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Special Focus
May 2003

Enabling NATs And Firewalls With SIP

BY Karl Erik St�hl

It is expected that real-time person-to-person communication, like IP telephony (VoIP), presence, instant messaging, voice, video, and data collaboration will be the next big step of Internet usage. The Internet standard for such communication is SIP. To be part of this accelerating SIP user community, it is important that your network is prepared for it. To have universal connectivity across the Internet, NATs and Firewalls need to be SIP capable, which is currently uncommon.

Person-To-Person Communication
Internet started as a defense, research, and university network. There are two applications that have spread the Internet usage to almost all companies and persons in the world: e-mail and Web surfing. However, these applications do not direct real-time communication between individuals, a capability that is becoming highly useful as more and more individuals have broadband or a fixed connection to the Internet.

Therefore, the next big step of Internet usage will include person-to-person communication such as:

� Voice (of which IP telephony is but one component);
� Video;
� Presence;
� Instant Messaging;
� Conferencing with voice, video and data collaboration, and more�

Several forms of person-to-person communication over the Internet have already been in use for a few years. However, it is just now, when a general standard has been established, that these types of applications will become more available and more widely used. SIP is the Internet standard for such applications and currently has a strongly accelerating growth.

A powerful driving force for SIP is that Microsoft has announced that all future real time communication (RTC) will be based on the SIP standard. Windows Messenger, which can be downloaded at no charge, already has a SIP mode that provides the user with telephony, voice, video, presence, and instant messaging. And, Microsoft is set to launch Greenwich, the RTC services for the Windows 2003 server. Greenwich includes a SIP server for safe enterprise usage and a programming API, which is expected to result in numerous SIP applications. With the market impact of Microsoft, there could easily be tens of millions of SIP users.

SIP is also used for �ordinary telephony,� i.e., voice with 3 kHz bandwidth and common number dialing, over IP networks. For this application, the SIP standard is taking over from the earlier H.323, which is a protocol from the standardisation organisation of the telecom world, the ITU-T. H.323 has been used to build islands of VoIP, but most often without interoperability on the IP level between the different operators. Another protocol is MGCP, or the related H.248/MEGACO, that sometimes is used to control IP phones on a low level in order for operators to connect these to the old telephone network, the PSTN.

It should be noted that IP telephony -- where ordinary telephony is emulated over IP -- is only a small part of person-to-person communication for which SIP was created. Real time person-to-person communication is expected to be the next big step in Internet usage following e-mail and Web surfing.

When connecting a PC to the Internet, you do not want it to be accessible for everyone or vulnerable to attacks from hackers. That is especially important if you are constantly connected, for example via broadband or a fixed line. A firewall protects the PC by only allowing approved traffic and by rejecting attacks and illegal data packets.

On a local-area network (LAN), where several PCs or other equipment is connected, it is common to have private IP addresses on the LAN and a single common public IP address to the Internet. That is called NAT (Network Address Translation) and is often an integrated part of the firewall.

Firewalls and NAT-routers are designed for data traffic that is initiated from the inside of the private network. If instead the data traffic is initiated from the outside, and even worse, must reach a specific user on the private network, serious problems occur.

This is exactly what is happening with person-to-person communication via SIP. Therefore, it is highly important that all new firewalls and NAT routers now being installed are designed to support SIP properly and securely.

Most firewalls installed today do not handle SIP in an adequate way. The problem occurs for all similar protocols (e.g., H.323) where a person on a private LAN is to be contacted. Ordinary firewalls are simply not designed for such data traffic. It is a common misunderstanding that well known firewalls can be configured to handle SIP traffic, but that is not the case. One problem is that the media streams (e.g., voice and video packets) are transferred over dynamically assigned UDP ports that are generally closed. Another problem is that the SIP clients inside the firewall cannot be reached by IP addresses since these most often are private and local to the LAN. It simply does not work, unless there is specific SIP support in the firewall.

The same applies to routers that are switching the address space, NATs. NAT routers are used when several users share a common Internet connection with a single IP address. There are also operators only offering private IP addresses to their customers.

It is of course a fundamental problem that person-to-person communication does not reach the users on the LANs. Various methods and equipment have been suggested to solve this problem in a number of situations, but the most general one is to eliminate the problem where it occurs -- in the firewall itself. Firewalls including a SIP server (with a SIP proxy and SIP registrar) that dynamically controls the firewall are currently available.

A number of firewall vendors are planning to introduce models including a SIP ALG (Application Layer Gateway). These ALGs usually work at a lower level than a proxy, adjusting the data packets �on-the-fly.� Cisco is planning to introduce such ALGs that also handle incoming calls to multiple users, while other more simple implementations may only support a single SIP user on the LAN. A common limitation of the ALG architecture is that it cannot handle secure SIP signaling via TLS (Transport Layer Security). TLS is strongly recommended by Microsoft to be used with their Greenwich SIP enterprise solution.

SIP capable firewalls are not more expensive than ordinary firewalls and should be considered for all new installations of firewalls and NATs. If not, there is a high risk that even newly installed firewalls and NATs have to be exchanged in the near future.

Other methods are also proposed for SIP firewall and NAT traversal.

STUN is a method for getting SIP through existing NATs. It works through keeping holes open in the NAT by dummy traffic and having the SIP clients emulate their �looks� from the outside of the protected LAN. STUN will not work for all NATs and not for real secure firewalls, and may have some scalability and security issues. The SIP client has to implement STUN and integrate it in the SIP stack to make it work.

There are also various tunneling approaches, creating a tunnel through the firewall and then having an ALG in a central place at the �SIP operator� to cope with the separate address space of the private LANs and their individual users. Special equipment is therefore required at the SIP operator and sometimes, special equipment and software are required on the LAN or in the SIP clients. With this approach, the users get locked into a specific SIP operator. This approach can normally not handle complex configurations, such as inter-working between an operator and the Microsoft Greenwich architecture, where a local SIP server on the LAN is used.

For home users, Microsoft has suggested an extension to UPnP (Universal Plug and Play) to allow Windows to control the NAT or firewall. Several small inexpensive NATs have implemented these UPnP extensions, and thus allow SIP traversal for Windows Messenger (which is SIP based). However, it is not secure to allow every PC on the LAN to open the firewall, so UPnP is not acceptable for a proper firewall that should protect the LAN (In the Greenwich architecture, even Microsoft recommends that UPnP be disabled for high security). Another limitation is of course that UPnP control from Windows clients will not help other SIP products (e.g., SIP phones) to traverse a NAT or firewall.

In all new installations of firewalls and NAT routers, proper SIP support should be assured to allow the users on the LANs to utilize real time person to person communication, the next big usage of the Internet. Of the various methods proposed for firewall and NAT traversal, the most general and reliable is to solve the problems where it occurs, in the firewall or NAT itself. By including a SIP proxy and a SIP registrar for controlling the NAT and firewall, it is possible to handle complex SIP scenarios and to use TLS for secure and private signaling.

Karl Erik St�hl is president, Intertex Data AB, Sweden, a Swedish company with 20 years� experience in telecommunications and the development of high-quality communication and security products. Currently, Intertex is developing next generation broadband access products focusing on the SIP standard and person-to-person IP communication. For more information, please visit the company online at

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