ENUM Scott Bradner Internet-Draft Harvard University Intended status: Standards Track Lawrence Conroy Roke Manor Research Kazunori Fujiwara Japan Registry Service Co., Ltd. 4 May 2009 The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM) Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 4, 2009. Abstract This document discusses the use of the Domain Name System (DNS) for the storage of E.164 numbers, and for resolving them into URIs that can be used for (for example) telephony call setup. This document also describes how the DNS can be used to identify the services associated with an E.164 number. This document obsoletes RFC 3761. Copyright and License Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of Bradner, Conroy & Fujiwara [Page 1] Internet-Draft 3761bis 4 May 2009 publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Use of These Mechanisms for Private Dialing Plans . . . . . 2. The ENUM Application Specifications . . . . . . . . . . . . . 2.1. Application Unique String . . . . . . . . . . . . . . . . . 2.2. First Well Known Rule . . . . . . . . . . . . . . . . . . . 2.3. Expected Output . . . . . . . . . . . . . . . . . . . . . . 2.4. Valid Databases . . . . . . . . . . . . . . . . . . . . . . 2.4.1. Initial Key Construction . . . . . . . . . . . . . . . . . 2.4.2. Optional Name Server Additional Section Processing . . . . 2.4.3. Flags . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4. Services Parameters. . . . . . . . . . . . . . . . . . . . 2.4.4.1. ENUM Services. . . . . . . . . . . . . . . . . . . . . . 2.4.4.2. Compound NAPTRs and Implicit ORDER/PREFERENCE Values . . 2.5. The ENUM Algorithm Always Returns a Single Rule . . . . . . 2.6. Case Sensitivity in ENUM . . . . . . . . . . . . . . . . . . 3. ENUM Clients . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Unsupported NAPTRs . . . . . . . . . . . . . . . . . . . . . 3.2. ENUM NAPTR Processing . . . . . . . . . . . . . . . . . . . 3.2.1. Use of Order and Preference Fields . . . . . . . . . . . . 3.2.2. NAPTRs With Identical ORDER/PRIORITY Values . . . . . . . 3.2.3. Processing Order Value Across Domains . . . . . . . . . . . 3.3. Non-Terminal NAPTR Processing . . . . . . . . . . . . . . . . 3.3.1. Non-Terminal NAPTRs - Necessity . . . . . . . . . . . . . . 3.3.2. Non-Terminal NAPTRs - Considerations . . . . . . . . . . . 3.3.2.1. Non-Terminal NAPTRs - General . . . . . . . . . . . . . . 3.3.2.2. Non-Terminal NAPTRs - Loop Detection and Response . . . . 3.3.2.3. Field content in Non-Terminal NAPTRs . . . . . . . . . . 3.3.2.3.1. Flags Field Content With Non-Terminal NAPTRs . . . . . 3.3.2.3.2. Services Field Content with Non-Terminal NAPTRs . . . 3.3.2.3.3. Regular Expression and Replacement Field Content with Non-terminal NAPTRs . . . . . . . . . . . . . . . 3.4. Backwards Compatibility . . . . . . . . . . . . . . . . . . 3.4.1. Services Field Syntax . . . . . . . . . . . . . . . . . . 3.5. Collected Implications for ENUM Clients . . . . . . . . . . . 3.5.1. Non_terminal NAPTR Processing . . . . . . . . . . . . . . . 4. ENUM Service Example . . . . . . . . . . . . . . . . . . . . . 5. Collected Implications for ENUM Provisioning . . . . . . . . . 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7.1. DNS Security . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Caching Security . . . . . . . . . . . . . . . . . . . . . . 7.3. Call Routing Security . . . . . . . . . . . . . . . . . . . Bradner, Conroy & Fujiwara [Page 2] Internet-Draft 3761bis 4 May 2009 7.4. URI Resolution Security . . . . . . . . . . . . . . . . . . 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9. Changes from RFC 3761 . . . . . . . . . . . . . . . . . . . . 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10.1. Normative References . . . . . . . . . . . . . . . . . . . 11.2. Informative References . . . . . . . . . . . . . . . . . . Editor's Address . . . . . . . . . . . . . . . . . . . . . . . . . Copyright Statement . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction This document discusses the use of the Domain Name System (DNS) for the storage of E.164 [E164] numbers, and for resolving them into URIs that can be used for (for example) telephony call setup. This document also describes how the DNS can be used to identify the services associated with an E.164 number. This document includes a Dynamic Delegation Discovery System (DDDS) Application specification, as detailed in the document series described in [RFC3401]. This document obsoletes [RFC3761]. Using the process defined in this document, International Public Telecommunication Numbers in the international format defined in ITU Recommendation E.164 [E164] (called here "E.164 numbers") can be transformed into DNS names. Using existing DNS services (such as delegation through NS records and queries for NAPTR resource records), one can look up the services associated with that E.164 number. This takes advantage of standard DNS architectural features of decentralized control and management of the different levels in the lookup process. The domain "e164.arpa" has been assigned to provide the infrastructure in DNS for storage of E.164 numbers. In order to facilitate distributed operations, this domain is divided into subdomains. Holders of E.164 numbers which want the numbers to be listed in the DNS should contact the appropriate zone administrator as listed in the policy attached to the zone. One should start looking for this information by examining the SOA resource record associated with the zone, just like in normal DNS operations. Of course, as with other domains, policies for such listings will be controlled on a subdomain basis and may differ in different parts of the world. 1.1. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 [RFC2119]. Bradner, Conroy & Fujiwara [Page 3] Internet-Draft 3761bis 4 May 2009 All other capitalized terms are taken from the vocabulary found in the DDDS algorithm specification found in RFC 3402 [RFC3402]. 