Stun - AudioCodes MediaPack MP-11X User Manual

The following figure illustrates the gateway's supported NAT architecture.
The design of SIP creates a problem for VoIP traffic to pass through NAT. SIP uses IP
addresses and port numbers in its message body and 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 has problems in receiving incoming signaling
responses (they are blocked by the NAT server). Furthermore, the initiating gateway must
notify the receiving gateway where to send the media.
To resolve these issues, the following mechanisms are available:
STUN (refer to 'STUN' on page 366)
First Incoming Packet Mechanism (refer to 'First Incoming Packet Mechanism' on
page 367)
RTP No-Op packets according to the avt-rtp-noop draft (refer to 'No-Op Packets' on
page 367)
For information on SNMP NAT traversal, refer to the SIP Series Reference Manual.
8.2.1

STUN

Simple Traversal of UDP through NATs (STUN), based on 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 types and does not require any special behavior.
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).
On startup, the gateway sends a STUN Binding Request. The information received in the
STUN Binding Response (IP address:port) is used for SIP signaling. This information is
updated every user-defined period (NATBindingDefaultTimeout).
At the beginning of each call and if STUN is required (i.e., not an internal NAT call), the
media ports of the call are mapped. The call is delayed until the STUN Binding Response
(that includes a global IP:port) for each media (RTP, RTCP and T.38) is received.
SIP User's Manual
Figure 8-1: Nat Functioning
366
MediaPack Series
Document #: LTRT-65409