UDP connection setting

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Hi there,

I am trying to get the video stream from camera of robot via UDP connection. I use follow subscriber;

ros::Subscriber sub_image = nh.subscribe("get/image", 1, imageCallback, ros::TransportHints().udp());

This code should select UDP connection.
However, bellow message sometimes appears.

a message of over a gigabyte was predicted in tcpros...

And, I checked the connection status by ping, but UDP connection was not used.
How do I set connection as UDP and check connection setting? I think ros::TransportHints().udp() is not enough to do that.

Thanks in advance,
Keiya