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IP Telephony Is On the Line -- Can the Telephone Directory join the Internet Age?
by Chris Risley, CEO
1/13/2006
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The Internet is poised for its next major burst of growth and usage as billions of telephones, fax machines and PDAs join the desktops, laptops and servers already communicating on the Internet. This is a critical moment for traditional telephone carriers and Internet Service Providers (ISPs) developing next-generation voice, data and multimedia services; missteps or slowdowns can be enormously costly in the race for market and mindshare.
No one questions the inevitability of the convergence of voice, data and multimedia on IP networks worldwide. We probably can't even foresee all the ways that this convergence will affect our daily lives. What we can and should question is exactly how we are getting there. Are today's IP networks really ready for dial-tone?
Protocol maturity is not the issue. Open standards such as the Session Initiation Protocol (SIP) are becoming de facto methods for connecting IP telephony devices. The various telephony system components are coming into place as VoIP services roll out around the globe. Session border controllers and gateways are connecting VoIP services to the world at large. Even bandwidth and routing capacity are not in question.
The questions are more basic -- how are all of these calls going to connect to each other, particularly in a time of convergence and transition? How do the VoIP "islands" find each other to connect calls most efficiently, without requiring users to change their calling
behaviors?
At issue is the seemingly simple process of dialling the phone.
Connecting Two of the World's Largest
Databases
The Public Switched Telephone Network (PSTN) uses an ITU standard known as E.164 to define the international telephone numbering system that we are all familiar with. This system provides a globally unique international number to one (or more) telephones. If you know someone's E.164 number, that is, their telephone number, you can dial them from anywhere in the world. We typically know our own phone numbers and those of friends, family and associates, and when we don't, we use directory services.
The Internet uses IP addresses to direct traffic. These addresses are either 4 numbers from 0 to 255 separated by the "." character in the case of IPv4 or, in the case of IPv6, a string of hexadecimal digits separated by colons (:). Like E.164 numbers, IP addresses are globally unique international numbers, however, within the Internet, people generally do not use IP addresses directly. Instead, we use easy-to-remember words in the form of "Uniform Resource Identifiers" (URIs) like www.yahoo.com or email addresses such as david@nominum.com.
When we use one of these easily remembered identifiers by sending an email, opening a web page, or clicking on a link, our computer sends a query to the global Domain Name System (DNS) to resolve the name of the computer referenced in the identifier to its numerical IP address. This happens in real time, behind the scenes with no other user interaction. We take this translation service for granted, but if the DNS isn't working, we are typically stuck. While a few technical individuals might have
memorized the IP addresses of some of the computers they want to communicate with, the vast majority of people have not. As a result, like plumbing, if the DNS breaks, people including the press, generally notice quickly. DNS problems have been responsible for widespread and lengthy outages of major websites, including those of Microsoft, Google, and numerous other organisations.
In a sense, the mapping of IP addresses from (and to) computer names provided by DNS servers around the globe constitutes one enormous, globally distributed database that is constantly changing. The network of E.164 phone numbers maintained by governmental organisations and telephone carriers is another. The interconnection of VoIP and the PSTN is making us find ways to connect these two databases efficiently.
Bridging the VoIP Gap
Phone numbers are an entrenched standard that are unlikely to disappear anytime soon. VoIP services need to be able to exist in a world with phone numbers and devices designed to work with only those numbers. Routing voice calls between PSTN and VoIP networks is relative easy: just let a gateway between the PSTN and the Internet look up the phone number and determine where to connect the call. But without a global directory system for VoIP users, the only way to connect calls initiated with telephone numbers between VoIP networks is to fall back to the PSTN. SIP proxies route calls to the PSTN when they don't recognise the destination phone number. This relatively simple approach degrades call quality -- as the digital stream is converted to analogue and back to digital -- and introduces additional delays and costs. This approach might work in the short term for voice, but it will fall apart when convergence goes beyond voice to include video and other services.
To deliver on the promise of convergence, communications between IP-based devices must be able to travel solely over IP networks, without traversing multiple protocol conversions ( e.g., IP-to-PSTN gateways). For this to happen in the telephony world, however, we need to be able to direct calls using phone numbers to the appropriate VoIP servers or session border controllers, without using the PSTN. Essentially, we need to link phone numbers to IP addresses.
