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18 • Architecture
[ S N O M 4 S N A T F I L T E R ]
a=rtpmap:0 pcmu/8000
a=rtpmap:8 pcma/8000
a=rtpmap:3 gsm/8000
a=rtpmap:18 g729/8000
a=rtpmap:2 g726-32/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=sendrecv
a=silenceSupp:off - - - -
The NAT Filter changes the private address to a globally routable
address and inserts the local port. It also inserts a hint that tells the other
user agent that it should not do silence suppression. This reduces the risk
that the connection is closed during a talk spurt of one of the parties.
2.4 Scaling and Redundancy
The NAT Filter product was designed to support distributed server
farms. That means that the servers act stateless in principle; user agents
may randomly pick a server in a server farm. This feature allows opera-
tors to set up very large and robust networks.
2.
Table 1: NAT conbimations
To/From Public IP Full Cone
NAT
Restricted
NAT
Symmetrical
NAT
Public IP Always When
STUN
support is
available
When STUN
and ICE are
available
When STUN
and ICE are
available
Full Cone NAT When
STUN is
available
When STUN
and ICE are
available
When STUN
and ICE are
available
Restricted
NAT
When STUN
and ICE are
available
When STUN
and ICE are
available
and no port
checking
Symmetrical
NAT
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