SIP User's Manual 368 Document #: LTRT-68806
Mediant 2000 & TP-1610 & TP-260/UNI
Notes:
• STUN only applies to UDP (doesn’t support TCP and TLS).
• STUN can’t be used when the gateway is located behind a symmetric
NAT.
• Use either the STUN server IP address (STUNServerPrimaryIP) or
domain name (STUNServerDomainName) method, with priority to the
first one.
8.3.2 First Incoming Packet Mechanism
If the remote gateway resides behind a NAT device, it’s possible that the gateway can
activate the RTP/RTCP/T.38 streams to an invalid IP address / UDP port. To avoid such
cases, the gateway automatically compares the source address of the incoming
RTP/RTCP/T.38 stream with the IP address and UDP port of the remote gateway. 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. The
two parameters EnableIpAddrTranslation and EnableUdpPortTranslation allow you to
specify the type of compare operation that occurs on the first incoming packet. To compare
only the IP address, set EnableIpAddrTranslation to 1, and EnableUdpPortTranslation to 0.
In this case, if the first incoming packet arrives with only a difference in the UDP port, the
sending addresses won’t change. If both the IP address and UDP port need to be
compared, then both parameters need to be set to 1.
8.3.3 No-Op Packets
The gateway's No-Op packet support can be used to verify Real-Time Transport Protocol
(RTP) and T.38 connectivity, and to keep NAT bindings and Firewall pinholes open. The
No-Op packets are available for sending in RTP and T.38 formats.
You can control the activation of No-Op packets by using the ini file parameter
NoOpEnable. If No-Op packet transmission is activated, you can control the time interval in
which No-Op packets are sent in the case of silence (i.e., no RTP or T.38 traffic). This is
performed using the ini file parameter NoOpInterval. For a description of the RTP No-Op ini
file parameters, refer to 'Networking Parameters' on page 273.
 RTP No-Op: The RTP No-Op support complies with IETF’s draft-wing-avt-rtp-noop-
03.txt (titled ‘A No-Op Payload Format for RTP’). This IETF document defines a No-
Op payload format for RTP. The draft defines the RTP payload type as dynamic. You
can control the payload type with which the No-Op packets are sent. This is performed
using the RTPNoOpPayloadType ini parameter (refer to 'Networking Parameters' on
page 273). AudioCodes’ default payload type is 120.
îš„ T.38 No-Op: T.38 No-Op packets are sent only while a T.38 session is activated. Sent
packets are a duplication of the previously sent frame (including duplication of the
sequence number).
Note: Receipt of No-Op packets is always supported.
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