Re: [MMUSIC] [Technical Errata Reported] RFC4566 (3278)

Riccardo Bernardini <riccardo.bernardini@uniud.it> Fri, 06 July 2012 09:39 UTC

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Date: Fri, 06 Jul 2012 11:39:34 +0200
From: Riccardo Bernardini <riccardo.bernardini@uniud.it>
To: mmusic@ietf.org
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Subject: Re: [MMUSIC] [Technical Errata Reported] RFC4566 (3278)
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"Stach, Thomas" <thomas.stach@siemens-enterprise.com> ha scritto:

> All,
>
> I think this errata should be rejected.
>
> The text on page 9 is pretty clear and I don't see a conflict with the ABNF.
>
> Under "Session description" it is explicitly stated that you can  
> have zero or more media descriptions (as well as time descriptions)
> Further down under "Media description, if present" it is elaborated  
> what a media description includes.
> It must at least include exactly one m-line. The a,b,k,i,c-lines are  
> optional.
> Adding the asterisk as proposed would remove this subtlety.
> The same technique is used for "Time description". The t-line is  
> mandatory, the r-lines are optional.

OK, let me try to rephrase it: the media-description BLOCK is OPTIONAL  
and you can have zero media-description blocks; but the "m=" line is  
MANDATORY whenever you have a media-description block. Maybe this was  
the reason of my confusion: I interpreted the missing asterisk as  
``you cannot have SDP descriptions without m= lines,''  while I should  
have read ``m= lines are mandatory within a media block, but an SDP  
without m= lines is OK as long as you have no media blocks.'' I guess  
that "if present" part should clarify this, but it escaped me, sorry.   
Note that this type of subtlety does not happen with t= lines since,  
according to the ABNF, at least one "time-fields" block must be present.

OK, I can see why the text on page 9 is correct.

Regards

Riccardo

>
> BTW: Being able to provide zero media descriptions is crucial for  
> certain implementations. An example is tunneling QSIG in SIP.
>
> Regards
> Thomas
>
>> -----Ursprüngliche Nachricht-----
>> Von: mmusic-bounces@ietf.org [mailto:mmusic-bounces@ietf.org]
>> Im Auftrag von RFC Errata System
>> Gesendet: Dienstag, 03. Juli 2012 20:01
>> An: M.Handley@cs.ucl.ac.uk; van@packetdesign.com;
>> csp@csperkins.org; gonzalo.camarillo@ericsson.com;
>> rjsparks@nostrum.com; fandreas@cisco.com; miguel.a.garcia@ericsson.com
>> Cc: mmusic@ietf.org; rfc-editor@rfc-editor.org
>> Betreff: [MMUSIC] [Technical Errata Reported] RFC4566 (3278)
>>
>>
>> The following errata report has been submitted for RFC4566,
>> "SDP: Session Description Protocol".
>>
>> --------------------------------------
>> You may review the report below and at:
>> http://www.rfc-editor.org/errata_search.php?rfc=4566&eid=3278
>>
>> --------------------------------------
>> Type: Technical
>> Reported by: Riccardo Bernardini <riccardo.bernardini@uniud.it>
>>
>> Section: 5
>>
>> Original Text
>> -------------
>> At page 9:
>>
>>
>>       Media description, if present
>>          m=  (media name and transport address)
>>
>> Corrected Text
>> --------------
>>
>>       Media description, if present
>>          m=* (media name and transport address)
>>
>>
>>
>> Notes
>> -----
>> A '*' is added making "m=" lines optional
>>
>> Rationale:
>> The description about media lines in section 5, page 9 is
>> conflicting with the ABNF syntax of media-description in
>> Section 9, page 40
>>
>> media-descriptions =  *( media-field
>>                          information-field
>>                          *connection-field
>>                          bandwidth-fields
>>                          key-field
>>                          attribute-fields )
>>
>> According to the ABNF grammar, an SDP description can have
>> zero "m=" lines, but according to the description at page 9,
>> at least one line must be present.  Note that the conflict
>> could be solved also by replacing the media-descriptions
>> definition with
>>
>>    media-descriptions =  1*( media-field ... )
>>
>> but at page 8 (still Section 5) it is said
>>
>>    An SDP session description consists of a session-level section
>>    followed by zero or more media-level sections. ...
>>                ^^^^^^^^^^^^^^^^^^^^^^^^
>>
>> so, I guess that the ABNF grammar is correct.
>>
>> Instructions:
>> -------------
>> This errata is currently posted as "Reported". If necessary, please
>> use "Reply All" to discuss whether it should be verified or
>> rejected. When a decision is reached, the verifying party (IESG)
>> can log in to change the status and edit the report, if necessary.
>>
>> --------------------------------------
>> RFC4566 (draft-ietf-mmusic-sdp-new-26)
>> --------------------------------------
>> Title               : SDP: Session Description Protocol
>> Publication Date    : July 2006
>> Author(s)           : M. Handley, V. Jacobson, C. Perkins
>> Category            : PROPOSED STANDARD
>> Source              : Multiparty Multimedia Session Control
>> Area                : Real-time Applications and Infrastructure
>> Stream              : IETF
>> Verifying Party     : IESG
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>>
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-- 
Riccardo Bernardini
DIEGM -- University of Udine
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33100 Udine
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