Snom 4S Media Server Use and care manual

Administrator‘s
Manual
snom 4S
SIP Proxy/Registrar
Version 2.21

snom technology AG • 3
snom 4S Registrar Proxy Version 2.21 User Manual
© 2002 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 registrar proxy 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
specically 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 technology AG • 3
Welcome to the
SIP World!
SIP is becoming more and more accepted in the area of VoIP.
Many companies are working on SIP solutions and making great products
that will make telephony much easier and better.
SIP without a proxy makes no sense. Entering IP addresses to nd
another user cannot be the future of telephony. Locating http web content
and nding email users has become a part of todays communication
behavior and sure will be part of telephony in the near future. However,
for the small and medium size ofce, we are missing a simple yet powerful
solution that is available for a standard-software price. That is why we are
making this simple and straightforward proxy available.
snom 4S stands for “snom soft switch for small and medium
enterprises“. That means, the snom 4S proxy was designed for
environments handling up to 1000 users with normal trafc. In
environments where you need more features and better scalability, we
would be happy to refer you to other companies offering carrier grade and
feature-rich proxies that solve these problems.
This product is a proxy/locations server with built-in registrar
which means this software is responsible for locating users. Features
like follow me and group calling are therefore supported; however media
services like mailbox and music on hold are not part of the software. You
should use a media server for this.
Interoperability is important to us. We have tried to stick to the
SIP standard as well as possible and tested the phones of other vendors.
We hope that this will help to build up a ourishing VoIP telephone
industry in which 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 and therefore good for us.
This manual gives you a brief introduction to VoIP and SIP,
explains the installation process for Windows and Linux and shows how

4 • Welcome
[ S N O M 4S PR O X Y / R E G I S T R A R M ANUAL ]
snom technology AG • 5
to run the SIP proxy. For additional snom 4S information, please visit our
Web site at http://www.snomag.de and if you have any comments and
suggestions about snom 4S, please contact us through snom technology
AG’s support link Web site. We would appreciate your feedback.
And, of course, you can reach us at sip:[email protected].
Thank you and have fun using the snom 4S!
Dr. Christian Stredicke Nicolas Peter-Pohland

4 • Welcome
[ S N O M 4S PR O X Y / R E G I S T R A R M ANUAL ]
snom technology AG • 5
Table of Contents
Welcome to the SIP World! ........................................3
The Voice over Internet Protocol................................7
Why SIP?................................................................................... 7
Open Standards ......................................................................... 8
What You Can Expect and What Not .............................................. 9
The SIP Architecture ................................................11
User Agents ............................................................................. 11
Proxies.................................................................................... 12
Location Server ........................................................................ 12
Registrars ................................................................................ 12
Media Server............................................................................ 13
Gateways ................................................................................ 13
snom 4S Framework.................................................15
Proxy ...................................................................................... 15
SIP NAT Gateway...................................................................... 16
Media Server............................................................................ 17
Installation ..............................................................19
Windows Installation ................................................................. 19
Uninstalling in Windows ............................................................. 23
Linux Installation ...................................................................... 24
General Concepts .....................................................27
Security ................................................................................. 27
Reliabilty ................................................................................. 28
State ...................................................................................... 29
Overlap Dialling........................................................................ 30
Sequential Forking .................................................................... 30
Network Address Translation ...................................................... 31
Routing ................................................................................... 32

6 • Contents
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Conguration ...........................................................33
Licensing ................................................................................. 33
General Settings....................................................................... 35
Registration ............................................................................. 38
Routing ................................................................................... 40
Domain Administration .............................................................. 43
User Administration .................................................................. 44
Dial Plan.................................................................................. 45
Error-Information ..................................................................... 49
Welcome Message..................................................................... 52
DNS........................................................................................ 53
Server Farm Operation .............................................................. 54
Registering Phones ..................................................57
snom 100 Registration............................................................... 57
Microsoft Messenger Registration ................................................ 58
Maintenance.............................................................61
Registered Users ...................................................................... 61
Call Logs ................................................................................. 62
SIP Message Flow ..................................................................... 65
Logging ................................................................................... 67
More Information .....................................................69
Open Issues............................................................................. 69
Standards................................................................................ 69
Other useful information ............................................................ 70
Footnotes ................................................................................ 70
Index .......................................................................71

