Subject: [AVT] Interpreting RFC2327: SDP whitespace problem in "s=" line

From: Thomas Zeng <[email-masked]>
Date: Tue, 26 Aug 2003 17:29:52 -0700

Hello All:

During recent interop testing, one issue has come up regarding whether it is
valid to have "s=[single space]" in
an SDP as the subject of a presentation.

In section 6 of RFC2327 (see
http://www.ietf.org/rfc/rfc2327.txt?number=2327):

   An SDP session description consists of a number of lines of text of
   the form <type>=<value> <type> is always exactly one character and is
   case-significant.  <value> is a structured text string whose format
   depends on <type>.  It also will be case-significant unless a
   specific field defines otherwise.  Whitespace is not permitted either
   side of the `=' sign. In general <value> is either a number of fields
   delimited by a single space character or a free format string.

Taken off context, this paragraph seems to be prohibiting
'<type>[space]=<value>'  or
'<type>=[space]<value>' .  However, we don't think the intent was to place
restrictions on <value>.  If <value> is a free-format string, then it should
be able to start with a space. (And this is what the ABNF definition says,
for the value in the 's=' case).

We checked
http://www.ietf.org/internet-drafts/draft-ietf-mmusic-sdp-new-13.txt as
well, which clearly states that
if there is no meaningful subject, a single space should be used in "s="
line, yet it apparently has contracdictory statement in section 5.

We hereby would like to confirm with this mailing list, that "s=[single
space]" is a legal subject line. 
We would also appreciate clarification in
http://www.ietf.org/internet-drafts/draft-ietf-mmusic-sdp-new-13.txt , 
section 5, to indicate that:

   " An SDP session description consists of a number of lines of text of
   the form:

      <type>=<value>

   where <type> MUST be exactly one case-significant character and
   <value> is structured text whose format depends on <type>.  In
   general <value> is either a number of fields delimited by a single
   space character, or a free format string.   [modified text]  Whitespace
MUST NOT be used either side of the "=" sign, except if the free    format
<vallue> string  begins with a space or contains a single space. An exampel
is: "s=[single space]", which is a   legal SDP line."

Thanks in advance for your feedback.

thomas
PacketVideo Corp.

www.pv.com

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

---

From: Colin Perkins <[email-masked]>
Date: Wed, 27 Aug 2003 17:55:25 +0100

> space]" is a legal subject line.

I would consider it to be legal. RFC 3264 (offer/answer) also 
recommends either a single space or a single dash ("-") character in 
the s= line.

-- 
Colin Perkins
http://csperkins.org/