Middleboxes are defined as any intermediary box performing functions
apart from normal, standard functions of an IP router on the data
path between a source host and destination host. Two such functions
are network address translation and firewalling.
When Application Layer Gateways, such as SIP entities, interact with
NATs and firewalls, as described in the MIDCOM architecture, then
problems may occur in the transport of media traffic when signaling
protocol interaction takes place along the media path, as it is the
case for recent key exchange proposals (such as DTLS-SRTP). This
document highlights problems that may arise. Unfortunately, it is
difficult for the end points to detect or predict problematic
behavior and to determine whether the media path is reliably
available for packet exchange.
This document aims to summarize the various sources and effects of
NAT and firewall control, the reasons that they exist, and possible
means of improving their behavior to allow protocols that rely upon
signaling along the media path to operate effectively.