1. Using NPAC as the ENUM Registry
IPNNI
Using NPAC as the ENUM Registry
December 10, 2013
Penn Pfautz

2. Why an ENUM Registry?
Routing mechanisms need to evolve beyond reliance on central office codes
ENUM technology is already deployed within many networks and in some interconnection
Looking for consensus on a common ENUM registry as is needed for ENUM to ultimately replace existing routing mechanisms

3. NPAC as ENUM Registry
NPAC capabilities can be used to instantiate a combined Tier 0/1 ENUM registry for the US
Existing VOICEURI field is re-purposed to contain NS record(s) which point to the Tier 2 ENUM server of the service provider of record for number
NS records are populated into NPAC by the serving provider SOA and distributed to each provider LSMS
Providers then populate the NS records in their internal ENUM servers
Originating provider queries its ENUM server and obtains the NS record resulting in a query to the Tier 2 ENUM server of the destination carrier.
The response to the Tier 2 query provides NAPTR records that identify the ingress SBC to be used to deliver the call to terminating provider.

5. NS Records in the VOICEURI Field
Populate NS info in VOICEURI field as just FQDN, tier2enum.serviceprovider.com, rather than full form e164enum.net IN NS tier2enum.provider.net
Full record form reconstituted by the service provider for provisioning in its ENUM server
Multiple NS records could be populated in the NPAC VOICEURI field using separator character.
allows for redundant name server instances
An apex domain needs to be agreed upon

6. Tier 2 ENUM Server
Query to the service provider of record’s Tier 2 server has the normal (RFC 6116) form
Response would be the set of NAPTR records associated with the queried number
Tier 2 server is not constrained to be a vanilla DNS server; behind the ENUM query/response interface it might support whatever application of policy that the destination carrier desired
Query over same interconnection fabric that a subsequent SIP INVITE would traverse

7. Why use the NPAC? Providers already interface with NPAC
No additional development in the NPAC per se is required
Already pay for NPAC which can’t go away until PSTN sunset complete
Putting IP and TDM routing data in one platform facilitates synchronization with number portability and eases transition
Post sunset, data can be migrated to new platform if desired.

8. Why a Tiered Architecture?
“Thin” registry minimizes the amount of common infrastructure for which the industry must share costs
Delegates policy to decisions to the Tier 2, allowing providers flexibility
Policy in the NPAC would require new development and cost and agreement among providers on the scope and nature of policy to be supported.
Current capabilities of the NPAC URI fields are limited
Tiered approach provides for definition of enumservices and new NAPTRs as desired without NPAC development

9. How does the NPAC ENUM approach affect the number of queries needed to route a call?
In the end state, inter-provider routing will require a query to the originator’s internal ENUM server and a query to the destination provider’s Tier 2
In the interim when there is still TDM interconnection, different providers may adapt different strategies
ENUM only for IP routing (e.g., in IMS):
calls for which there is IP interconnection require just the internal ENUM server and external Tier 2 query
Calls without an IP route might be routed to the BGCF for a conventional LNP query if the internal ENUM query received a null response
LNP integrated into ENUM (RFC 4769):
single ENUM query (followed by a Tier 2 query where there is IP interconnection) suffices for all call types

10. How does the NPAC ENUM approach affect Reliability?
Destination carriers have strong incentives to keep their customers reachable and to engineer their Tier 2 arrangements to provide high reliability.
Path used to send the query will generally be the same used to send the subsequent SIP INVITE
In initial implementation, legacy TDM interconnection is likely to be present as a fallback providing some opportunity for any issues to be worked out.