Snom 4S NAT Filter User manual

snom 4S NAT Filter
Admin Manual
snom 4S
NAT Filter
Version 2.11

snom 4S NAT Filter Version 2.11
© 2004-2005 snom technology Aktiengesellschaft. All Rights Reserved.
This document is supplied by snom technology AG for information purposes only to licensed
users of the snom 4S NAT filter and is supplied on an “AS IS” basis, that is, without any
warranties whatsoever, express or implied.
Information in this document is subject to change without notice and does not represent any
commitment on the part of snom technology AG. The software described in this document
is furnished under a license agreement and may be used only in accordance with the terms
of that license agreement. It is against the law to copy or use this software except as
specifically allowed in the license. No part of this document may be reproduced, republished
or retransmitted in any form or by any means whatsoever, whether electronically or
mechanically, including, but not limited to, by way of photocopying, recording, information
recording or through retrieval systems, without the express written permission of snom
technology AG.
snom® is a registered trademark in the United States of America, Germany and some other
contries.

snom technology AG • 3
Table of Contents
1 Overview ..........................................................5
1.1 Applications ...................................................................... 6
1.2 Features ........................................................................... 6
2 Architecture .....................................................9
2.1 The NAT Filter and SIP ........................................................ 9
2.2 NAT ............................................................................... 10
2.2.1 How does NAT work? ..............................................................................................................................................11
2.2.2 Symmetrical RTP ............................................................................................................................................................11
2.2.3 Signalling SIP .......................................................................................................................................................................11
2.2.4 Media RTP ...................................................................................................................................................................................12
2.2.5 Classification of User Agents ....................................................................................................................12
2.2.6 Probing Media Paths .................................................................................................................................................13
2.2.7 The Role of the NAT Filter ..............................................................................................................................13
2.2.8 Optimizing the Media Path for Symmetrical NAT ..................................................14
2.3 SBC Behaviour ................................................................ 15
2.3.1 Registering ................................................................................................................................................................................15
2.3.3 RTP Relay ....................................................................................................................................................................................16
2.4 Scaling and Redundancy ................................................... 18
2.5 Detecting the right NAT Filter ............................................ 19
2.6 Requirements on User Agents ............................................ 20
2.6.1 Non NAT-Aware User Agents .....................................................................................................................20
2.6.2 STUN/ICE-Aware User Agents ................................................................................................................20
2.7 Defining the Maximum Session Time .................................. 21
3 Installation .....................................................23
3.1 Windows ......................................................................... 23
3.2 Linux ............................................................................. 28
4 Configuration ..................................................31
4.1 Logging In ...................................................................... 31
4.2 Port Binding .................................................................... 31
4.3 System Settings .............................................................. 33
4.3.1 Logging ...........................................................................................................................................................................................33
4.3.2 Preparing Recovery ...................................................................................................................................................34
4.3.3 General Outound Proxy ......................................................................................................................................34

4 • Contents
[ S N O M 4S NAT FI L T E R ]
4.3.4 Media Ports ..............................................................................................................................................................................35
4.3.5 Port Budgets ........................................................................................................................................................................35
4.3.6 Media Relay .............................................................................................................................................................................35
4.3.7 Controlling Routing ....................................................................................................................................................35
4.3.8 Multiple 2xx Handling ............................................................................................................................................36
4.3.9 Challenging ..............................................................................................................................................................................37
4.3.10 Trusted Addresses .......................................................................................................................................................37
4.3.11 Maximum Packet Size ...........................................................................................................................................37
4.3.12 Silence Suppression .................................................................................................................................................38
4.3.13 Connection Oriented Media .........................................................................................................................38
4.3.14 Removing Headers ......................................................................................................................................................38
4.3.15 Codec Control ......................................................................................................................................................................39
4.3.16 Web Server Integration ......................................................................................................................................39
4.3.17 CLIR Addresses .................................................................................................................................................................39
4.4 Timeout Settings ............................................................. 40
4.4.1 Register Timeouts ........................................................................................................................................................40
4.4.2 Call Timeouts ........................................................................................................................................................................41
4.5 Security Settings ............................................................. 42
4.6 Outbound Proxy List ......................................................... 44
4.7 System Information ......................................................... 45
4.8 Server Log ...................................................................... 45
4.9 Trace .............................................................................. 46
4.10 Call History ..................................................................... 47
4.11 Current Ports .................................................................. 48
4.12 Currently Handled UA ....................................................... 49
4.13 Memory Statistics ............................................................ 49
5. Web Server Integration ..................................51
5.1 Interface to the Web Server .............................................. 52
5.2 Authentication ................................................................. 52
5.3 Registration .................................................................... 55
5.4 Call Initiation .................................................................. 56
5.5 Call Termination .............................................................. 58
6. SNMP ..............................................................61
6.1 Setup of the SBC ............................................................. 61
6.2 Setup of the Tools ............................................................ 61
6.3. Available OID .................................................................. 62
7 Checklist for Installation ................................65
6.1 Linux ............................................................................. 65
6.2 Windows ......................................................................... 65

