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

[Roman Shpount <[e-mail]>]

On Mon, Mar 7, 2016 at 11:52 AM, Paul Kyzivat <[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 / "+" / "/"



> - 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.

- 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