1.2. Use of These Mechanisms for Private Dialing Plans This document describes the operation of these mechanisms in the context of numbers allocated according to the ITU-T recommendation E.164. The same mechanisms might be used for private dialing plans. If these mechanisms are re-used, the suffix used for the private dialing plan MUST NOT be e164.arpa, to avoid conflict with this specification. Parties to the private dialing plan will need to know the suffix used by their private dialing plan for correct operation of these mechanisms. Further, the application unique string used SHOULD be the full number as specified, but without the leading '+', and such private use MUST NOT be called "ENUM". 2. The ENUM Application Specifications This template defines the ENUM DDDS Application according to the rules and requirements found in [RFC3402]. The DDDS database used by this Application is found in [RFC3403], which is the document that defines the NAPTR DNS Resource Record type. ENUM is only applicable for E.164 numbers. ENUM compliant applications MUST only query DNS for what it believes is an E.164 number. Since there are numerous dialing plans which can change over time, it is probably impossible for a client application to have perfect knowledge about every valid and dialable E.164 number. Therefore a client application, doing everything within its power, can end up with what it thinks is a syntactically correct E.164 number which in reality is not actually valid or dialable. This implies that applications MAY send DNS queries when, for example, a user mistypes a number in a user interface. Because of this, there is the risk that collisions between E.164 numbers and non-E.164 numbers can occur. To mitigate this risk, the "E2U" token MUST NOT be provisioned into the services field of NAPTRs in domains associated with non-E.164 numbers. 2.1. Application Unique String The Application Unique String is a fully qualified E.164 number minus any non-digit characters except for the '+' character which appears at the beginning of the number. The "+" is kept to provide a well understood anchor for the AUS in order to distinguish it from other telephone numbers that are not part of the E.164 namespace. For example, the E.164 number could start out as "+44-116-496-0348". To ensure that no syntactic sugar is allowed into the AUS, all non- digits except for "+" are removed, yielding "+441164960348". Bradner, Conroy & Fujiwara [Page 4] Internet-Draft 3761bis 4 May 2009 2.2. First Well Known Rule The First Well Known Rule for this Application is the identity rule. The output of this rule is the same as the input. This is because the E.164 namespace and this Application's database are organized in such a way that it is possible to go directly from the name to the smallest granularity of the namespace directly from the name itself. Take the previous example, the AUS is "+441164960348". Applying the First Well Known Rule produces the exact same string, "+441164960348". 2.3. Expected Output The output of the last DDDS loop is a Uniform Resource Identifier in its absolute form according to the 'absoluteURI' production in the Collected ABNF found in [RFC3986]. 2.4. Valid Databases At present only one DDDS Database is specified for this Application. "Dynamic Delegation Discovery System (DDDS) Part Three: The DNS Database" [RFC3403] specifies a DDDS Database that uses the NAPTR DNS resource record to contain the rewrite rules. The Keys for this database are encoded as domain names. 2.4.1 Initial Key Construction The output of the First Well Known Rule for the ENUM Application is the E.164 number minus all non-digit characters except for the "+". In order to convert this to a unique key in this Database the string is converted into a domain name according to this algorithm: 1. Remove all characters with the exception of the digits. For example, example, given the E.164 number "+44-20-7946-0148" the First Well Known Rule produces the string "+442079460148". This step would simply remove the leading "+", producing "442079460148". 2. Put dots (".") between each digit. Example: 4.4.2.0.7.9.4.6.0.1.4.8 3. Reverse the order of the digits. Example: 8.4.1.0.6.4.9.7.0.2.4.4 4. Append the string ".e164.arpa." to the end. Example: 8.4.1.0.6.4.9.7.0.2.4.4.e164.arpa. This domain name is used to request NAPTR records which may contain the end result or, if the flags field is empty, produces new keys in the form of domain names from the DNS. The character set used to encode the substitution expression is UTF-8. The allowed input characters are all those characters that are allowed anywhere in an E.164 number. The characters allowed to Bradner, Conroy & Fujiwara [Page 5] Internet-Draft 3761bis 4 May 2009 be in a Key are those that are currently defined for DNS domain names. 2.4.2. Optional Name Server Additional Section Processing Some nameserver implementations attempt to be intelligent about items that are inserted into the additional information section of a given DNS response. For example, BIND will attempt to determine if it is authoritative for a domain whenever it encodes one into a packet. If it is, then it will insert any A records it finds for that domain into the additional information section of the answer until the packet reaches the maximum length allowed. It is therefore potentially useful for a client to check for this additional information. It is also easy to contemplate an ENUM enhanced nameserver that understands the actual contents of the NAPTR records it is serving and inserts more appropriate information into the additional information section of the response. Thus, DNS servers MAY interpret Flag values and use that information to include appropriate resource records in the Additional Information portion of the DNS packet. Clients are encouraged to check for additional information but are not required to do so. See the Additional Information Processing section of [RFC3403], Section 4.2 for more information on NAPTR records and the Additional Information section of a DNS response packet. 