RE: [rohc] [SigComp]Shared mode and some other SigComp-state rela ted stuff.

["Surtees, Abigail" <abigail.surtees at roke.co.uk>]

Hi Zacco, 

Sorry for the delay.  I've got some questions about the PoC related use of
usds.

> 
> > 3. USD. I fully understand the usage of a USD. However, I do 
> > not understand 
> > if a USD-state should be treated any differenty then any 
> > other state created 
> > at the remote end. Is a USD per definition a shared state? 
> 
> No, it isn't. RFC 3321 says it's a regular state. POC USD is 
> something different. I would say it's a regular and a shared 
> state at the same time.

What does that actually mean?  As you say 3321 says it's a regular state, so
in SigComp terms how can it be a shared state as well?

> 
> > I'm guessing that 
> > it must be if the sending compressor is to have any 
> advantage of the 
> > USD-state at the remote end. Then, again, should the 
> > USD-state (local state) 
> > at A be considered to occupy State-memory? Should it be 
> > considered any 
> > differently then any other shared state?
> 
> It depends on which USD you are talking about. A regular 
> state is not saved in a state memory at the compressor side, 
> so if you go for the RFC 3321 USD, it decreases the available 
> memory at B in the state memory of compresssor at A. In case 
> POC USD is used, it must be saved as a local state, otherwise 
> the compressor at B would not be able to access it.

However, we believe that the POC definition of using USD as shared state is
not well defined and can lead to loss of synchronisation between the two
endpoints.

When you say it must be saved as 'local state', do you mean in the state
memory of the compressor at B or not in a compartment at all?
> 
> > 5. (Little bit off-topic, PoC specific question): In the PoC 
> > NEMS Signalling 
> > Spec. "Signalling flows (UNI) v2.06", a procedure for 
> > establishing a USD is 
> > described. Among others it states that the four first bytes of the 
> > state-value should be "usd1". Can any helpful individual 
> > explain to me what 
> > the purpose might be? Should either local or remote 
> endpoint take any 
> > specific action here?
> > The spec says:
> > "For USD identification it is important that the identifier 
> > string is placed 
> > in the beginning of the state_value of the USD, therefore 
> > special attention 
> > must be paid in the case of large USD as the UDVM has a 
> > circular buffer."
> > I do not understand the purpose here..
> 
> Suppose Endpoint 2 supports USD (without the optional 
> requirement of moving it to the local state location) and the 
> explicit acknowledgement specified in RFC 3321, too. If 
> Endpoint 2 explicitlly acks the uploaded USD state ID, 
> Endpoint 1 will think USD is moved to the local states 
> location (indication is the same as if it was an explicit 
> ack) and will not update (decrease) its remote available SMS. 
> Later on it may result in decompression failure if a state is 
> saved deleting another one in use, due to the remote 
> available SMS out of synchron.
> 

I'm a bit confused here:

By 'moving it to the local state location' I take it that you mean that the
state created at Endpoint 2's decompressor would be moved to Endpoint 2's
compressor.  Is this correct?  Would it be duplicated (i.e. still exist in
the decompressor) or actually moved (and therefore the compressor at A has
no control over or knowledge of when it is deleted)?

Are you saying that the presence of the string "usd1" is an indicator to
endpoint 2 that it should move/copy the state into the compressor
compartment?

If so, this seems to be at odds with the SigComp conceptual model of state
that it is just 'some bytes'.  

If my understanding above is correct, then the piece of state presumably has
minimum retention priority?  Care must once again be taken due to the
possibility that the creation of multiple pieces of minimum retention
priority state can lead to loss of synchronisation between endpoints.

How does this mechanism interact with the use of shared mode as defined in
3321?

I also think there are some implications for interoperability between
endpoints that do this and endpoints that don't.  As you showed in your
example above, if there is disagreement between endpoints about whether or
not state is moved to 'local state location' (whatever that is) it can lead
to loss of synchronisation between endpoints.

Is this concept mandatory in PoC?  If not, or if a non PoC endpoint receives
a state create request and an explicit advertisement, it will not know that
it should check the first 4 bytes for "usd1" and move the state so
synchronisation will be lost as you explained.

Best regards,

Abbie

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