Re: QUESTION : Feature length about the non-negotiable feature

[Eddie Kohler <kohler@cs.ucla.edu>]

Implementing this option as variable length is not RFC compliant.  If you 
would like to be forwarded the discussion about fixed vs. variable length -- 
the option was originally variable length -- please let me know.  Until then 
it is best to follow the RFC, assuming that you are trying to implement DCCP 
rather than something else.

Eddie


Gerrit Renker wrote:
> Quoting Wei Yongjun:
>> Now non-negotiable feature does not has a fix size, it's length is  
>> decide by the value of the non-negotiable feature. For example, the  
>> client send Sequence Window Feature as the following: 0x23 0x04 0x03  
>> 0x64, the Sequence Window Feature has the length of 1-byte only.
>>
>> But RFC4340 said:
>>
>> 7.5.2.  Sequence Window Feature
>>   Sequence Window has feature number 3 and is non-negotiable.  It takes  
>> 48-bit (6-byte) integer values, like DCCP sequence numbers.  Change  and 
>> Confirm options for Sequence Window are therefore 9 bytes long.
>>
>> Is this correct? Or there are some RFCs said the non-negotiable feature  
>> has variable length?
>>
> You are correct - the RFC states fixed lenghts, even if values can be
> communicated in much smaller options.
> 
> The implementation does not implement the fixed lengths suggested by the
> RFC, since this does not make sense. The implementation will always chose
> the smallest-possible container length.
> 
> I believe that this is the preferrable approach. In particular since
> DCCP is a protocol which has a special header flag (`X') to allow saving
> 3 bytes. Why use 6 bytes when the value fits comfortably in a single
> byte?
> 
> Moreover, a lot of problems are generated by all these copious option
> lengths - unless using very small payloads, it is easily possible to
> exceed the maximum packet size.
> 
> Thus, although the RFC says otherwise, I think that using the smallest
> option size for a given value is the right thing to do.
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