[MMUSIC] Benjamin Kaduk's No Objection on draft-ietf-mmusic-data-channel-sdpneg-25: (with COMMENT)

Benjamin Kaduk via Datatracker <noreply@ietf.org> Thu, 11 April 2019 02:53 UTC

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Subject: [MMUSIC] Benjamin Kaduk's No Objection on draft-ietf-mmusic-data-channel-sdpneg-25: (with COMMENT)
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Benjamin Kaduk has entered the following ballot position for
draft-ietf-mmusic-data-channel-sdpneg-25: No Objection

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----------------------------------------------------------------------
COMMENT:
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Section 4

   The mechanism in this document only applies to the Session
   Description Protocol (SDP) [I-D.ietf-mmusic-rfc4566bis] when used
   together with the SDP offer/answer mechanism [RFC3264].  Declarative
   usage of SDP is out of scope of this document, and is thus undefined.

nit: this text doesn't actually scope itself to "this mechanism" or the
RTCWeb data channel, and seems to be saying that all declarative SDP is
undefined, globally.

Section 5.1.7

                                The ordered parameter is optional and
   takes two values: "true" for ordered and "false" for unordered
   delivery with "true" as the default value.  Any other value is
   ignored and default "ordered=true" is assumed.  [...]

This seems to be saying that offers or answers that do not conform to
the ABNF will be special-cased as being ignored (instead of rejected)
when they appear where ordering-value should appear.  It's not clear to
me why we need a special case here, since in Section 8 we explicitly say
that peers MUST close the relevant data channel when such error cases
occur.

Section 5.1.8

If we chase links to draft-ietf-rtcweb-data-channel we find that larger
values indicate higher priority.  Do we want to short-circuit that and
say so explicitly here?

Section 5.2

                                                                   Each
   of these attributes is represented by one new attribute line, and it
   includes the contents of a media-level SDP attribute already defined
   for use with this (sub)protocol in another IETF (Internet Engineering
   Task Force) document.  [...]

Do we need to limit the data-channel subprotocols to those specified by
the IETF?  Or is it just the media-level SDP attributes that we are
trying to maintain change control over?

   The detailed offer/answer procedures for the dcsa attribute are
   dependent on the associated sub-protocol.  If no offer/answer
   procedures exist for the sub-protocol when used outside of the dcsa
   attribute, no specification is needed for use with dcsa.  The IANA
   registration procedures for the WebSocket Subprotocol Name Registry
   (Section 9.1) do not strictly require a specification of the offer/
   answer procedures for the sub-protocol when used with dcsa.  If the
   sub-protocol has defined offer/answer procedures when used outside of
   dcsa, such a specification is encouraged to ensure interoperability.
   If the sub-protocol has defined offer/answer procedures when used
   outside of dcsa, but no specification exists for the offer/answer
   procedures for the sub-protocol when used with dcsa, implementations
   SHOULD assume the use of the default values for all otherwise-
   negotiable and applicable sub-protocol parameters.

I'm not entirely sure I understand what this paragraph is trying to say.
The first sentence is clear, but the second sentence seems to be saying
that if there are no O/A procedures for a given subprotocol outside
the datachannel, it's just a free for all and I can use it in the data
channel and make up my own procedures for using the dcsa attribute.
I understand that IANA doesn't require a specification for the
subprotocol when allocating the subprotocol name, but that just means
that there may not be a specification available whether or not I want to
use it with dcsa -- the IANA registry doesn't care directly about use
with dcsa.  If a subprotocol has defined O/A procedures outside of dcsa,
doesn't that basically mean there's a specification?  Why do we need to
go out of our way to encourage a specification in that case (and is it a
specification of the O/A semantics when the subprotocol is used outside
dcsa or with dcsa)?  In the last sentence, I think we're talking about
using defaults for parameters that are not explicitly negotiated using
dcsa, but we don't actually say that.

Section 5.2.1

                                               Note however that not all
   SDP attributes are suitable as a "a=dcsa:" parameter.  IANA SDP
   parameters contains the lists of IANA (Internet Assigned Numbers
   Authority) registered session and media level or media level only SDP
   attributes.

Am I supposed to infer that these are the attributes that are suitable
for a=dcsa: parameters?

   As opposed to the data channel "a=dcmap:" attribute parameters, these
   parameters are subject to offer/answer negotiation following the
   procedures defined in the subprotocol specific documents.

I am not sure where we really say that the a=dcmap: parameters are not
subject to O/A negotiation.

Section 6.1

   SCTP stream identifiers associated with data channels that have been
   negotiated using DCEP MUST NOT be included in SDP offers and answers.

This sentence appears as a dedicated paragraph and is duplicated at the
end of the previous paragraph; presumably we only need one copy
thereof...

Section 6.2

                                         If an SDP answer contains both
   of these parameters then the offerer MUST treat the associated SDP
   offer/answer failed.

nit: "as failed", I think.

Section 6.4

We only mention that the dcmap-stream-id, max-retr, and max-time values
need to match the offer, leaving the ordering/subprotocol/label/priority
to be omitted because they are conveyed at the SCTP layer?  Perhaps we
should state explicitly that they need to be absent...

Section 6.6

Do we want to say anything about the appropriate response to receiving
changed values for the SDP attributes/attribute values is to close the
corresponding data channel?

Section 6.6.1

It seems like the second bullet point only makes sense after the first
has completed, as the tsvart reviewer noted?  Specifically, the "; or"
at the end of the first bullet point doesn't seem to make much sense.

Section 9.3

               If existing media attributes are used in a datachannel
   subprotocol specific way (Section 5.2.1), then a new dcsa usage level
   MUST be defined for the existing media attribute.  [...]

I'm not entirely sure that I understand what this means, though I'm not
really an expert in SDP.

Appendix A.1

I'm not sure that I know what "glare situations" are; is there a
reasonable reference to include?

   Which set of stream identifiers is owned by which endpoint is
   determined by convention or other means.

(specific to the application?)

      Note:For data channels negotiated via different protocol from
      DCEP, no convention is defined by default.

That seems a bit misleading, since Section 6.1 says that the even/odd
split applies to the SDP O/A negotiation case.