Re: [hybi] RSV bits in extensions

[Bruce Atherton <[e-mail]>]

On 5/21/2012 3:35 AM, Greg Wilkins wrote:
> In practise, I believe this means that the RSV bits are of very very 
> limited use to 3rd party extensions and really have  been reserved for 
> extensions developed by this WG.    For example RSV1 will effectively 
> become the compression bit once the per-frame-compession draft is 
> accepted and RSV1 will only be able to be used by other extensions 
> that are obviously mutually exclusive to per-frame-compression (such 
> as stream compression)
>
> I'm not saying this is a bad thing... it's probably good to have these 
> bits available for standard extensions without the need for the 
> complexity of dynamic bit allocations.     It just means that 3rd 
> party extension should really avoid using these bits and put their own 
> flags in the payload.... unless somebody wants to define the 
> unlimited-reserved-bit extension to be used with other extension :)
>

I think this is a little pessimistic. People writing extensions only 
have to accept that if they use a particular RSV bit, they are 
automatically incompatible with any other extension that uses it. So 
long as everyone is really clear in their documentation about what RSV 
bits they use, users can decide how to deploy them so that no conflicts 
occur.

Admittedly, though, if this WG defines any other RSV bits, that will 
effectively make them off-limits to others. I wouldn't recommend to any 
extension writer that they use the RSV1 bit for their extension, for 
example, unless it was an extension for compression.

So although users manually managing extensions for RSV bit conflicts is 
complicated, and although extension writers run a risk of being booted 
off their chosen RSV bit by the WG, I still think using them is doable 
for a couple of reasons. First, I don't see that there will be a flood 
of extensions that require an RSV bit, and second I don't see the WG 
rushing to create standard uses for them either.

Of course, I could be wrong. If there is a sudden rush for RSV bits then 
that might be a reason for the WG to define one of the RSV bits as an 
RSV bit extender, or to have an extension for reserve bits as you 
suggest. Let's see what happens.