Internet Engineering Task Force (IETF) M. Jones Request for Comments: 6408 Bridgewater Systems Updates: 3588 J. Korhonen Category: Standards Track Nokia Siemens Networks ISSN: 2070-1721 L. Morand Orange Labs November 2011
The Diameter base protocol specifies mechanisms whereby a given realm may advertise Diameter nodes and the supported transport protocol. However, these mechanisms do not reveal the Diameter applications that each node supports. A peer outside the realm would have to perform a Diameter capability exchange with every node until it discovers one that supports the required application. This document updates RFC 3588, "Diameter Base Protocol", and describes an improvement using an extended format for the Straightforward-Naming Authority Pointer (S-NAPTR) application service tag that allows for discovery of the supported applications without doing Diameter capability exchange beforehand.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6408.
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Copyright Notice
Copyright (c) 2011 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
The Diameter base protocol [RFC3588] specifies three mechanisms for Diameter peer discovery. One of these involves the Diameter implementation performing a Naming Authority Pointer (NAPTR) query [RFC3403] for a server in a particular realm. These NAPTR records
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provide a mapping from a domain to the DNS Service Locator (SRV) record [RFC2782] or A/AAAA record [RFC1035] [RFC3596] for contacting a server with the specific transport protocol in the NAPTR services field.
The extended NAPTR usage for Diameter peer discovery defined by this document is based on the Straightforward-NAPTR (S-NAPTR) Dynamic Delegation Discovery System (DDDS) application defined in [RFC3958]. This document updates the Diameter peer discovery procedure described in Section 5.2 of [RFC3588] and defines S-NAPTR application service and application protocol tag values that permit the discovery of Diameter peers that support a specific Diameter application and transport protocol.
The Diameter base protocol specification (Section 1.3 of [RFC3588]) and the Straightforward-NAPTR (S-NAPTR) DDDS application (Section 2.1 of [RFC3958]) define the terminology used in this document.
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 [RFC2119].
The NAPTR service field format defined by the S-NAPTR DDDS application in [RFC3958] follows this Augmented Backus-Naur Form (ABNF) [RFC5234]:
service-parms = [ [app-service] *(":" app-protocol)] app-service = experimental-service / iana-registered-service app-protocol = experimental-protocol / iana-registered-protocol experimental-service = "x-" 1*30ALPHANUMSYM experimental-protocol = "x-" 1*30ALPHANUMSYM iana-registered-service = ALPHA *31ALPHANUMSYM iana-registered-protocol = ALPHA *31ALPHANUMSYM ALPHA = %x41-5A / %x61-7A ; A-Z / a-z DIGIT = %x30-39 ; 0-9 SYM = %x2B / %x2D / %x2E ; "+" / "-" / "." ALPHANUMSYM = ALPHA / DIGIT / SYM ; The app-service and app-protocol tags are limited to 32 ; characters and must start with an alphabetic character. ; The service-parms are considered case-insensitive.
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This specification refines the "iana-registered-service" tag definition for the discovery of Diameter agents supporting a specific Diameter application as defined below.
iana-registered-service =/ aaa-service aaa-service = "aaa+ap" appln-id appln-id = 1*10DIGIT ; Application Identifier expressed as ; a decimal integer without leading ; zeros.
The appln-id element is the Application Identifier used to identify a specific Diameter application. The Diameter Application Identifier is a 32-bit unsigned integer, and values are allocated by IANA as defined in [RFC3588].
This specification also refines the "iana-registered-protocol" tag definition for the discovery of Diameter agents supporting a specific Diameter transport protocol as defined below.
The S-NAPTR application protocol tags defined by this specification MUST NOT be parsed in any way by the querying application or resolver. The delimiter (".") is present in the tag to improve readability and does not imply a structure or namespace of any kind. The choice of delimiter (".") for the application protocol tag follows the format of existing S-NAPTR application protocol tag registry entries, but this does not imply that it shares semantics with any other specifications that create registry entries with the same format.
The S-NAPTR application service and application protocol tags defined by this specification are unrelated to the IANA "Service Name and Transport Protocol Port Number Registry" (see [RFC6335]).
The maximum length of the NAPTR service field is 256 octets, including a one-octet length field (see Section 4.1 of [RFC3403] and Section 3.3 of [RFC1035]).
