This indicates an implicit reverse path, which will be provided by the underlying protocol.
该值表示使用隐式反向路径，该路径将由底层协议提供。
A fault should be generated when the forwarding process fails or the reverse path information cannot be provided.
当转发过程失败或者无法提供反向路径信息时，应该生成错误。
Traditionally, such a circuit was provisioned through the use of two copper pairs, one for transmission ( forward path , or upstream path ) and one for reception ( reverse path , or downstream path );
传统上，这种电路通过使用两个铜电缆对提供，一个用于发送（前向路径或上游路径）、一个用于接收（反向路径或下游路径）。
Traditionally, such a circuit was provisioned through the use of two copper pairs, one for transmission ( forward path , or upstream path ) and one for reception ( reverse path , or downstream path );
传统上，这种电路通过使用两个铜电缆对提供，一个用于发送（前向路径或上游路径）、一个用于接收（反向路径或下游路径）。
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