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RFC 7075 - Realm-Based Redirection In Diameter

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Internet Engineering Task Force (IETF)                           T. Tsou
Request for Comments: 7075                     Huawei Technologies (USA)
Updates: 6733                                                     R. Hao
Category: Standards Track                                  Comcast Cable
ISSN: 2070-1721                                           T. Taylor, Ed.
                                                     Huawei Technologies
                                                           November 2013

                  Realm-Based Redirection In Diameter


   The Diameter protocol includes a capability for message redirection,
   controlled by an application-independent "redirect agent".  In some
   circumstances, an operator may wish to redirect messages to an
   alternate domain without specifying individual hosts.  This document
   specifies an application-specific mechanism by which a Diameter
   server or proxy (node) can perform such a redirection when the
   Straightforward-Naming Authority Pointer (S-NAPTR) is not used for
   dynamic peer discovery.  A node performing this new function is
   referred to as a "Realm-based Redirect Server".

   This memo updates Sections 6.13 and 6.14 of RFC 6733 with respect to
   the usage of the Redirect-Host-Usage and Redirect-Max-Cache-Time
   Attribute-Value Pairs (AVPs).

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

Copyright Notice

   Copyright (c) 2013 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Support of Realm-Based Redirection Within Applications  . . .   4
   3.  Realm-Based Redirection . . . . . . . . . . . . . . . . . . .   5
     3.1.  Configuration of the Realm-Based Redirect Server  . . . .   5
     3.2.  Behavior of Diameter Nodes  . . . . . . . . . . . . . . .   6
       3.2.1.  Behavior at the Realm-Based Redirect Server . . . . .   6
       3.2.2.  Proxy Behavior  . . . . . . . . . . . . . . . . . . .   6
       3.2.3.  Client Behavior . . . . . . . . . . . . . . . . . . .   7
     3.3.  The Redirect-Realm AVP  . . . . . . . . . . . . . . . . .   7
     3.4.  DIAMETER_REALM_REDIRECT_INDICATION Protocol Error Code  .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Diameter base protocol [RFC6733] specifies a basic redirection
   service provided by a redirect agent.  The redirect indication
   returned by the redirect agent is described in Section 6.1.8 and
   Sections 6.12 through 6.14 of [RFC6733].  It provides one or more
   individual hosts to the message sender as the destination of the
   redirected message.

   However, consider the case where an operator has offered a specific
   service but no longer wishes to do so.  The operator has arranged for
   an alternative domain to provide the service.  To aid in the
   transition to the new arrangement, the original operator maintains a
   redirect server to indicate to the message sender the alternative
   domain to which the redirect the request should be sent.  However,
   the original operator should not have to configure the redirect
   server with a list of hosts to contact in the alternative operator's
   domain; the original operator should simply be able to provide
   redirect indications to the domain as a whole.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Within this specification, the term "realm-based redirection" is used
   to refer to a mode of operation where a realm, rather than an
   individual host, is returned as the redirect indication.

   The term "Realm-based Redirect Server" denotes the Diameter node
   (Diameter server or proxy) that returns the realm-based redirection.
   The behavior of the Realm-based Redirect Server itself is a slight
   modification to the behavior of a basic redirect agent as described
   in Section 6.1.8 of [RFC6733].

   The use of a number of terms in this document is consistent with the
   usage in [RFC6733]: "Diameter client", "Diameter node", "Diameter
   peer", "Diameter server", "proxy", "realm" or "domain", "redirect
   agent", and "session" as defined in Section 1.2, and "application" as
   defined implicitly by Sections 1.3.4, 2.3, and 2.4.

2.  Support of Realm-Based Redirection Within Applications

   The DNS-based dynamic peer discovery mechanism defined in the
   Diameter base protocol [RFC6733] provides a simple mechanism for
   realm-based redirection using the S-NAPTR DDDS application [RFC3958].
   When S-NAPTR is used for peer discovery, redirection of Diameter
   requests from the original realm to a new realm may be performed by
   updating the existing NAPTR resource records (RRs) for the original
   realm as follows: the NAPTR RR for the desired application(s) and
   supported application protocol(s) provided by the new realm will have
   an empty FLAG field and the REPLACEMENT field will contain the new
   realm to use for the next DNS lookup.  The new realm can be
   arbitrary; the restriction in [RFC6733] that the NAPTR replacement
   field match the domain of the original query does not apply for
   realm-based redirect purposes.

   However, the use of DNS-based dynamic peer discovery is optional for
   Diameter implementations.  For deployments that do not make use of
   S-NAPTR peer discovery, support of realm-based redirection needs to
   be specified as part of the functionality supported by a Diameter
   application.  In this way, support of the considered Diameter
   application (discovered during capabilities exchange phase as defined
   in Diameter base protocol [RFC6733]) indicates implicit support of
   the realm-based redirection mechanism.  A new application
   specification can incorporate the mechanism specified here by making
   it mandatory to implement for the application and referencing this
   specification normatively.

