What a fun problem. There are a few different ways to think about a solution. You were vague on the details of the exact needs you seek to meet, though. Despite that, here are some ideas.

You are concerned with reliable delivery of your data. Think about TCP/IP and how it solves the problems it addresses.

You want to deliver a chunk of data from the handheld to the server. Define a "protocol" for how the server handles the data you send it.

1. Split your data up in to &ltsome byte size&gt chunks.
2. Create a "transaction number" to which the chunks belong.
3. Notify the server that you are sending &ltX&gt chunks in the
transmission number, the "start" message for the "transaction number".
3. For each chunk, calculate the checksum and append it to the chunk. Also, append the "transaction number". Finally, append a "sequence" number to the server knows how to reassemble the chunks.
4. Send the chunk.
5. The server will receive the message, and analyze the check sum. If it is wrong, it will discard it. If it is correct, it will cache the chunk in the queue for that "transaction number".
6. Send all of your chunks.
7. Send a "stop" message for that "transaction number".
8. The server will then attempt to reassemble the chunks. If it is missing any chunks, it can tell you what chunks it is missing and you can simply send those chunks.
9. If you wish, you could even send a final "checksum" message for the entire set of data. You get the point.

You could look for solutions in all sorts of networking areas.