NAME

       LCRQ - C library implementation of RFC6330 (RaptorQ) for Librecast

DESCRIPTION

       IP  Multicast is based on UDP, which is inherently unreliable. Packets may arrive out of order, or not at
       all. TCP provides unicast with a reliable messaging layer  on  top  of  this  unreliable,  connectionless
       medium.

       Unicast, however, is one-to-one only. Multicast could, in theory, use all of the same reliability options
       (ACKs etc.) as TCP, at the cost of not being scalable any more.

       Fortunately there are other ways to achieve similar reliability.   RFC3208  describes  Pragmatic  General
       Multicast (PGM) based on NAKs (negative acknowledgements). This, too, has scaling issues.

       Forwards Error Correction (FEC) offers us another approach.

       Thanks  to  parity  checking  in  the network stack, we don't generally need to worry about errors within
       packets. Every packet has a checksum, and if that doesn't match, the packet is dropped before it  reaches
       us.  Our  encryption  provides  further  checking  of data received.  We need only concern ourselves with
       erasures.  ie. dropped packets.

       RaptorQ is an implementation of a class of systematic erasure codes called fountain codes.

       The data we want to send is split into blocks, and then pre-encoded into a set of  intermediate  symbols.
       From  these  intermediate  symbols  we can generate both our original source symbols, and also additional
       repair symbols.

       Provided the recipient receives at least a minimum value K' of these symbols (any unique  combination  of
       source and repair) the intermediate symbols can be reconstituted, and the original data recovered.

       RaptorQ is what is called a systematic encoding, because the set of symbols we send includes our original
       data as plain text. Provided all source symbols are received, the original data has been transmitted with
       no  decoding overhead.  It is only in the case where we need to supplement the source symbols with repair
       symbols that we must perform the decoding process.

CONFORMING TO

       RFC6330 (IETF) describes the RaptorQ proposed standard, which  LCRQ  more-or-less  follows.  The  primary
       focus  has been on building a fast, simple and dependency-free FEC implementation for use with Librecast,
       and not on strict standards compliance. The code does, however, fairly closely follow the RFC.

EXAMPLES

       There is example code in the examples/ directory of the source repository.

BUGS

       If you find one, email bugs@librecast.net

SEE ALSO

       rq_init(3),   rq_free(3),   rq_query(3),   rq_encode(3),   rq_decode(3),   rq_symbol(3),   rq_pid2sbn(3),
       rq_pid2esi(3), rq_pidsetsbn(3), rq_pidsetesi(3)

       https://datatracker.ietf.org/doc/html/rfc6330