Messages, which are the x-kernel 's abstract data type for network packets, are defined by the Msg structure. Loosely speaking, this structure consists of a tree of buffers that collectively hold the bytes contained in the message. The leftmost buffer in this tree is called the header stack because it holds the headers that are pushed onto the front of a packet. This data structure is fairly complex, however, and so we do not describe it in this document. The interested reader is referred to a companion report [4]. In addition, there is a MsgWalk structure that is used by msgWalkNext to traverse the buffers that make up a message (see Section 3.3.12). This structure is also defined in [4]. The fields of neither structure should not be directly accessed by the protocol developer.