2.4.3. Flags This Database contains a field that contains flags that signal when the DDDS algorithm has finished. At this time only one flag, "U", is defined. This means that this Rule is the last one and that the output of the Rule is a URI [RFC3986]. See [RFC3404]. If a client encounters a record with an unknown flag, it MUST ignore it and move to the next Rule. This test takes precedence over any ordering since flags can control the interpretation placed on fields. A novel flag might change the interpretation of the regexp and/or replacement fields such that it is impossible to determine if a record matched a given target. If this flag is not present then this rule is non-terminal. If a Rule is non-terminal then clients MUST use the Key produced by this Rewrite Rule as the new Key in the DDDS loop (i.e., causing the client to query for new NAPTR records at the domain name that is the result of this Rule). 2.4.4. Services Parameters Bradner, Conroy & Fujiwara [Page 6] Internet-Draft 3761bis 4 May 2009 Service Parameters for this Application take the following form and are found in the Service field of the NAPTR record that holds a terminal rule. Where the NAPTR holds a non-terminal Rule, the Services field SHOULD be empty, and clients SHOULD ignore its content. service-field = "E2U" 1*(servicespec) servicespec = "+" enumservice enumservice = type 0*(subtypespec) subtypespec = ":" subtype type = 1*32(ALPHA / DIGIT / "-") subtype = 1*32(ALPHA / DIGIT / "-") In other words, a non-optional "E2U" (used to denote ENUM only Rewrite Rules in order to mitigate record collisions) followed by one or more Enumservices which indicate the class of functionality a given end point offers. Each Enumservice is indicated by an initial '+' character. 2.4.4.1. ENUM Services Enumservices may be specified and registered via the process defined in "Guide and Template for IANA Registrations of Enumservices" [SV_GUIDE]. This registration process is not open to any Enumservice that has '-' as the second character in its type string. In particular, this registration process is not open to Enumservice types starting with the facet "X-". This "X-" facet is reserved for experimental or trial use, and any such Enumservices cannot be registered using the normal process. Finally, any Enumservice type that starts with the facet "P-" is intended for use exclusively on private networks. As such, NAPTRs containing Enumservice types starting "P-" should not be seen on the global Internet. Even if an ENUM client recognizes and can engage in the Enumservice, it may be incapable of resolving the URI generated by the containing NAPTR. These Enumservices WILL NOT be registered. Such Enumservices MUST NOT be provisioned in any system that provides answers to DNS queries for NAPTR resource record sets from entities outside the private network context in which these Enumservices are intended for use. Unless an ENUM client is sure that it is connected to the private network for which these NAPTRs are provisioned and intended, it MUST discard any NAPTR with an Enumservice type that starts with the "P-" facet. 2.4.4.2. Compound NAPTRs and Implicit ORDER/PREFERENCE Values It is possible to have more than one Enumservice associated with a single NAPTR. These Enumservices share the same Regexp field and so Bradner, Conroy & Fujiwara [Page 7] Internet-Draft 3761bis 4 May 2009 generate the same URI. Such a "compound" NAPTR could well be used to indicate a mobile phone that supports both "voice:tel" and "sms:tel" Enumservices. The Services field in that case would be "E2U+voice:tel+sms:tel". A compound NAPTR can be treated as a set of NAPTRs that each hold a single Enumservice. These reconstructed NAPTRs share the same ORDER and PREFERENCE/PRIORITY field values but should be treated as if each had a logically different priority. ENUM clients SHOULD process the Enumservices within a compound NAPTR in a left-to-right sequence. ENUM provisioning systems SHOULD assume that such a processing order will be used and provision the Enumservices within a compound NAPTR accordingly. 2.5. The ENUM Algorithm Always Returns a Single Rule The ENUM algorithm always returns a single rule. Specific applications may have application-specific knowledge or facilities that allow them to present multiple results or speed selection, but these should never change the operation of the algorithm. 2.6. Case Sensitivity in ENUM The only place where NAPTR field content is case sensitive is in any static text in the Repl sub-field of the Regexp field. Everywhere else, case-insensitive processing SHOULD be used. 3. ENUM Clients 3.1. Unsupported NAPTRs An ENUM client MAY discard a NAPTR received in response to an ENUM query because: o the NAPTR is syntactically or semantically incorrect, o the NAPTR has a different (non-empty) DDDS Application identifier from the 'E2U' used in ENUM, o the NAPTR's Extended Regular Expression (ERE) does not match the Application Unique String for this ENUM query, o the ENUM client does not recognize any Enumservice in that NAPTR, o this NAPTR (only) contains an Enumservice that is unsupported. These conditions SHOULD NOT cause the whole ENUM query to terminate, and processing SHOULD continue with the next NAPTR in the returned Resource Record Set (RRSet). When an ENUM client encounters a compound NAPTR (i.e., one containing more than one Enumservice -- see Section 2.4.4.2) and cannot process or cannot recognize one of the Enumservices within it, that ENUM client SHOULD ignore this Enumservice and continue with the next Enumservice within this NAPTR's Services field, discarding the NAPTR only if it cannot handle any of the Enumservices contained. These Bradner, Conroy & Fujiwara [Page 8] Internet-Draft 3761bis 4 May 2009 conditions SHOULD NOT be considered errors. ENUM uses regular-expression processing when generating URIs from the Regexp field of "terminal" NAPTRs. Just as with all uses of regular expressions, there is a potential for buffer overrun when generating this output. There may be repeated back-reference patterns in a NAPTR's Repl sub-field, and the output these generate may consume a considerable amount of buffer space. Even if an ENUM client would normally encounter only NAPTRs with short URIs, it may also receive NAPTRs with repeated back-reference patterns in their Repl sub-fields that could generate strings longer than the client's buffer. Such NAPTRs may have been misconfigured accidentally or by design. The client MUST NOT fail in this case. It SHOULD NOT discard the entire ENUM query, but instead just discard the NAPTR that would otherwise have caused this overrun. If a problem is detected when processing an ENUM query across multiple domains (by following non-terminal NAPTR references), then the ENUM query SHOULD NOT be abandoned, but instead processing SHOULD continue at the next NAPTR after the non-terminal NAPTR that referred to the domain in which the problem would have occurred. See Section 3.3.2.2 for more details. 