A Diameter agent MUST be capable of using the extended S-NAPTR application service tag for dynamic discovery of a Diameter agent supporting Standards Track applications. Therefore, every IETF Standards Track Diameter application MUST be associated with a "aaa-service" tag formatted as defined in this specification and allocated in accordance with IANA policy (see Section 7).
For example, a NAPTR service field value of:
'aaa+ap6:diameter.sctp'
means that the Diameter node in the SRV or A/AAAA record supports the Diameter Session Initiation Protocol (SIP) application ('6') and the Stream Control Transmission Protocol (SCTP) as the transport protocol.
S-NAPTR application service and application protocol tag values can also be used to discover Diameter peers that support a vendor- specific Diameter application. In this case, the vendor-specific Diameter application MUST be associated with a "aaa-service" tag formatted as defined in this specification and allocated in accordance with IANA policy (see Section 7).
For example, a NAPTR service field value of:
'aaa+ap16777251:diameter.sctp'
means that the Diameter node in the SRV or A/AAAA record supports the Diameter Third Generation Partnership Project (3GPP) S6a application ('16777251') and SCTP as the transport protocol.
Domain Name System (DNS) administrators SHOULD also provision legacy NAPTR records [RFC3403] in the RFC 3588 style in order to guarantee backwards compatibility with legacy Diameter peers that are RFC 3588 compliant. If the DNS administrator provisions both extended S-NAPTR records as defined in this specification and legacy RFC 3588 NAPTR records, then the extended S-NAPTR records MUST have higher priority (e.g., lower order and/or preference values) than legacy NAPTR records.
The Diameter Peer Discovery principles are described in Section 5.2 of [RFC3588]. This specification updates the NAPTR query procedure in the Diameter peer discovery mechanism by allowing the querying node to determine which applications are supported by resolved Diameter peers.
The extended-format NAPTR records provide a mapping from a domain to the SRV record or A/AAAA record for contacting a server supporting a specific transport protocol and Diameter application. The resource record will contain an empty regular expression and a replacement value, which is the SRV record or the A/AAAA record for that particular transport protocol.
The assumption for this mechanism to work is that the DNS administrator of the queried domain has first provisioned the DNS with extended-format NAPTR entries. The steps below replace the NAPTR query procedure steps in Section 5.2 of [RFC3588].
a. The Diameter implementation performs a NAPTR query for a server in a particular realm. The Diameter implementation has to know in advance in which realm to look for a Diameter agent, and in which Application Identifier it is interested. For example, the realm could be deduced from the Network Access Identifier (NAI) in the User-Name attribute-value pair (AVP) or extracted from the Destination-Realm AVP.
b. If the returned NAPTR service fields contain entries formatted as "aaa+apX:Y" where "X" indicates the Application Identifier and "Y" indicates the supported transport protocol(s), the target realm supports the extended format for NAPTR-based Diameter peer discovery defined in this document.
If "X" contains the required Application Identifier and "Y" matches a supported transport protocol, the Diameter implementation resolves the "replacement" field entry to a target host using the lookup method appropriate for the "flags" field.
If "X" does not contain the required Application Identifier or "Y" does not match a supported transport protocol, the Diameter implementation abandons the peer discovery.
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c. If the returned NAPTR service fields contain entries formatted as "aaa+apX" where "X" indicates the Application Identifier, the target realm supports the extended format for NAPTR-based Diameter peer discovery defined in this document.
If "X" contains the required Application Identifier, the Diameter implementation resolves the "replacement" field entry to a target host using the lookup method appropriate for the "flags" field and attempts to connect using all supported transport protocols following the order specified in Section 2.1 of [RFC3588].
If "X" does not contain the required Application Identifier, the Diameter implementation abandons the peer discovery.
d. If the returned NAPTR service fields contain entries formatted as "aaa:X" where "X" indicates the supported transport protocol(s), the target realm supports Diameter but does not support the extended format for NAPTR-based Diameter peer discovery defined in this document.
