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Buffering
---------
This document outlines the buffering policy used in the GStreamer
core that can be used by plugins and applications.
The purpose of buffering is to accumulate enough data in a pipeline so that
playback can occur smoothly and without interruptions. It is typically done
when reading from a (slow) and non-live network source but can also be used for
live sources.
We want to be able to implement the following features:
- buffering up to a specifc amount of data, in memory, before starting playback
so that network fluctuations are minimized.
- download of the network file to a local disk with fast seeking in the
downloaded data. This is similar to the quicktime/youtube players.
- caching of semi-live streams to a local, on disk, ringbuffer with seeking in
the cached area. This is similar to tivo-like timeshifting.
- progress report about the buffering operations
- easy (backward compatible) application notification of buffering
- the possibility for the application to do more complex buffering
Some use cases:
* Stream buffering:
+---------+ +---------+ +-------+
| httpsrc | | buffer | | demux |
| src - sink src - sink ....
+---------+ +---------+ +-------+
In this case we are reading from a slow network source into a buffer element
(such as queue2).
The buffer element has a low and high watermark expressed in bytes. The
buffer uses the watermarks as follows:
- The buffer element will post BUFFERING messages until the high watermark
is hit. This instructs the application to keep the pipeline PAUSED, which
will eventually block the srcpad from pushing while data is prerolled in
the sinks.
- When the high watermark is hit, a BUFFERING message with 100% will be
posted, which instructs the application to continue playback.
- When during playback, the low watermark is hit, the queue will start posting
BUFFERING messages again, making the application PAUSE the pipeline again
until the high watermark is hit again.
- during playback, the queue level will fluctuate between the high and the
low watermark as a way to compensate for network irregularities.
This buffering method is usable when the demuxer operates in push mode.
Seeking in the stream requires the seek to happen in the network source.
It is mostly desirable when the total duration of the file is not know, such
as in live streaming or when efficient seeking is not possible/required.
* Incremental download
+---------+ +---------+ +-------+
| httpsrc | | buffer | | demux |
| src - sink src - sink ....
+---------+ +----|----+ +-------+
V
file
In this case, we know the server is streaming a fixed length file to the
client. The application can choose to download the file on disk. The buffer
element will provide a push or pull based srcpad to the demuxer to navigate in
the downloaded file.
This mode is only suitable when the client can determine the length of the
file on the server.
In this case, buffering messages will be emited as usual when the requested
range is not within the downloaded area + buffersize. The buffering message
will also contain an indication that incremental download is being performed.
This flag can be used to let the application control the buffering in a more
intelligent way, using the BUFFERING query, for example.
The application can use the BUFFERING query to get the estimated download time
and match this time to the current/remaining playback time to control when
playback should start to have a non-interupted playback experience.
* Timeshifting
+---------+ +---------+ +-------+
| httpsrc | | buffer | | demux |
| src - sink src - sink ....
+---------+ +----|----+ +-------+
V
file-ringbuffer
In this mode, a fixed size ringbuffer is kept to download the server content.
This allows for seeking in the buffered data. Depending on the size of the
buffer one can seek further back in time.
This mode is suitable for all live streams.
As with the incremental download mode, buffering messages are emited along
with an indication that timeshifting download is in progress.
* Live buffering
In live pipelines we usually introduce some latency between the capture and
the playback elements. This latency can be introduced by a queue (such as a
jitterbuffer) or by other means (in the audiosink).
Buffering messages can be emited in those live pipelines as well and serve as
an indication to the user of the latency buffering. The application usually
does not react to these buffering messages with a state change.
Messages
--------
A GST_MESSAGE_BUFFERING must be posted on the bus when playback temporarily
stops to buffer and when buffering finishes. When percentage field in the
BUFFERING message is 100, buffering is done. Values less than 100 mean that
buffering is in progress.
The BUFFERING message should be intercepted and acted upon by the application.