ENUM: Connecting VoIP Islands
ENUM is the name for an Internet Engineering Task Force (IETF) standard for directing traffic over the Internet using traditional phone numbers. It integrates E.164 phone numbers with IP addressing using the Domain Name System (DNS).
The DNS works by using a worldwide network of caching and authoritative DNS servers. When you click on a web page, your computer automatically requests the IP address for that page from your local, caching DNS server. If the server has the address cached locally -- and it often does for frequently-accessed sites -- it responds immediately. Otherwise, it queries the hierarchical DNS infrastructure to find a DNS server that can provide the definitive (‘authoritative') information for the URI you have requested by your click.
ENUM uses the DNS domain structure, delegation, and protocols for resolving international phone numbers to IP addresses. A carrier or other organisation maintains the "authoritative" data for its subscribers, directing traffic destined for a specific phone number to the appropriate IP address.
The ENUM standard basically takes an international phone number, flips it around and puts periods between each number -- resulting in a very odd-looking domain name that takes advantage of the hierarchical organisation of the DNS domains. The Public ENUM standard, meant to create a global directory of phone numbers, appends the "e164.arpa." domain to the end of the reversed and period-separated number. This fully-qualified domain name can then be sent to the local caching DNS server.
For example, when using public ENUM, the phone number
+1-650-381-6000
becomes the domain name:
0.0.0.6.1.8.3.0.5.6.1.e164.arpa.
The objective of the IETF ENUM standard (RFC 3761) is to create a global, publicly-available ENUM directory. Public ENUM trials and commercial services are underway in several countries, but adoption varies between countries and there are significant political and privacy issues to resolve. In some countries, a
nationalized telephony system makes ENUM implementations easier, while in countries like the U.S. the success of ENUM will require participation and commitment from the government as well as major telephony service providers.
The pressure to interconnect VoIP and PSTN is increasing rapidly and carriers are taking matters into their own hands by adopting private or semi-private variants of the ENUM concept:
Private ENUM publishes DNS information internal to an enterprise's or telco's telephone dial plan. For example, a telephony service carrier might use private ENUM to route calls within a voice plan or MultiMedia Messaging Service (MMS) traffic between phones within its network.
Carrier ENUM is gaining popularity as VoIP carriers, particularly VoIP over cable service providers, cooperate to route voice and other traffic via Internet protocols without going through the PSTN. As with Private ENUM, this inter-carrier traffic remains outside the E164.arpa root domain and is not publicly available.
Regardless of the eventual progress of the public ENUM effort, the various private and carrier variants will undoubtedly coexist for some time.
How ENUM Strains the Domain Name System
The hard fact is that most carriers do not have the DNS infrastructure in place to handle even a fraction of the projected ENUM traffic. Even for carriers with diverse IP-based services and IP addressing expertise, ENUM puts new and unusual demands on their existing DNS servers.
For starters, ENUM will push the scalability limits on DNS servers by several orders of magnitude. Today, the majority of the authoritative DNS servers manage a few thousands entries, sometimes hundreds of thousands, rarely millions (One of the largest domains is .com with approximately 30 million entries). ENUM will blow past these numbers, requiring the support for hundreds of millions of phone numbers.
ENUM NAPTR (Naming Authority PoinTeR) records, the DNS database entries used to facilitate the mapping between ENUM formatted domains and IP addresses, are much larger than typical DNS records for computers, with hundreds of bytes instead of dozens. A NAPTR record doesn't just correlate a domain name with an IP address, it can also provide mapping instructions and priorities for contacting an individual through a variety of devices: home, work, and cell phones, PDAs, fax machines, desktop computers; and services: voice, e-mail, text message, instant message, fax, and so on. This functionality greatly compounds the already-heavy load placed on the domain name system.
Maintaining ENUM data over time poses another challenge. Phone numbers are constantly created, deleted, and transferred and these updates have to take place in real time or close to real time. Integration with provisioning systems is essential, however, few DNS servers are capable of handling mass, real-time updates without service interruption.