snom technology AG • 5
1 Overview
Network address translation (NAT) is a reality today. There have
been many discussions about the evil and the good of this network topol-
ogy and the replacement by IP version 6. However, operators and busi-
ness want to offer VoIP services today and therefore must address the
problem.
The snom 4S NAT Filter is a SIP session border controller (SBC).
It enables non-NAT aware devices to operate in private networks. It also
allows operating the data center in a private network. It takes care about
translation of SIP messages with private network identities into identities
that can be addressed from the data center. When necessary or explicitly
required, it forwards media and changes the SDP attachments in the SIP
messages accordingly.
The SBC offers recording capabilities (depending on the licens-
ing). Through a separate management interface, operators can define
numbers and patterns that are silently recorded. Users may explicitly re-
quest the recording of a call by pressing a key on the phone; in this case
the whole call will be recorded (even parts of the call before pressing the
key). The filter records in a compressed format where only the voice part
of the conversation is recorded in highly-compressed audio format (13.2
kBit/s). By using an appropriately configured web server, operators can
manage very large numbers of calls and, for example, forward them per
e-mail to users or authorities.
The SBC does not translate signaling, e.g., from SIP to H.323 or
to MGCP. It is a semi-transparent SIP proxy that takes care only about
known methods and leaves the rest unchanged. If all user agents are
fully NAT-compliant or on public Internet, the filter can transparently be
removed from the network without changing the call flows or functional-
ity. Also, the filter does not interfere with unknown applications. This is
a advantage over back-to-back user agents (B2BUA) that operate on the
application level.
1.

6 • Overview
[ S N O M 4S NAT FI L T E R ]
1.1 Applications
The filter can be used in the following scenarios:
• Corporations. Corporations which operate their infrastructure be-
hind NAT and/or firewalls can talk to the public Internet through the
filter.
• Operators. Operators offer the NAT traversal feature to their cus-
tomers. Using the scalability feature of the filter, the operation of
large networks becomes possible.
• Record specific calls for legal purposes. In many countries, opera-
tors must provide the possibility to record certain calls on request.
The filter can perform this task.
• Recording can be used for legal proofing (brokers, etc). The filter is
fully compliant with other SIP equipment and can, for example, put
between a PSTN gateway and SIP phones.
1.2 Features
The filter offers powerful features based on modern VoIP technol-
ogy:
• The built-in RFC3261-compliant SIP proxy makes additional exter-
nal SIP logic superfluous and simplifies the system setup.
• A built-in RFC3489-compatible STUN server for single IP addresses
allows clients to self-refresh their bindings
• Support for instant messaging, presence and all other SIP-compli-
ant applications.
• Rich logging features allow easy maintenance.
• Recording functions based on number lists and expressions offer a
flexible way of filtering out information.
• Recordings can be saved in WAV file format (the data rate is 6 MB
per hour).
• Almost stateless operation allows the filter to be used in server
farms. This offers a tremendous scalability and redundancy making
the product suitable for large operators.
1.

