[CAP] Presidential CAP questiion

[Rex Buddenberg]

I think the question really has to do with the nature of packet
switching and the archaic circuit switched notion of pre-emption.  (With
Presidential as just one instantiation)????

In circuit-switched networks, a channel can carry only one application
at a time.  Which translates to user terms of 'gotta hang up on one
conversation to pick up another'.  Pre-emption is a means of forcing the
first conversation to close so one can barge in with the second.  In the
interior of the network, pre-emption is represented by a parallel 'grab
the circuit'.  In abstract terms, circuit switched networks are
connection-oriented.  
    
But in packet switched systems things change.  Packets arrive at routers
from multiple applications in no particular order or organization.
Layer 3 plumbing in the internet is expressly connectionless and
stateless.  Therefore, pre-empting' a connection makes no sense -- there
aren't any.  (*more below)

This doesn't translate quite as intuitively to end system applications.
It may make sense to interrupt one human conversation to get a
converser's attention.  But this has nothing to do with the underlying
plumbing any more. 

There was a working group in IETF a few years ago thrashing emergency
services issues.  Great amounts of flame and heat trying to get some of
the members to understand that 'pre-empt' belonged in places like SIP
servers, not routers.  




*in most of the internet infrastructure, there's enough overprovisioning
that fiddling with packet priorities within a router makes no sense
either -- there's nothing you can do to 'improve' service if there's no
congestion.  The place where this does become important is at the parts
of the internet where we reach to mobile platforms -- the radio-WANs.
But that's a different conversation, so I won't chase it here unless
somebody rings back (it's my research area).  Here the issue is not
pre-emption in the customary sense, but a packet prioritization sense
where 'the most important packets get handled first'.  The definition of
'most important' is always disputed along with the means of marking
packets with appropriate labeling.  



On Mon, 2007-10-15 at 14:16 -0400, matt hoffman wrote:
> Art is certainly the authority on this, but I'll second that impresson. 
> I've worked on IPAWS-related prototypes implementing CAP interfaces, but 
> I have never heard mention of any system that disseminated Presidential 
> messages via anything other than the existing EAS framework.
> 
> Matt
> 
> 
> Art Botterell wrote:
> > Jim -
> >
> > I can't speak for FEMA, but I would expect that presidential messages will go from WACA to FEMA for distribution through the IPAWS framework to EAS, cellular and other dissemination media.  Of course, presidential alerts have been, to date, vanishingly rare.
> >
> > - Art
> >
> >
> > Art Botterell, Manager
> > Community Warning System
> > Contra Costa County Office of the Sheriff
> > 50 Glacier Drive
> > Martinez, California 94553
> > (925) 313-9603
> > fax (925) 646-1120
> >
> >   
> >>>> "Jim Trawick" <JimTrawick at viaRadio.com> 10/15/2007 7:45 AM >>>
> >>>>         
> > Although there's been a lot of talk about the Presidential messaging
> > possibilities in DEAS and at least three different versions of what that
> > might be (PBS, NOAA and DHS variations, that I'm aware of), is anyone aware
> > of a specific, currently available Presidential source (i.e., pre-empting
> > all others, even those in progress, per various Executive Orders), which is
> > available in CAP format (or any other digital, text-oriented format), and if
> > so, what that current source might be, and through whom it might be
> > currently available? Surely one must have been employed in the DMIS EAN test
> > back in June.
> >
> >  
> > Jim Trawick
> > Senior Software Engineer
> > viaRadio Logo Scaleable smaller no tag no background
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