   The result of making realm-based redirection an application-specific
   behavior is that it cannot be performed by a redirect agent as
   defined in [RFC6733], but MUST be performed instead by an
   application-aware Diameter node (Diameter server or proxy) (hereafter
   called a "Realm-based Redirect Server").

   An application can specify that realm-based redirection operates only
   on complete sessions beginning with the initial message or on every
   message within the application, even if earlier messages of the same
   session were not redirected.  This distinction matters only when
   realm-based redirection is first initiated.  In the former case,
   existing sessions will not be disrupted by the deployment of realm-
   based redirection.  In the latter case, existing sessions will be
   disrupted if they are stateful.

3.  Realm-Based Redirection

   This section specifies an extension of the Diameter base protocol
   [RFC6733] to achieve realm-based redirection.  The elements of this
   solution are:

   o  a new result code, DIAMETER_REALM_REDIRECT_INDICATION (3011);

   o  a new attribute-value pair (AVP), Redirect-Realm (620); and

   o  associated behavior at Diameter nodes implementing this

   This behavior includes the optional use of the Redirect-Host-Usage
   and Redirect-Max-Cache-Time AVPs.  In this document, these AVPs apply
   to the peer discovered by a node acting on the redirect server's
   response, an extension to their normal usage as described in Sections
   6.13 and 6.14 of [RFC6733].

   Section 3.2.2 and Section 3.2.3 describe how a proxy or client may
   update its routing table for the application and initial realm as a
   result of selecting a peer in the new realm after realm-based
   redirection.  Note that as a result, the proxy or client will
   automatically route subsequent requests for that application to the
   new realm (with the possible exception of requests within sessions
   already established with the initial realm) until the cached routing
   entry expires.  This should be borne in mind if the rerouting is
   intended to be temporary.

3.1.  Configuration of the Realm-Based Redirect Server

   A Diameter node (Diameter server or proxy) acting as a Realm-based
   Redirect Server MUST be configured as follows to execute realm-based

   o  configured with an application that incorporates realm-based

   o  the Local Action field of the routing table described in
      Section 2.7 of [RFC6733] is set to LOCAL;

   o  an application-specific field is set to indicate that the required
      local action is to perform realm-based redirection;

   o  an associated application-specific field is configured with the
      identities of one or more realms to which the request should be

3.2.  Behavior of Diameter Nodes

3.2.1.  Behavior at the Realm-Based Redirect Server

   As mentioned in Section 2, an application can specify that realm-
   based redirection operates only on complete sessions beginning with
   the initial message (i.e., to prevent disruption of established
   sessions) or on every message within the application, even if earlier
   messages of the same session were not redirected.

   If a Realm-based Redirect Server configured as described in
   Section 3.1 receives a request to which realm-based redirection
   applies, the Realm-based Redirect Server MUST reply with an answer
   message with the 'E' bit set, while maintaining the Hop-by-Hop
   Identifier in the header.  The Realm-based Redirect Server MUST
   include the Result-Code AVP set to
   DIAMETER_REALM_REDIRECT_INDICATION.  The Realm-based Redirect Server
   MUST also include the alternate realm identifier(s) with which it has
   been configured, each in a separate Redirect-Realm AVP instance.

   The Realm-based Redirect Server MAY include a copy of the Redirect-
   Host-Usage AVP, which SHOULD be set to REALM_AND_APPLICATION.  If
   this AVP is added, the Redirect-Max-Cache-Time AVP MUST also be
   included.  Note that these AVPs apply to the peer discovered by a
   node acting on the Realm-based Redirect Server's response as
   described in the next section.  This is an extension of their normal
   usage as described by Sections 6.13 and 6.14 of [RFC6733].

      Realm-based redirection MAY be applied even if a Destination-Host
      AVP is present in the request, depending on the operator-based

3.2.2.  Proxy Behavior

   A proxy conforming to this specification that receives an answer
   message with the Result-Code AVP set to
   original request to a server in a realm identified by a Redirect-
   Realm AVP instance in the answer message, and if it fails MUST
   forward the indication toward the client.  To reroute the request, it
   MUST take the following actions:

   1.  Select a specific realm from amongst those identified in
       instances of the Redirect-Realm AVP in the answer message.

   2.  If successful, locate and establish a route to a peer in the
       realm given by the Redirect-Realm AVP, using normal discovery
       procedures as described in Section 5.2 of [RFC6733].

   3.  If again successful:

       A.  update its cache of routing entries for the realm and
           application to which the original request was directed,
           taking into account the Redirect-Host-Usage and Redirect-Max-
           Cache-Time AVPs, if present in the answer.

       B.  Remove the Destination-Host (if present) and Destination-
           Realm AVPs from the original request and add a new
           Destination-Realm AVP containing the realm selected in the
           initial step.

       C.  Forward the modified request.

   4.  If either of the preceding steps 2-3 fail and additional realms
       have been identified in the original answer, select another
       instance of the Redirect-Realm AVP in that answer and repeat
       steps 2-3 for the realm that it identifies.