snom technology AG • 7
[ S N O M 4S NAT FI L T E R ]
• Both http and https as web interface for simple access from any-
where on the Internet.
• The filter supports Interactive Connectivity Establishment (ICE).
User agents that support this feature will optimize the media path
for the shortest possible delay.
• Media relay is established using connection-oriented media. User-
agents that are not NAT-aware inherently support this feature. This
makes the operation of the NAT filter backward-compatible.
• Call-alive polling. During calls, the filter checks if the call is still alive
and terminates the call if this should not be the case. With this fea-
ture, charging users for broken calls can be avoided.
• Reliable and unreliable transport layers. The filter supports both UDP
and TCP transport layers. Full TLS support will be added soon.
• To and From headers may be changed for calls. The filter talks to a
web application server to get this information.
• Application Server Integration. The web application server can also
change the request-URI. This makes simple routing possible, which
can be used for least cost routing, for example.
• Call Duration Limit. The web application server can define the du-
ration of the call. This makes it possible to implement prepaid ser-
vices.
• HTTP Registrar. The SBC may convert SIP register requests into
HTTP requests and send the response back via SIP. This makes it
possible to use the web server as registrar.
• Authentication. The SBC may authenticate incoming requests. It
keeps a authentication cache updated by the web server. Trust rela-
tionships defined by IP address define exceptions.
• CLIR support. When a request leaves the data center, the SBC can
change the From-header of the outgoing request and make it anon-
ymous. This way, the caller ID can be hidden.
Usually, the filter acts as stateless proxy. That means, by default
it just forwards the packets and does not change the content of the at-
tachments or the headers themselves. That means that the filter will not
interfere with applications (instant messaging, presence, weather report,
etc).
There are three exceptions to this rule:
1.

8 • Overview
[ S N O M 4S NAT FI L T E R ]
• The first exception is a REGISTER request. When a user agent tries
to register and needs the support of the filter, the filter will set up a
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 destined to the user agents will be forwarded
properly.
• The second exception is an 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.
• The third exception occurs when the filter queries a web server for
routing information. In this case, it will send a provisional response
to stop the UAC from repeating messages.
These three 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
media path to the other party (peer-to-peer).
1.

snom technology AG • 9
2 Architecture
2.1 The NAT Filter and SIP
In the SIP architecture, the SBC acts as the first proxy that is
contacted by user agents. There are two ways to make sure that the rel-
evant traffic gets routed trough the filter:
• User agents can be set up to use the filter as outbound proxy. When
using this method, all SIP traffic will flow through the SBC, whether
it is destined to the operator or not. That means that service for calls
outside of the operator’s domain may also be serviced by the SBC.
However, by redirecting all outgoing traffic of the SBC to a proxy
the operator can make sure that the authentication, authorization
and accounting (AAA) requirements for requiring the service are
fulfilled. Alternatively, you can use the application server interface
to do the job on the SBC itself.
• User agents resolve the SBC though the RFC3263 DNS resolving
process. That means that only the traffic that is destined to the
operator’s domain will use the service of the NAT Filter. However,
users might be annoyed if they place a call to a domain that does
not properly support NAT services. In this case, the SBC can also
redirect the traffic to another proxy.
We recommend using the first alternative and to only choose the
second alternative if it is too difficult to provision user agents with the
outbound proxy or when there are concerns about providing service for
foreign operators.
Usually, the SBC acts as stateless proxy. This means, that it just
forwards the packets by default and that it does not change the content
of the attachments or the headers themselves. The SBC will not interfere
with applications (instant messaging, presence, weather report, etc).
There are three exceptions to this rule:
• The first exception is a REGISTER request. When a user agent tries
2.

10 • Architecture
[ S N O M 4S NAT FI L T E R ]
to register and needs the support of the SBC, the SBC will set up a
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 destined to the user agents will be forwarded
properly.
• The second exception is an SDP attachment. The SBC checks if the
user agent needs support (or must be recorded) and, in that case,
will add a local contact to the SDP that can be used for media re-
lay.
• The third exception occurs when the SBC queries a web server for
routing information. In this case, it will send a provisional response
to stop the UAC from repeating messages.
These three 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
media 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 on how to deal with the problem. This document shows how the
snom 4S NAT Filter can be used to solve the problems.
Although snom also makes user agents, the snom 4S NAT Filter
works with most SIP user agents from other companies. The require-
ments on these user agents are described below.
If you want to use the SBC just for recording purposes, you don’t
need to bother about NAT. The SBC also works when no NAT is present.
2.

