1. RTP Payload Format for Reed-Solomon FEC draft-galanos-fecframe-rtp-reedsolomon-01 Sarit Galanos, RADVISION sarit@radvision.com IETF 77 – March 2010 Orly Peck, RADVISION orlyp@radvision.com Vincent Roca, INRIA vincent.roca@inria.fr

2. 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

3. 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.

4. 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

5. 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).

6. 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.

7. 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.

8. Questions & Comments