Re: [MMUSIC] An error in SDP syntax regarding r= lines?

[Federico Montesino Pouzols <fedemp@suidzer0.org>]

On Sun, Jun 30, 2002 at 02:58:25PM -0400, Colin Perkins wrote:
> --> Federico Montesino Pouzols writes:
> >Hello everyone,
> >
> >  On section 6 in draft-ietf-mmusic-sdp-new-10.txt the following
> >examples of r= lines are given:
> >
> ># r=604800 3600 0 90000
> ># r=7d 1h 0 25h
> >
> >  From this examples, I understand that "0" is a valid value for the
> >offsets. However on appendix A r= lines are defined like this:
> >
> ># repeat-fields =       "r=" repeat-interval SP typed-time
> >#                      1*(SP typed-time)
> >
> ># repeat-interval =     typed-time
> >
> ># typed-time =          POS-DIGIT *DIGIT [fixed-len-time-unit]
> >
> ># POS-DIGIT =           %x31-39 ; 1 - 9
> >
> >  The two examples above would violate this syntax, since the first
> >offset is "0", which is not a POS-DIGIT. In RFC 2327, typed-time
> >started with DIGIT, so there was no problem.
> >
> >  It seems to me, correct me please if I am wrong, that the first
> >offset, "0", is not necessary and could be assumed. Anyway, to keep
> >backward compatibility it has to be there. Would the following rule
> >solve the problem?
> >
> ># repeat-fields =       "r=" repeat-interval SP typed-time SP "0"
> >#                      1*(SP typed-time)
> 
> I agree that it looks like a bug in the ABNF. A better fix might be to
> change typed-time to be
> 
> 	typed-time = 1*DIGIT [fixed-len-time-unit]
> 
> to avoid mandating a leading zero?
> 
> Colin

	I understand the draft mandates the leading zero from the 
following paragraph:

o    ``r='' fields specify repeat times for a session.  For example, if
    a session is active at 10am on Monday and 11am on Tuesday for one
    hour each week for three months, then the <start time> in the
    corresponding ``t='' field would be the NTP representation of 10am
    on the first Monday, the <repeat interval> would be 1 week, the
    <active duration> would be 1 hour, and the offsets would be zero and
    25 hours. The corresponding ``t='' field stop time would be the NTP
    representation of the end of the last session three months later. By
    default all fields are in seconds, so the ``r='' and ``t='' fields
    might be:


	So I guess the first offset could be non zero only if the start
time given in the corresponding t= line were shifted from the actual
beginning of the session. I believe the first offset is always 0 and it
could be omitted if backward compatibility were not a concern. Is it
true?


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