1. Introduction

   The Simple Middlebox Configuration (SIMCO) protocol is used to
   control firewalls and Network Address Translators (NATs).  As defined
   in [RFC3234], firewalls and NATs are classified as middleboxes.  A
   middlebox is a device on the datagram path between the source and
   destination that performs other functions than just IP routing.  As
   outlined in [RFC3303], firewalls and NATs are potential obstacles to
   packet streams, for example, if dynamically negotiated UDP or TCP
   port numbers are used, as in many peer-to-peer communication
   applications.

   SIMCO allows applications to communicate with middleboxes on the
   datagram path in order to request a dynamic configuration at the
   middlebox that enables datagram streams to pass the middlebox.
   Applications can request pinholes at firewalls and address bindings
   at NATs.

   The semantics for the SIMCO protocol are described in [RFC3989].

1.1. Terminology

   The terminology used in this document is fully aligned with the
   terminology defined in [RFC3989].  In the remainder of the text, the
   term SIMCO refers to SIMCO version 3.0.  The term "prefix-length" is
   used as described in [RFC4291] and [RFC1519].  With respect to
   wildcarding, the prefix length determines the part of an IP address
   that will be used in address match operations.

1.2. Binary Encodings

   Previous experimental versions of SIMCO used simple ASCII encodings
   with augmented BNF for syntax specification.  This encoding requires
   more resources than binary encodings do for generation and parsing of
   messages.  This applies to resources for coding agents and
   middleboxes as well as to resources for executing a SIMCO stack.

   Low resource requirements are important properties for two main
   reasons:

      - For many applications (for example, IP telephony), session setup
        times are critical.  Users do accept setup times only up to some
        limit, and some signaling protocols start retransmitting
        messages if setup is not completed within a certain time.

      - Many middleboxes are rather small and relatively low-cost
        devices.  For these, support of resource-intensive protocols
        might be a problem.  The acceptance of a protocol on these
        devices depends, among other things, on the cost of implementing
        the protocol and of its hardware requirements.

   Therefore, we decided to use a simple and efficient binary encoding
   for SIMCO version 3.0, which is described in this document.