Version 6.2 617 February 2011
SIP User's Manual 9. VoIP Networking Capabilities
9 VoIP Networking Capabilities
This section provides an overview of the device's VoIP networking capabilities.
9.1 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:
First Incoming Packet Mechanism (see ''First Incoming Packet Mechanism'' on page
617)
RTP No
-Op packets according to the avt-rtp-noop draft (see ''No-Op Packets'' on page
618)
For info
rmation on SNMP NAT traversal, refer to the Product Reference Manual.
9.1.1 First Incoming Packet Mechanism
If the remote device resides behind a NAT device, it’s possible that the device can activate
the RTP/RTCP/T.38 streams to an invalid IP address / UDP port. To avoid such cases, the
device automatically compares the source address of the incoming RTP/RTCP/T.38
stream with the IP address and UDP port of the remote device. If the two are not identical,
the transmitter modifies the sending address to correspond with the address of the
incoming stream. The RTP, RTCP and T.38 can thus have independent destination IP
addresses and UDP ports.
You can disable the NAT mechanism by setting the ini file parameter DisableNAT to 1.
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