Session-Specific Explicit Diameter Request Routing
draft-tsou-diameter-explicit-routing-05

Abstract

This document describes a mechanism to enable specific Diameter proxies to remain in the path of all message exchanges constituting a Diameter session. This document is being published to provide the basis for a standardized solution to a problem raised by some architectures (e.g., WLAN 3GPP IP access, 3GPP TS23.234) that use Diameter. The intended use will be as a reference within the non-IETF specification of a Diameter application that meets the needs of these architectures.

Status of this Memo

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Table of Contents

1.  Introduction
2.  Terminology
3.  The 3GPP Wireless LAN (WLAN) Access Architecture
    3.1.  Maintaining the Routing Path
4.  Diameter Explicit Routing (ER)
    4.1.  Originating a request (ER-Originator)
    4.2.  Relaying and Proxying Requests (ER-Proxy)
    4.3.  Receiving Requests (ER-Destination)
    4.4.  Diameter answer processing
    4.5.  Failover and Failback Considerations
    4.6.  Attribute-Value Pairs
        4.6.1.  Explicit-Path-Record AVP
            4.6.1.1.  Proxy-Realm AVP
        4.6.2.  Explicit-Path AVP
    4.7.  Error Handling
    4.8.  Example Message Flow
5.  RADIUS/Diameter Protocol Interactions
6.  IANA Considerations
7.  Security Considerations
8.  Acknowledgements
9.  References
    9.1.  Normative References
    9.2.  Informative References
§  Authors' Addresses

1.  Introduction

In the Diameter base protocol [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.), the routing of request messages is based solely on the routing decisions made separately by each node along the path. [RFC5729] (Korhonen, J., Jones, M., Morand, L., and T. Tsou, “Clarifications on the Routing of Diameter Requests Based on the Username and the Realm,” December 2009.) has added the ability to force messages to pass through a specified set of realms through the use of NAI decoration. However, no other specification provides the ability to force routing through a specific set of agents. Therefore, in a topology where multiple paths exist from source to destination, there is no guarantee that all messages relating to a given session will take the same path. In general, this has not caused problems, but some architectures (e.g., WLAN 3GPP IP access [TS23.234] (3GPP, “3GPP system to Wireless Local Area Network (WLAN) interworking; System description,” 2006.)) require that once certain agents become engaged in a session, they are able to process all subsequent messages for that session.

While the solution presented in this document is valid, it violates one of the basic premises of Diameter, the robustness of its architecture. With normal Diameter routing, sessions will survive failures of agents along the routing path. With the proposals in this document, routing becomes pinned to specific agents whose failure will terminate the session. The IETF does not endorse this specification because of its impact on Diameter session survivability, but do not object to its publication for use in specialized situations where the loss of robustness is acceptable.

The authors see no interaction between explicit routing and the specific applications with which it is employed. Hence in principle it can be added to existing applications if they support the necessary extensibility, and equally can be used with new applications.

2.  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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

The following terms are used to define the functionality and participants in the routing extensions described in this document.

ER

Explicit routing, the mechanism provided by this specification to allow proxies traversed by the initial message of a session to ensure that they remain on the messaging path for all subsequent request messages of a session.

ER-proxy

A proxy that implements the ER mechanism and can therefore use it to remain in the path for subsequent messages of a session.

ER-Destination

A Diameter node which is capable of participating in ER and which will ultimately consume the request sent by an ER-Originator.

ER-Originator

A Diameter node initiating a session and sending the requests. The ER- Originator can be any Diameter node sending a request, i.e. client, server or proxy capable of initiating sessions and participating in ER.

AAA Relays

Other Diameter nodes interspersed between the ER-Originator, ER-Proxies, and the ER-Destination. These nodes represent existing Diameter agents and proxies that do not participate in ER and do not recognize Explicit-Path AVPs.

3.  The 3GPP Wireless LAN (WLAN) Access Architecture

One example of a system requiring that certain agents (stateful proxies, in this case) remain in the forwarding path of all session messages is the 3GPP WLAN IP access architecture [TS23.234] (3GPP, “3GPP system to Wireless Local Area Network (WLAN) interworking; System description,” 2006.). The 3GPP WLAN interworking architecture extends 3GPP services to the WLAN access side, enabling a 3GPP subscriber to use a WLAN to access 3GPP services.

WLAN AAA provides access to the WLAN to be authenticated and authorized through the 3GPP system. This access control can permit or deny a subscriber the access to the WLAN system and/or the 3GPP system.