3.2. ENUM NAPTR Processing ENUM is a DDDS Application, and the way in which NAPTRs in an RRSet are processed reflects this. The details are described in Section 3.3 of RFC3402]. The client is expected to sort the records it receives into a sequence and then process those records in that sequence. The sequence reflects the ORDER field value ("lowest value is first") and PREFERENCE/PRIORITY field value (again, lowest value first) in each of the NAPTRs. The ORDER field value is the major, or most significant, sort term and the PREFERENCE/PRIORITY field value is the minor, or least significant, sort term. The combination of ORDER and PREFERENCE/PRIORITY field values indicates the sequence chosen by the publisher of this data, and NAPTRs will be considered in this sequence. Subsequent NAPTRs with worse ORDER values MUST only be dealt with once the current ones with a better ORDER value have been processed. Once sorted into a sequence reflecting ORDER and PREFERENCE/PRIORITY values, other fields are also considered during evaluation of retrieved NAPTRs. ENUM clients will take into account the Flags field value, the Services field value, and the Regexp ERE sub-field, along with the ORDER and PREFERENCE/PRIORITY field values. Local policies or local knowledge may play a factor in the decision process, once a Bradner, Conroy & Fujiwara [Page 9] Internet-Draft 3761bis 4 May 2009 NAPTR has reached that point in the sequence at which it is considered. 3.2.1. Use of Order and Preference fields NAPTRs in ENUM zones that hold incorrect ORDER values can cause major problems. [RFC3403] highlights that having both ORDER and PREFERENCE/PRIORITY fields is a historical artifact of the NAPTR resource record type. It is reasonable to have a common default value for the ORDER field, relying on the PREFERENCE/PRIORITY field to indicate the preferred sort. The ORDER field value is the major sort term, and the PREFERENCE/PRIORITY field value is the minor sort term. Thus, one should expect to have a set of NAPTRs in a zone with identical ORDER field values and different PREFERENCE/PRIORITY field values; not the other way around. To avoid common interoperability issues, it is recommended that ENUM NAPTRs SHOULD hold a default value in their ORDER field. 3.2.2. NAPTRs with Identical ORDER/PRIORITY Values From experience, it has been learned that there are zones that hold discrete NAPTRs with identical ORDER and identical PREFERENCE/ PRIORITY field values. This will lead to indeterminate client behavior and so SHOULD NOT normally occur. Such a condition indicates that these NAPTRs are truly identical in priority and that there is no preference between the services these NAPTRs offer. Implementers SHOULD NOT assume that the DNS will deliver NAPTRs within an RRSet in a particular sequence. 3.2.3. Processing Order Value Across Domains Using a different ORDER field value in different domains is unimportant for most queries. However, DDDS includes a mechanism for continuing a search for NAPTRs in another domain by including a reference to that other domain in a "non-terminal" NAPTR. The treatment of non-terminal NAPTRs is covered in the next section. If they are supported, then the way that ORDER and PREFERENCE/PRIORITY field values are processed is affected. ENUM implementations MUST consider the ORDER and PREFERENCE/PRIORITY values only within the context of the domain currently being processed in an ENUM query. These values MUST be disregarded when processing other RRSets in the query. 3.3. Non-Terminal NAPTR Processing Bradner, Conroy & Fujiwara [Page 10] Internet-Draft 3761bis 4 May 2009 3.3.1. Non-Terminal NAPTRs - Necessity Consider an ENUM RRSet that contains a non-terminal NAPTR record. This non-terminal NAPTR holds, as its target, another domain that has a set of NAPTRs. In effect, this is similar to the non-terminal NAPTR being replaced by the NAPTRs contained in the domain to which it points. It is possible to have a non-terminal NAPTR in a domain that is, itself, pointed to by another non-terminal NAPTR. Thus, a set of domains forms a "chain", and the list of NAPTRs to be considered is the set of all NAPTRs contained in all of the domains in that chain. Given that, in principle, a non-terminal NAPTR can be replaced by the NAPTRs in the domain to which it points, support of non-terminal NAPTRs is not needed and non-terminal NAPTRs may not be useful. Furthermore, some existing ENUM clients do not support non-terminal NAPTRs and ignore them if received. As current support is limited, non-terminal NAPTRs SHOULD NOT be used in ENUM unless it is clear that all of the ENUM clients this environment supports can process these. 3.3.2. Non-Terminal NAPTRs - Considerations The following specific issues need to be considered if non-terminal NAPTRs are to be supported in a particular environment. These issues are gleaned from experience and indicate the kinds of conditions that should be considered before support for non-terminal NAPTRs is contemplated. 3.3.2.1. Non-Terminal NAPTRs - General A non-terminal NAPTR in one RRSet refers to the NAPTRs contained in another domain. The NAPTRs in the domain referred to by the non- terminal NAPTR may have a different ORDER value from that in the referring non-terminal NAPTR. See Section 3.2.3 for details. 