[GitHub] ccollins476ad commented on issue #6: BLE reassembly support

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ccollins476ad commented on issue #6: BLE reassembly support
URL: https://github.com/apache/mynewt-mcumgr/issues/6#issuecomment-457310275
 
 
   @bgiori
   
   Re: Write Long Characteristic Values procedure- My "BLE" is a bit rusty, so please correct me if I am wrong.  During a long write procedure, each fragment must be acknowledged by the server (Prepare Write Response).  The protocol would still effectively be stop-and-wait, so I am not sure this would help much with performance.
   
   I think we would want to do this at the application layer.  Then we can still use unacknowledged writes.  I can think of two ways we may want to achieve this (my apologies if I am rehashing things that have already been discussed):
   
   **1. Generic fragmentation mechanism.**  This is probably what you'd expect: the server buffers each fragment until the entire packet is received, then reassembles and processes.
   
   *Good:* Works for all SMP command types.
   *Bad:* Requires a lot of buffer space on the server.
   
   **2. DFU-specific mechanism.**  The image upload requests are not fragmented.  Instead, the requests are selectively acknowledged.  The server writes image chunks to flash as it receives them, but only acknowledges every X requests (or every X bytes).  Both sides understand the acknowledgement policy, so the client can rapidly send X requests, then listen for a response.
   
   *Good:* Does not require extra buffer space on the server.
   *Bad:* a) Only helps with DFU; b) probably more complicated to implement.
   
   Thoughts?

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