On 10/16/12 2:13 PM, Frank S. wrote:
>
> Hi, Suppose offer contains codec a,b,c. Answering party supports b.c.
> Based on what criteria Answering party choose b or c?Answer will contain only
> one codec. What is the selection criteria between b and c. ThanksFrank S
>
Why do you say the answer will contain only one codec?
That is not required.
The special case is telephone-events. Nominally it is a codec, but you
wouldn't choose just that one. You will choose that *and* another one,
or more. And then you will use *both*, not just one of those chosen.
It is recommended that the offer contains codecs in preference order.
Then for answer you would presumably choose the highest priority one(s)
that you also support. The issue arises if the answerer supports more
than one, and had differing preferences than the offerer for those.
The answerer can impose its choice by selecting only one. Or it can put
more than one in the answer, in the order it prefers. Then each end can
decide which it wants to send, and can switch among them if desired.
However there are implementations that can support any *one* of several
codecs, but cannot support more than one of them for a single rtp
session. Those may *assume* that only the first codec in the answer will
be used. (With exception of telephone-events.) A more thorough
implementation with that limitation would send a second offer reducing
to a single codec.
Then there are applications that support multiple RTP streams per RTP
session. Those are being discussed a lot recently. In that case you may
negotiate multiple codecs, and mix them up - one per RTP stream within
the session.
Bottom line: there is no simple answer. You need to understand your
limitations and the environments you expect to work with.
Thanks,
Paul
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