Canonical way to transfer protocol buffers over a network?

I am trying to use Google protocol buffers (protobuf) with Python in a network program using bare sockets. My question is: after the sending party sends the message, how does the receiving party know which message was sent? For example, let's say I have message definitions:

message StrMessage {
    required string str = 1;
}

message IntMessage {
    required int32 num = 1;
}

Now the transmitter does StrMessage, serializes it and sends serialized bytes over the network. How does the receiver know byte deserialization using StrMessage rather than IntMessage? I tried to do 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;
}

None of them seem very clean, and both require me to list all types of messages manually. Is there a canonical / best way to handle this problem?

Thank!