Subject: [rohc] Question about SMS size zero

From: "Carrie Zhang" <[email-masked]>
Date: Thu, 22 Sep 2005 10:45:06 -0500

Hi, experts,

Reading Implementer's guide for sigcomp, section 9 as below:
=============================
9.  Feedback when SMS is zero

   If an endpoint receives a request for feedback then it should return
   the feedback even if its SMS is zero.  The storage overhead of the
   requested feedback is NOT part of the SMS.
=============================
I got confused. I agree that "storage overhead of the requested feedback
is NOT part of the SMS".

The first question is : when SMS is zero, the sate creation request will
fail. If the state creation fails, will UDVM still forward the feedback
to State Handler?

   +----------------+                                 +----------------+
   |                |     Request compressed data     |                |
   |                |-------------------------------->|                |
   |                |<--------------------------------|                |
   |                |     Provide compressed data     |                |
   |                |                                 |                |
   |                |    Output decompressed data     |  Decompressor  |
   |                |-------------------------------->|   dispatcher   |
   |                |                                 |                |
   |                |     Indicate end of message     |                |
   |                |-------------------------------->|                |
   |                |<--------------------------------|                |
   |      UDVM      | Provide compartment identifier  |                |
   |                |                                 +----------------+
   |                |
   |                |                                 +----------------+
   |                |    Request state information    |                |
   |                |-------------------------------->|                |
   |                |<--------------------------------|                |
   |                |    Provide state information    |     State      |
   |                |                                 |    handler     |
   |                |   Make state creation request   |                |
   |                |-------------------------------->|                |
   |                |  Forward feedback information   |                |
   +----------------+                                 +----------------+
         Figure 6: Interfaces between the UDVM and its environment

The 2nd question is, if the feedback is returned to far compressor, in
case of dynamic compression, the far compressor will take this returned
feedback as an acknowledgement for the state it requested the decompression
to create, but actually the state is NOT created in decompressor due to
the zero SMS size. When compressor makes use of this kind of state,
decompression failure will occur, because "STATE_NOT_FOUND", right?

I think when the SMS is zero, no feedback shall be returned.

Thanks in advance,
Carrie

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---

From: Pekka Pessi <[email-masked]>
Date: Thu, 22 Sep 2005 22:10:08 +0300

>fail. If the state creation fails, will UDVM still forward the feedback

It should. In general case, the compressor can not know the
contents of the state item created at remote end. That is because
a single state item is truncated at state memory size - 64 bytes.
SMS zero is just a special case.

>I think when the SMS is zero, no feedback shall be returned.

The remote end should return parameters along with the feedback
item. In your case, you should discard feedback if you are not
returned parameters. Of course, you can assume you have 2048 bytes
of SMS for SIP, for instance.

--Pekka

---

From: "Carrie Zhang" <[email-masked]>
Date: Thu, 22 Sep 2005 15:34:39 -0500

Pekka,

Thanks a lot for your response.

So, if I am decompressor, I got state creation request, if my
SMS is zero, I should still put whatever data from the requested
feedback item into my returned feedback item in the header
of next SigComp message, and at the same time I should put
the SigComp parameters including DMS, SMS, CPB in the same
SigComp message?

If so, then every time the returned feedback item is received by
SigComp, the SMS needs to be checked, if SMS is zero, then
the state corresponding to the returned feedback item shall not
be used for future compression, because the far decompression
had been failed to create this state. Is my understanding correct?

Another question related to feedback is that, if the DMS size is not
enough in decompression side, what should the decompressor
return to compressor? If NACk is supported, what would be the
proper error code; if NACK is not supported, can feedback
handle this case?

Thanks a lot,
Carrie

-----Original Message-----
From: Pekka Pessi [mailto:[email-masked]]
Sent: Thursday, September 22, 2005 2:10 PM
To: [email-masked]
Cc: Rohc
Subject: Re: [rohc] Question about SMS size zero

--Pekka