VoIP and ENUM

ENUM (Carrier and User ENUM) is an applet to IP -Interconnect. To be able to use an applet you need to be able to use the applications the applet is running on.

You cannot use e.g. an applet integrating your Skype buddies in MS Outlook without having Skype and MS Outlook installed and running on your laptop.

Same with ENUM:

ENUM is useless without the possibility to enter an E.164 number in DNS and the capability to use the resulting URI, e.g. in case of real-time communications a SIP URI.

Using a SIP URI means you take it and establish a communication with the destination server. Therefore this SIP URI must be a public identity. Of course a group of carriers may decide to use "private" SIP URIs to route calls between them, but then these SIP URIs are not public user identities. A public user identity is something you may put on your business card without additional context e.g. +1 301 123 4567 or sip:name@foo.com. Skype:hugo or ICQ:detlef are NO public user identities in this definition.

Any national IP Interconnect solution is equivalent to the group of carriers approach described above using "private" user identities. They would require a super IP - Interconnect solution above.

Back to ENUM:

If you are not able to resolve the resulting SIP URI in the DNS AND if you are not able contact the server running on the given IP-address in most or all cases, the result of the ENUM query and therefore ENUM itself is useless for you.

A SIP URI given in User ENUM MUST be usable by any other end-user globally, like e-mail.

Note: this implies that a SIP URI given in User ENUM is also usable by any "Carrier" if he decides to query User ENUM. IP Interconnect between End-Users is defined via the IETF Standards, so no additional work is required (although a BCP may be helpful here ;-)

A SIP URI given in Carrier ENUM MUST at LEAST be usable by any other carrier (whatever that means in contrast to end-user) globally.

Note: this does NOT imply that an entry in Carrier ENUM is usable by any end-user.

The basic requirement for Carrier ENUM to work is a global IP Interconnect (VoIP peering) solution for "Carriers".

Again, a group of carriers may decide to use a private ENUM tree to resolve all E.164 numbers used within these group of carriers, but if you really want to use IP Interconnect for ALL E.164 numbers reachable via IP, you are shifting the problem only one level upwards. You need another Super ENUM tree to resolve the rest - or you dump the call to the PSTN, which is not the basic idea of IP Interconnect.

One problem remains for Carrier ENUM:
Who has the right to enter a E.164 number in Carrier ENUM?
Or reformulated: what is a "Carrier" in the context of "Carrier" ENUM?

Since this is connected to the assignement and distribution of E.164 numbers, and since this is a national matter, the definition of a "Carrier" and who has the right to enter E.164 numbers in "Carrier" ENUM is also a national matter.

On the other hand, the minimum requirements for IP Interconnect cannot be defined nationally, this has to be defined globally.

To summarize:

1. The minimum requirements and rules for IP Interconnect based on routing via SIP URIs have to be established globally. Note: if the minimum requirements are fullfilled (e.g. TLS), the calling server must be able to resolve the SIP AoR to the IP-address of the called server and also to be able to contact the called server. The called user MAY have additional agreements in his profile, e.g. to reject all anonymous calls.

2. Only if this is in place, Carrier ENUM makes sense on a global scale. This does not imply that Carrier ENUM may not be trialed and used commercially earlier, but only in a limited way.

3. Carrier ENUM must be available on a global scale. This implies that Carrier MUST be available for any carrier who wants to use it. This implies that although it is a national matter how to implement Carrier ENUM in detail and to define e.g. the entity operating the Tier 1, there should be no opt-out possible (this is IMHO the most problematic point).

4. It is a national matter to define who is entitled to enter which numbers and the corresponding NAPTRs in "Carrier" ENUM,

e.g. an entity is only allowed to enter numbers in Carrier ENUM if it is hosting the final destination of the E.164 number on behalf of the end-user (that means no signalling transit is allowed)

or if it is providing a direct gateway/SBC to the own destination network hosting the E.164 number on a private IP-based network (NGN) or on the PSTN. National number portability issues have also be taken into account here.

If an entity is assigned an E.164 number directly (e.g. an 800 number or a corporate number), and if the entity is running the SIP server on its own, it may also be allowed to enter this number and the NAPTRs in Carrier ENUM.

4. There is no user opt-in or opt-out possible in Carrier ENUM. The entities allowed to enter NAPTRs in Carrier ENUM MUST be aware that the data in Carrier ENUM is public, therefor end-user privacy MUST be assured. This implies that either the E.164 number or a random number (userid) is used in the user-part of the SIP URI. The mapping from this number to the end-user identity used further (even if it is a public identity) MUST be done within the end-systems.

5. All SIP URIs entered in Carrier ENUM must be reachable via the minimum requirements of IP Interconnect as defined in 1.