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.
Yes, you can save some space for store more data, but this break the
compatibility. If the other system implement follow the RFC, since those
feature will be treat as invalid value. Also if it is set as mandatory
option, assoc will be reset. endpoint with different implement can not
talk to each other. That is the problem.
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