snom technology AG • 11
[ S N O M 4S NAT FI L T E R ]
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
from the same origin. This fact is used by STUN (RFC3489) to set up an
association between a public IP address and a private IP address.
In symmetrical NAT, the router stores the address where the
packet was sent. Only packets coming from this address are forwarded to
the private address. This algorithm increases the security as it is harder
to guess the source IP and port for attackers. Full cone NAT does not per-
form this check.
There are some mixed variants between full cone NAT and sym-
metrical NAT. Restricted port NAT works similar to symmetrical NAT, but
uses only one port association.
Hairpinning is the ability of the NAT to route packets coming from
the private network and addressed towards a public IP address binding
back to the private network. Not all routers support this feature.
2.2.2 Symmetrical RTP
Real time protocol (RTP) is used to transport media. Symmetrical
RTP is a trick to extend the number of cases when communication can
be established. A SIP user agent supporting symmetrical RTP waits for
the first RTP packet coming in and then sends its media stream back to
the IP address from which it received that packet. Symmetrical RTP al-
ways works when the user agent doing symmetrical RTP is on a globally
routable address. However, this algorithm can easily be cheated (port
spraying) and therefore implies a certain security risk.
2.2.3 Signalling SIP
SIP traffic is relatively unproblematic because SIP typically is not
as time critical as media. Usually, it is ok to route SIP packets through a
longer path than media.
2.

12 • Architecture
[ S N O M 4S NAT FI L T E R ]
In SIP it is legal to send from a different port than the receiving
port. When this is being done, there is no way of supporting these de-
vices behind NAT. However, some phones offer an option that disables this
mechanism so that the sending port is the same as the receiving port.
Typically, the SIP proxy will run on a public IP address where it
is possible to deal with all kinds of NAT. Keep-Alive messages may keep
the NAT binding open (for example, short registration periods or non-SIP
messages).
2.2.4 Media RTP
Media is much more problematic than SIP because users are sen-
sitive 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 SBC 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 to allocate a public IP address. Often, they are “legacy”
2.

snom technology AG • 13
[ S N O M 4S NAT FI L T E R ]
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.
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 in-
clude the private address, an address allocated via STUN, one or more
addresses allocated with the TURN protocol or an address allocated with
UPnP. Because in practice it is hard to predict which of these addresses
are visible to the other user agent, all of the possible addresses are pro-
posed to the other user agent.
The other user agent sends test packets to the possible address-
es. 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 ad-
dress 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.
2.

14 • Architecture
[ S N O M 4S NAT FI L T E R ]
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 mes-
sages 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 iden-
tity and reroute the packets to this destination.
While this approach has the huge advantage that it works with
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 an
operator in New York and wants to call his colleague B (who is registered
with a service provider in Sydney and sitting in the same office in a private
network), the media would have to flow first from Tokyo to New York, then
to Sydney and then back to Tokyo. The delay would at least be around one
second even though the user agents are located in the same network.
Unfortunately, it is not trivial to make the media path shorter.
There have been some attempts to reduce the problem, but it is much
easier to address the problem starting at the user agent. If the user agent
uses ICE, it will try all addresses listed in the SDP attachment, including
the port allocated by the NAT Filter. If there is a shorter path, it will switch
to this shorter path. If there is no other way or if the other side does not
support ICE, it will fall back to the NAT Filter-allocated port which will work
in all cases.
2.2.8 Optimizing the Media Path for Symmetrical
NAT
In the cases where both user agents are behind symmetrical NAT,
the NAT Filter approach will ensure that media will flow between the user
agents. However, the Tokyo example shows that this might result in intol-
erable media delay.
To address this problem, TURN [5] comes into play. The idea be-
hind this approach is to allocate identities on several places in the Internet
and to propose all of the allocated ports to the other user agent. If the
ports are allocated on all continents, the other user agent will automati-
cally pick the TURN server with the shortest delay. In the Tokyo example,
a TURN server located in Japan will reduce the delay to a tolerable level
(even when there is no direct path between the user agents).
2.