6 • Contents
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The Voice over
Internet Protocol
Today there is a wide choice of different switched network
products. Telephones have now been being built for more than a century,
and their technology is well-understood and proven. Why choose a
different technology?
Modern communication infrastructures transport much more than
just one application: email, http, les, instant messages, videos, music,
so it is only natural to include voice in the list of applications and use one
infrastructure for all of them.. Voice is a real time application. Sending
voice over the Internet Protocol is called “VoIP”. The delay between
sending a packet to the network and receiving it needs to be minimal and
constant and this makes specic demands on this application.
Most network equipment can already full this real time
requirement. Virtually all switches currently on sale support a VLAN with
different priorities in the network, and the vast majority of higher layer
network equipment supports some means of transporting packets with
different qualities (DiffSrv). The LAN usually supports a bandwidth of
100 MBit/s, which is more than enough to allow voice to ow through
the network, and adherence to a certain set of rules ensures that this
bandwidth is enough to supply superior telephone quality. The Internet
backbone’s ability to transport large loads is increasing on a daily basis,
and global communications are now ruled by the Internet.
Why SIP?
There has been a “protocol war” regarding the “best” way to set
up a phone call. In the mid 90s, H.323 was the rst attempt to unify the
VoIP industry under a common standard,and move the world of telephony
into the computer industry, using most of the methods known from ISDN.
Seen however, from today’s perspective, the resulting technology was
1

8 • About VoIP
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[ S N O M 4S PR O X Y / R E G I S T R A R M ANUAL ]
far too complex, so products based on this technology did not work well
together. The late introduction of “supplementary services” (H.450.x) not
only introduced another level of complexity, but was also simply too late.
By the late 90s, the Session Initial Protocol (SIP) had been
proposed (RFC 25431). SIP follows the paradigms of the Internet, and is
built upon the same principles used by http and email. Moreover, it has
found an enthusiastic community of researchers and developers who like
the idea of applying Internet technology to real time communications.
More and more applications are being put into SIP, telephony being just
one of them.
So far more than 150 drafts have been proposed for extending
the SIP protocol. All kinds of solutions are being addressed in these
documents, and the highly dynamic eld of this new real time
communication technology is resulting in evolutionary pressure to nd
the best common denominator.
Most of the “big players” have jumped on the SIP train. Microsoft
Messenger is based on SIP and Cisco Systems offers SIP extensions
to most of its products. International organizations like ETSI host SIP
interoperability events, and next generation mobile technology will be
integrated with, if not based on, SIP (see the 3GPP for more details).
Open Standards
Open standards dene the rules of the game. Interoperability
allows customers to choose between the products of different vendors and
opens up competition below the system level. This can be advantageous
for the customer, as the computer hardware industry has shown.
Many vendors therefore advertise their usage of an “open
standard”, dening this term as “we make the way our standard works
public”. However, this cannot really be called “standard” if only one vendor
is using it. The disadvantage is that customers still have a limited choice
of products they can buy.
There is no one objective denition of an open standard. However,
something approaching an open standard could be reached if a signicant
number of vendors offered products using the same standard, giving
customers the possibility of combining products to create a system. SIP
is just such a standard.
1

8 • About VoIP
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[ S N O M 4S PR O X Y / R E G I S T R A R M ANUAL ]
What You Can Expect and What Not
Telephony is more than making calls from A to B. SIP supports
all kinds of transfers, call parking and call picking, user searches (Follow-
me), mailbox support, and all the other features known from traditional
telephony. In addition to this, telephones can now indicate their willingness
to receive calls and the probability of nding a specic user.
You can call a PSTN number from a SIP phone just as you did ten
years ago. The network will usually be set up to terminate these calls on
a gateway which translates the packet stream into a switched network
signal. You can also dial email-like numbers like “sip:fred.intstone@
megaportal.com”, and you can reach your sales team under the same
telephone number and email address.
Internet telephony is still a “best effort” communications
technology and does not always necessarily support the quality of
transport telephony requires. If you are placing a phone call over the
public Internet, there is no guarantee that a packet will be transported
within a reasonable time. Usually there is acceptable quality, but it may
happen that calls suddenly break off, that there is signicant delay, or
that packet loss causes stuttering. It is important that users know what
to expect: Cell phone users know that driving through a tunnel may break
the call, and Internet telephony users must be aware that talking for free
may compromise call quality.
1

