User`s manual

H.323 User's Manual 9. Networking Capabilities
Version 5.0 281 December 2006
To solve these problems the following mechanisms are available:
 First Incoming Packet Mechanism (refer to Section 9.3.1 on page 281)
 RTP / T.38 No-Op packets according to the avt-rtp-noop draft (refer to Section 9.3.2
on page 281).
 For SNMP NAT traversal, refer to Section 15.10 on page 364.
9.3.1 First Incoming Packet Mechanism
If the remote gateway resides behind a NAT device, it’s possible that the Mediant 1000 can
activate the RTP/RTCP/T.38 streams to an invalid IP address / UDP port. To avoid such
cases, the Mediant 1000 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.
Users can choose to disable the NAT mechanism by setting the ini file parameter
‘DisableNAT’ to 1. The two parameters ‘EnableIpAddrTranslation’ and
‘EnableUdpPortTranslation’ enable users to specify the type of compare operation that
takes place on the first incoming packet. To compare only the IP address, set
‘EnableIpAddrTranslation = 1’ and ‘EnableUdpPortTranslation = 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.
9.3.2 RTP / T.38 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. No-Op
packets are available for sending in RTP and T.38 formats.
Users can control the activation of No-Op packets by using the ini file parameter
NoOperationSendingMode. If No-Op packet transmission is activated, users 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 NoOpInterval ini parameter.
Note: Receipt of No-Op is always supported.
 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. Users can control the payload
type with which the No-Op packets are sent. This is performed using the
RTPNoOpPayloadType ini parameter. AudioCodes’ default payload type is 120.
 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).