这个层(位于顶端)用于向终端用户展现可视化应用程序。
This layer (shown on top) is used to visualize applications to end users.
终端用户不与这个层进行交互。
应用层组播树是由终端用户构造而成，其稳定性不能得到保证。
The application level multicast tree is composed of the end hosts which can not ensure the stability of the multicast tree.
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