ULE, and CRC-32

[Gorry Fairhurst <gorry@erg.abdn.ac.uk>]

After some-off list discussions with Alain, I think we can now propose 
some modified text for the section in ULE on CRC usage. Basically, this 
says follow the well-known practice, and in particular this converges on 
what ISO DSM-CC specifies.

New text reads:


SNDU Trailer CRC

Each SNDU MUST carry a 32-bit CRC field in the last four bytes of the 
SNDU. This position eases CRC computation by hardware.  The CRC-32 
polynomial is to be used. This is a 32 bit value calculated according to 
the generator polynomial represented 0x04C11DB7 in hexadecimal:

x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x^1+x^0.

Examples where this polynomial is also employed include Ethernet, DSM-CC 
section syntax [ISO-DSMCC} and AAL5 [ITU356]. The use resembles, but is 
different to that in SCTP [RFC3309].

The Encapsulator initialises the CRC-32 accumulator register to the 
value 0xFFFF FFFF.  It then accumulates a transmit value for the CRC32 
including all bytes from the start of the SNDU header to the end of the 
SNDU (excluding the 32-bit trailer), and places this in the CRC Field.  
The receiver performs an integrity check by independently calculating 
the same CRC value and comparing this with the transmitted value in the 
SNDU trailer. SNDUs that do not have a valid CRC, are discarded, causing 
the Receiver to enter the Idle State.

This description may be suited for hardware implementation, but this 
document does not imply any specific implementation.  Software-based 
table-lookup or hardware-assisted software-based implementations are 
also possible.

The primary purpose of this CRC is to protect the SNDU payload from 
undetected resassembly errors and errors introduced by unexpected 
software / hardware operation while the SNDU is in transit across the 
MPEG-2 subnetwork and during processing at the encapsulation gateway 
and/or the receiver. It may also detect the presence of uncorrected 
errors from the physical link (these may however, in some cases, also be 
detected by other means).

Alain:
> Some questions about the CRC-32, because I'm a little bit confused.
> I think many of you are familiars with that CRC-32 stuff, so I
> need their help ;-)
<SNIP>