sip不分配多波地址、也不保留资源，但它能把做到这点所必要的信息传递给被邀请的系统。
SIP does not allocate multicast addresses and it does not reserve resources, but it can convey to the invited system the information necessary to do this.
在多播中，信息包能被送到网络组里的各个地址，而不是单个的地址。
In multicasting, packets can be sent to groups of Network addresses, instead of individual addresses.
在多播中，信息包能被送到网络组里的各个地址，而不是单个的地址。
In multicasting, packets can be sent to groups of Network addresses, instead of individual addresses.
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