rosbag play cannot return back?

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Hello,

I use rosbag play -s 10 -u 10 a.bag to publish the recorded messages. But I find that when I use the command before, I cannot use rosbag play -s 1 -u 1 a.bag to get the right result, which means if I use the parameter -s to set a particular time slot, I cannot play back to a time slot before.

I want to ask if it is the limitation of rosbag, or if I use it in the wrong way. Thank you.