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V6100 and V7122 User Guide
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Figure 88 NAT Functioning
The way SIP is designed creates a problem for VoIP traffic to pass through NAT. SIP uses IP
addresses and port numbers in its message body. The NAT server can’t modify SIP
messages and therefore, can’t change local to global addresses.
Two different streams traverse through NAT: signaling and media. A gateway (located
behind a NAT) that initiates a signaling path will have problems in receiving incoming
signaling responses (they will be blocked by the NAT). Furthermore, the initiating gateway
must notify the receiving gateway where to send the media to.
To solve these problems the following mechanisms are available:
ƒ
STUN (see STUN ).
ƒ
First Incoming Packet Mechanism (see First Incoming Packet Mechanism )
ƒ
RTP NO-OP packets according to the avt-rtp-noop draft (see RTP NO-OP ).
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For SNMP NAT traversal, see SNMP NAT Traversal .
STUN
Simple Traversal of UDP through NATs (STUN) (according to RFC 3489) is a client / server
protocol that solves most of the NAT traversal problems. The STUN server operates in the
public 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
existing
NAT
type
s, and does not require any special behavior from them.
STUN enables the gateway to discover the presence (and types) of NATs and firewalls
located between it and the public Internet. It provides the gateway with the capability to
determine the public IP address and port allocated to it by the NAT. This information is later
embedded in outgoing SIP/SDP messages and enables remote SIP user agents to reach the
gateway. It also discovers the binding lifetime of the NAT (the refresh rate necessary to keep
NAT ‘Pinholes’ open).