3.2.3.  Client Behavior

   A client conforming to this specification MUST be prepared to receive
   either an answer message containing a Result-Code AVP set to
   DIAMETER_REALM_REDIRECT_INDICATION, or, as the result of proxy
   action, some other result from a realm differing from the one to
   which it sent the original request.  In the case where it receives
   steps prescribed in the previous section for a proxy, in order to
   both update its routing table and obtain service for the original

3.3.  The Redirect-Realm AVP

   The Redirect-Realm AVP (620) is of type DiameterIdentity.  It
   specifies a realm to which a node receiving a redirect indication
   containing the result code value DIAMETER_REALM_REDIRECT_INDICATION
   and the Redirect-Realm AVP SHOULD route the original request.


   The DIAMETER_REALM_REDIRECT_INDICATION (3011) Protocol error code
   indicates that a server has determined that the request within an
   application supporting realm-based redirection could not be satisfied
   locally, and the initiator of the request SHOULD direct the request
   directly to a peer within a realm that has been identified in the
   response.  When set, the Redirect-Realm AVP MUST be present.

4.  Security Considerations

   The general recommendations given in Section 13 of the Diameter base
   protocol [RFC6733] apply.  Specific security recommendations related
   to the realm-based redirection defined in this document are described

   Realm-based redirection implies a change in the business relationship
   between organizations.  Before redirecting a request towards a realm
   different from the initial realm, the client or proxy MUST ensure
   that the authorization checks have been performed at each connection
   along the path toward the realm identified in the realm-based
   redirect indication.  Details on Diameter authorization path set-up
   are given in Section 2.9 of [RFC6733].  Section 13 of [RFC6733]
   provides recommendations on how to authenticate and secure each peer-
   to-peer connection (using TLS, DTLS, or IPsec) along the way, thus
   permitting the necessary hop-by-hop authorization checks.

   Although it is assumed that the administrative domains are secure, a
   compromised Diameter node acting as a Realm-based Redirect Server
   would be able to redirect a large number of Diameter requests towards
   a victim domain that would then be flooded with undesired Diameter
   requests.  Such an attack is nevertheless discouraged by the use of
   secure Diameter peer-to-peer connections and authorization checks,
   since these would enable a potential victim domain to discover from
   where an attack is coming.  That in itself, however, does not prevent
   such a DoS attack.

   Because realm-based redirection defined in this document implies that
   the Destination-Realm AVP in a client-initiated request can be
   changed by a Diameter proxy in the path between the client and the
   server, any cryptographic algorithm that would use the Destination-
   Realm AVP as input to the calculation performed by the client and the
   server would be broken by this form of redirection.  Application
   specifications that would rely on such cryptographic algorithms
   SHOULD NOT incorporate this realm-based redirection.

5.  IANA Considerations

   This specification allocates a new AVP code Redirect-Realm (620) in
   the "AVP Codes" registry under "Authentication, Authorization, and
   Accounting (AAA) Parameters".

   This specification allocates a new Result-Code value
   Values (code 268) - Protocol Errors" registry under "Authentication,
   Authorization, and Accounting (AAA) Parameters".

6.  Acknowledgements

   Glen Zorn, Sebastien Decugis, Wolfgang Steigerwald, Mark Jones,
   Victor Fajardo, Jouni Korhonen, Avi Lior, and Lionel Morand
   contributed comments that helped to shape this document.  As
   shepherd, Lionel contributed a second set of comments that added
   polish to the document before it was submitted to the IESG.  Benoit
   Claise picked up additional points that were quickly resolved with
   Lionel's help.  During IETF Last Call Review, Enrico Marocco picked
   up some important editorial corrections.  Stefan Winter contributed
   text on the use of S-NAPTR as an alternative method of realm-based
   redirection already specified in [RFC6733].  Derek Atkins performed a
   review on behalf of the Security Directorate.  Lionel noted one more

   Finally, this document benefited from comments and discussion raised
   by IESG members Stewart Bryant, Stephen Farrell, Barry Leiba, Pete
   Resnick, Jari Arkko, and Sean Turner during IESG review.

   The authors are very grateful to Lionel Morand for his active role as
   document shepherd.  At each stage, he worked to summarize and resolve
   comments, making the editor's role easy.  Thank you.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6733]  Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
              "Diameter Base Protocol", RFC 6733, October 2012.

7.2.  Informative References

   [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.

Authors' Addresses

   Tina Tsou
   Huawei Technologies (USA)
   2330 Central Expressway
   Santa Clara, CA  95050

   Phone: +1 408 330 4424
   EMail: Tina.Tsou.Zouting@huawei.com
   URI:   http://tinatsou.weebly.com/contact.html

   Ruibing Hao
   Comcast Cable
   One Comcast Center
   Philadelphia, PA  19103

   Phone: +1 215 286 3991(O)
   EMail: Ruibing_Hao@cable.comcast.com

   Tom Taylor (editor)
   Huawei Technologies

   EMail: tom.taylor.stds@gmail.com


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