3.3.2.2. Non-Terminal NAPTRs - Loop Detection and Response Where a chain of non-terminal NAPTRs refers back to a domain already traversed in the current query, a "non-terminal" or referential loop is implied. An implementation MAY treat a chain of more than 5 domains traversed during a single ENUM query as an indication that a self-referential loop has been entered. There are many techniques that can be used to detect such a loop, but the simple approach of counting the number of domains queried in the current ENUM query suffices. Where a loop has been detected, processing SHOULD continue at the next NAPTR in the referring domain (i.e., after the non-terminal NAPTR that included the reference that triggered the loop detection). Bradner, Conroy & Fujiwara [Page 11] Internet-Draft 3761bis 4 May 2009 3.3.2.3. Field content in Non-Terminal NAPTRs The set of specifications defining DDDS and its applications are complex and multi-layered. This reflects the flexibility that the system provides but does mean that some of the specifications need clarification as to their interpretation, particularly where non- terminal rules are concerned. 3.3.2.3.1. Flags Field Content with Non-Terminal NAPTRs The Flags field will be empty in non-terminal NAPTRs encountered in ENUM processing. ENUM does not have any other way to indicate a non- terminal NAPTR. 3.3.2.3.2. Services field Content with Non-Terminal NAPTRs In a non-terminal NAPTR encountered in an ENUM query, the Services field SHOULD be empty, and clients SHOULD ignore any content it contains. Non-terminal NAPTRs with an empty Services field are not specific to any DDDS Application. Thus, other means must be used to ensure a non-terminal NAPTR that is intended only for a particular DDDS Application cannot be encountered during a lookup for another DDDS Application (for example, by ensuring that the same domain is not used to host NAPTRs for more than one such DDDS Application). 3.3.2.3.3. Regular Expression and Replacement Field Content with Non- Terminal NAPTRs [RFC3403] is specific; Regexp and Replacement fields are mutually exclusive. This means that if the Regexp element is not empty, then the Replacement element must be empty, and vice versa. However, [RFC3403] does not specify which is used with terminal and non- terminal rules. A Replacement element cannot be used in ENUM for terminal rules, as only non-terminal rules ("non-terminal NAPTRs") have a domain as their output in ENUM: terminal ENUM rules generate a URI instead. The alternative Regexp element may be used either to generate a domain name as the next key to be used in the non-terminal case or to generate the output of the DDDS Application. However, generating an ENUM domain name from the Regexp field is difficult at best and impossible for the general case of a variable- length telephone number, or one that has more than 9 digits. Thus, the Regexp field is inappropriate for non-terminal rules encountered during ENUM processing. To clarify, the target domain of a non-terminal ENUM NAPTR MUST be placed in the (non-empty) Replacement field. This field MUST be interpreted as holding the domain name that forms the next key output Bradner, Conroy & Fujiwara [Page 12] Internet-Draft 3761bis 4 May 2009 from this non-terminal rule. Conversely, the Regexp field MUST be empty in a non-terminal NAPTR encountered in ENUM processing, and ENUM clients MUST ignore its content. 3.4. Backwards Compatibility 3.4.1. Services Field Syntax [RFC2915] and [RFC2916] have been obsoleted by [RFC3401] - [RFC3404] and by this document. However, [RFC3824] suggests that ENUM clients should be prepared to accept NAPTRs with the obsolete syntax. Thus, an ENUM client implementation may have to deal with both forms. This need not be difficult. For example, an implementation could process the Services field into a set of tokens and expect exactly one of these tokens to be "E2U". In this way, the ENUM client might be designed to handle both the old and the current forms without added complexity. To facilitate this method, IANA should reject any request to register an Enumservice with the label "E2U", and this request is included in the IANA considerations of [SV_GUIDE]. ENUM clients MUST support ENUM NAPTRs according to the service field syntax specified in Section 2.4.2. ENUM clients SHOULD also support ENUM NAPTRs according to the obsolete syntax of [RFC2916]; there are still zones that hold "old" syntax NAPTRs. ENUM zones MUST NOT be provisioned with NAPTRs according to the obsolete form defined in [RFC2916], and MUST be provisioned with NAPTRs in which the Services field is according to the specification of Section 2.4.2 of this document. 3.5. Collected Implications for ENUM Clients ENUM clients SHOULD NOT discard NAPTRs in which they detect characters outside the US-ASCII printable range (0x20 to 0x7E hexadecimal). ENUM clients MAY discard NAPTRs that have octets in the Flags, Services, or Regexp fields that have byte values outside the US-ASCII equivalent range (i.e., byte values above 0x7F). Clients MUST be ready to encounter NAPTRs with such values without failure. ENUM clients SHOULD NOT assume that the delimiter is the last character of the Regexp field. Unless they are sure that in their environment this is the case, in general an ENUM client may still encounter NAPTRs that have been provisioned with a following 'i' (case-insensitive) flag, even though that flag has no effect at all in an ENUM scenario. Bradner, Conroy & Fujiwara [Page 13] Internet-Draft 3761bis 4 May 2009 ENUM clients SHOULD discard NAPTRs that have more or less than 3 unescaped instances of the delimiter character within the Regexp field. In the spirit of being liberal with what it will accept, if the ENUM client is sure how the Regexp field should be interpreted, then it may choose to process the NAPTR even in the face of an incorrect number of unescaped delimiter characters. If it is not clear how the Regexp field should be interpreted, then the client must discard the NAPTR. Where the ENUM client presents a list of possible URLs to the end user for his or her choice, it MAY present all NAPTRs -- not just the ones with the highest currently unprocessed ORDER field value. The client