Re: [MMUSIC] 4566bis outstanding issues

"Ali C. Begen (abegen)" <> Tue, 05 August 2014 14:33 UTC

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From: "Ali C. Begen (abegen)" <>
To: Fredrik Bergholtz <>
Thread-Topic: [MMUSIC] 4566bis outstanding issues
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Date: Tue, 5 Aug 2014 14:32:55 +0000
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Subject: Re: [MMUSIC] 4566bis outstanding issues
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Hi Fredrik

There were some changes in the bis document regarding these attributes in -08:

Do they help with your issue? If not, what else would you like to see?

I remember seeing a discussion regarding the use of the same port at some point but it was another draft (sorry I dont remember which one it was).


On Aug 5, 2014, at 3:35 PM, Fredrik Bergholtz <> wrote:

> Hi. Excellent to see a summarised list of what is considered for 4566bis.
> Personally I would like to see ABNF definitions for all the attributes. I do realise the amount of work that is needed but, as someone else mentioned, the 4566 is the base line for all extensions and I think that it is important to make the base as well-defined as possible.
> One item that is not in your list that I would like to see is a clarification of how to signal asymmetric sessions. By asymmetric sessions I mean using the a=sendonly and a=recvonly in separate media definitions, specifically addressing the "same port" issue of section 5.14. 
> I realise that this is a more use-case oriented addition than an improvement of the examples for the various attributes that has been discussed. Perhaps this kind of clarification belong in something like rfc4317 rather than in 4566. Do you have any thoughts on if and how such clarifications could and should be made?
> Best regards,
> Fredrik Bergholtz
> Swedish Radio
>> -----Original Message-----
>> From: mmusic [] On Behalf Of Paul Kyzivat
>> Sent: den 7 juli 2014 19:32
>> To:
>> Subject: Re: [MMUSIC] 4566bis outstanding issues
>> Trimming again.
>> On 7/7/14 12:16 PM, Ali C. Begen (abegen) wrote:
>>> On Jul 7, 2014, at 11:02 PM, Paul Kyzivat <> wrote:
>>>> On 7/7/14 4:07 AM, Ali C. Begen (abegen) wrote:
>>>>> Hi Paul,
>>>>> OK, no one else seems to be paying attention to this. I have some
>> comments inline and lets see whether we can reach an agreement:
>>>> OK.
>>>> I've trimmed to a few points to comment on...
>>>>>>> On 6/6/14 5:19 PM, Ali C. Begen (abegen) wrote:
>>>>>>>> 1) 4566bis needs to provide ABNF syntax for all of the attributes 4566
>> defined.
>>>>>>>> Comments: This was discussed again in a side meeting in London and
>> there was not a consensus whether we actually needed this.
>>>>>>> IIRC the argument against doing this is that we might get it wrong.
>>>>>>> But IMO we should do it because if we can't get it right how can we
>> expect readers to get it right?
>>>>> I personally don't wanna take on this task. It is a tedious task and I am not
>> such an ABNF lover to get these definitions right. If you wanna volunteer, by all
>> means, please do so.
>>>> I'm willing to contribute to this, but won't have time until after the Toronto
>> meeting.
>>> That is fine.
>>>> Also, I'm not intimately familiar with all of the attributes.
>>> Nobody is I am afraid.
>> There are a lots of people who at least *use* them, and so are familiar with
>> them at that level. I don't "do" media, so when I'm trying to create examples I
>> can get as far as "m=audio", and I know there should be an "a=rtpmap", but
>> beyond that I need to find an example to copy.
>>>>> OK, this goes back to the issue #1. Personally, I suggest we do not define the
>> abnfs for every sdp attribute in 4566, but then you have a good point. We can
>> seek a format and provide a few good examples to show prospective authors.
>>>>> To do this, obviously as you said, we need to agree on that format. Has
>> there been a proposal in the past or does anyone have a good proposal to start
>> with?
>>>> Not a formal one. There has been some discussion about this during review
>> of individual attribute definitions. My take has been that there is not agreement.
>>> If you can recall, can you point us to that proposal and maybe briefly explain
>> the disagreements as well?
>> I think it will take a lot of searching to find it. So I can't do so quickly/easily.
>>>> One question is whether the definition of a new attribute should:
>>>> 1) define the full syntax of the line (starting with "a="),
>>>> 2) should provide additional definitions of <attribute> (via "/=")
>>>> 3) should provide additional definitions of <att-field> and <att-value>
>>>>   (via "/=")
>>>> 4) provide definitions of the name and value that are consistent with
>>>>   <att-field> and <att-value> without formally extending the
>>>>   definitions of those.
>>>> Note that 4566 loosely follows (4) but gives the definitions informally rather
>> than via ABNF. So it gives no guidance on how to use ABNF for this. A number of
>> drafts have chosen to follow (2). Some have started to do (1) but I don't know of
>> any that ended up that way.
>>>> A question I have raised is: if (2) or (3) is followed, does that mean that the
>> draft doing so must be considered as "updating" 4566?
>>> I dont see why it should. Personally most of the attributes I defined followed
>> (4). That seemed the easiest option.
>> I've been outnumbered on the discussions of this. ISTM that whenever I write
>> 	attribute =/ something new
>> then I have at least extended the rfc that defines <attribute> and arguably have
>> updated it. The reason it might be considered an update is that what I am
>> defining implicitly includes the definition of <attribute> and thus probably all the
>> abnf that includes the definition of <attribute>. To verify my new abnf, or to
>> generate a parser, I need to create a combined abnf. And abnf doesn't have
>> namespaces. So all the rulenames I newly define must be distinct from those
>> defined with <attribute>.
>> And that is true for *every* extension. And for them to be compatible with one
>> another they all must define rulenames that are unique among them all.
>> So effectively each one is an update.
>> It is subtly different if I follow (4), even if I use abnf to define the syntax for my
>> new attribute, and even if I reuse rule definitions (like
>> <token>) from the base sdp definition.
>>>> One thing that people sometimes forget when defining new attributes is that
>> the syntax of the new attribute MUST also be valid when parsed by the "generic"
>> attribute syntax. AFAIK there is no way in ABNF to specify that. I think that (4)
>> comes closest.
>>> Yes, but this must be checked during the last call by actual humans not some
>> abnf parser.
>> For now that is true. I have found such errors while doing SDP directorate
>> reviews.
>> 	Thanks,
>> 	Paul
>>> -acbegen
>>>> 	Thanks,
>>>> 	Paul
>>> _______________________________________________
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