snom technology AG • 15
[ S N O M 4S NAT FI L T E R ]
2.3 SBC Behaviour
2.3.1 Registering
When a user agent registers, it puts its IP address in the top
Via. If the user agent is on public Internet or properly supports NAT, this
Via will match the perceived IP address. In this case the SBC does not
interfere with the registering process and just forwards this packet to the
registrar.
If the top Via does not contain the perceived address, the SBC
will take care of the request. It will replace the provided contact with a
locally generated contact and forward the request to the registrar (see
below).
REGISTER sip:snomag.de SIP/2.0
Via: SIP/2.0/UDP 203.145.183.113:12975;branch=z9hG4bK-
abx3au3mxb01;rport
From: <sip:[email protected]>;tag=k9p6fmeg7h
To: <sip:[email protected]>
Call-ID: 3c26701d7cb9-pady07b5783t@203-145-183-113
CSeq: 14 REGISTER
Max-Forwards: 70
Contact: <sip:[email protected]:12975;line=lhynyb3y>;q=1.0
Supported: gruu
Expires: 86400
Content-Length: 0
REGISTER sip:snomag.de SIP/2.0
Via: SIP/2.0/UDP 217.115.141.99:5082;branch=z9hG4bK-e8d1feb8138c3d85
0637ced821ef40a3;ua=c9b140ab598290e5bb491e9c3aaca440
Via: SIP/2.0/UDP 203.145.183.113:12975;branch=z9hG4bK-
abx3au3mxb01;rport=17401
From: <sip:[email protected]>;tag=k9p6fmeg7h
To: <sip:[email protected]>
Call-ID: 3c26701d7cb9-pady07b5783t@203-145-183-113
CSeq: 14 REGISTER
Max-Forwards: 69
Contact: <sip:217.115.141.99:5082;ua=c9b140ab59829bb491e9c3aaca440>
Supported: gruu
Expires: 86400
Content-Length: 0
SIP/2.0 200 Ok
Via: SIP/2.0/UDP 217.115.141.99:5082;branch=z9hG4bK-e8d1feb8138c3d85
2.

16 • Architecture
[ S N O M 4S NAT FI L T E R ]
0637ced821ef40a3;ua=c9b140ab598290e5bb491e9c3aaca440
Via: SIP/2.0/UDP 203.145.183.113:12975;branch=z9hG4bK-
abx3au3mxb01;rport=17401
From: <sip:[email protected]>;tag=k9p6fmeg7h
To: <sip:[email protected]>;tag=epuy85kzm5
Call-ID: 3c26701d7cb9-pady07b5783t@203-145-183-113
CSeq: 14 REGISTER
Contact: <sip:217.115.141.99:5082;ua=c9b140ab598290e5bb491e9c3aaca44
0>;expires=3600;gruu=”sip:[email protected];gruu=hobiv52b”
Date: Wed, 26 May 2004 16:03:33 GMT
Content-Length: 0
SIP/2.0 200 Ok
Via: SIP/2.0/UDP 203.145.183.113:12975;branch=z9hG4bK-
abx3au3mxb01;rport=17401
From: “denny” <sip:[email protected]>;tag=k9p6fmeg7h
To: “denny” <sip:[email protected]>;tag=epuy85kzm5
Call-ID: 3c26701d7cb9-pady07b5783t@203-145-183-113
CSeq: 14 REGISTER
Contact: <sip:[email protected]:12975;line=lhynyb3y>;expires=360
0;gruu=”sip:[email protected];gruu=hobiv52b”
Date: Wed, 26 May 2004 16:03:33 GMT
Content-Length: 0
The SBC will make the registration short enough to keep the con-
nection alive. The UA will reregister shortly after. However, because the
registration binding time in the registrar is longer, the SBC will not forward
the request to the registrar and answer it locally.
2.3.3 RTP Relay
When initiating a call, user agents usually include a Session De-
scription 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 SBC 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 opera-
tion, it reduces the overall load on the network and at the same time in-
creases 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
2.

snom technology AG • 17
[ S N O M 4S NAT FI L T E R ]
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 SBC will make sure that
media will always be delivered properly.
The media filter supports the “interactive connectivity establish-
ment” (ICE) method that has been published recently in the IETF. Using
this method, user agents may probe several addresses and decide which
address they use for communication. In this case, the SBC will just add
another contact to the ICE list.
Table 1 shows the cases when the SBC needs to interfere if STUN
and ICE support are available from the user agents. The support of the
SBC is necessary only in cases when both sides have symmetrical NAT
and in the case when talking from symmetrical NAT to restricted NAT. If
the user agents don’t support STUN and ICE, the number of cases goes
up significantly.
If the user agent operates without NAT support, it will send a SDP
like the one below:n
v=0
o=root 19387 19387 IN IP4 192.168.1.10
s=call
c=IN IP4 192.168.1.10
t=0 0
m=audio 58146 RTP/AVP 0 8 3 18 2 101
a=rtpmap:0 pcmu/8000
a=rtpmap:8 pcma/8000
a=rtpmap:3 gsm/8000
a=rtpmap:18 g729/8000
a=rtpmap:2 g726-32/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=sendrecv
The NAT Filter will detect that the user agents needs help and
allocates local ports for relaying media. It will forward the request with
changed SDP:
v=0
o=root 19387 19387 IN IP4 217.115.141.99
s=call
c=IN IP4 217.115.141.99
t=0 0
m=audio 49170 RTP/AVP 0 8 3 18 2 101
2.