SHOULD keep to the ORDER and PREFERENCE/PRIORITY values specified by the Registrant. ENUM clients SHOULD accept all NAPTRs with identical ORDER and identical PREFERENCE/PRIORITY field values, and process them in the sequence in which they appear in the DNS response. (There is no benefit in further randomizing the order in which these are processed, as intervening DNS Servers might have done this already). ENUM clients receiving compound NAPTRs (i.e., ones with more than one Enumservice) SHOULD process these Enumservices using a left-to-right sort ordering, so that the first Enumservice to be processed will be the leftmost one, and the last will be the rightmost one. ENUM clients SHOULD consider the ORDER field value only when sorting NAPTRs within a single RRSet. The ORDER field value SHOULD NOT be taken into account when processing NAPTRs across a sequence of DNS queries created by traversal of non-terminal NAPTR references. ENUM clients MUST be ready to process NAPTRs that use a different character from '!' as their Regexp Delimiter without failure. ENUM clients MUST be ready to process NAPTRs that have non-trivial patterns in their ERE sub-field values without failure. ENUM clients MUST be ready to process NAPTRs with a DDDS Application identifier other than 'E2U' without failure. ENUM clients MUST be ready to process NAPTRs with many copies of back-reference patterns within the Repl sub-field without failure (see also Section 3.1). If a NAPTR is discarded, this SHOULD NOT cause the whole ENUM query to terminate and processing SHOULD continue with the next NAPTR in Bradner, Conroy & Fujiwara [Page 14] Internet-Draft 3761bis 4 May 2009 the returned Resource Record Set (RRSet). When an ENUM client encounters a compound NAPTR (i.e., one containing more than one Enumservice) and cannot process or cannot recognize one of the Enumservices within it, that ENUM client SHOULD ignore this Enumservice and continue with the next Enumservice within this NAPTR's Services field, discarding the NAPTR only if it cannot handle any of the Enumservices contained. These conditions SHOULD NOT be considered errors. ENUM clients MUST support ENUM NAPTRs according to syntax defined in Section 2.4.2. ENUM clients SHOULD also support ENUM NAPTRs according to the obsolete syntax of [RFC2916]; there are still zones that hold "old" syntax NAPTRs. Unless an ENUM client is sure that it is connected to the private network for which these NAPTRs are provisioned and intended, it MUST discard any NAPTR with an Enumservice type that starts with the "P-" facet. 3.5.1. Non-terminal NAPTR Processing ENUM clients MUST be ready to process NAPTRs with an empty Flags field ("non-terminal" NAPTRs) without failure. More generally, non- terminal NAPTR processing SHOULD be implemented, but ENUM clients MAY discard non-terminal NAPTRs they encounter. ENUM clients SHOULD ignore any content of the Services field when encountering a non-terminal NAPTR with an empty Flags field. ENUM clients receiving a non-terminal NAPTR with an empty Flags field MUST treat the Replacement field as holding the domain name to be used in the next round of the ENUM query. An ENUM client MUST discard such a non-terminal NAPTR if the Replacement field is empty or does not contain a valid domain name. By definition, it follows that the Regexp field will be empty in such a non-terminal NAPTR. If present in a non-terminal NAPTR, a non-empty Regexp field MUST be ignored by ENUM clients. If a problem is detected when processing an ENUM query across multiple domains (by following non-terminal NAPTR references), then the ENUM query SHOULD NOT be abandoned, but instead processing SHOULD continue at the next NAPTR after the non-terminal NAPTR that referred to the domain in which the problem would have occurred. If all NAPTRs in a domain traversed as a result of a reference in a non-terminal NAPTR have been discarded, then the ENUM client SHOULD continue its processing with the next NAPTR in the "referring" RRSet (i.e., the one including the non-terminal NAPTR that caused the traversal). Bradner, Conroy & Fujiwara [Page 15] Internet-Draft 3761bis 4 May 2009 ENUM clients MAY consider a chain of more than 5 "non-terminal" NAPTRs traversed in a single ENUM query as an indication that a referential loop has been entered. Where a domain is about to be entered as the result of a reference in a non-terminal NAPTR, and the ENUM client has detected a potential referential loop, then the client SHOULD discard the non-terminal NAPTR from its processing and continue with the next NAPTR in its list. It SHOULD NOT make the DNS query indicated by that non- terminal NAPTR. 4. ENUM Service Example $ORIGIN 3.8.0.0.6.9.2.3.6.1.4.4.e164.arpa. NAPTR 100 50 "u" "E2U+sip" "!^.*$!sip:info@example.com!" . NAPTR 100 51 "u" "E2U+h323" "!^.*$!h323:info@example.com!" . NAPTR 100 52 "u" "E2U+email:mailto" "!^.*$!mailto:info@example.com!" . This describes that the domain 3.8.0.0.6.9.2.3.6.1.4.4.e164.arpa. is preferably contacted by SIP, secondly via H.323 for voice, and thirdly by SMTP for messaging. Note that the Enumservice tokens "sip", "h323", and "email" are Enumservice Types registered with IANA, and they have no implicit connection with the protocols or URI schemes with the same names. In all cases, the next step in the resolution process is to use the resolution mechanism for each of the protocols, (specified by the URI schemes sip, h323 and mailto) to know what node to contact. 