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。
A fault should be generated when the forwarding process fails or the reverse path information cannot be provided.
对于转发消息路径和反向消息路径来说，消息交换规则是相同的。
The rules for the exchange of messages remain the same for both forward and reverse message paths.
如果出现了rev元素，当消息沿转发消息路径传送时，动态构建反向消息路径。
If the rev element is present, the reverse message path is built dynamically when the message travels along the forward message path.
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