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1

10 • About VoIP
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The SIP
Architecture
User Agents
In a SIP network, the phones2 make up most of the brain power,
unlike traditional telecoms equipment which can not scale so well:
• they play and record audio,
• they compress and uncompress the digital audio,
• they do echo compensation,
• they compensate for packet jitter and packet loss,
• they look for the destination,
• they retrieve their conguration information,
• they keep track of phones that offer a call pickup,
• they publish their state upon request,
• they determine and publish the probability of nding somebody,
• they terminate one or more identities,
• they redirect calls when nobody picks up,
• they are part of a virtual LAN,
• they search address books (LDAP),
• they search internet addresses (DNS A, DNS SRV),
• they usually include a web server,
• they send an receive instant messaging information,
• they publish network management information (SNMP),
• they behave like normal computers on the network (DHCP, DNS).
Phones are also called “user agents” and behave in a client/server
manner (somebody being the user agent client, UAC and somebody the
user agent server, UAS). In SIP, there is no conceptual difference between
a hard phone and a soft phone. The snom 100 VoIP phone or Microsoft
Messenger are examples of this kind of system.
2

12 • SIP Architecture
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[ S N O M 4S PR O X Y / R E G I S T R A R M ANUAL ]
Proxies
Proxies forward requests and help the user agent carry out its
tasks. Stateless proxies just forward messages and serve as a “hop” on
the path from a user agent client to a user agent server. The rules for
hopping may depend on all kinds of rules, e.g. traversing NAT using a
stateless proxy.
Stateful proxies keep a list of pending requests. This way proxies
can forward requests to different destinations at the same time. When
the responses come back from the destinations, the proxy merges the
responses, determines the best result and passes it down to the user
agent that sent the request (UAC). The snom 4S proxy is a stateful
proxy.
Location Server
From RFC3261 we read: „A location service is used by a SIP
redirect or proxy server to obtain information about a callee‘s possible
location(s). It contains a list of bindings of address-of-record keys to zero
or more contact addresses. The bindings can be created and removed in
many ways; this specication denes a REGISTER method that updates
the bindings.“
In other words, the location server nds out where a request to a
domain the proxy feels responsible for should go.
Registrars
User agents register with a registrar. When a request for the user
agent arrives at the registrar, it redirects the request to the location that
was previously stored in the internal database.
The registrar is the part of the location server that can be
controlled with REGISTER requests. So snom 4S Proxy/Location Server
would be a better name for the product.
2

12 • SIP Architecture
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[ S N O M 4S PR O X Y / R E G I S T R A R M ANUAL ]
Media Server
Strictly speaking, the media server is just a special kind of user
agent. Typically it is able to deal with several calls at the same time and
is a located on a PC or workstation.
The media server has the following tasks:
• Implement mailbox function. When a user is absent, the user
agent of the stateful proxy redirects the call to the mailbox, so that
the caller may leave a message. The owner of the mailbox calls the
mailbox directly to listen to messages.
• Implement music on hold. Using a fat client, all kinds of music
tastes can be played with highest possible quality.
• Implement call parking. Calls can be parked on the media server
until a user picks the call for processing. In the meantime, the
caller can enjoy the music on hold, using DTMF keys to select his
favourites.
• Implement conferencing services. Three or more persons dial
into the conference server, which mixes the audio streams for
each participant and also noties them of participants joining and
leaving the conference. The conference server also checks the
credentials of participants joining the conference. The snom 4s is
an example of this technology.
Gateways
From a SIP perspective, the gateway is also just a user agent.
Instead of playing the audio stream on a speaker, it sends it to the PSTN
network and instead of getting voice from a microphone it retrieves
signals from the switched network.
There are three kinds of gateways; PSTN, proxy signalling and
NAT gateways.
Depending on the nature of the gateway, it may serve one, two,
four, thirty, sixty or more channels at the same time.
Other gateways may translate the signal to existing H323
networks or other proprietary technology networks. These gateways
are sometimes called signalling gateways. snom does not produce SIP
gateways. Examples of such gateways are manufactured by Cisco,
2

14 • SIP Architecture
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snom technology AG • 15
Mediatrix, Sonus and Vegastream. The snom 4s gateway is a SIP NAT
gateway software enabling Linux computers to be SIP-aware.
2