The ENUM server should be flexible as well. For example, a private or carrier ENUM implementation might use different "DNS views" to present data differently depending on whether you are connecting from outside or inside the network. Queries originating inside an
organization's network might receive the IP address for the internal SIP server,, while queries coming from beyond the organisation might be directed to a session border controller, hiding internal network addresses.
Finally, ENUM requires security. The IETF ENUM standard encourages the use of the DNSSEC protocol, which helps DNS servers verify the integrity of DNS information using public key cryptography to prevent cache poisoning (also called Pharming attacks). DNSSEC is not widely adopted today, in part because of the performance degradation it creates on most DNS servers and the increase in the amount of data the DNS server has to store and use in responses. However its deployment becomes more and more critical as the threats on the Internet grows in sophistication.
10 Megabit DNS in a Gigabit World
The reality is that most of the DNS servers around the world are provisioned for yesterday's Internet. They simply won't scale to handle the hundreds of millions of phone numbers that are required for today's and tomorrow's IP networks.
Performance problems will be a hallmark of increased DNS traffic loads. Further, when incidental and malicious spikes from worms, denial-of-service-attacks, and viruses hit networks, this will have an increasingly detrimental impact on network availability and performance.
Slower response to DNS queries is somewhat acceptable in a world of web browsing and email delivery but not in a telephony world where end-to-end call connection must be completed in less than 500 milliseconds.
Finally, configuration and provisioning changes place DNS servers offline for seconds, minutes, and even hours, unthinkable when service level agreements require 99.999% availability.
At Nominum, as a vendor of commercial-grade network addressing solutions, we are seeing organisations making the shift to DNS servers capable of handling the new infrastructure demands ENUM raises. Not surprisingly, most of the demand for carrier-grade DNS software so far has come from traditional IP service providers, well aware of the limitations of existing DNS servers and with experiences of outages. But telcos, network equipment providers, and companies interested in ENUM are finally
realizing that they too have to upgrade their DNS in short order.
Strengthening the DNS for ENUM -- and Convergence
Without a concerted effort to upgrade the DNS servers that will be handling ENUM services, something as simple as addressing threatens to become a bottleneck in VoIP call placement and, ultimately, adoption. Problems encountered now, during this critical period of change and convergence, can have long-term effects on market share and reputation.
As with most challenges, this situation also represents an opportunity for carriers and service providers. By upgrading the DNS infrastructure with industrial-strength software capable of handling the demands of ENUM, they can also improve DNS services for other Internet services. Adding DNS capacity makes IP networks more resilient, as the DNS servers remain functioning under increased loads caused by denial of service attacks or virus/worm propagation. Providing real-time add/change/delete capabilities enables self-service provisioning for all kinds of new services. With a highly scalable, flexible and secure DNS infrastructure in place, carriers are better able to deploy a whole range of innovative IP-based services while addressing the opportunities opened by voice over IP.
About
the Author
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Chris Risley is President and Chief Executive Officer of Nominum, Inc, an IP name and address software
company.Mr. Risley joins Nominum with over 25 years of management experience, including service as CEO of
NewChannel, an e-business service provider, as Entrepreneur-in-residence at Bessemer Venture Partners, and as Chairman and CEO of ON Technology Corp., a network software provider. He lead the ON Technology IPO in 1995. On was the #36 on the INC 100 Fastest Growing Public Companies List in 1996 growing revenues 2700% over 5 years.
Prior to his experience in the software industry, Mr. Risley spent thirteen years with European industrial companies including BTR plc (now
Invensys) and Advisor to the Chairman of British Aerospace during BAe's turnaround.
Mr. Risley earned a B.A. Magna Cum Laude from Cornell and a PMD from Harvard Graduate School of
Business
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About Nominum
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Nominum is the leading provider of IP address infrastructure solutions for Global 2000 companies. Nominum provides the industry's only high scalability and high availability Internet name and address solutions for the always-on network. Nominum's Foundation product family includes the most scalable, reliable and secure DNS and DHCP servers, with open architectures for extensibility and real-time integration with external systems. These solutions allow service providers to deliver a carrier-grade telecommunications infrastructure that supports the rapidly growing demand for new services, and enterprises to create resilient and reliable networks that are responsive to changing business conditions.
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