Re: Determining p value for WLSB of RTP SN.

You are correct, the interpretation interval for k=5 doesn't cover
the whole interpretation interval for k=4.

This have been discussed before on the rohc list. Please search the
mailing list archive for more information about this issue.

Colorful language by the way..

/Calle

On Sat, 2 May 2009, Ang Way Chuang wrote:

> Dear ROHC list subscribers,
>
> According to section 5.7 of RFC 3095:
>     SN: The compressed RTP Sequence Number.
>
>         Compressed with W-LSB.  The interpretation intervals, see section
>         4.5.1, are defined as follows:
>
>              p = 1                   if bits(SN) <= 4
>              p = 2^(bits(SN)-5) - 1  if bits(SN) >  4
>
>     where bits(x) is the number of bits in the compressed header.
>
> At first, when I read that the change in the value of p depends on
> bits(SN), I was very perplexed. If the algorithm determines k <= 4 can
> be used, but due to other factors packet type that is to be used only
> have bits(SN) > 4, then we may have problem with the following situation:
>
> Say, compressor receives reordered RTP, then:
> vref_c = 32
> val = 31
> then k=1 is more than enough to cover val because the interpretation
> interval is
> [32 - 1, 32 + (2 - 1) - 1 ] = [31, 32]
>
> But if the packet type only have bits(SN) = 5, then we are screwed:
> [32 - (2^0 - 1), 32 + 2^5 - 1 - (2^0 - 1)] = [32, 63]
>
> 31 is not covered!!
> Thus, the logic for getting k for SN becomes very hairy because it has
> to take care of this situation. k > 5 are okay because whatever the
> interpretation interval for these k bits are superset of interpretation
> interval for k <= 4.
>
> But when I look at all packet types for RTP, none of them has bits(SN)
> == 5. Thus, I assume that whatever calculated k bit of SN are generally
> safe to be expanded to the available bits(SN). Is my understanding on
> this matter correct? Do tell me if I get it wrong or miss out something
> important.
>
> Thank you in advance.
>
> Regards,
> Ang Way Chuang
>
>
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