Subject: [MMUSIC] Representing Boolean MIME-type parameters in SDP?

From: Ross Finlayson <[email-masked]>
Date: Mon, 24 Jan 2005 00:30:10 -0800

I recently encountered a MPEG-4 video + AMR audio RTSP/RTP session whose 
SDP lines contained the following (for the AMR audio stream):

	a=rtpmap:97 AMR/8000
...
	a=fmtp:97 octet-align

Is that last line legal?  I had thought that only the following would be legal:
	a=fmtp:97 octet-align=1

RFC 3267 (which defines the RTP payload format for AMR) defines the 
"octet-align" MIME parameter as follows:
	"octet-align: Permissible values are 0 and 1."

So, for Boolean MIME parameters like this, is it permissible to omit the 
value assignment in the SDP "a=fmtp:" line, thereby assuming the value 
1?  I had assumed not (and in fact our own RTSP client code got confused by 
the example above), but it would be nice to get some clarification of this 
question, in case examples like the one above become commonplace.

	Ross Finlayson
	LIVE.COM
	<http://www.live.com/>

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---

From: Magnus Westerlund <[email-masked]>
Date: Mon, 24 Jan 2005 10:48:26 +0100

Hi Ross,

It is required for RTP media types to have values. This is due to the 
following in RFC 3555:

"3. Mapping to SDP Parameters

    The representation of a MIME media type is specified in the syntax of
    the Content-Type header field in RFC 2045 [6] as follows:

       type "/" subtype  *(";" parameter)

    Parameters may be required for a particular type or subtype or they
    may be optional.  For media types which represent RTP payload
    formats, the parameters "rate", "channels", "ptime", and "maxptime"
    have general definitions (given above) that may apply across types
    and subtypes.  The format for a parameter is specified in RFC 2045 as

       attribute "=" value

    where attribute is the parameter name and the permissible values are
    specified for each parameter.  The value may need to be a quoted
    string if it contains any of the special characters listed in RFC
    2045."

So the usage you have witnessed is ilegal.

Cheers

Magnus

Ross Finlayson wrote:
> Audio/Video Transport Working Group
> [email-masked]
> https://www1.ietf.org/mailman/listinfo/avt
> 

-- 

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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