If a TCP or UDP IP datagram is fragmented, only the first fragment will
contain the port information. When a fragment without port information
is received, we have no idea which "stream" this fragment belongs to,
thus we no idea how to forward this packet.
To solve this problem, we use "forward twice" method. When IP datagram
which needs fragmentation is received, we forward it using the
frag_id(src_ip, dst_ip, identification, protocol) hash. When all the
fragments are received, we forward it using the connection_id(src_ip,
src_port, dst_ip, dst_port) hash.
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