What is a new modification? This is another good question people once asked.
Without clear definition for extension, anything or its combination defined in chapter 8 of RFC3264 MUST be treated as a new modification, that is:
Adding a Media Stream,
Removing a Media Stream,
Modifying a Media Stream,
Putting a Unicast Media Stream on Hold(or from on hold to active)
"Modifying a Media Stream" can be further divided into:
Modifying Address, Port or Transport,
Changing the Set of Media Formats,
Changing Media Types,
Changing Attributes.
And if we can say something form a nested transaction, there MUST be correspond definition. In RFCs, there are two of such definition:
1. Precondition
It is defined in RFC3312(Quality-of-Service precondition)\4032(Precondition Framework)\5027(Security Preconditions), and the suspending\resuming semantic of session modification is clear. So, it is a typical nested transaction.
2. Intuitionistic(user point of view) requirement for rejecting session modification
It is defined in RFC3261. From users's point of view, it is a kind of late committment. And it is useful for users to rejecting session modification.
3. 3GPP usage of Offer/Answer to refine(reduce) codec set
In chapter 8.3.2 of RFC3264, refining(reduce) codec set is a kind of modification, and there is no definition as RFC3312 to define sub-transaction concept. So, I do not think this kind of Offer/Answer can be treated as sub-transaction of the original one in semantics of current RFCs.
And if one side send a new UPDATE during the pending Re-INVITE, the other side can has two choose:
o if the UA can accept the UPDATE, and after Offer/Answer, the modification MUST be committed.
o if the UA can not accept the UPDATE(for example, can not accept it without the user's permition), the UA MUST reject UPDATE.
I MUST point out that in RFCs semantics, the third rule should be omitted.
But 3GPP really has such usage. And I think the original requirement of obeys the concept of nested transaction, so I list it here.
And the classification of a new modification is quit simple. At list, it is not harder than the judgement of whether Precondition has been satisfied.
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