[SIPForum-discussion] Is "fmtp" line needed in 200 OK answer?

Randell Jesup rjesup at wgate.com
Thu May 29 23:28:10 UTC 2008

"Herve Jourdain" <herve.jourdain at mstarsemi.com> writes:
>As far as I remember of RFC 3264, the devices should be prepared to receive
>ANY of the codecs it sends in its offer/response

>So basically, if you respond with 3 codecs, you should be able to receive at
>any time on any of those 3 codecs, and switch between them

Answer to an ancient question, but...

That's correct.  If you accept more than one codec, the other side can use
any of them, and switch codecs at *any* time.  If it wants to, it could in
theory switch codecs on every packet.  Not very smart.... but it could.
And there are some very good reasons for accepting multiple codecs, but
some devices just can't handle it - and they should only accept one.

And note, if multiple codecs are selected, the two sides can be sending
different codecs to each other at any given point, if they want to.

>But even if it’s stated this way in the specs, I still do think that before
>switching to another codec, the UAs should at least use the codec that was
>agreed for in the negotiation at first (the “highest priority” one, in your
>case G729).
>Experience shows, unfortunately, that even if it’s usually the case, it’s
>NOT ALWAYS the case

Quite true.  Sometimes they simply ignore the preference, in other cases
they may choose the actual codec based on other concerns or user preference.

Randell Jesup, Worldgate (developers of the Ojo videophone), ex-Amiga OS team
rjesup at wgate.com
"The fetters imposed on liberty at home have ever been forged out of the weapons
provided for defence against real, pretended, or imaginary dangers from abroad."
		- James Madison, 4th US president (1751-1836)

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