Reading Protobuf objects using boost :: asio :: read_async

I am writing an application using Boost asio in which the client and server exchange messages that are serialized using Google protobuffers. I do not know what size serialized message is sent over the network. It seems that proto-buf objects do not have a separator.

Here is the contents of the .proto file.

 package tutorial;

message Person {
        required string name = 1;
        required int32 id = 2;
        optional string email = 3;
}

This is how I write from the server

        tutorial::Person p;
        p.set_name("abcd pqrs");
        p.set_id(123456);
        p.set_email("abcdpqrs@gmail.com");
        write(p);

        boost::asio::streambuf b;
        std::ostream os(&b);
        p.SerializeToOstream(&os);
        boost::asio::async_write(socket_, b,
                        boost::bind(&Server::handle_write, this,
                                boost::asio::placeholders::error));

In the client, I read the message sent above using boost :: asio :: async_read. How to find the value argas an argument to boost::asio::transfer_at_leastin the code below?

 boost::asio::async_read(socket_, response_,
                            boost::asio::transfer_at_least(arg),
                            boost::bind(&Client::handle_read_header, this,
                                    boost::asio::placeholders::error));

Or how can I make sure boost :: async_read is returned after reading the entire object?