RTP Payload Format for ReedSolomon FEC

1
RTP Payload Format for Reed-Solomon FEC
IETF 76 November 2009

• draft-galanos-fecframe-rtp-reedsolomon-00

Sarit Galanos, RADVISION sarit_at_radvision.com
Orly Peck, RADVISION orlyp_at_radvision.com
2
Background

• Splitting draft-galanos-fecframe-rtp-reedsolomon-m
  f-00 to 2 drafts
• Searched for a method of protecting RTP packets
  against random, bursty losses which enables
• Backward compatibility
• Low recovery latency
• Dynamic on/off switching
• This draft
• Describes FEC scheme based on Reed Solomon code
  and FEC framework
• Specifies RTP payload format for such FEC packets

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RTP 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
• -----------------------
  ---------
• V2PX CC M PT
  sequence number
• -----------------------
  ---------
• timestamp
  
• -----------------------
  ---------
• synchronization source (SSRC)
  identifier
• 
  
• contributing source (CSRC)
  identifiers
• ....
  
• -----------------------
  ---------
• Marker bit (M) not used
• Payload Type (PT) dynamically determined in SDP
• Sequence Number
• initiated randomly rfc3550
• One higher than number in the previously
  transmitted repair packet
• Timestamp corresponds to repair packet transmit
  time

4
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-K i SN Base
  
• -----------------------
  ---------
• Num Packets
  Reserved
• -----------------------
  ---------
• General fields
• N-K the number of FEC packets used to protect
  this block
• i 0 based packet index in the N-K FEC packets
  sequence
• SN base the lowest sequence number (taking
  wraparound into account) of the source packets
  protected by this repair packet
• Num Packets The number of consecutive source
  packets protected by this repair packet
• Reserved - for future use

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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
• symbol-size a non-negative integer indicating
  the length of each encoding symbol in bits.

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SDP example

• v0
• osarit 1122334455 1122334466 IN IP4
  fec.example.com
• s Reed Solomon FEC Example
• t0 0
• agroupFEC S1 R1
• mvideo 30000 RTP/AVP 100
• cIN IP4 224.1.1.1/127
• artpmap100 MP2T/90000
• amidS1
• mapplication 30000 RTP/AVP 110
• cIN IP4 224.1.2.1/127
• artpmap110 reed-solomon-fec /90000
• afmtp110 max_N5 repair-window 200000
  symbol-size8
• amidR1

7
Open issues

• Reed-Solomon primitive polynomial
• Defined by protocol (hard definition)
• Negotiated in SDP
• Combination (unless specified in SDP use default
  value from standard).

8

• Comments are welcome!