This documents describes functions implemented in Session Initiation Protocol (SIP) intermediaries known as Session Border Controllers (SBCs). Although the goal of this document is to describe all the functions of SBCs, a special focus is given to those practices that are viewed to be in conflict with SIP architectural principles. It also explores the underlying requirements of network operators that have led to the use of these functions and practices in order to identify protocol requirements and determine whether those requirements are satisfied by existing specifications or additional standards work is required.

This work is being discussed on the sipping@ietf.org mailing list.