RE: ULE encryption Support.

["Allison, Art" <AAllison@nab.org>]

It is always a tough call to decide where the tradeoff point is between
general capability and efficiency... when one sees that there might be such
extensions in the future one certainly should allow for the expansion to
avoid the problem of versioning the recommendation. 

For a more flexible method we could use the DSM-CC data carousel.. but that
is too flexible and has too much overhead...
 
In this case adding extension headers seems to be overkill because it seems
very remote that they will ever be used. The functional capability of this
layer <should> be stable. 

So, 1 bits can be escape.. and then define more in that...but a different
structure would follow.
But if you want more, two bits would provide ability to add 2 new unforeseen
extensions and then an escape to a different structure.  Three bits provides
6 new explicitly defined versions, then the escape.
.....

Even conservatively, how many bits do we need for this just in case? Is 2
enough? 3?


Art
::{)>
Art Allison
Director Advanced Engineering
NAB
1771 N St NW
Washington DC 20036
202 429 5418


-----Original Message-----
From: Gorry Fairhurst [mailto:gorry@erg.abdn.ac.uk]
Sent: Monday, March 01, 2004 3:18 PM
To: ip-dvb@erg.abdn.ac.uk
Subject: Re: ULE encryption Support.


Art, I think the scheme being talked about is rather like IPv6
extension headers. Devices that don't understand the extension
headers, simply ignore the extensions and continue to process the packet. 
A similar thing could be done at the link layer for specific ULE payloads
that would benefit from this. As you say, no requirements have yet been
written for ULE that match this need, although they may appear sooner
(or even later).

The question that needs to be addressed by this WG is should ULE allow
such extension headers to be defined in future?

We don't have to define them now, but we will have to define the 
MECHANISM by
which they can be later added. If we don't include this possibility now,
we'd have to version the whole protocol to get this functionality later
(this would have a serious deployment issues).

Gorry

Allison, Art wrote:

>As to backwards compatibility, no mater how it is defined, the initial
>products will not process and use newly defined data. So, even if the
>original product can process the larger header, it can do nothing with data
>with a new meaning defined after the product is built: 
>With a bit = 0 it is the 'Rev0' standard device Gen0.
>With a bit = 1 a Gen0 device cannot process the <new> data, but can process
>all Rev0 data.
>With a series of bits, a Gen0 device cannot process the <new> data, but can
>process all Rev0 data.
>At each header instance a different type can be signaled. 
>
>Bits are bucks for broadcasters, and each one has a fixed pipe size, so we
>must resist sending any more than absolutely needed.  I can make a case for
>NO expansion bit, as the new service can be defined when the application is
>defined and new devices can process it -- but 1 bit per header is ok. 
>
>Please help me see what I must be missing that justifies use of more
bits...
>Art
>::{)>
>Art Allison
>Director Advanced Engineering
>NAB
>1771 N St NW
>Washington DC 20036
>202 429 5418
>
>
>-----Original Message-----
>From: Alain RITOUX [mailto:alain.ritoux@6wind.com]
>Sent: Monday, March 01, 2004 12:08 PM
>To: ip-dvb@erg.abdn.ac.uk
>Subject: Re: ULE encryption Support.
>
>
>
>
>Allison, Art wrote:
>
>  
>
>>So, if we should not encrypt at this layer [agree with Alain] and we
>>    
>>
>should
>  
>
>>not put additional error correction at this layer.. it seems we no longer
>>have a reason to add more than a just-in-case escape bit just in case a
>>yet-to-be-invented function is needed someday.  
>>    
>>
>
>Not totally agree. This "reserve" may have no usage now, but the
>mechanism is not that heavy, and once it is done, it will allow
>introduciton of "possible" ext headers in a backward-compatible
>way, which seems to me worth the price.
>
>Cheers
>Alain.
>  
>