Re: [AVT] draft-kerr-avt-vorbis-rtp-04

[Phil Kerr <[e-mail]>]

Hi Magnus,

Thanks for clarifying this point.  To avoid the problems you've 
mentioned should I include polynomial function and a C code snippet?

Cheers

Phil


Magnus Westerlund wrote:

> Hi Phil,
>
> I think the point Colin tries to make is that the CRC polynom and how 
> it is calculated must be very well defined. This is an important to 
> get correct to be interoperable, however history in many groups has 
> shown that definitions of checksums are often not sufficiently explicit.
>
> Cheers
>
> Magnus
>
> Phil Kerr wrote:
>
>> Hi Colin,
>>
>> Colin Perkins wrote:
>>
>>
>>> Actually, I meant what is the CRC algorithm, not which data is it 
>>> calculated using?
>>
>>
>>
>> I'm probably not the best person to discuss polynomial functions in 
>> depth with, but the CRC32 routine I'm using in the implementations is 
>> a basic, non-table lookup algorithm. As the CRC is fixed for the 
>> duration it's only calculated once per chained file, and as the 
>> codebooks are limited to 64K for RTP, it doesn't need to be highly 
>> optimised.
>>
>>
>>>> The chance of collision should be quite low, MD5 would be a lot 
>>>> safer in this regard but it does add quite a lot of overhead to 
>>>> each packet.  There may need to be an additional identifier, 
>>>> possibly a host id which ties the CRC32 value to the host, which is 
>>>> sent via SDP to reduce the chance of collision further.
>>>
>>>
>>>
>>> Host identifiers are difficult to pick too, so I'm not sure that'd 
>>> help.
>>>
>>> Can you expand on why the CRC was chosen, and why a table-indexing 
>>> to onto a list of URLs wouldn't work?
>>
>>
>>
>> The decoding codebook associated with the stream can change at the 
>> boundary of a chained file, which can be at any point in the session 
>> (new audio tracks or station jingles for example).  Having a CRC of 
>> the associated codebook in each packet means there is a hard 
>> association between the audio and decoder.
>>  
>> As the CRC is derived from the codebook itself we have a mechanism 
>> for verifying the correct decoder is being used at any particular 
>> time independent of any table.
>>
>> Cheers
>>
>> Phil
>>
>>
>>> Cheers,
>>> Colin
>>>
>>>
>>> _______________________________________________
>>> Audio/Video Transport Working Group
>>> [e-mail]
>>> https://www1.ietf.org/mailman/listinfo/avt
>>>
>>
>>
>>
>> _______________________________________________
>> Audio/Video Transport Working Group
>> [e-mail]
>> https://www1.ietf.org/mailman/listinfo/avt
>>
>
>


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