I'm currently writing a dissector for a protocol where the server can fragment its data within a single frame as well as across multiple consecutive frames (if necessary). No fragment will ever begin in one frame and end in the next, so frames will always contain whole fragments.
Also, the total length of the data is not known ahead of time, so I cannot use tcp_dissect_pdus(). There are also no sequence numbers, ids, or anything else in the fragment headers, all I have is a byte containing some flags (indicating fragment or termination -- the final fragment) and the fragment length, which is present at the beginning of each fragment. There is no interleaving of application-level packets from the server to the client, so it is safe to keep reading fragments/frames until I find that a termination flag is set.
I started out using fragment_add_multiple_ok() and process_reassembled_data(), but then I became a bit confused about the meaning of the offset parameter. Since there can be multiple fragments per frame in this case, I assumed that the frag_offset is simply the offset within the current tvbuff_t. However, what should I then be using for the offset parameter?
If there's an easier way to go about defragmenting for this type of protocol, please let me know as well.