14 • SIP Architecture
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snom technology AG • 15
snom 4S
Framework
snom has set up a SIP-based solution targeted at small and
medium sized installations. This solution may be installed on Windows®
as well as on Linux computers. The registar proxy is the core part of this
framework.
Proxy
The snom 4S registrar proxy is a SIP registrar and proxy with the
following features:
• Stateful forking. Requests are forked to one or more destinations
and the responses are ltered before passing them back to the
user agent client.
• Sequential forking: Users are searched according to the probability
that was provided with the registration.
• Full functionality: All SIP methods are supported, that includes
transfers, call parking, call picking, notications, instant messaging
and other SIP features.
• Dial plan: You can set up dial plans that will determine whether
specic users may call specic destinations, whether numbers are
complete, or whether numbers are to be redirected to one or more
gateways.
• Authentication: You can force clients to authenticate their
identity.
• NAT handling: Requests leaving the private network may be
redirected to a NAT gateway.
• Support of path registrations. This way user agents may register
with a path that may contain proxies that must be passed.
• Failure recovery: Even after a reboot, the proxy keeps the state of
the registrations.
3

16 • snom 4S Framework
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[ S N O M 4S PR O X Y / R E G I S T R A R M ANUAL ]
• TCP and UDP transport layer support: Both unreliable and reliable
transport layers are supported.3
• Web Access: The proxy can be managed remotely via a web
browser.
• Interoperability: The proxy is interoperable with the SIP equipment
of other vendors; you are not limited to snom products.4
SIP NAT Gateway
The snom 4S Network Address Translation (NAT) gateway is a
stateless proxy that transports SIP messages between private and public
networks. This makes it possible to share one public Internet address
amongst several SIP elements. The NAT gateway supports:
• Forwarding of RTP packets. Both incoming and outgoing packets
may be forwarded by the NAT gateway. The SDP attachments of
SIP messages are patched according to the local ports. This allows
usage of the NAT gateway together with a rewall.
• Path registrations. Registration messages passing the proxy are
tagged with the proxy path.
• Default destination: Packets destined for the NAT gateway may be
forwarded to a xed address. This way a publicly accessible proxy
may reside inside a private network.
• PPPoE device support. In Linux, the NAT gateway automatically
detects the public IP address and changes the address when the
PPPoE device changes the IP address.
• Assignment of RTP port range. To comply with available rewalls,
a range of ports may be assigned.
• Codec preference reordering. The available codecs are reordered
according to their bandwidth requirements. This reduces the
bandwidth used when talking over the NAT gateway and makes
usage in DSL environments easier.
• Linking to Linux ipchains. This way packets destined at SIP port
5060 can be redirected to the NAT gateway without setting up the
user agents in the private network.
3

16 • snom 4S Framework
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[ S N O M 4S PR O X Y / R E G I S T R A R M ANUAL ]
Media Server
There are situations when there is nobody available to handle a
call. In these cases the media server helps out.
• Mailbox. When nobody picks up a call, the caller can leave a message
on a mailbox. The owner of the mailbox receives notication on his
phones and an email with voice mail as an attachment.
• Music on hold. When a call is put on hold, the waiting party can
listen to some music or announcements. Calls can also be parked
on a music on hold server.
• Conferencing services. When more than two people want to talk
in a telephone conference, the media server can introduce new
participants, ask for pass codes, and mix the audio streams in such
a way that participants do not hear themselves.
• Error explanations. When something goes wrong, the media server
explains what it was .
3

18 • snom 4S Framework
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3

18 • snom 4S Framework
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Installation
Windows Installation
Important: If you are doing an update, you need to stop and
uninstall the old proxy rst (see below).
After double clicking on the setup executable, the installations
program starts up (see g. x-1). Press Next to begin the installation. 4
Figure 4-1: Installation Welcome Screen

20 • Installation
[ S N O M 4S PR O X Y / R E G I S T R A R M ANUAL ]
snom technology AG • 21
[ S N O M 4S PR O X Y / R E G I S T R A R M ANUAL ]
At the beginning of the installation the setup program asks you
to accept the license conditions (see g. x-2). Please read them carefully,
then select the “accept” button and press “next” to accept the conditions.
If you decline, the installation will be aborted.
After accepting the license agreement, the next screen asks you
to enter your personal information. Enter your name and the name of
organization.
You can then select the location where the proxy’s les will be put.
The installation program proposes a reasonable location but if you want
to you can change it. After this, the installation asks you for the location
where the registration information will be put. This directory needs
write access and will contain the information for registered users. The
installation program proposes a location relative to the proxy installation
directory, but it might be useful to specify a different location for this, e.g.
4Figure 4-2: Installation License Agreement