The message contains at least one field that is sufficient for basic
functionality:
"buffer-percent", G_TYPE_INT, between 0 and 100
Several more clever ways of dealing with the buffering messages can be used when
in incremental or timeshifting download mode. For this purpose additional fields
are added to the buffering message:
"buffering-mode", GST_TYPE_BUFFERING_MODE,
enum { "stream", "download", "timeshift", "live" }
- gives the buffering mode in use. See above for an explanation of the
different modes of buffering. This field can be used to let the
application have more control over the buffering process.
"avg-in-rate", G_TYPE_INT
- gives the average input buffering speed in bytes/second. -1 is unknown.
This is the average number of bytes per second that is received on the
buffering element input (sink) pads. It is a measurement of the network
speed in most cases.
"avg-out-rate", G_TYPE_INT
- gives the average consumption speed in bytes/second. -1 is unknown.
This is the average number of bytes per second that is consumed by the
downstream element of the buffering element.
"buffering-left", G_TYPE_INT64
- gives the estimated time that bufferring will take in milliseconds.
-1 unknown.
This is measured based on the avg-in-rate and the filled level of the
queue. The application can use this hint to update the GUI about the
estimated remaining time that buffering will take.
Application
-----------
While data is buffered, the pipeline should remain in the PAUSED state. It is
also possible that more data should be buffered while the pipeline is PLAYING,
in which case the pipeline should be PAUSED until the buffering finished.
BUFFERING messages can be posted while the pipeline is prerolling. The
application should not set the pipeline to PLAYING before a BUFFERING message
with 100 percent value is received, which might only happen after the pipeline
prerolled.
An exception is made for live pipelines. The application may not change
the state of a live pipeline when a buffering message is received. Usually these
buffering messages contain the "buffering-mode" = "live".
The buffering message can also instruct the application to switch to a periodical
BUFFERING query instead to more precisely control the buffering process. The
application can, for example, choose to not act on the BUFFERING message with
100 percent fill level to resume playback but instead use the estimated download
time to resume playback to get uninterrupted playback.
Buffering Query
---------------
In addition to the BUFFERING messages posted by the buffering elements we want
to be able to query the same information from the application. We also want to
be able to present the user with information about the downloaded range in the
file so that the GUI can react on it.
In addition to all the fields present in the buffering message, the BUFFERING
query contains the following field, which indicate the available downloaded
range in a specific format and the estimated time to complete:
"busy", G_TYPE_BOOLEAN
- if buffering was busy. This flag allows the application to pause the
pipeline by using the query only.
"format", GST_TYPE_FORMAT
- the format of the "start" and "stop" values below
"start", G_TYPE_INT64, -1 unknown
- the start position of the available data
"stop", G_TYPE_INT64, -1 unknown
- the stop position of the available data
"estimated-total", G_TYPE_INT64
- gives the estimated download time in milliseconds. -1 unknown.
When the size of the downloaded file is known, this value will contain
the latest estimate of the remaining download time. This value is usualy
only filled for the "download" buffering mode. The application can use
this information to estimate the amount of remaining time to download the
complete file.
For the "download" and "timeshift" buffering-modes, the start and stop positions
specify the ranges where efficient seeking in the downloaded media is possible.
Seeking outside of these ranges might be slow or not at all possible.
For the "stream" and "live" mode the start and stop values describe the oldest
and newest item (expressed in "format") in the buffer.
Defaults
--------
Some defaults for common elements:
A GstBaseSrc with random access replies to the BUFFERING query with:
"buffer-percent" = 100
"buffering-mode" = "stream"
"avg-in-rate" = -1
"avg-out-rate" = -1
"buffering-left" = 0
"format" = GST_FORMAT_BYTES
"start" = 0
"stop" = the total filesize
"estimated-total" = 0
A GstBaseSrc in push mode replies to the BUFFERING query with:
"buffer-percent" = 100
"buffering-mode" = "stream"
"avg-in-rate" = -1
"avg-out-rate" = -1
"buffering-left" = 0
"format" = a valid GST_TYPE_FORMAT
"start" = current position
"stop" = current position
"estimated-total" = -1
|
