I have some general questions about reinvite:
- exactly when do the changed media descriptions take effect?
- isn't there a period of ambiguity when it is possible that
either the old or the new descriptions might be operative?
- if so, is it necessary to support the union of the two
descriptions for that period of time?
To be more specific:
In a first invite, ignoring early media, the caller may
begin to send media after sending the ACK. But because the
media may overtake the ACK, the callee needs to be prepared
to receive media at the time it sends an OK to the invite.
In a *successful* reinvite, this should also be true. So at
the time the callee sends an OK to a reinvite, it needs to be
prepared to receive according to the new media description it
sent.
But until the reinvite is known to be successful, I believe
the old media description remains in effect. So the caller,
while awaiting a response to a reinvite, may be sending
media according to the old description. These could or course
be received by the callee for some time after the OK has been
sent.
To avoid losing data, it would seem that the callee needs to
be prepared to receive according to either description starting
from when the OK is sent and ending at some indeterminate time
later. This could be difficult if the intent is to switch between
mutually exclusive codecs. It could also be a problem if the
intent is to put media on hold from one source so the same
receiving resources can be used for a different call.
Am I missing something?
Paul Kyzivat
Cisco Systems
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