If "X" matches a supported transport protocol, the Diameter implementation resolves the "replacement" field entry to a target host using the lookup method appropriate for the "flags" field.
e. If the returned NAPTR service fields contain entries formatted as "aaa", the target realm supports Diameter but does not support the extended format for NAPTR-based Diameter peer discovery defined in this document. The Diameter implementation resolves the "replacement" field entry to a target host using the lookup method appropriate for the "flags" field and attempts to connect using all supported transport protocols following the order specified in Section 2.1 of [RFC3588].
f. If the target realm does not support NAPTR-based Diameter peer discovery, the client proceeds with the next peer discovery mechanism described in Section 5.2 of [RFC3588].
As an example, consider a client that wishes to discover a Diameter server in the ex1.example.com realm that supports the Credit Control application. The client performs a NAPTR query for that domain, and the following NAPTR records are returned:
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;; order pref flags service regexp replacement IN NAPTR 50 50 "s" "aaa:diameter.sctp" "" _diameter._sctp.ex1.example.com IN NAPTR 50 50 "s" "aaa+ap1:diameter.sctp" "" _diameter._sctp.ex1.example.com IN NAPTR 50 50 "s" "aaa+ap4:diameter.sctp" "" _diameter._sctp.ex1.example.com
This indicates that the server supports NASREQ (ID=1) and Credit Control (ID=4) applications over SCTP. If the client supports SCTP, it will be used, targeted to a host determined by an SRV lookup of _diameter._sctp.ex1.example.com.
That SRV lookup would return:
;; Priority Weight Port Target IN SRV 0 1 3868 server1.ex1.example.com IN SRV 0 2 3868 server2.ex1.example.com
As an alternative example, a client wishes to discover a Diameter server in the ex2.example.com realm that supports the NASREQ application over SCTP. The client performs a NAPTR query for that domain, and the following NAPTR records are returned:
;; order pref flags service regexp replacement IN NAPTR 150 50 "a" "aaa:diameter.sctp" "" server1.ex2.example.com IN NAPTR 150 50 "a" "aaa:diameter.tls.tcp" "" server2.ex2.example.com IN NAPTR 150 50 "a" "aaa+ap1:diameter.sctp" "" server1.ex2.example.com IN NAPTR 150 50 "a" "aaa+ap1:diameter.tls.tcp" "" server2.ex2.example.com
This indicates that the server supports NASREQ (ID=1) over SCTP and Transport Layer Security (TLS)/TCP via hosts server1.ex2.example.com and server2.ex2.example.com, respectively.
Diameter is a peer-to-peer protocol, whereas most of the applications that extend the base protocol behave like client/server applications. The role of the peer is not advertised in the NAPTR tags and not even communicated during Diameter capability negotiation (Capabilities-Exchange-Request and Capabilities-Exchange-Answer message exchange). For this reason, NAPTR-based Diameter peer discovery for an application defining client/server roles should only be used by a client to discover servers.
IANA has reserved a value of "aaa" for Diameter in the "(S-NAPTR) Application Service Tag" registry created by [RFC3958]. IANA has also reserved the following S-NAPTR application service tags for existing IETF Diameter applications in the same registry.
Future IETF Diameter applications MUST reserve the S-NAPTR application service tag corresponding to the allocated Diameter Application ID as defined in Section 3.
IANA has reserved the following S-NAPTR application service tags for existing 3GPP Diameter applications in the "S-NAPTR Application Service Tag" registry created by [RFC3958].
Future 3GPP Diameter applications can reserve entries in the "S-NAPTR Application Service Tag" registry created by [RFC3958] that correspond to the allocated Diameter Application IDs as defined in Section 3.
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7.3. WiMAX Forum Diameter Application Service Tags
IANA has reserved the following S-NAPTR application service tags for existing Worldwide Interoperability for Microwave Access (WiMAX) Forum Diameter applications in the "S-NAPTR Application Service Tag" registry created by [RFC3958].
Future WiMAX Forum Diameter applications can reserve entries in the "S-NAPTR Application Service Tag" registry created by [RFC3958] that correspond to the allocated Diameter Application IDs as defined in Section 3.
