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Middleboxes, such as packet filtering firewalls or network address translators (NATs), are an obstacle to the dynamic setup of connections required by many applications in today's Internet, for example IP telephony. The problem can be solved if the application obtains a means for configuring middleboxes dynamically. The paper discusses the design of the simple middlebox configuration (SIMCO) protocol for this purpose. We describe related standardization activities in the IETF and discuss our protocol design's advantages, which mainly concern simplicity of syntax, semantics and state machines.