5. Collected Implications for ENUM Provisioning ENUM NAPTRs SHOULD NOT include characters outside the printable US- ASCII equivalent range (U+0020 to U+007E) unless it is clear that all ENUM clients they are designed to support will be able to process such characters correctly. If ENUM zone provisioning systems require non-ASCII characters, these systems SHOULD encode the non-ASCII data to emit only US-ASCII characters by applying the appropriate mechanism ([RFC3492], [RFC3987]). Non-printable characters SHOULD NOT be used, as ENUM clients may need to present NAPTR content in a human-readable form. The case-sensitivity flag ('i') is inappropriate for ENUM, and SHOULD NOT be provisioned into the Regexp field of E2U NAPTRs. ENUM zone provisioning systems SHOULD use '!' (U+0021) as their Regexp delimiter character. Bradner, Conroy & Fujiwara [Page 16] Internet-Draft 3761bis 4 May 2009 If the Regexp delimiter is a character in the static text of the Repl sub-field, it MUST be "escaped" using the escaped-delimiter production of the BNF specification shown in Section 3.2 of [RFC3402] (i.e., "\!", U+005C U+0021). Note that when a NAPTR resource record is entered in DNS master file syntax, the backslash itself must be escaped using a second backslash. If present in the ERE sub-field of an ENUM NAPTR, the literal character '+' MUST be escaped as "\+" (i.e. U+005C U+002B). Note that, as always, when a NAPTR resource record is entered in DNS master file syntax, the backslash itself must be escaped using a second backslash. The Registrant and the ENUM zone provisioning system he or she uses SHOULD NOT rely on ENUM clients solely taking account of the value of the ORDER and the PREFERENCE/PRIORITY fields in ENUM NAPTRs. Thus, a Registrant SHOULD place into his or her zone only contacts that he or she is willing to support; even those with the worst ORDER and PREFERENCE/PRIORITY values MAY be selected by an end user. Many apparent mistakes in ORDER and PREFERENCE/PRIORITY values have been detected in provisioned ENUM zones. To avoid these common interoperability issues, provisioning systems SHOULD NOT use different ORDER field values for NAPTRs in a Resource Record Set (RRSet). To generalize, all ENUM NAPTRs SHOULD hold a default value in their ORDER field. A value of "100" is recommended, as it seems to be used in most provisioned domains. Multiple NAPTRs with identical ORDER and identical PREFERENCE/ PRIORITY field values SHOULD NOT be provisioned into an RRSet unless the intent is that these NAPTRs are truly identical and there is no preference between them. Implementers SHOULD NOT assume that the DNS will deliver NAPTRs within an RRSet in a particular sequence. An ENUM zone provisioning system SHOULD assume that, if it generates compound NAPTRs, the Enumservices will normally be processed in left- to-right order within such NAPTRs. ENUM zone provisioning systems SHOULD assume that, once a non- terminal NAPTR has been selected for processing, the ORDER field value in a domain referred to by that non-terminal NAPTR will be considered only within the context of that referenced domain (i.e., the ORDER value will be used only to sort within the current RRSet and will not be used in the processing of NAPTRs in any other RRSet). Whilst this client behavior is non-compliant, ENUM provisioning systems and their users should be aware that some ENUM clients have been detected with poor (or no) support for non-trivial ERE sub-field Bradner, Conroy & Fujiwara [Page 17] Internet-Draft 3761bis 4 May 2009 expressions. ENUM provisioning systems SHOULD be cautious in the use of multiple back-reference patterns in the Repl sub-field of NAPTRs they provision. Some clients have limited buffer space for character expansion when generating URIs (see also Section 3.1). These provisioning systems SHOULD check the back-reference replacement patterns they use, ensuring that regular expression processing will not produce excessive-length URIs. As current support is limited, non-terminal NAPTRs SHOULD NOT be provisioned in ENUM zones unless it is clear that all ENUM clients that this environment supports can process these. When populating a set of domains with NAPTRs, ENUM zone provisioning systems SHOULD NOT configure non-terminal NAPTRs so that more than 5 such NAPTRs will be processed in an ENUM query. In a non-terminal NAPTR encountered in an ENUM query (i.e., one with an empty Flags field), the Services field SHOULD be empty. A non-terminal NAPTR MUST include its target domain in the (non- empty) Replacement field. This field MUST be interpreted as holding the domain name that forms the next key output from this non-terminal rule. The Regexp field MUST be empty in a non-terminal NAPTR intended to be encountered during an ENUM query. ENUM zones MUST NOT be provisioned with NAPTRs according to the obsolete form, and MUST be provisioned with NAPTRs in which the Services field is according to Section 2.4.2 of this document. Enumservices in which the Enumservice type starts with the facet "P-" MUST NOT be provisioned in any system that provides answers to DNS queries for NAPTR resource record sets from entities outside the private network context in which these Enumservices are intended for use. 6. IANA Considerations RFC 2916 and then RFC 3761 (which this document replaces) requested IANA to delegate the E164.ARPA domain following instructions to be provided by the IAB. The domain was delegated according to those instructions. Names within this zone are to be delegated to parties according to the ITU-T Recommendation E.164. The names allocated should be hierarchic in accordance with ITU-T Recommendation E.164, and the codes should be assigned in accordance with that Recommendation. Bradner, Conroy & Fujiwara [Page 18] Internet-Draft 3761bis 4 May 2009 The IAB is to coordinate with ITU-T TSB if the technical contact for the domain e164.arpa is to change, as ITU-T TSB has an operational working relationship with this technical contact which needs to be reestablished. Delegations in the zone e164.arpa (not delegations in delegated domains of e164.arpa) should be done after Expert Review, and the IESG will appoint a designated expert. See [SV_GUIDE] for Enumservice-related IANA Considerations. 