Ansari, Mohammed wrote:
I am a bit unclear on the use of re-Invite to "modify" a specific sip
header value. (example is the case of OMA POC where session-expires
header is used to inform the server about the request session expires
interval). UPDATE can be used but it is discouraged to be used after
session establishment in favor of re-Invite per UPDATE specification.
Using re-Invite for session refresh brings out the issue of the sdp that
was previously negotiated:
A) If no sdp is provided in the re-INVITE (since the purpose of this
type of re-INVITE is not negotiating sdp parameters but modify sip
header only), RFC3261 indicates that an offer will come in the response
to the re-INVITE. This is not desired. Hence this option is not taken.
B) Is it ok to re-offer identical sdp offer as the first INVITE? Is
there a GUARANTEED way to ensure that the receiving end will NOT process
is this identical sdp and send a different answer? The specifications
are not clear on the no-op case.
Can someone comment on the use of re-INVITE to modify a sip header and
not have to deal with sdp offer/answer re-negptiation?
If you use reinvite then you MUST be prepared to deal with offer/answer
re-negotiation - there will be an offer/answer.
The offer MAY include the same SDP currently in use by the offerer, but
it is not required to be the same. IF the offer is the same, then the
answer MAY be the same, but need not be.
But I don't understand why this is such a big deal. A reinvite that
changes things can come at any time, whether you want it or not, so you
must be prepared to deal with it. At the same time, such a reinvite may
not change anything, so you probably want to have an easy path through
your code if important things don't change.
So, when you want to do something that uses a reinvite, just send it
off, changing the things you want to change and proposing to keep other
things unchanged. Then just deal with whatever happens.
Paul
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