The VoIP Peering Puzzle�Part 15: Digium’s DUNDi

In our several previous tutorials, we examined various implementations of Electronic Numbering, or ENUM, technology, which provide a means to translate between telephone numbers and Internet Protocol (IP) addresses. Thus far, we have examined NetNumber’s TITAN , the SPIDER Registry, Verisign’s ATLAS, Neustar’s SIP-IX, Telcordia’s Service Interconnection Registry, and Nominum’s Navitas. While product and service distinctions certainly abound in this group of solutions, at the core of each you find some type of a centralized database that is keeping track of all of those numbers and addresses.

Digium, Inc., founded in 1999, is the original creator and primary developer of Asterisk, the industry’s first open-source telephony platform written for the Linux operating system. Digium provides hardware and software products—including the Asterisk Business Edition, its professional grade version of Asterisk—to enterprises and telecommunications providers worldwide.

The code for Asterisk, originally written by Mark Spencer of Digium, has incorporated contributions from open-source software engineers around the world. Currently boasting over one million users, Asterisk supports a wide range of TDM protocols for the handling and transmission of voice over traditional telephony interfaces, and VoIP packet protocols such as the Inter-Asterisk Exchange (IAX)—which enables VoIP connections between Asterisk servers—SIP and H.323. Asterisk supports U.S. and European standard signaling types used in business phone systems, allowing it to bridge between next-generation voice/data integrated networks and existing infrastructure.

From this Asterisk research and development comes a system called DUNDi, which stands for the Distributed Universal Number Discovery protocol. This protocol was originally designed to allow for simple system-to-system number location, such as locating Internet gateways to telephony services. In this way, DUNDi is like ENUM. However, DUNDi is designed so as not to require a central system to control it: It is fully distributed, with no centralized authority. This feature has advantages both in large-scale situations, such as external lookups to peered servers, and in small environments, with anywhere from five to thousands of telephones. DUNDi is not a VoIP signaling or media protocol. Instead, it publishes routes, which are then accessed via the well-known signaling protocols, such as the IAX, SIP, or H.323. In other words, unlike ENUM, which is built around the hierarchical Domain Name System (DNS), DUNDi is designed to adapt to whatever architecture is given.

DUNDi users collaborate based on a common agreement called the Digium General Peering Agreement, which establishes a base standard for the quality and the use of the network. This agreement identifies guidelines under which the parties establish peering relationships, such as only publishing routes for valid telephone numbers (route accuracy), and honoring acceptable use policies for those routes (avoiding abuse of the information such as VoIP spam). Thus, there is no central authority for the database (in contrast to many ENUM implementations). As a result, the pricing monopoly—and the resultant costing structure—that can accompany a monopolistic model simply goes away.

DUNDi is a very lightweight protocol in that it doesn’t send anything more than an encrypted text stream containing the dial pattern needed to contact the number being searched for. In fact, admins often run the DUNDi protocol over the same network segment that the voice calls run over. This is in contrast to the traditional practice of running the voice traffic over a separate network from other (non-voice) data, separating out things like database lookups to avoid interfering with the quality of the calls.

DUNDi has a number of possible applications, Perhaps foremost is peering between enterprises. Another is for voice networks that span multiple WANs, allowing for a dynamic extension lookup across the system so that a user is reachable no matter what part of the network s/he is on. This is a novel approach to simple provisioning across systems, as you can have a configuration database replicated to remote locations, and have a phone registry based on location, and the whole time, users will never know they aren’t talking through their “usual” server.

DUNDi can be an extremely versatile and disruptive technology, since the protocol itself is open and can be implemented in a multitude of environments. Information on Asterisk and the Digium products is available at Further details on the DUNDi protocol are available at, including a mailing list devoted to DUNDi implementation at Our next tutorial will continue our examination of some of the commercial enterprises that are offering ENUM and other directory services.

Copyright Acknowledgement: © 2007 DigiNet Corporation®, All Rights Reserved

Author’s Biography
Mark A. Miller, P.E. is President of DigiNet Corporation®, a Denver-based consulting engineering firm. He is the author of many books on networking technologies, including Voice over IP Technologies, and Internet Technologies Handbook, both published by John Wiley & Sons.

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