18 • Architecture
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S N O M
4 S N A T F
I L T E R
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2.3.3 RTP Relay
When initiating a call, user agents usually include a Session
Description Protocol (SDP) attachment that describes where they expect
media. If the user agent operates on a public Internet address, there is
no need to interfere in this process. In this case the filter will just forward
the request.
Operators should encourage customers to use equipment that
operates on a public Internet address or properly allocates a globally
routable Internet address. Because media relay is an expensive operation,
it reduces the overall load on the network and at the same time increases
the quality of the service.
However, when a user agent is behind NAT, it might not be able
to receive media directly. In some cases this is because the user agent is
simply not programmed to allocate an address properly or because it is
behind symmetrical NAT, which makes it impossible to properly allocate
this address. In this case, the help of the media filter will make sure that
media will always be delivered properly.
The media filter supports the “interactive connectivity
establishment” (ICE) method that has been published recently in the IETF.
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
Needs filter
