Nat (Network Address Translation) Support - AudioCodes Mediant 1000 User Manual

When the CPU module loses all Ethernet connectivity, a Critical alarm is generated:
When MIIRedundancyEnable is disabled: the alarm is generated when the single
physical connection is lost. The alarm is cleared when the single physical connection
is restored.
When MIIRedundancyEnable is enabled: the alarm is generated when both physical
connections are lost. The alarm is cleared when one or both of the physical
connections are restored.
8.3

NAT (Network Address Translation) Support

Network Address Translation (NAT) is a mechanism that maps a set of internal IP
addresses used within a private network to global IP addresses, providing transparent
routing to end hosts. The primary advantages of NAT include (1) Reduction in the number
of global IP addresses required in a private network (global IP addresses are only used to
connect to the Internet); (2) Better network security by hiding its internal architecture.
The following figure illustrates the device'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 device (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 device must notify the receiving device
where to send the media.
To resolve these issues, the following mechanisms are available:
STUN (refer to ''STUN'' on page 447)
First Incoming Packet Mechanism (refer to ''First Incoming Packet Mechanism'' on
page 448)
RTP No-Op packets according to the avt-rtp-noop draft (refer to ''No-Op Packets'' on
page 448)
For information on SNMP NAT traversal, refer to the Product Reference Manual.
SIP User's Manual 446
Mediant 1000 & Mediant 600
Document #: LTRT-83303