This memo specifies semantics for a Middlebox Communication (MIDCOM) protocol to be used by MIDCOM agents for interacting with middleboxes, such as firewalls and NATs. The semantics discussion does not include any specification of a concrete syntax or a transport protocol. However, a concrete protocol is expected to implement the specified semantics or - more probably - a superset of it. The MIDCOM protocol semantics is derived from the MIDCOM requirements, from the MIDCOM framework, and from working group decisions. Multiparty Multimedia Session Control (mmusic) ----------------------------------------------