Independent of the syntax discussion ("allows present ABNF grammar an omitted
"m=" line"),
there's a semantic mentioned in revised SDP Offer/Answer.
See ยง 3.2 in
http://tools.ietf.org/html/draft-ietf-mmusic-sdp-media-capabilities-07
...
o A new attribute ("a=lcfg") specifies latent media stream
configurations when no corresponding media line ("m=") is offered.
An example is the offer of a latent configuration for video even
though no video is currently offered. If both parties indicate
support for one or more latent configurations, the corresponding
media stream(s) may be added via a new offer/answer exchange.
There seems to be thus use cases behind initially omitted "m=" lines.
-Albrecht
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On
> Behalf Of Frank Shearar
> Sent: Mittwoch, 22. April 2009 10:00
> To: sip fourm
> Subject: Re: [Sip-implementors] media line (m line) is
> mandatory in SDP?
>
> Ivar Lumi wrote:
> > Hi,
> >
> > According RFC 4566.
> >
> > media-descriptions = *( media-field information-field
> > *connection-field bandwidth-fields key-field attribute-fields )
> >
> > And if remember right in RFC text too: There may be zero
> media lines.
> > With 0 lines just session created and new offer can add
> stream to it.
> >
> Bearing in mind that RFC 4566 obsoletes RFC 2327, RFC 2327
> still says this in its ABNF:
>
> media-descriptions = *( media-field
> information-field
> *(connection-field)
> bandwidth-fields
> key-field
> attribute-fields )
>
> (just like RFC 4566's, in other words).
>
> Friend friend's snippet covers the first field of a media
> description, not the media description in its entirety.
>
> In other words, this is an illegal media-field because it has
> no media:
>
> m= 0 RTP/AVP 0
>
> frank
> >
> > friend friend wrote:
> >
> >> Hi,
> >> Thanks for your response.
> >>
> >> but RFC 2327 Grammar says like
> >>
> >> media-field = "m=" media space port ["/" integer]
> >> space proto 1*(space fmt) CRLF
> >>
> >>
> >> media = 1*(alpha-numeric)
> >> ;typically "audio", "video", "application"
> >> ;or "data"
> >>
> >>
> >> Which means, one or more m line is mandatory rite.
> >>
> >> please correct me, if i am wrong.
> >>
> >> Regards,
> >> vijay
> >>
> >>
> >>
> >>
> >>
> >> --- On Tue, 21/4/09, karthik karthik
> <[email protected]> wrote:
> >>
> >>
> >> From: karthik karthik <[email protected]>
> >> Subject: Re: [Sip-implementors] media line (m line) is
> mandatory in SDP?
> >> To: [email protected]
> >> Cc: "sip fourm" <[email protected]>
> >> Date: Tuesday, 21 April, 2009, 7:36 PM
> >>
> >>
> >> Hi,
> >> Hope this thread has more info about this topic
> >>
> https://lists.cs.columbia.edu/pipermail/sip-implementors/2008-October
> >> /020798.html
> >>
> >> Karthik
> >>
> >>
> >> On Tue, Apr 21, 2009 at 7:31 PM, Frank Shearar
> <[email protected]> wrote:
> >>
> >>
> >> friend friend wrote:
> >>
> >>
> >>> Dear Friends,
> >>> In SDP, media (m line) is mandatory? if so, if
> the UAC doesnt support video, audio, can the UAC send SDP
> like m: 0(zero)?
> >>>
> >>> Please clarify.
> >>>
> >>>
> >>>
> >> The m header is not mandatory. RFC 4566 section 5 says
> >>
> >> An SDP session description consists of a session-level section
> >> followed by zero or more media-level sections.
> >>
> >> If the UAC supports no kinds of media it'll send no media-level
> >> sections. Not very _useful_, since the UAS must respond
> with the same
> >> number of media-level sections!
> >>
> >> frank
> >>
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