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ALC track

M. Luby, Digital Fountain

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ALC track overall status

ALC PI – RFC 3450 pub as Experimental LCT BB – RFC 3451 pub as Experimental FEC BB – RFC 3452 pub as Experimental FEC INFO – RFC 3452 pub as Informational WEBRC BB – past WG last call – to IESG?? Compact FEC BB

• Scoped by the FEC BB• Draft 00• Should go to WG last call soon Experimental RFC

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Compact FEC BB

A supplement to RFC 3452 (FEC building block) Introduces two new FEC Encoding IDs

• FEC Encoding ID 0 (Fully-specified)– No coding– Useful for interoperability testing– Requested by several entities

• FEC Encoding ID 130 (Under-specified)– Similar to FEC Encoding ID 128– More compact payload header

• Relaxes requirements from FEC Encoding IDs 128 and 129– Object length not required as part of FEC OTI

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FEC Payload ID

FEC Payload ID for existing 128 -------+-------+-------+-------+-------+-------+-------+-------+ | Src. Blk. No. | Enc. Sym. ID | -------+-------+-------+-------+-------+-------+-------+------ 32 bits 32 bits

FEC Payload ID for new 0 and 130 -------+-------+-------+-------+ | SrcBlkNo | EncSymID | -------+-------+-------+-------+ 16 bits 16 bits

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FEC Object Transmission Info.

FEC Encoding ID (0 or 130) FEC Instance ID (only for 130 since it is Under-Specified) Encoding Symbol Length Source Block Length

Not the object length (different than RFC 3452)• Allows for delivery of objects of unknown length• May not be able to obtain object length before transmission starts• On-line compression or encryption is used

– Transmitted object length != Delivered object length• Object is a stream

– Object has no a priori length• Recommend for others when RFC 3452 Proposed Standard

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FEC Encoding ID 0 Definitions

• L = Encoding Symbol Length (in bytes)• X = Source Block Length (in bytes)• N = X/L rounded up (recommended to be integral)• Source Block consists of N pieces each of length L

Sender• Sender chooses Y randomly in range [0,…,N-1]• Sends carousels pieces Y, Y+1, Y+2, … (mod N)

Receiver• For a Source Block with Source Block Number I, do the following:• Allocate and initializes a boolean array RECEIVED[0,…,N-1] to all false• Allocate a byte array SRCBLK[0,…,N*L-1]• When piece Y arrives:

– RECEIVED[Y] is set to true– The piece is placed into bytes SRCBLK[L*Y,…,L*(Y+1)-1]

• Source Block I is recovered when RECEIVED[0,…,N-1] are all true

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Available FEC codes

Fully-Specified• No Coding (FEC Encoding ID 0)

Under-Specified• Luigi Rizzo’s implementation of Reed-Solomon codes• Embedded Fountain Core Technology

– Digital Fountain’s most powerful encoder/decoder– Development and Product License programs– www.digitalfountain.com/products/licensing/index.cfm – Licensing@digitalfountain.com