Packet wrangling from C to C++ or... how Neither Variant nor Any is a Union

The problem in a nutshell: I'm considering a rewrite of some packet handling code for a UDP protocol ... from C to C++. Obviously, I can just use the structures that I had with C to model the data, but then what would be the point? So, for the sake of argument, we have:

struct { uint8:2 type; uint8:6 con_id; uint16 seq_num } packet_header;
struct { packet_header ph; uint16 command; } control_packet;
struct { packet_header ph; uchar8 data[DATASIZE]; } data_packet;

In this case, "type" is 0 for data, 1 for control, 2 for encrypted data and 3 for encrypted control. There would also be structs reprenting each of the control packets, for argument, lets say "open" and "close".

Now... I would normally have a type that the function that fetches the packet returns and the function that sends accepts:

struct { size_t pkt_len;
  union { uchar8 raw_packet[DATASIZE]; packet_header ph; control_packet cp;
          control_open_packet cop; control_close_packet ccp;
          data_packet dp; 
  } p; 
}; packet;

... this is all simplified for argument... but the point is: when sending a packet, we want to create a packet of a certain type and send a generic packet. Conversely, when receiving a packet, you want to receive a generic packet and and cast it to more specific types as you classify it.

So where's the question? My question is: if Variant and Any are not appropriate ways to express this, what are? Am I stuck with union in this case? Certainly I don't violate the stipulation of POD types (so far) for the union. I've been scouring both C++11 and Boost (and to some extent looking at C++14 ... although I'm stuck with C++11) and I don't see offered solutions.