IANA Registration for an Enumservice Containing Public Switched Telephone Network (PSTN) Signaling Information
updated by
- rfc-6118 — Update of Legacy IANA Registrations of Enumservices
Extracted elements (16)
Number portability data distribution is often highly restricted by contract or National Regulatory Authority regulation. Records may be distributed privately within service providers, openly, or on a national basis subject to regulatory policy, and may not be part of the e164.arpa public DNS tree.
Tel URIs may be easier and more efficient to use than SIP URIs and result in somewhat smaller NAPTR records. When adding hundreds of millions of records to DNS, the size difference could have a substantial impact on processing and storage requirements.
The 'pstn' Enumservice enables service providers to consolidate all telephone number lookups (ported numbers, pooled numbers, number blocks) into a single ENUM lookup, simplifying call routing and network operations into either an on-net IP-based or off-net PSTN-based response.
The subtype is needed to identify the specific Enumservice intended because when only one URI scheme is associated with a given service, an additional URI scheme may be added later; the subtype thus future-proofs the registration by distinguishing between 'pstn:tel' and 'pstn:sip'.
Both E2U+sip and E2U+pstn Enumservice type records are likely to be returned for a given query, representing on-net (IP) and off-net (PSTN) routes respectively. Application logic must select appropriately from multiple records based on business, network, or other rules.
When both E2U+pstn and E2U+sip records are returned, a typical calling application prioritizes the on-net SIP record and may fall back to the E2U+pstn record to route via a PSTN-connected media gateway if the IP call route fails.
A Number Portability Dip Indicator (npdi) should be used in practice with both the 'pstn:tel' and 'pstn:sip' Enumservice subtypes. The npdi field is included to prevent subsequent lookups in legacy-style PSTN databases.
Implementations receiving URIs from the 'pstn:tel' Enumservice must be prepared to accept TEL URI parameters defined in RFC 4694, which include number portability parameters such as the routing number (rn) and number portability dip indicator (npdi).
The 'pstn' Enumservice type indicates PSTN routing data, including number portability data, non-ported telephone number data (individually or in number blocks), and other PSTN-oriented data associated with E.164 telephone numbers. It provides routing for numbers only routable via the traditional PSTN.
The service parameters convention follows RFC 3761 where 'type' defines the service (more generic) and 'subtype' defines the URI scheme. For 'pstn', the subtype 'tel' maps to the tel: URI scheme and 'sip' maps to the sip: URI scheme.
This document registers the Enumservice type 'pstn' with subtype 'sip' using the URI scheme 'sip:' under the Enumservice registry described in RFC 3761. The service is intended for COMMON usage to facilitate PSTN call routing including number portability.
Although E.164 telephone numbers reveal less identity information than SIP or email URIs with usernames, PSTN Enumservice data is still publicly available and carries risk of unwanted communication. DNSSEC applicability and DNS threat analysis are covered in RFC 3761 and RFC 3833 respectively.
Carriers and service providers may choose not to publish PSTN Enumservice data in the public e164.arpa tree, instead maintaining an internal ENUM routing database queryable only by trusted network elements such as softswitches and SIP proxy servers.
Publishing PSTN Enumservice data in the public e164.arpa tree makes it anonymously visible to anyone. While comparable to telephone directory publication, it provides no indication of when or by whom data has been accessed, enabling data harvesting for purposes such as unsolicited telemarketing calls.
A NAPTR record for a ported number using 'pstn:sip' takes the form: NAPTR 10 100 "u" "E2U+pstn:sip" "!^.*$!sip:+1-215-555-0123;npdi;rn=+[email protected];user=phone!". Conversion from tel to SIP URI follows RFC 3261 Section 19.1.6 and RFC 4694 Section 6.
Regular expressions may be used in NAPTR records to reduce record size; for example, "!(^.*)$!tel:\1;npdi!" dynamically replaces the expression with the telephone number plus leading '+'. This reduces individual record size and has significant impact on database storage at scale.