Definitions¶
The following defines the various terms used in Rely.
Payload¶
A unit of data originating from the source that is used as input to to the Rely encoder. Conversely it is also the output of the Rely decoder. Examples of this could be video/audio data or similar.
Source symbol¶
A unit of data that is used as input to the sliding window encoder and output of the sliding window decoder. One Payload may be fragmented into multiple source symbols.
Repair symbol¶
The result of performing encoding over the set of available Source symbols. Used by the decoder to reconstruct lost source symbols.
Stream¶
The stream queue that controls the memory held by either the encoder or decoder. Source symbols can be pushed and popped from the stream. Each symbol is assigned a unique position in the stream.
Maximum stream size¶
The maximum size (max_stream_size) of the stream directly corresponds to the
maximum number of Source symbols stored in the stream.
A higher value will result in a more effective codec, whereas a lower value will
result in a lower memory footprint.
Maximum payload bytes¶
The maximum size of the Payload, consumed by the encoder and later reconstructed by the decoder.
The maximum size equals 65536 bytes.
Packet¶
A packet is the output of the encoder and later the input of the decoder. A packet can contain either a Source symbol or a Repair symbol.
Maximum packet bytes¶
The maximum size of a Packet in bytes. Specified with
the max_packet_bytes parameter in encoder::configure(...)
and decoder::configure(...). An encoder and its corresponding decoder should
be configured with the same max_packet_bytes value.
Source symbol repair count¶
The number of times a specific Source symbol was included in a Repair symbol.
Source symbol timestamp¶
The timestamp associated with each source symbol.
Source symbol timeout¶
The timeout of a specific source symbol. This is based on the timeout parameter
given in encoder::configure(...) and decoder::configure(...), and the
latest seen argument for the now parameter.
Expired source symbol¶
If a source symbol’s Source symbol timeout is less than or 0 the source symbol is expired.
Protected source symbol¶
A source symbol is protected if it has been part of one or more Repair phases.
Unprotected source symbol¶
A source symbol is unprotected if it has never been part of a Repair phase. One exception to this statement is if the repair target is 0, i.e., if Repair disabled. If so the source symbol is marked protected immediately.
Decoded source symbol¶
Source symbols are implicitly decoded, Repair symbols are decoded once the internal sliding window decoder has converted it into a missing Source symbol.
Released source symbol¶
A source symbol is said to be released when:
- the source symbol has been decoded and forwarded to the defragmentation component,
- the decoder has given up decoding the source symbol and will not forward the source symbol to the defragmentation component.
A released source symbol may still be used by the decoder to continue decoding of other source symbols.
Source symbol are always released in-order.
Repair interval¶
The repair interval (repair_interval) is the number of
source symbols added between each Repair phase.
The following values are valid for the repair_interval:
1 <= repair_interval <= max_stream_size
This can ensure that all source symbols are part of minimum one repair interval before being popped off the Stream.
Repair position¶
Tracks the position in the Repair interval, i.e., how many source symbols have been added so far.
This number will never exceed the repair_interval, as it’s reset after each
Repair phase.
Repair target¶
The number of repair symbols that will be produced when the Repair position reaches the Repair interval.
The following condition must hold: 0 <= repair_target
repair_target == 0 is equivalent to repair disabled.
Repair rate¶
The rate of repair symbols equal:
repair_rate = repair_target / (repair_interval + repair_target)
Repair phase¶
The encoder enters the repair phase if one of the following conditions are satisfied:
- The Repair position reaches the Repair interval.
- The Repair interval is changed to be less than or equal to the Repair position.
- An Unprotected source symbol expires.
When the encoder enters the repair phase, a number of Repair symbols are produced and the Repair position is reset.
The following calculation can be used for determining the number of repair symbols produced:
repair_produced = ceil(repair_position / (1 - repair_rate)) - repair_position
Example:
repair_interval = 10
repair_target = 2
repair_rate = 2 / 12 = 0.166
repair_position = 1
repair_produced = ceil(1 / (1 - 0.166)) - 1 = 1
repair_position = 3
repair_produced = ceil(3 / (1 - 0.166)) - 3 = 1
repair_position = 6
repair_produced = ceil(6 / (1 - 0.166)) - 6 = 2
repair_position = 10
repair_produced = ceil(10 / (1 - 0.166)) - 10 = 2
Repair enabled¶
Repair is enabled when repair_rate > 0, i.e., repair symbols for protection
against packet loss are being produced.
Repair disabled¶
When repair is disabled no repair symbols are produced to protect against
packet loss, i.e. only Source symbols are produced. This happens when
repair_target == 0 and thus repair_rate == 0.
Systematic mode¶
A strategy used in ECC/FEC schemes. When enabled the encoder will always initially produce each source symbols as a Source packet. The class encoder is a systematic encoder.
Coding window¶
The number of source symbols that is being included in the produced repair symbols(s). The coding window of the class encoder will always follow the size of the Stream.