Subject: [MMUSIC] RFC 3264 media format negotiation

From: "Sumin Seo" <[email-masked]>
Date: Mon, 12 Mar 2007 11:42:00 -0700

Per section 6.1 of RFC 3264, we can add additional media formats , not
listed in the corresponding stream in the offer.
Following examples contain additional media formats in answer. only
difference is the order. First example has the codec not listed in the offer
as its highest preference. Second example has the codec listed in the offer
as its highest preference.
Offer        Answer                     Offer               Answer
      ---------->                                    --------------->
    (a, b, c)                                        (a, b ,c)
     <----------                                    <----------------
    (d, b, c, e, f)                                 (b, c, d, e, f)

 If the example on left side is also correct, this means that highest
preference from the answer can be a codec which is not in the offer.
In this case, it seems that we violate following sentence in RFC 3264.
"The media formats in the "m=" line MUST be listed in order of preference, with
the first format listed being preferred.  In this case, preferred means that
*the offerer SHOULD use the format with the **highest preference from the
answer *."
In above context, what does "SHOULD" mean?
In case that highest preference format in answer is not listed in the offer,
this session should be disconnected by offerer or offerer should choose the
highest preference format from amongst common formats in answer?

Thanks in advance.

Regards,
Sumin
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