18 • Architecture
[ S N O M 4S NAT FI L T E R ]
a=rtpmap:0 pcmu/8000
a=rtpmap:8 pcma/8000
a=rtpmap:3 gsm/8000
a=rtpmap:18 g729/8000
a=rtpmap:2 g726-32/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=sendrecv
a=silenceSupp:off - - - -
The NAT Filter changes the private address to a globally routable
address and inserts the local port. It also inserts a hint that tells the other
user agent that it should not do silence suppression. This reduces the risk
that the connection is closed during a talk spurt of one of the parties.
2.4 Scaling and Redundancy
The NAT Filter product was designed to support distributed server
farms. That means that the servers act stateless in principle; user agents
may randomly pick a server in a server farm. This feature allows opera-
tors to set up very large and robust networks.
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

snom technology AG • 19
[ S N O M 4S NAT FI L T E R ]
The distribution of user agents to a server is performed using
DNS SRV (RFC 2782). This means that you need to list the available serv-
ers on DNS level; the user agents must perform DNS SRV look ups and
pick one of the servers (possible using the detection algorithms described
below).
The following table shows an example configuration for Linux
named(8):
_sip._udp IN SRV 3 5 5082 frankfurt1
_sip._udp IN SRV 3 5 5082 newyork1
_sip._udp IN SRV 3 5 5082 newyork2
_sip._udp IN SRV 3 5 5082 newyork3
_sip._udp IN SRV 3 5 5082 tokyo2
_sip._udp IN SRV 3 5 5082 tokyo1
If one of the servers should become unavailable, the SRV algo-
rithm will make sure that the other servers will be contacted. The user
agents that are refreshed by that particular server will become unreach-
able until the user agents initiate a new REGISTER request. Therefore,
you should make sure that your servers have a high uptime probability
and that the registration period is not too long. We think that registration
periods of thirty minutes up to one hour are a good balance between ser-
vice failure time and performance.
2.5 Detecting the right NAT Filter
User agents must detect which server in the server farm is near-
est to the user agent. This is an important feature for a company or
operator that has user agents scattered around the globe. Example: A
company has offices in Berlin, Tokyo and Dallas and locally operates NAT
Filter servers. When a user agent is located in Tokyo, it should use the
Tokyo server. This could be set up manually; howeve, it is also possible to
automatically pick the best server.
To detect the nearest server, the user agent sends STUN packets
to all possible servers (the servers with the lowest priority in the SRV list).
The user agent picks the server that responds first. Alternatively, the user
agent could send more test packets and take the mean response time for
making the decision.
2.

20 • Architecture
[ S N O M 4S NAT FI L T E R ]
The snom 4S NAT Filter includes a STUN server that operates on
the SIP UDP port. User agents should send their test packets to the SIP
port.
2.6 Requirements on User Agents
Generally, there are two categories of user agents: The non NAT
aware user agents and the STUN/ICE capable user agents.
2.6.1 Non NAT-Aware User Agents
Non-NAT aware user agents must have at least the following fea-
tures:
1. Must send SIP UDP packets from the port where they receive SIP
traffic
2. Must start sending media immediately after receiving SDP
Requirement 1 is not fulfilled by the default configuration of the
Cisco 7960; however there is a setting that enables this feature. Most
known user agents support this feature, however.
Requirement 2 sometimes creates problems with user agents
who don’t send media during silent periods. In this case, users have to
start speaking before two-way audio can be established.
In any case, customers are asked to contact their vendor in case
of problems and explanations. In general, snom recommends using NAT-
aware user agents to reduce the network and support overhead.
2.6.2 STUN/ICE-Aware User Agents
STUN/ICE-Aware User Agents must implement the two IETF stan-
dards. It is ok if the user agents use the built-in STUN server for refresh-
ing the bindings and learning the public IP address.
snom phones starting with version 2.05a fall into this category.
2.