"draft-gaoyang-sipping-session-state-criterion-01.txt" has given out a precise definition of session state. And what forms a nested transaction is application level issue, and it is opened for evolution and extension.
Precondition forms a famous nested transaction which can have more than one Offer/Answer pair.
3GPP usage of offer/answer in PRACK/200OK or UPDATE/200OK pair to refine(reduce) the codec set is also a famous nested transaction which can have more than one Offer/Answer pair.
But we should point out that UPDATE/200OK pair changing codec set beyond the refine use-case MUST be treated as a new modification.
I think that considering current SIP/SDP usage in RFCs and 3GPP, the classification(a new modification or part of previous one) of Offer/Answer pairs is clear.
But if we do not adopt the precise definition of session state, it will be confused.
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