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

["Lajos Zaccomer (IJ/ETH)" <lajos.zaccomer at ericsson.com>]

Hi Abbie,

See inline ...

br/Zacco

> -----Original Message-----
> From: Surtees, Abigail [mailto:abigail.surtees at roke.co.uk]
> Sent: Thursday, January 13, 2005 3:12 PM
> To: Lajos Zaccomer (IJ/ETH); 'Gustav E'; rohc at ietf.org
> Subject: RE: [rohc] [SigComp]Shared mode and some other 
> SigComp-state rela ted stuff.
> 
> 
> 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?

[zacco] In PoC, the terminal uploads the USD after saving it in the state memory. The special advertisement lets the server know it received a USD that it also saves in the state memory (or outside the state memory, which must be signalled backwards). Thus, both endpoints will have the USD and can use it for compression. That is why I say PoC USD is a dynamic (or regular) and shared state at the same time.

> 
> > 
> > > 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?

Not in a compartment at all (in A). However, in B it is up to the endpoint if it saves in the state memory or not in the compartment.

> > 
> > > 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)?
> 
It is not duplicated. I meant it was moved to the "non-compartment" state memory. It must be remembered somehow (implementation stuff) that the state actually belongs to a specific compartment, so when the compartment is deleted, the USD must also be 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?

Besides the state is saved and advertised at the same time. The reason why the usd tag was added is that this situation might easily happen with a well-known bytecode.

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

Why do you say that? 'usd1' is not something that appears at the beginning of a state, especially in a first message, and especially not in a bytecode.

> 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.
> 

USD is just one state, which is absolutely in line with the "one shared state at a time" concept.

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

Nothing special, as the shared state is not saved as a state in B, only in A. 

> 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.

As far as I know it is mandatory, but this I should check.

> 
> Best regards,
> 
> Abbie
> 
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