Canonical way to transmit protocol buffers over network?

I'm trying to use Google's protocol buffers (protobuf) with Python in a networked program, using bare sockets. My question is: after the transmitting side sends a message, how does the receiving side knows what kind of message was transmitted? For example, say I have message definitions:

message StrMessage {
    required string str = 1;
}

message IntMessage {
    required int32 num = 1;
}

Now the transmitter makes a StrMessage, serializes it, and sends the serialized bytes over the network. How does the receiver know to deserialize the bytes with StrMessage rather than IntMessage? I've tried doing two things:

// Proposal 1: send one byte header to indicate type
enum MessageType {
    STR_MESSAGE = 1;
    INT_MESSAGE = 2;
}

// Proposal 2: use a wrapper message
message Packet {
    optional StrMessage m_str = 1;
    optional IntMessage m_int = 2;
}

Neither of these seems very clean, though, and both require me to list all the message types by hand. Is there a canonical/better way to handle this problem?

Thanks!