dissect_tcp_pdus with truncated segments

I created a lua script to dissect DoIP messages. The header is pretty simple: A version byte, a byte with the bitwise invert of the version, the payload type as two bytes and the payload length as 4 bytes.

function get_doip_length(tvb, pinfo, offset)
    -- Callback function to decide how long a PDU is, so that
    -- Wireshark can reassemble it for us
    local version = tvb(offset, 1):uint()
    local inverse_version = tvb(offset+1, 1):uint()
if version == 0x02 and inverse_version == 0xFD then
    local payload_length = tvb(offset+4, 4):uint()
    return payload_length + 8 -- Payload length + header size
else
    return 8 -- Unknown protocol, assume only header is to be reassembled
    -- According to https://www.wireshark.org/lists/wireshark-dev/201610/msg00057.html
end

end
function doip.dissector(tvb, pinfo, tree)
dissect_tcp_pdus(tvb, tree, 8, get_doip_length, dissect_doip_pdu)
end

This works well when the trace is complete. I have payloads of about 16K, which is 12 TCP frames. The 11 first TCP frames are marked [TCP segment of a reassembled PDU] and the last one contains the proper protocol name and all the data.

However, I sometimes trace also on a small device using tcpdump -s128, because the throughtput is too low (over UART…) and it’s better for me to not have the full payload but not drop packets.

In this case, the dissector doesn’t work as expected. The first frame of the PDU is marked as the reassembly, and the length is indeed 16K. But all the 11 remaining frames are not taken into account. Rather, dissect_tcp_pdus tries to match them to a new PDU. I would expect that dissect_tcp_pdus could at least reassemble all the segments, even if the data is not captured (Tvb has indeed a :len() and a :reported_len() functions).

Is there someting I do wrong?