Protocol in LAN, way to go when I will not author the client?

I need help to point me in the right direction, please.

So far I have been writing both the client and server applications for my product.
I have been using EasyTCPSocket and SocketServer and have been happily comfortable with that.

Now, for a project, a customer wants to poll some data, via the LAN, from my product, but using a client of his own.
It is about 300 bytes at the time.

He says, that he can work with most protocols and it is up to me to chose.
What is the way to go?

Thanks in advance.