Rebuffering

The size of the buffer depends on two factors:

Desired latency: The larger the buffer, the longer it takes for the stream to start.

Stream stability: Larger buffers allow the stream to deal with noise and interruptions better on the network. Making

the buffer bigger increases the chance that the stream will play through without any stutters, gaps, or other interruptions.

These two factors are conflicting: Ideally the stream should both start quickly and be stable. One way to deal with this situation is to have two different buffer settings, one for when the stream starts and one for when the playback is in progress. This strategy significantly reduces the conflict. It allows a small buffer to be set for fast start, and a longer one to be used to ensure stream stability when playback begins. Because network issues can still affect the initial buffer size, the initial buffer size cannot be arbitrarily small.

Live applications that use the Live Aggregate Messages feature on Flash Media Server to improve performance need to use a buffer that is at least as long as the delay introduced by the live aggregation. The delay is 500ms (0.5s) by default but can be configured on a per-application basis. If the buffer isn't long enough, the stream will not play back smoothly.

For dynamic bitrate switching, Adobe recommends using a buffer length value that is equal to the keyframe frequency of the encoded video.

I think the information would be helpful to you:

http://www.adobe.com/devnet/flashmediaserver/articles/fms_dual_buffering.html