What Is RaptorQ?
RaptorQ is a rateless erasure code (also known as fountain code), and provides two functions:
- Redundantly encoding a message into practically infinite number (~2**24) of symbols;
- Reliably decoding the original message from any subset of encoded symbols with high probability, provided that the cumulative size of the symbols received is equal to or slightly greater than the original message size.
RaptorQ is useful for a variety of purposes, including but not limited to:
- Transmitting a message reliably over a lossy and/or adversarial network path, without employing acknowledgement (feedback) mechanism or suffering from round-trip delays incurred thereby.
- Reliable object storage, where redundancy/fault-tolerence level—such as number of parity disks in RAID arrays—can be scaled up or down on demand without having to re-code contents on the existing disks.
We at Harmony are developing and using
go-raptorq in order to implement a
near-optimal, adversary-resilient, and stable-latency message broadcasting
mechanism for use in our highly scalable and performant blockchain network.
go-raptorq in your Go application:
$ CGO_CXXFLAGS='-std=c++11' go get simple-rules/go-raptorq
go-raptorq contains two main interfaces,
In order to send a binary object, termed source object, the sender creates
Encoder for the source object. The
Encoder first splits the source
object into one or more contiguous blocks, termed source blocks, then for
each source block, the
Encoder generates encoded and serially numbered
chunks of data, termed encoding symbols. For each source block, the sender
Encoder to generate as many encoding symbols as needed by the
receiver to recover the source block. The sender chooses the size of the
encoding symbol, in octets, when creating an
On the other side, the receiver creates a
Decoder for the same source
object, then keeps feeding the
Decoder with the encoding symbols received
from the sender, until the
Decoder is able to recover the source object from
the encoding symbols. Once the source object has been recovered, the receiver
Close()-s and discards the
An encoding symbol is a unit data of transmission. Each encoding symbol is
identified by a (
esi) pair, where:
sbn, or source block number, is the 8-bit, zero-based serial number of the source block from which the symbol was generated;
esi, or encoding symbol identifier is the 24-bit, zero-based serial identifier of the encoding symbol within the source block.
Upon receipt of an encoding symbol, the receiver first needs to identify the
Decoder to use for the source object to which the received encoding symbol
belongs, then feed the
Decoder with the encoding symbol, along with its
In order to recover each source block with high probability, the receiver needs as many encoding symbols as needed to fulfill the original size (in octets) of the source block. For example, a 1.8MB source block with 1800-byte encoding symbols, the receiver would need 1000 encoding symbols with 99% probability. Each additional symbol adds roughly “two nines” to the decoding success probability. In the example above:
- 1000 encoding symbols would mean 99.99%,
- 1001 encoding symbols would mean 99.9999%,
- 1002 encoding symbols would mean 99.999999%, and so on.
It is okay for an encoding symbol to be completely lost (erased) during transit. For each encoding symbol lost, the encoder simply needs to generate and send another encoding symbol. The sender need not know which encoding symbol was lost; a brand new encoding symbol would be able to replace for any previously sent-and-lost encoding symbol. The sender may even anticipate losses and send additional encoding symbols in advance without having to wait for negative acknowledgements (NAKs) from the receiver.
(Proactively sending redundant symbols this way is called forward error correction (FEC), and is useful to reduce or even eliminate round-trip delays required for reliable object transmission. It can be seen as a form of “insurance,” with the amount of extra, redundant data being its “insurance premium.”)
It is NOT okay for an encoding symbol to be corrupted—accidentally or
maliciously—during transit. Feeding a
Decoder with such corrupted symbols
WILL jeopardize recovery of the source object, so the receiver MUST detect and
discard corrupted encoding symbols, e.g. using checksums calculated and
transmitted by the sender along with the encoding symbol.
Decoder for the same source object should share the
following information: Total source object size (in octets), symbol size (chosen
by the sender), number of source blocks, number of sub-blocks (an internal
detail), and the symbol alignment factor (another internal detail). The IETF
Forward Error Correction (FEC) Building Block specification (RFC 5052) and the
RaptorQ specification (RFC 6330) encapsulate these into two parameters: 64-bit
Common FEC Object Transmission Information, and 32-bit Scheme-Specific FEC
Object Transmission Information. They are available from the sender's
Encoder; the receiver should pass them when creating a
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