RE: [rohc] SigComp state creation request, requested feedback, and SMS = 0

["Price, Richard" <richard.price@roke.co.uk>]

> Hi,
> 
> (This is not specified on RFC 3320, so I want to double check.)
> 
> If a message carries state creation request but the receiving 
> endpoint has SMS = 0, I understand that the message can still be
> accepted but the state creation request will be rejected.

Hi Zhigang,

That's right - state creation requests can be rejected due to lack
of memory, even if the decompressed message is accepted.

> Now, what about the requested feedback carried in the message?
> Will it be returned?

The requested feedback can still be returned even if SMS = 0.

> If yes, that means the sender must not rely on feedback to determine
> if the state has been successfully created. Then, is the answer no?

The sender can still rely on the feedback to determine if the state
has been successfully created, but only if it takes into account the
returned SigComp parameters as well. In particular it needs to check
that the receiver has advertised SMS > 0.

SigComp-Extended has the following to say on the subject:

   Note: There is a possibility that state(S) is discarded due to lack
   of state memory even though the announcement information is
   successfully forwarded.  This possibility must be taken into account
   (otherwise a decompression failure may occur); this can be done by
   using the SigComp parameter state_memory_size which is announced by
   the SigComp feedback mechanism.  The endpoint can use this parameter
   to infer if a state creation request has failed due to lack of
   memory.

Hope this helps!

Regards,

Richard
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