Stun - 3Com VCX v7111 User Manual

Figure 105 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 cannot modify SIP
messages and therefore, cannot 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:
STUN (see STUN).
First Incoming Packet Mechanism (see
RTP No-Op packets according to the avt-rtp-noop draft (see
For SNMP NAT traversal, see

STUN

Simple Traversal of UDP through NAT's (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
type
NAT s, and does not require any special behavior from them.
STUN enables the gateway to discover the presence (and types) of NAT's 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).
284
First Incoming Packet
SNMP NAT Traversal.
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3Com
Mechanism)
RTP NO-OP).
VCX V7111 VoIP Gateway User Guide
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