[MMUSIC] IANA registration of SDP attributes

[Paul Kyzivat <[e-mail]>]

[splitting off from the thread on data-channel-sdpneg]

Currently IANA has five(!) separate registries for sdp attributes:

att-field (session level)
att-field (both session and media level)
att-field (media level only)
att-field (source level)
att-field (unknown level)

They all have the same format:

*Type**
* 	*SDP Name**
* 	*Reference**
*
att-field (session level) 	cat 	[RFC4566]
att-field (both session and media level) 	recvonly
	[RFC4566]
att-field (both session and media level) 	mediaclk
	[RFC7273]
att-field (media level only) 	accept-types
	[RFC4975]
att-field (media level only) 	fmtp
	[RFC4566]
att-field (source level) 	fmtp
	[RFC5576]


This format is a pain, because it is hard to look an attribute up if you 
don't know at what level(s) it is valid. It also has the potential to 
allow an attribute name to be registered for unrelated purposes if the 
type is different. (IMO that would be bad.)

A long time ago (several years now), as part of the 4566bis work, I 
proposed that these tables be merged into one. It was my impression that 
this was agreed and would be done. But I don't recall any agreement on 
the logistics of doing so.

My thought was that the combined table would look like:

*SDP Name* 	*Level(s)* 	*Reference(s)*
accept-types
	media
	[RFC4975]
cat
	session
	[RFC4566]
fmtp
	media,source 	[RFC4566][RFC5576]
mediaclk
	session,media,source
	[RFC7273]
ptime
	media
	[RFC4566]
recvonly
	session,media
	[RFC4566]



Then we get to data channel attributes. My thought is to incorporate 
them into this table structure, as yet another "level". E.g.,

*SDP Name* 	*Level(s)* 	*Reference(s)*
accept-types
	media,dcsa
	[RFC4975][draft-ietf-mmusic-msrp-usage-data-channel]
cat
	session
	[RFC4566]
fmtp
	media,source 	[RFC4566][RFC5576]
mediaclk
	session,media,source
	[RFC7273]
ptime
	media
	[RFC4566]
recvonly
	session,media,dcsa
	[RFC4566][RFC4975][draft-ietf-mmusic-msrp-usage-data-channel]


(And this could also be extended for websockets if somebody proposes a 
way to negotiate attributes for data channels too.)

Using this format, if you want to know more than the name and the 
level(s) at which it can be used you need to consult the references. And 
when there are multiple references you don't know which one(s) you need 
to consult. This can be "fixed" by including more information from the 
reference into the registry. Conversely, we could strip it down further 
and remove the levels from the registry - so you need to consult the 
references for that too.

For instance, if we wanted to simplify finding the right reference for 
the level you are interested in, we could do:

*SDP Name* 	*Level(s)* 	*Reference(s)*
accept-types
	media,
dcsa
	[RFC4975]
[draft-ietf-mmusic-msrp-usage-data-channel]
cat
	session
	[RFC4566]
fmtp
	media,source 	[RFC4566][RFC5576]
mediaclk
	session,media,source
	[RFC7273]
ptime
	media
	[RFC4566]
recvonly
	session,media,
dcsa
	[RFC4566][RFC4975]
[draft-ietf-mmusic-msrp-usage-data-channel]


Or we could go further, and break the dcsa level down by subprotocol:

*SDP Name* 	*Level(s)* 	*Reference(s)*
accept-types
	media,
dcsa(MSRP)
	[RFC4975]
[draft-ietf-mmusic-msrp-usage-data-channel]
cat
	session
	[RFC4566]
fmtp
	media,source 	[RFC4566][RFC5576]
mediaclk
	session,media,source
	[RFC7273]
ptime
	media
	[RFC4566]
recvonly
	session,media,
dcsa(MSRP)
	[RFC4566][RFC4975]
[draft-ietf-mmusic-msrp-usage-data-channel]


Thoughts?

     Thanks,
     Paul