RTP Payload Format for Reed-Solomon FEC

draft-galanos-fecframe-rtp-reedsolomon-01

Sarit Galanos, RADVISIONsarit@radvision.com

IETF 77 – March 2010

Orly Peck, RADVISIONorlyp@radvision.com

Vincent Roca, INRIAvincent.roca@inria.fr

Main Changes from draft 00

• Using terminology as defined in RFC 5510• Removing constraint for protection of consecutive RTP

packets• Added implementation considerations section• Added clarifying sentences and examples.• Added a comprehensive security section

FEC header for repair packets 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| n_r | i | SN_base | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| reserved | BML | pkt_span |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+. .. bit-mask (optional, variable length) .. .+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

– n_r : the number of FEC repair packets used to protect this source block.– i – the 0-based index in the sequence of n_r FEC repair packets– SN_base – the lowest RTP sequence number (taking wraparound into account) of the source

packets protected by this repair packet. This SN_base also identifies the source block.

– pkt_span –The number of consecutive source packets protected by this repair packet– Reserved - for future use– BML - Bit Mask Length: indicates the length of the bit-mask in units of 32-bit words. For case of

non-consecutive RTP packets protected.– Bit-mask – set at the length indicated by BML. Indicates which RTP packets are protected.

About the bit-mask…

• used in situations where RTP packet erasures happen prior to reaching FECFRAME encoder or when non-consecutive packets are protected.– need a way to signal gaps in RTP SN numbering, i.e.

packets that are not in the source block– use a variable length bit-mask array – no mask if BML=0

IANA Registration

• Register subtype name reed-solomon-fec for audio/video/text/application

• Required parameters– Max_N – upper limit for N, which is the total

number of source and repair packets in a block– Repair window (us) – time span of the source and

repair packets– element-size: a non-negative integer indicating

the length of each encoding symbol in bits. Equals to the m parameter in the GF (2^m).

Repair-Window

• The sender knows the time that spans between the first RTP packet sent, and the repair packets. Defining the RW size impacts the maximum number of source packets in a FEC block.

• Defines the minimum time the receiver should wait.

• May be negotiated between sender and receiver.

Implementation considerations

• Set FEC code-rate dynamically according to network packet loss. Improves bandwidth utilization.

• Reduce protected media bandwidth, in order not to increase total bandwidth consumption and worsen network conditions.

• Appropriate design – choosing k, n• Packetization – protecting RTP packets with

similar sizes if possible.

Questions & Comments