Subject: RE: [rohc] ROHC implementer's guide - error in mode transition di agram?

From: "Lars-Erik Jonsson (LU/EAB)" <[email-masked]>
Date: Tue, 2 Sep 2003 09:59:49 +0200

Alan,

> There seems to be an error in the mode transition diagram for
> transitions to U-mode in the ROHC Implementer's Guide, version 3.
> Is this true?

No, it is exactly as in the RFC, and it is correct. I'll explain below...

> The diagram on page 5 (part 3.3) shows no change of mode to mode U in
> the decompressor even though an IR/IR-DYN/UOR-2(SN,U) packet 
> is received.

The mode change must not happen until the compressor has completed its
transition. The decompressor must continue to send feedback as long as
the compressor expects to get feedback, i.e. as long as the compressor
has not completed transition.

> Question 2:
> Suppose the decompressor initiates the mode transition U -> O 
> when the feedback link is down or non-existent. Then the
> compressor is stuck in U mode forever and the decompressor is
> stuck in O mode forever. Is this true?

The only difference between U and O mode is whether the decompressor
sends feedback or not, and if the compressor expects feedback or not.
If there is really a feedback link so that the decompressor can send
feedback, then the compressor will at one point "get it". If there is
no feedback link, feedback can not be sent. The only important thing 
is that the compressor do not expect to get feedback (O-mode), while
the decompressor do not really use feedback (U-mode).

> Since D_TRANS is not changed in this transition, the 
> decompressor will not
> be aware of the fact that the compressor has not changed mode.
> So maybe the decompressor will be free to initiate the change O -> R?

If it does, that will work (see U->R)!

/L-E 

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