On 6/22/2011 12:40 PM, Nauman Sulaiman wrote:
> Hi
>
> Some UAs do not support PRACK or UPDATE and still work fine with various
> Proxies, PBX, Softswitches etc. Are there any scenarios or well known
> Proxies,Softswitches that require mandatory PRACK and UPDATE support for a UA
> to be interoperable? Or maybe some features will not be available if support
> for these methods is not there.
>
> Just trying to understand where PRACK and UPDATE are used.
Its been a *really* long time since 3262 and 3311 were published. There
is no reason for UAs not to support PRACK and UPDATE.
There are devices/systems with constraints that may make it necessary to
to multiple (typically 2) offer/answer exchanges before establishing a
call.
The use case that initially motivated PRACK was QoS, but that is not the
only one. For instance there are some UAsthat can only support one codec
at a time. Such a UA will offer multiple codecs because it doesn't know
which ones the answerer can support. But then if the answerer answers
with multiple codecs, there is need of another o/a cycle to select one
from among those common to offer and answer.
(However this is often handled without a second o/a, using "wishful
thinking" to assume that only the first of the codecs in the answer will
actually be used. I guess if others are then used it just doesn't work.)
Thanks,
Paul
> Thanks
>
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