Media Rtp; Classification Of User Agents - Snom 4S NAT Filter Admin Manual

2.2.4 Media RTP

Media is much more problematic than SIP because users are
sensitive to delay in a voice conversation. When the delay is too long, the
speakers need to be disciplined not to interrupt the other person when
starting to speak. Also, the ear is much more sensitive to echo when the
media delay becomes too long. The effect is known from intercontinental
calls where the speed of light increases the delay for voice transmission.
SIP was designed for peer-to-peer communication. That means
the user agents (telephones) send the media directly to the other user
agent. This approach is the best way to minimize the delay; however it
becomes a problem when NAT is involved.

2.2.5 Classification of User Agents

From a filter point of view, available user agents can be classified
into the following categories:
•
Public IP devices. These devices operate on public IP addresses and
don't need any specific support regarding NAT. The true location of
these devices may be in a private network, as they might have al-
located a public identity using mechanisms like UPnP™ [3]. These
devices are most welcome as they don't cause any additional re-
quirements.
• STUN devices. Phones that operate behind full cone NAT and allo-
cate public IP addresses themselves fall into this category. The only
support that the proxy needs to give is a STUN server. Apart from
that they act like public IP devices.
• Non NAT-aware devices. These devices don't attempt to check the
NAT type or allocate a public IP address. Often, they are "legacy"
devices that have been designed without having NAT in mind. These
devices can register only for a short period of time, so that the REG-
ISTER messages keep the port association open (the SIP messages
are used to keep the port association). Also, these devices need a
NAT-aware media server or other device that forward the RTP pack-
ets of these devices.
• Symmetrical NAT devices. These devices may be NAT-aware; how-
ever, because they operate behind symmetrical NAT, there is little
that they can do. They essentially behave like non NAT-aware SIP
devices and hope for the support of the proxy.
10 • Architecture
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