Re: [MMUSIC] Thoughts on draft-ietf-mmusic-dtls-sdp-10 semantics

[Paul Kyzivat <[e-mail]>]

On 3/7/16 12:12 PM, Roman Shpount wrote:
> On Mon, Mar 7, 2016 at 11:52 AM, Paul Kyzivat <[e-mail]
> <mailto:[e-mail]>> wrote:
>
>     On 3/7/16 11:26 AM, Christer Holmberg wrote:
>
>         I suggest dtls-association-id. In my opinion there is no longer
>         the same
>         need to be aligned with the name of the connection attribute.
>
>
>     This seems plausible. But I want to reserve judgement until the
>     detailed semantics of the attribute are worked out:
>
> I wanted to model this attribute on ICE ufrag
>
>     - is it numeric, or a token?
>
>
> I was thinking something like this:
> dtls-association-id-attr = "dtls-association-id" ":" dtls-association-id
>
> dtls-association-id = 4*256dtls-association-id-char
>
> dtls-association-id-char = ALPHA / DIGIT / "+" / "/"

Why this in particular? Is it your intent to reuse values that were 
generated for another purpose? Or to reuse code that is also used for 
another purpose?

>     - any rules for how the value chosen?
>
>
> The dtls-association-id attributes MUST be chosen randomly when new DTLS
> association is requested.  The dtls-association-id attribute MUST
> contain at least 24 bits of randomness.  This means that
> the dtls-association-id  attribute will be at least 4 characters
> long since the grammar for this attribute allows for 6 bits of
> randomness per character.  The attribute MAY be longer than 4
> characters, respectively, of course, up to 256 characters.  The upper
> limit allows for buffer sizing in implementations.  Its large upper
> limit allows for increased amounts of randomness to be added over time.

What is the reason for using random values, rather than some well 
defined sequence of values? (Where the sequence might start with a 
random value, or perhaps every sequence would start with 0.)

	Thanks,
	Paul

>     - is it tied to an actual property of the association,
>         or is it just an arbitrary extra piece of state?
>
>
> It is not tied to the association and it is an arbitrary piece of state.
> DTLS protocol can be extended in the future to use dtls-association-id
> to demux multiple DTLS associations over the same transport but this is
> out of scope for now.
>
>     - any rules for how the *next* value is chosen when
>         a new (replacement) association is being proposed?
>
>
> New values is generated randomly.
>
>     - when (if ever) can an id be reused?
>
>
> The dtls-association-id should be reused while end point want to
> maintain the same DTLS association. If either end point changes the
> dtls-association-id value, this will indicate that new DTLS association
> MUST be established.
>
>     It is *possible* (though not likely) that the details may suggest a
>     particular name that is more meaningful than "id".
>
>
> Regards,
> _____________
> Roman Shpount