of continuous playout and periods of freezing.
❖ When x > r starting at time, the buffer increases from Q to B at rate x > r since bits
are being depleted at rate r but are arriving at rate x, the time the client buffer
becomes full. when the available rate in the network is more than the video rate,
after the initial buffering delay, the user will enjoy continuous playout until the
video ends.

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❖ Early Termination and Repositioning the video
HTTP streaming systems often make use of the HTTP byte-range header in the HTTP
GET request message, which specifies the specific range of bytes the client currently
wants to retrieve from the desired video.
When the user wants to reposition (that is, jump) to a future point in time in the video , the
client sends a new HTTP request, indicating with the byte-range header from which byte in
the file should the server send data. When the server receives the new HTTP request, it can
forget about any earlier request and instead send bytes beginning with the byte indicated in
the byte-range request.
when a user repositions to a future point in the video or terminates the video early, some
prefetched-but-not-yet-viewed data transmitted by the server will go unwatched a waste of
network bandwidth and server resources.

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Example: When the client buffer is full with B bits at some time t0 into the
video, and at this time the user repositions to some instant t > t0 + B/r into the
video, and then watches the video to completion from that point on. In this case, all
B bits in the buffer will be unwatched and the band width and server resources that
were used to transmit those B bits have been completely wasted. which can be
quite costly, particularly for wireless links.
For this reason, many streaming systems use only a moderate-size client
application buffer, or will limit the amount of prefetched video using the byte-
range header in HTTP requests. Repositioning and early termination are analogous
to cooking a large meal, eating only a portion of it, and throwing the rest away,
thereby wasting food.
But, of course, both food and bandwidth are not to be wasted!

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Streaming Stored Video: Adaptive Streaming
Although HTTP streaming, as described in the previous subsection, has been
extensively deployed in practice (for example, by YouTube since its inception), it has
a major shortcoming: All clients receive the same encoding of the video, despite the
large variations in the amount of bandwidth available to a client, both across different
clients and also over time for the same client. This has led to the development of a
new type of HTTP-based streaming, often referred to as Dynamic Adaptive
Streaming over HTTP (DASH).

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● server:
○ divides video file into multiple chunks
○ each chunk stored, encoded at different rates
○ manifest file: provides URLs for different chunks
● client:
○ periodically measures server-to-client bandwidth
○ consulting manifest, requests one chunk at a time
○ chooses maximum coding rate sustainable given current bandwidth
○ can choose different coding rates at different points in time (depending on
available bandwidth at time)
Streaming Stored Video: DASH
Dynamic, Adaptive Streaming over HTTP

● “intelligence” at client: client determines
○ when to request chunk (so that buffer starvation, or overflow does not occur)
○ what encoding rate to request (higher quality when more bandwidth available)
○ where to request chunk (can request from URL server that is “close” to client or
has high available bandwidth)
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Streaming Stored Video: DASH

Streaming Stored Video- Computer Networking

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Streaming Stored Video- Computer Networking