User`s manual

MediaPack SIP
MediaPack SIP User’s Manual 194 Document #: LTRT-65405
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 Robust Reception of RTP Streams
This mechanism filters out unwanted RTP streams that are sent to the same port number on the
gateway. These multiple RTP streams can result from traces of previous calls, call control errors
and deliberate attacks.
When more than one RTP stream reaches the gateway on the same port number, the gateway
accepts only one of the RTP streams and rejects the rest of the streams. The RTP stream is
selected according to the following procedure:
The first packet arriving on a newly opened channel sets the source IP address and UDP port
from which further packets are received. Thus, the source IP address and UDP port identify the
currently accepted stream. If a new packet arrives whose source IP address or UDP port are
different to the currently accepted RTP stream, there are two options:
• The new packet has a source IP address and UDP port which are the same as the remote IP
address and UDP port that were stated during the opening of the channel. In this case, the
gateway reverts to this new RTP stream.
• The new packet has any other source IP address and UDP port, in which case the packet is
dropped.
9.4 Multiple Routers Support
Multiple routers support is designed to assist the media gateway when it operates in a multiple
routers network. The gateway learns the network topology by responding to ICMP redirections
and caches them as routing rules (with expiration time).
When a set of routers operating within the same subnet serve as gateways to that network and
intercommunicate using a dynamic routing protocol (such as OSPF), the routers can determine
the shortest path to a certain destination and signal the remote host the existence of the better
route. Using multiple router support the media gateway can utilize these router messages to
change its next hop and establish the best path.
Note: Multiple Routers support is an integral feature that doesn’t require configuration.
9.5 Simple Network Time Protocol Support
Simple Network Time Protocol (SNTP) client functionality generates requests and reacts to the
resulting responses using the NTP version 3 protocol definitions (according to RFC 1305).
Through these requests and responses, the NTP client is able to synchronize the system time to
a time source within the network, thereby eliminating any potential issues should the local system
clock 'drift' during operation. By synchronizing time to a network time source, traffic handling,
maintenance, and debugging actions become simplified for the network administrator.
The NTP client follows a simple process in managing system time; the NTP client requests an
NTP update, receives an NTP response, and updates the local system clock based on a
configured NTP server within the network.
The client requests a time update from a specified NTP server at a specified update interval. In
most situations this update interval should be every 24 hours based on when the system was