Nat; How Does Nat Work - Snom 4S NAT Filter Admin Manual

local data structure representing the user agents. It will make sure
that the connection to the user agents stays alive. It will also make
sure that requests that are destined to the user agents will be for-
warded properly. The same applies to SUBSCRIBE requests.
• The second exception is a SDP attachment. The filter checks if the
user agent needs support (or must be recorded) and will in that case
add a local contact to the SDP that can be used for media relay.
These two exceptions make sure that all user agents will work
behind NAT, no matter what NAT-type or how many NAT-levels are being
used. If user agents support ICE, they will automatically find the shortest
path to the other party (peer-to-peer).

2.2 NAT

Network Address Translation (NAT) is a reality in today's networks.
Many operators save IP addresses by providing only one IP address for a
number of devices, sometimes companies. Firewall manufacturers make
NAT a feature by performing inspection of packets that go though NAT.
Even for IPv6 networks, the fundamental problem will remain as there will
also be a need for firewalls and private networks.
The Session Initiation Protocol (SIP) has neglected this problem
in the beginning. However, in some recent RFC there have been useful
proposals how to deal with the problem. This document shows how the
snom 4S filter can be used to solve the problems.
Although snom also makes user agents, the snom 4S filter works
with most SIP user agents from other companies. The requirements on
these user agents are described below.
If you want to use the filter just for recording purposes, you don't
need to bother about NAT. The filter also works when no NAT is present.

2.2.1 How does NAT work?

NAT is essentially a translation table that maps public IP address
and ports combinations to private IP address and port combinations.
The translation table is implicitly set up when a packet is sent
from the private network to the public network. The association is kept
alive for a certain time and is refreshed every time a new packet is sent
8 • Architecture
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