Fragmentation layer. Fragments messages larger than frag_size into smaller packets.
Reassembles fragmented packets into bigger ones. The fragmentation number is prepended
to the messages as a header (and removed at the receiving side).
Each fragment is identified by (a) the sender (part of the message to which the header is appended),
(b) the fragmentation ID (which is unique per FRAG2 layer (monotonically increasing) and (c) the
fragement ID which ranges from 0 to number_of_fragments-1.
Requirement: lossless delivery (e.g. NAK, ACK). No requirement on ordering. Works for both unicast and
multicast messages.
Compared to FRAG, this protocol does
not need to serialize the message in order to break it into
smaller fragments: it looks only at the message's buffer, which is a byte[] array anyway. We assume that the
size addition for headers and src and dest address is minimal when the transport finally has to serialize the
message, so we add a constant (1000 bytes).