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

From: Federico Montesino Pouzols <[email-masked]>
Date: Wed, 3 Jul 2002 16:02:59 +0200

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?

_______________________________________________
mmusic mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/mmusic

---

From: Colin Perkins <[email-masked]>
Date: Thu, 04 Jul 2002 09:01:54 -0400

>could be omitted if backward compatibility were not a concern. Is it

The ABNF in RFC 2327 does not require the leading zero.

Colin

---

From: "Federico Montesino Pouzols" <[email-masked]>
Date: Sat, 6 Jul 2002 15:08:27 +0200 (CEST)

> The ABNF in RFC 2327 does not require the leading zero.

I see, the explicit zero does not fit the RFC ABNF. However
I keep wondering the same question: is actually the first offset
always zero? I mean, although the RFC ABNF does not mandate it, the
text describing r= lines (the same in both RFC 2327 and
draft-ietf-mmusic-sdp-new-10) seems to imply that the first offset is
always zero. The examples and explanations given in both documents
make me think that the first offset correponds to the beginning of the
session (as opposed to the first time the session is resumed), thus
being zero.

	Is this interpretation right or is there any case in which the
first offset can be non zero?

---

From: Colin Perkins <[email-masked]>
Date: Sat, 06 Jul 2002 10:10:02 -0400

>first offset can be non zero?

The first offset does correspond to the beginning of the session, and I
would expect it to be zero. There's no requirement for this though, and
a non-zero offset would change the start time of the session.

Colin

---

From: "Federico Montesino Pouzols" <[email-masked]>
Date: Sat, 6 Jul 2002 19:15:00 +0200 (CEST)

> a non-zero offset would change the start time of the session.

Another question is if specifying the first offset is redundant. After
all, any value of the first offset may be added to the start time of
the corresponding t= line. Are the session start time and the repeat
sequence start time distinguished for any particular reason/purpose?

---

From: Colin Perkins <[email-masked]>
Date: Mon, 08 Jul 2002 11:40:29 -0400

>sequence start time distinguished for any particular reason/purpose?

Huh? One indicates the start of the session, one indicates the repeat
offset. They're separate things.

Colin