Only when the control and media flow traverse NAT, can people use private IP address to make sessions with SIP protocol.
要让使用私有ip地址的用户通过SIP协议进行会话，就必须使SIP的信令流和媒体流穿越NAT，而其中的难点是媒体流的穿越。
Though TRUN can traverse symmetrical NAT, it causes network delay because packet needs to transmit through TRUN server.
TRUN虽然能够穿越对成型的NAT，但由于报文要通过TRUN服务器转发，这又增加了网络延迟。
Though TRUN can traverse symmetrical NAT, it causes network delay because packet needs to transmit through TRUN server.
TRUN虽然能够穿越对成型的NAT，但由于报文要通过TRUN服务器转发，这又增加了网络延迟。
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