Internet Engineering Task Force (IETF) T. Tsou

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

Abstract

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
http://www.rfc-editor.org/info/rfc7075.

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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
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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

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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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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.

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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.

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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
specification.

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
redirection:

o configured with an application that incorporates realm-based
redirection;

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
redirected.

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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
policy.

3.2.2. Proxy Behavior

A proxy conforming to this specification that receives an answer
message with the Result-Code AVP set to
DIAMETER_REALM_REDIRECT_INDICATION MUST attempt to reroute the
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].

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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
DIAMETER_REALM_REDIRECT_INDICATION, the client SHOULD follow the same
steps prescribed in the previous section for a proxy, in order to
both update its routing table and obtain service for the original
request.

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.

3.4. DIAMETER_REALM_REDIRECT_INDICATION Protocol Error Code

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.

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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
below.

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
DIAMETER_REALM_REDIRECT_INDICATION (3011) in the "Result-Code AVP
Values (code 268) - Protocol Errors" registry under "Authentication,
Authorization, and Accounting (AAA) Parameters".

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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
correction.

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.

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Authors' Addresses

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

Phone: +1 408 330 4424
EMail: [email protected]
URI: http://tinatsou.weebly.com/contact.html

Ruibing Hao
Comcast Cable
One Comcast Center
Philadelphia, PA 19103
USA

Phone: +1 215 286 3991(O)
EMail: [email protected]

Tom Taylor (editor)
Huawei Technologies
Ottawa
Canada

EMail: [email protected]

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