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Re: [rohc] ROHC-FN tutorial: scaled_seq_no compression formula

Hi Klaus,

Thanks for the clarification. There is certainly nothing wrong with designing the "how" rather than the "what" - the main point is to understand that the FN only intends to specify the "what". This is intentional, to allow the implementer as much freedom as possible. Originally we developed an engine which would take an FN description and automatically provide compression and decompression, meaning that you could get a reference solution without specifying the "how" part. However, this was a lot of work to maintain so we abandoned it.

For reference, with real protocols where the operators used are non-invertible, look-up tables are very useful (and also fast). Although the individual operators used may be many-to-one as with modulo, the overall mapping must be one-to-one, so a lookup table is a nice solution. Of course scalability is an issue - for this example though the tables are small enough to sit in L1 cache so offer a good solution. Partly this depends on your architecture, but typically modulo consumes a lot of clock cycles.

Thanks again for the input.

All the best

Raffles

Klaus Warnke wrote: 
Hi Raffles,

I tried to implement this encoding:

  ENFORCE(sequence_no.UVALUE == (scaled_seq_no.UVALUE * 3) % 16);

I saw, it is easy to calculate the sequence_no.UVALUE from then
scaled_seq_no.UVALUE, because the formula can be used 1:1.
But if I want to calculate the scaled_seq_no.UVALUE from the
sequence_no.UVALUE, I can't see how here. 

Did I oversaw something?

So I restructured the formula, which was not so easy to me, because
there is no inverse for the "%" operator. So I wanted to share this,
maybe it is helpful for other developers.

  
You specify "what" rather than "how"
    
My intention was to explain "how", not "what".
Could you please explain what was wrong about that?

br
Klaus

At Thu, 03 Sep 2009 21:37:39 +0100,
Raffles wrote:
  
Hi Klaus,

Thanks for the post. I think you have probably already seen the 
following line in the example:

         ENFORCE(sequence_no.UVALUE
                   == (scaled_seq_no.UVALUE * 3) % 16);

Which is to say, a similar function to the one which you have described, 
but expressed in the inverse form. The bindings created by this are two 
way. That is, this single line defines both compression and 
decompression. One of the key principles of the FN is that all bindings 
are two way, so that you only define the profile once for both 
compression and decompression. You specify "what" rather than "how". See 
section 3.2.1 for more details: 
http://tools.ietf.org/html/rfc4997#section-3.2.1

Thanks for taking the time to read the RFC and send in a post.

If you have any questions feel free to ask.

Regards

Raffles


Klaus Warnke wrote:
    
Hello RoHC'ers 2.0,

the example B.9. in the RFC 4997 ROHC-FN introduces a scaled_seq_no,
but provides no formula for compression. 

Here is my compression formula, maybe it is helpful for other.

scaled_seq_no.UVALUE = (sequence_no.UVALUE + (m*16)) / 3

where m is:

m = 0 if (U%3 == 0)
m = 2 if (U%3 == 1)
m = 1 if (U%3 == 2)

Checked with oo calc:

U	U%3	m	M*16	U+m*16	C=(U+m*16)/3	C*3	U=(C*3)%16
-       ---     -       ----    ------  ------------    ---     ----------
0	0	0	0	0	0	        0	0
1	1	2	32	33	11	        33	1
2	2	1	16	18	6	        18	2
3	0	0	0	3	1	        3	3
4	1	2	32	36	12	        36	4
5	2	1	16	21	7	        21	5
6	0	0	0	6	2	        6	6
7	1	2	32	39	13	        39	7
8	2	1	16	24	8	        24	8
9	0	0	0	9	3	        9	9
10	1	2	32	42	14	        42	10
11	2	1	16	27	9	        27	11
12	0	0	0	12	4	        12	12
13	1	2	32	45	15	        45	13
14	2	1	16	30	10	        30	14
15	0	0	0	15	5	        15	15

br
Klaus
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