Randy Stewart -
You described a "checkpointing" mechanism for DDP during last week's robustness call but it is not in the current DDP draft. However, you have evidently thought about this some and stated (in private mail) that you had built such mechanism on top of DDP using the SEND_TO_ALL mechanism. How would you envision something like this being done to synch state for a pool of H.323 entities? Would this be a application level library that could be called from H.323 software at appropriate checkpoint times? Could you describe this checkpointing idea in some more detail?
It would seem preferable to either break up state information into a number of subsets (to avoid resending large amounts each time) or to only send differences. Since DDP gives reliable delivery to the other members of the pool, perhaps only changes need be sent. Did your trials solve this problem. Perhaps each call would be a separate subset.
One issue is how to solve reliable end-to-end delivery. SCTP would guarantee delivery to other end of connection but the H.225 and H.245 protocols may pass through intermediate nodes (e.g., routing GKs). If a node fails after acknowledging the message but before sending it on, end-to-end delivery fails and no live element knows. Either we need far end acknowledgement or backup elements must accept delivery reponsibility once acknowledgement has been sent by its later failed peer. DDP would guarantee delivery but the recipient would have to checkpoint this message before acknowledgement to guarantee that a peer taking over whould know that there was an outstanding message that it must deliver and receive acknowledgement for. This mechanism would require checkpoint after receipt of but before sending acknowledgement of a message and again after receiving acknowledgement from its next neighbor. Does this seem reasonable? Unless the amount of data sent in such checkpointing is kept very small this would seem to add too much network activity to be acceptible.
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