7. Security Considerations 7.1. DNS Security As ENUM uses DNS, which in its current form is an insecure protocol, there is no mechanism for ensuring that the data one gets back is authentic. As ENUM is deployed on the global Internet, it is expected to be a popular target for various kind of attacks, and attacking the underlying DNS infrastructure is one way of attacking the ENUM service itself. There are multiple types of attacks that can happen against DNS that ENUM implementations should consider. See Threat Analysis of the Domain Name System [RFC3833] for a review of the various threats to the DNS. Because of these threats, a deployed ENUM service SHOULD include mechanisms to ameliorate these threats. Most of the threats can be solved by verifying the authenticity of the data via mechanisms such as DNS Security (DNSSEC) [RFC4033]. Others, such as Denial Of Service attacks, cannot be solved by data authentication. It is important to remember that these threats include not only the NAPTR lookups themselves, but also the various records needed for the services to be useful (for example NS, MX, SRV and A records). Even if DNSSEC is deployed, a service that uses ENUM for address translation should not blindly trust that the peer is the intended party as DNSSEC deployment cannot protect against every kind of attack on DNS. A service should always authenticate the peers as part of the setup process for the service itself and never blindly trust any kind of addressing mechanism. Finally, as an ENUM service will be implementing some type of security mechanism, software which implements ENUM MUST be prepared to receive DNSSEC and other standardized DNS security responses, including large responses, EDNS0 signaling, unknown RRs, and so on. Bradner, Conroy & Fujiwara [Page 19] Internet-Draft 3761bis 4 May 2009 7.2. Caching Security The caching in DNS can make the propagation time for a change take the same amount of time as the time to live for the NAPTR records in the zone that is changed. The use of this in an environment where IP-addresses are dynamically assigned (for example, when using DHCP [RFC2131]) must therefore be done very carefully. 7.3. Call Routing Security There are a number of countries (and other numbering environments) in which there are multiple providers of call routing and number/name- translation services. In these areas, any system that permits users, or putative agents for users, to change routing or supplier information may provide incentives for changes that are actually unauthorized (and, in some cases, for denial of legitimate change requests). Such environments should be designed with adequate mechanisms for identification and authentication of those requesting changes and for authorization of those changes. 7.4. URI Resolution Security A large amount of Security Issues have to do with the resolution process itself, and use of the URIs produced by the DDDS mechanism. Those have to be specified in the registration of the Enumservice used, as specified in "Guide and Template for IANA Registrations of Enumservices" [SV_GUIDE]. 8. Acknowledgements This document is an update of RFC 3761, which was edited by Patrik Faltstrom and Michael Mealling. Please see the Acknowledgements section in that RFC for additional acknowledgements. 9. Changes from RFC 3761 Two sections have been added explaining the implied protocol requirements for use of NAPTRs according to this specification. These have been collected from experience of ENUM deployment. Clarifications include the required use of Replacement field in non- terminal NAPTRs (Section 3.3.2.3.3) and that string matching is case insensitive (Section 2.6). Substantive changes include removing the discussion of registration mechanisms, (now specified in "Guide and Template for IANA Registrations of Enumservices" [SV_GUIDE]), adding "-" as a valid character in the type and subtype fields in the Services Parameters (Section 2.4.4) and adding the "P-" private service type (Section 3.4). 10. References Bradner, Conroy & Fujiwara [Page 20] Internet-Draft 3761bis 4 May 2009 10.1. Normative References [E164] ITU-T, "The International Public Telecommunication Number Plan", Recommendation E.164, February 2005. [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [RFC3402] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part Two: The Algorithm", RFC 3402, October 2002. [RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part Three: The Domain Name System (DNS) Database", RFC 3403, October 2002. [RFC3404] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part Four: The Uniform Resource Identifiers (URI)", RFC 3404, October 2002. [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, March 2003. [RFC3761] Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM)", RFC 3761, April 2004. [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, January 2005. [SV_GUIDE] Hoeneisen, B., Mayrhofer, A., and J. Livingood, "Guide and Template for IANA Registrations of Enumservices", draft-ietf-enum- enumservices-guide-06.txt (work in progress), November 2007. 10.2. Informative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [RFC3401] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part One: The Comprehensive DDDS", RFC 3401, October 2002. [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain Name System (DNS)", RFC 3833, August 2004. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. Author's Addresses Scott Bradner Harvard University 29 Oxford St. Cambridge MA 02138 Phone: +1 617 495 3864 Email: sob@harvard.edu Bradner, Conroy & Fujiwara [Page 21] Internet-Draft 3761bis 4 May 2009 Lawrence Conroy Roke Manor Research Roke Manor Old Salisbury Lane Romsey United Kingdom Phone: +44-1794-833666 Email: lconroy@insensate.co.uk URI: http://www.sienum.co.uk Kazunori Fujiwara Japan Registry Service Co., Ltd. Chiyoda First Bldg. East 13F 3-8-1 Nishi-Kanda Chiyoda-ku Tokyo 101-0165 JAPAN Email: fujiwara@jprs.co.jp URI: http://jprs.jp/en/ Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. change log - RFC Editor - please remove this section for publication version 01 -> 02 clean up English - many places removed Registration mechanism for Enumservices section removed IANA considerations - point to draft-ietf-enum-enumservices- guide ,br replace DNS Security Threats in section 6.1 with a pointer to RFC 3833 fold in text from the ENUM Experiences ID - many places version 02 -> 03 fixed minor typos revised section 2.4.4.1, added P- expanded IANA Considerations - Section 6 version 03 -> 04 Many changes to bring into sync with RFC 5483 Bradner, Conroy & Fujiwara [Page 22]