图2是一个表示PurchaseOrder的简单状态机的流程图。

Figure 2 is a flow chart representing a simple state machine for a PurchaseOrder.

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因为SIP协议标准本身（RFC3261）并没有给出实现方法，本文通过采用指数后退算法设计SIP客户事务的有限状态机及其流程图。

Since there are no implementation methods in SIP standards (RFC3261), a finite state machine of client's transaction and its dataflow diagram are designed with rollback pointers algorithm.

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