OPC UA is a relatively new standard worth a look for those in the market for a stateful application protocol that provides both an information model and transport layer. One downside is that, presently, only member organizations of the OPC Foundation are given full access to the specifications and reference implementations. Membership is certainly affordable, but interest and adoption might be accelerated if the specs and reference clients/servers were publicly accessible. Perhaps if the Foundation received enough enthusiastic inquiries they might change their policy in that regard.
http://en.wikipedia.org/wiki/OPC_Unified_Architecture
https://opcfoundation.org/about/opc-technologies/opc-ua/