Re: Fast SCP
David Rosenborg / Pantor Engineering AB
31 Jul 2008 3:28AM ETThe change rate of templates is very much application dependent. Some applications will have templates as static as the application protocol they are encoding. Otheres will change once a month or every other week. Yet others will change templates midstream to gain higher performance (better compression and/or lower latency) or to minimize manual administration and deployment overhead.
Since FAST is fairly new and SCP even newer, it is likely that early implementations will lean to the static side. However, as the technology matures and more implementations become available, the opportunities to use the dynamic features of SCP increases.
FAST actually says nothing about templates being common knowledge. The only thing it says is that a template must be known to the decoder before it decodes the first instance of that template. Templates may very well be short lived creatures, even generated on the fly, and only used in a single session between two peers.
In addition to defining the actual messages to encode templates, SCP also defines template interchange for stream oriented scenarios with reliable delivery (think TCP). Though, you're correct in that it doesn't define a template exchange model for unreliable, datagram oriented transports (think UDP). The reason is that multicast applications present a number of implementation strategies where none is superior in general. Different types of applications may have different types of optimal template interchange schemes.
There are challenges in multicast template distribution but they are not harder to solve than the similar challenges you have for the application data you intend to distribute.
And in the long run, I'd rather have the software solve the template synchronization problem for me than relying on manual intervention, e.g. perodically check for new templates on some web site and make sure I run on the latest and greatest.
/David
>
> >
> > I thinking of using Fast SCP 1.1 but I also would like to avoid
> > sending the same template information over the network too often.
> >
> Hi Dimitri
>
> What do you call too often?
>
> I would think that templates are fairly static objects. Think about what
> would occur if templates were broadcast on the fly? There would be
> syncronization problems hard to solve, such as a distributed consensus,
> or atomic broadcast (total order).
>
> For FAST assumes the current set of templates is *common knowledge* that
> is, everybody knows the set of templates, and everybody knows that
> everybody knows the set of templates, and so forth.
>
> I doubt that SCP is handling this problems, hence, there must be a
> priori rules for changing current set of templates.
>
> You should not worry about update frequencies.
