SIP中的四种状态机:ICT、NICT、IST、NIST
ICT:Invite Client (Outgoing) Transaction—处理INVITE客户事务的状态机NICT:Non-Invite Client (outgoing) Transaction—处理SIP中的非INVITE客户事务IST:Invite Server (incoming)Transaction—自身作为服务器的INVITE事务NIST: Non-Invite Server (incoming) Transaction—自身为服务器的非INVITE事务——————————