Snom 4S SIP NAT gateway, 4S User Manual

snom 4S SIP
NAT gateway

User Manual

Version 0.97

privatepublic

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for use as components in systems intended for applications in­tended to support or sustain life, or for any other application in
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For more information, mail [e-mail], Pascalstr. 10E, 10587
Berlin, Germany, sip:[e-mail].

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Preface

SIP is becoming more and more accepted in the VoIP
area. Many companies are working on SIP solutions
and prepare great products that will make telephony
much easier and better. However, in many installa­tions NAT is used and SIP messages and the associ­ated RTP cannot flow through the NAT gateway with­out additional overhead. This was the reason why we
decided to add a complementary product to our SIP
phones: a SIP NAT gateway.

With our experience in VoIP technology, adding this
soft product was easy. However, we implemented only
those features that we think are most useful and sim­ple in the current VoIP environments.

Interoperability is important for us. We tried to stick
to the SIP standard as good as possible and tested
with phones of others vendors. We hope that this
helps building up a flourishing VoIP telephony where
the products of the different vendors work together
like the products in the computer industry do today.
We believe that having a choice is good for you as a
customer and therefore it is good for us.

Let’s get VoIP up and running!

Dr. Christian Stredicke
snom technology AG

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Contents

Theory of Operation.....................................................5

The NAT Problem .....................................................5

Message Flow ..........................................................6

PPPoE.....................................................................7

Domain proxy behind NAT.........................................8

Starting......................................................................8

Manual Start ...........................................................8

Automatic Start .....................................................10

Proxy Chain..............................................................10

Outbound Proxy.....................................................11

IP Gateway ...........................................................11

Quality of Service......................................................12

Versions ...............................................................13

Open Issues..........................................................13

5

Theory of Operation

The NAT Problem

When the Internet was defined, only few computers
were connected to the network. The designers used
32 bit addresses for identifying the network elements
and introduced different classes for networks. Address
areas were assigned to important institutions and
regions.

Over time, the Internet community ran our of ad­dresses. That lead to the development of “IPv6”, Ver­sion 6 of the Internet Protocol. Instead of using 32
bits, 128 bits are used for addressing — more than the
number of atoms of the planet and surely enough for
the near and far future. However, the installations in
place is mostly not able to deal with the new protocol.

That was the reason why a trick was used to increase
the number of computers that can be attached to the
Internet: Network Address Translation (NAT).

The principle is simple. A computer may have up to
65535 ports for each protocol family, usually only a
few of them are actually used. By associating ports
with computers the number of computers associated
with one Internet address can be easily multiplied.
From the Internet, it seems that there is only one
computer, however this device just dispatches the
packets to the network behind.

The principle can easily be used for firewalls. The NAT
computer checks the packets for permission to trav­erse the NAT firewall. A whole industry grew around
this important problem.

For connection oriented protocols, the NAT principle
can easily be managed by the NAT gateway itself. It
keeps an internal list of the open connections and can
forward packets accordingly.

However, voice over IP mandates connectionless pro­tocols. The voice packets need to be transported over
UDP, so that packets can be transported in real time.
For the NAT gateway, there is no way to find out