User's Manual
146
Document #: LTRT-10375
Mediant 500 MSBR
15.2.2.2.2 No-Op Packets
The device can send No-Op packets to verify Real-Time Transport Protocol (RTP) and
T.38 connectivity, and to keep NAT bindings and Firewall pinholes open. The No-Op
packets can be sent in RTP and T.38 formats:
RTP No-Op:
The RTP No-Op support complies with IETF Internet-Draft draft-wing-
avt-rtp-noop-03 ("A No-Op Payload Format for RTP"). The IETF document defines a
No-Op payload format for RTP. The draft defines the RTP payload type as dynamic.
You can configure the payload type as described in the following procedure (default 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).
To configure the No-Op packet feature:
1.
Enable the feature, using the NoOpEnable
ini
file parameter.
2.
Configure the time interval during which the device sends No-Op packets when
silence occurs (i.e., no RTP or T.38 traffic), using the NoOpInterval
ini
file parameter.
3.
For RTP No-Op packets, configure the payload type of the No-Op packets, using the
RTPNoOpPayloadType
ini
file parameter.
Note:
•
The No-OP Packet feature requires DSP resources.
•
Receipt of No-Op packets is always supported.
15.2.2.2.3 Fax Transmission behind NAT
The device supports transmission from fax machines (connected to the device) located
inside (behind) NAT. Generally, the firewall blocks T.38 (and other) packets received from
the WAN, unless the device behind the NAT sends at least one IP packet from the LAN to
the WAN through the firewall. If the firewall blocks T.38 packets sent from the termination
IP fax, the fax fails.
To overcome this, the device sends No-Op (“no-signal”) packets to open a pinhole in the
NAT for the answering fax machine. The originating fax does not wait for an answer, but
immediately starts sending T.38 packets to the terminating fax machine upon receipt of a
re-INVITE with T.38 only in the SDP, or T.38 and audio media in the SDP. This feature is
configured using the T38FaxSessionImmediateStart parameter. The No-Op packets are
enabled using the NoOpEnable and NoOpInterval parameters.
15.3 Robust Receipt of Media Streams by Media Latching
The Robust Media mechanism (or media latching) is an AudioCodes proprietary
mechanism to filter out unwanted media (RTP, RTCP, SRTP, SRTCP, and T.38) streams
that are sent to the same port number of the device. Media ports may receive additional
multiple unwanted media streams (from multiple sources of traffic) as result of traces of
previous calls, call control errors, or deliberate malicious attacks (e.g., Denial of Service).
When the device receives more than one media stream on the same port, the Robust
Media mechanism detects the valid media stream and ignores the rest. Thus, this can
prevent an established call been stolen by a malicious attacker on the media stream.
For the involved voice channel, the device latches on to the first stream of the first received
packet. All packets (of any media type) received from the same IP address and SSRC are
accepted (for T.38 packets, the device considers only the IP address). If the channel
Summary of Contents for Mediant 500 MSBR
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Page 115: ...Part III General System Settings ...
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Page 137: ...Part IV General VoIP Configuration ...
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Page 455: ...Part V Gateway Application ...
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Page 757: ...Part IX Maintenance ...
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Page 837: ...Part X Status Performance Monitoring and Reporting ...
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Page 927: ...Part XI Diagnostics ...
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Page 977: ...Part XII Appendix ...
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