Probing Media Paths; The Role Of The Nat Filter - Snom 4S NAT Filter Admin Manual

2.2.6 Probing Media Paths

ICE is a method that has been proposed recently in the IETF [4].
The algorithm is simple: A user agent that supports ICE lists the possible
addresses where it could possibly be reached. These addresses may
include the private address, an address allocated via STUN, one or more
addresses allocated with the TURN protocol or an address allocated with
UPnP. Because practically it is hard to predict which of these addresses are
visible to the other user agent, all of the possible addresses are proposed
to the other user agent.
The other user agent sends test packets to the possible addresses.
Picking the first reply on the test packet will establish a working media
path and it will also probably be the fastest connection. STUN is being
used for these test packets.

2.2.7 The Role of the NAT Filter

When a user agent is not able to allocate a globally routable
address or it is not sure if it found enough possible addresses, the NAT
Filter can help out.
Again, the way the NAT Filter works, is simple. For the signalling,
the NAT Filter keeps the NAT alive with bogus messages (which can be SIP
messages or other non-SIP message). It patches the messages in such a
way, that other user agents will address the NAT Filter instead of the user
agent when they want to deliver a message. The NAT Filter then forwards
the message to the user agent using the connection which is kept open
with the keep-alive messages.
When the NAT Filter sees a message that contains information
about sending media (session description protocol, SDP), it opens a local
globally routable port on behalf of the user agent and patches these
messages in a way that the destination will send media via this port.
The NAT Filter will relay the media to the user agent like it relays SIP
messages. Using symmetrical RTP, it can detect the user agent's public
media identity and reroute the packets to this destination.
While this approach has the huge advantage that it does work
will all kinds of NAT, it has the disadvantage that it increases the media
path significantly. For example, when a user A in Tokyo is registered
with a operator in New York and wants to call his colleague B (which is
registered to a service provider in Sydney and who is sitting in the same
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