enumservice registration for Session Initiation Protocol (SIP) Addresses-of-Record
updated by
- rfc-6118 — Update of Legacy IANA Registrations of Enumservices
Extracted elements (11)
ENUM records SHOULD point to SIP addresses-of-record rather than SIP contact addresses because addresses-of-record represent user identity independent of device, preserving SIP's capability negotiation and discovery process. Provisioning contact addresses would compromise this process since NAPTR service fields cannot express the fine-grained capability detail (codecs, extensions) that SIP negotiation requires.
ENUM's capability management is one-way: the telephone number administrator expresses protocol preferences that the client must evaluate without negotiation. This is fundamentally unlike SIP's bidirectional capability negotiation, which is why ENUM should resolve to addresses-of-record (which have no capabilities) rather than contact addresses.
There is generally no need for more than one NAPTR record under a single telephone number pointing to a SIP address-of-record, because addresses-of-record do not have device-specific capabilities that would require multiple records.
Clients SHOULD be careful to avoid infinite loops when recursively performing ENUM queries on SIP URIs that result from an ENUM lookup, particularly when the user portion of the SIP URI contains a telephone number (e.g., 'sip:+[email protected]').
The public availability of ENUM records containing SIP addresses-of-record is a privacy concern, as these canonical user identities are exposed to all DNS inquirers without access control.
A SIP address-of-record is a long-term user identity URI of the form 'sip:user@domain', independent of any particular device. It differs from a SIP contact address, which is device-specific and may be a numeric IP address or device-qualified URI.
The 'E2U+SIP' enumservice is defined for use in NAPTR records to map E.164 telephone numbers to SIP addresses-of-record. The URI appearing in the regexp field of the NAPTR record may use either the 'sip:' or 'sips:' scheme.
When the SIPS URI scheme is used in an address-of-record provisioned in ENUM, it indicates that the user can only be reached over a secure TLS connection.
This document registers the 'E2U+SIP' enumservice in the IANA enumservice registry as described in RFC 3761. The enumservice name is 'E2U+SIP', type 'SIP', with URI schemes 'sip:' and 'sips:', intended usage COMMON.
All DNS records, including ENUM records containing SIP addresses-of-record, must be assumed publicly available to all inquirers at all times. Placing a SIP address-of-record in ENUM is comparable to placing an email address in DNS, with attendant privacy implications.
Unlike traditional telephone numbers, a SIP URI identified in ENUM may require callers to provide cryptographic credentials for authentication and authorization before alerting the user, potentially providing greater protection from unwanted callers than the PSTN despite the public availability of ENUM records.