284 3Com® VCX V7111 VoIP Gateway User GuideFigure 105 NAT FunctioningThe way SIP is designed creates a problem for VoIP traffic to pass through NAT. SIP uses IPaddresses and port numbers in its message body. The NAT server cannot modify SIPmessages and therefore, cannot change local to global addresses.Two different streams traverse through NAT: signaling and media. A gateway (locatedbehind a NAT) that initiates a signaling path will have problems in receiving incomingsignaling responses (they will be blocked by the NAT). Furthermore, the initiating gatewaymust notify the receiving gateway where to send the media to.To solve these problems the following mechanisms are available: STUN (see STUN). First Incoming Packet Mechanism (see First Incoming Packet Mechanism) RTP No-Op packets according to the avt-rtp-noop draft (see RTP NO-OP). For SNMP NAT traversal, see SNMP NAT Traversal.STUNSimple Traversal of UDP through NAT’s (STUN) (according to RFC 3489) is a client / serverprotocol that solves most of the NAT traversal problems. The STUN server operates in thepublic Internet and the STUN clients are embedded in end-devices (located behind NAT).STUN is used both for the signaling and the media streams. STUN works with many existingNAT type s, and does not require any special behavior from them.STUN enables the gateway to discover the presence (and types) of NAT’s and firewallslocated between it and the public Internet. It provides the gateway with the capability todetermine the public IP address and port allocated to it by the NAT. This information is laterembedded in outgoing SIP/SDP messages and enables remote SIP user agents to reach thegateway. It also discovers the binding lifetime of the NAT (the refresh rate necessary to keepNAT Pinholes open).