7.4. Vendor-Specific Diameter Application Service Tags
Vendor-Specific Diameter Application IDs are allocated by IANA according to the "First Come First Served" policy and do not require an IETF specification. However, the S-NAPTR application service tag registry created by [RFC3958] defines a registration policy of
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"Specification Required" with a further stipulation that the "specification" is an RFC (of any category). If a vendor-specific Diameter application requires the functionality defined in this document, an RFC of any category MUST be published that reserves the S-NAPTR Application Service Tag corresponding to the Vendor-Specific Diameter Application ID as defined in Section 3.
IANA has reserved the following S-NAPTR Application Protocol Tags for the Diameter transport protocols in the "S-NAPTR Application Protocol Tag" registry created by [RFC3958].
Future Diameter versions that introduce new transport protocols MUST reserve an appropriate S-NAPTR Application Protocol Tag in the "S-NAPTR Application Protocol Tag" registry created by [RFC3958].
This document specifies an enhancement to the NAPTR service field format defined in RFC 3588 and also modifications to the NAPTR processing logic defined in RFC 3588. The enhancement and modifications are based on the S-NAPTR, which is actually a simplification of the NAPTR, and therefore the same security considerations described in RFC 3588 [RFC3588] are applicable to this document. No further extensions are required beyond the security mechanisms offered by RFC 3588. However, a malicious host doing S-NAPTR queries learns applications supported by Diameter agents in a certain realm faster, which might help the malicious host to scan potential targets for an attack more efficiently when some applications have known vulnerabilities.
We would like to thank Glen Zorn, Avi Lior, Itsuma Tanaka, Sebastien Decugis, Dan Romascanu, Adrian Farrel, David Harrington, Pete Resnick, Robert Sparks, Stephen Farrell, Wesley Eddy, Ralph Droms, and Joe Touch for their comprehensive review comments.
[RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.
[RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part Three: The Domain Name System (DNS) Database", RFC 3403, October 2002.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, "DNS Extensions to Support IP Version 6", RFC 3596, October 2003.
[RFC3958] Daigle, L. and A. Newton, "Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS)", RFC 3958, January 2005.
[RFC4004] Calhoun, P., Johansson, T., Perkins, C., Hiller, T., Ed., and P. McCann, "Diameter Mobile IPv4 Application", RFC 4004, August 2005.
[RFC4006] Hakala, H., Mattila, L., Koskinen, J-P., Stura, M., and J. Loughney, "Diameter Credit-Control Application", RFC 4006, August 2005.
[RFC4072] Eronen, P., Ed., Hiller, T., and G. Zorn, "Diameter Extensible Authentication Protocol (EAP) Application", RFC 4072, August 2005.
[RFC4740] Garcia-Martin, M., Ed., Belinchon, M., Pallares-Lopez, M., Canales-Valenzuela, C., and K. Tammi, "Diameter Session Initiation Protocol (SIP) Application", RFC 4740, November 2006.
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[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5778] Korhonen, J., Ed., Tschofenig, H., Bournelle, J., Giaretta, G., and M. Nakhjiri, "Diameter Mobile IPv6: Support for Home Agent to Diameter Server Interaction", RFC 5778, February 2010.
[RFC5866] Sun, D., Ed., McCann, P., Tschofenig, H., Tsou, T., Doria, A., and G. Zorn, Ed., "Diameter Quality-of-Service Application", RFC 5866, May 2010.
[TS29.215] 3rd Generation Partnership Project, "3GPP TS 29.215; Technical Specification Group Core Network and Terminals; Policy and Charging Control (PCC) over S9 reference point; Stage 3 (Release 8)", <http://www.3gpp.org/ftp/Specs/html-info/29215.htm>.
[TS29.272] 3rd Generation Partnership Project, "3GPP TS 29.272; Technical Specification Group Core Network and Terminals; Evolved Packet System (EPS); Mobility Management Entity (MME) and Serving GPRS Support Node (SGSN) Related Interfaces Based on Diameter Protocol (Release 8)", <http://www.3gpp.org/ftp/Specs/html-info/29272.htm>.
[TS29.273] 3rd Generation Partnership Project, "3GPP TS 29.273; Technical Specification Group Core Network and Terminals; Evolved Packet System (EPS); 3GPP EPS AAA interfaces (Release 8)", <http://www.3gpp.org/ftp/Specs/html-info/29273.htm>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. Cheshire, "Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", BCP 165, RFC 6335, August 2011.