Packet is a binary serialization framework, similar to Google Protocol Buffers, Apache Thrift, and MessagePack. Packets offers granular control over underlying serialization format that enables you to define existing binary formats (such as network protocols) and generate binding for them in your favorite programming language.

Well, there are many binary protocols around: IP, TCP, and OpenFlow! Can you define an OpenFlow packet in protocol buffers or thrift? No, because you don't have control over the actual representation format. Their approach is elegant for most applications, but won't work for predefined binary wire protocols. If someone gives you a protocol definition in TLV, you're on your own to write the bindings. It gets even worse, when you have to support different languages. That's why I started the Packet project for these Odd (but important) applications.

Packet support polymorphism as long as all the packets in the hierarchy are generated in the same generation pass. The behaviour is undefined when Packets are separately generated.

This is a packet snippet that implements an OpenFlow packet_in:

# Packet received on port (datapath -> controller).
@type_selector(type = Type.PT_PACKET_IN)
packet PacketIn(Header10) {
  uint32 buffer_id;     # ID assigned by datapath.
  uint16 total_len;     # Full length of frame.
  uint16 in_port;       # Port on which frame was received.
  uint8 reason;         # Reason packet is being sent (one of.PR_*)
  uint8 pad;

  @repeated
  uint8 data;           # Ethernet frame, halfway through 32-bit word,
                        # so the IP header is 32-bit aligned.  The
                        # amount of data is inferred from the length
                        # field in the header.  Because of padding,
                        # offsetof(packet PacketIn, data) ==
                        # sizeof(packet PacketIn) - 2.
}

PacketIn derives from openflow header v1.0. The type field of a packet in always is always PT_PACKET_IN.

For a more comprehensive example take a look at: https://github.com/kandoo/beehive-netctrl/blob/master/openflow/of10.packet