Re: Question related to ROHC-TCP Profile

Igor, Raffles,

First, thank you Raffles for answering Igor's question and explaining
ROHC-FN. It is a tough subject, but think you guys did a great job on
RFC4997.

Igor, the binding between attributes that Robert explained below is
called two-way matching. It is one of the key concepts of ROHC-FN. I
advise you to study the RFC4997(ROHC-FN) before you start implementing
RFC4996. Implementing RFC4996 without understanding RFC4997 will be very
difficult.


Best Regards

/Carl Knutsson


Finking, Robert wrote:
> Hi Igor,
> 
> Thanks for the post and apologies for any lack of clarity in the RFCs.
> A key concept to understand when reading ROHC-FN is the idea of binding.
> You are right that the line you have quoted below only specifies the
> ULENGTH (32 bits).  However, if you look at the next line, you will find
> that the UVALUE is also bound by means of the field_scaling encoding
> method:
> 
>      seq_number_scaled                    [ 32 ];
>      seq_number_residue =:=
>          field_scaling(payload_size, seq_number_scaled.UVALUE,
>                        seq_number.UVALUE) [ 32 ];
> 
> The field_scaling encoding method is defined as follows:
> 
>  field_scaling(stride_value, scaled_value, unscaled_value)
>  {
>    UNCOMPRESSED {
>      residue_field [ 32 ];
>    }
> 
>    COMPRESSED no_scaling {
>      ENFORCE(stride_value == 0);
>      ENFORCE(residue_field.UVALUE == unscaled_value);
>      ENFORCE(scaled_value == 0);
>    }
> 
>    COMPRESSED scaling_used {
>      ENFORCE(stride_value != 0);
>      ENFORCE(residue_field.UVALUE == (unscaled_value % stride_value));
>      ENFORCE(unscaled_value ==
>              scaled_value * stride_value + residue_field.UVALUE);
>    }
>  }
> 
> So, the seq_number_scaled.UVALUE is bound to scaled_value within the
> field_scaling encoding method.  Assuming the stride_value (i.e. the
> payload_size) is non-zero, this binds the scaled_value (i.e. the
> seq_number_scaled.UVALUE) in an expression with stride_value,
> residue_field.UVALUE and unscaled_value (i.e. seq_number.UVALUE). This
> is done in such a way the scaled value is the unscaled_value divided by
> the stride_value. In short, the UVALUE of seq_number_scaled is set by
> the field_scaling encoding method (which also calculates the
> modulus/residue from the scaling and encodes it in 32 bits).
> 
> I am not sure if that has helped, it is often hard to put these things
> in words. Feel free to ask further questions if this does not answer
> your question.
> 
> Thanks for using ROHC-FN
> 
> Regards
> 
> Raffles 
> 
>> -----Original Message-----
>> From: [email protected] [mailto:[email protected]] On 
>> Behalf Of eggor
>> Sent: 11 March 2009 12:41
>> To: [email protected]
>> Subject: [rohc] Question related to ROHC-TCP Profile
>>
>> Hi,
>>
>> I've just started working on implementation of the ROHC-TCP 
>> profile, and I needed some clarifications.
>>
>> Question about "seq_number_scaled" field defined in the 
>> CONTROL format of the tcp(payload_size, ack_stride_value, 
>> ip_inner_ecn) encoding method.  It is defined as:
>>
>> CONTROL {
>>      seq_number_scaled                    [ 32 ];
>>      ...
>>    }
>>
>> and isn't appeared in other formats.  If I truly understood 
>> ROCH-FN, this definition specifies only ULENGTH attribute of 
>> this field, so the question is: where compressor can get its 
>> UVALUE?  And the second question is how decompressor can restore it?
>>
>> Thanks in advance,
>> Igor.
>> _______________________________________________
>> Rohc mailing list
>> [email protected]
>> https://www.ietf.org/mailman/listinfo/rohc
>>
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