Re: Comments on RFC 4997

Hi Lee,

from my point of view the definitions are correct.

A compressed value is a value that exist in a compressed format only.

The discriminator is used to distinguish the packets to choose
the correct format for decompression. Therefore this field does
not appear in a uncompressed format.

 > field_4 =:= '1010'; // set ULENGTH to zero

If a compressed packet start with '1010' the COMPRESSED format3
could be used. But the field_4 does not appear in a UNCOMPRESSED
format, so the ULENGTH is zero.

 > abc_flag_bits =:= uncompressed_value(3, 7) [0];

eg_header
{
UNCOMPRESSED {
abc_flag_bits [ 3 ];
}

COMPRESSED flags_set {
abc_flag_bits =:= uncompressed_value(3, 7) [ 0 ];
}
}

To compress a eg_header into the flags_set format, the
abs_flag_bits field in the uncompressed eg_header with the length
3 must have the value 7. If not, the flags_set format can not be
used, an other format must (irregular, for example). But the
value 7 itself does not appear in the flags_set format itself,
therefore the CLENTH is zero. It is compressed away.

The '[3]' in the UNCOMPRESSED format says, that this field is in
this format three bits in length. You can see this also in every
COMPRESSED format definiton:

abc_flag_bits =:= irregular(3) [ 3 ];
abc_flag_bits =:= uncompressed_value(3, 7) [ 0 ];
abc_flag_bits =:= static [ 0 ];

In irregular the field is send as it is with 3bits. For
uncompressed_value see above. In static, the value in the
eg_header must be the same as saved in the context. To
decompressed such format, a value in the context must exist.

The fn defines a two way mapping between uncompressed and
compressed format. You can use the eg_header UNCOMPRESSED ->
COMPRESSED flags_set mapping, if all definitions could be
fulfilled. Maybe more than one works for compression.
And vice versa.

This is how I understand the fn.

br
Klaus Warnke

Lee, Jiwoong wrote:
>
> Dear ROHC WG,
>
> I hope this comment was not discussed before. At least my quick search 
> did not find the related thread.
>
> These are some questions and comments about RFC 4997.
>
> 1. Definition of Single quotes
>
> According to pp 26 of the same document, a discriminator is defined
> discriminator =:= '01101';
> as equivalent to
> discriminator =:= compressed_value(5, 13);
>
> In pp 36 the last paragraph, COMPRESSED format3 was defined with
> field_4 =:= '1010'; // set ULENGTH to zero
>
> Here unless one do not impose different interpretation of the single
> quotes for the /field/ and the /discriminator/, wouldn’t it be better
> to change this into the following?
> field_4 =:= '1010'; // set ULENGTH to 4
>
> 2. Possible error in the Worked Example
>
> I suspected in Appendix B.7 pp. 56 “COMPRESSED flags_set” format,
>
> abc_flag_bits =:= uncompressed_value(3, 7) [0];
>
> shoud be corrected to
>
> abc_flag_bits =:= irregular(3) [3];
>
> Accordingly, subsequent examples till Appendix B.8 should be corrected.
>
> 3. Encoding function naming issue.
>
> To my impression, the ROHC documents are using the notion and the term 
> “compression” to mean both of compression and encoding, as if they are 
> the same. I believe it is not.
>
> To single out a case, the same document is defining encoding functions
>
> “uncompressed_value( ., . )” and “compressed_value( ., .)”
>
> These functions are indeed for inference functions. That is, it says 
> “the field value can be computed by using other explicit field 
> values”. Above naming does not carry this descriptive understanding at 
> all. I understand the author’s original idea of that naming – which is 
> possibly derived from the fact that the associated field is/is not 
> defined in UNCOMPRESSED/COMPRESSED format, or so, which is less 
> crucial in catching the notion of “inference” encoding.
>
> I will appreciate your inputs.
>
> Many thanks,
>
> Jwoong
>
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>
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-- 
Klaus Warnke                [email protected]
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