docs/design/part-buffering.txt - imx-gstreamer - Git at Google

 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
	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