There are two 3GPP WLAN interworking reference models:

1. In the non-roaming case, the model includes the WLAN access network and the 3GPP AAA server in the home network. The 3GPP AAA server is responsible for access control as well as charging.
2. In the roaming case, the model includes the WLAN access network, the 3GPP AAA proxy in the visited network and the 3GPP AAA server in the home network. The 3GPP AAA server is responsible for access control. Charging records may be generated by the AAA proxy and/or the AAA server. The AAA proxy relays access control and charging messages to the AAA server. The AAA proxy will also do offline charging, if required.

The roaming case presents two problems for which the Diameter routing mechanism described in [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.) does not offer any unambiguous and standard solution.

Network Selection

Selecting an initial message path for the Diameter session through (possibly many) alternative visited network(s) to the home network.

Explicit Routing

Maintaining the selected message path for all messages in the Diameter session.

The former is outside the scope of this document; the latter is described in detail below.

3.1.  Maintaining the Routing Path

After a successful authentication, a Diameter session is established involving (at least) the following stateful entities:

• the Diameter client in the WLAN access node,
• a Diameter proxy (the 3GPP AAA proxy) in the visited mobile network, and
• a Diameter server in the user's home realm.

The functions assigned to the 3GPP AAA proxy include:

• Reporting charging information to the offline charging system in the visited network
• Policy enforcement based on roaming agreements
• Service termination initiated by the visited network operator

These functions all require that state be maintained within the visited network. The 3GPP choice is to maintain that state at the 3GPP AAA proxy. This means that the latter must remain in the messaging path for all subsequent messages relating to the same session.

4.  Diameter Explicit Routing (ER)

This section outlines a Diameter ER mechanism by which Diameter nodes participating in ER can remain in the path of all request messages for a specific session. A new Explicit-Path AVP is defined to enable ER participants to manipulate the Destination-Host and/or Destination-Realm AVPs of request messages in order to ensure the correct routing behavior. The following sections describe the extensions to the request routing in [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.) to implement the ER mechanism. The proposed extensions utilize existing routing strategies in [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.) and do not mandate modifications to it. The scheme also differs from existing strict source routing schemes in which all hops in the path have to participate. In the ER mechanism, only Diameter nodes interested in participating in the ER scheme will be involved in it.

4.1.  Originating a request (ER-Originator)

A Diameter node acting as an ER-Originator for a particular session MUST maintain a local cache which enumerates all the Diameter identities of the ER- Proxies that the request messages must traverse along the path to the ER- Destination. The identity of a Diameter node is defined in [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.). The local cache may also include the node's realm. The data structure of the cache is left up to the implementation and should persist as part of the session attributes or properties.

An ER-Originator sending request messages MUST add an Explicit-Path AVP to these requests. The contents of the cache SHOULD be used to populate the Explicit-Path AVP where each cached entry is represented by a corresponding instance of the Explicit-Path-Record AVP. ER-Proxies along the path of the request message MUST examine the contents of the Explicit-Path AVP and make routing adjustments based on records it contains. An example of the message flow is shown in Section 4.8 (Example Message Flow). Note that the ER-Originator can be any Diameter node, i.e. client, server or proxy.

The ER-Originator can populate the cache either by pre-configuring its contents or by using the first request message of the session to gather identities of participating ER-Proxies along the routing path. The latter scheme is known as Explicit-Path discovery. The contents of the cache can be pre-configured if the ER-Originator has explicit knowledge of the ER-Proxies the request messages must traverse; otherwise it can use Explicit-Path discovery. It is recommended that Explicit-Path discovery be used whenever possible since pre-configuration is less flexible by nature.

Explicit-Path discovery is useful if the identities of the ER-Proxies are not known or if there are several ER capable proxies (a cluster of proxies) that can be dynamically chosen based on other routing policies. In Explicit-Path discovery, the cache of the ER-originator is initially empty. When the ER-Originator sends the first request message of a session, the Explicit-Path AVP will contain only one Explicit-Path-Record AVP with the identity and/or the realm of the ER-Originator. The Destination-Host and/or Destination-Realm AVP of the request message is set to the identity and/or the realm of the ER-Destination respectively as specified in [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.).

It should be noted that ER-Originator initial request message routing procedures and the population of Destination-Realm may be affected by the User- Name AVP NAI decoration [RFC5729] (Korhonen, J., Jones, M., Morand, L., and T. Tsou, “Clarifications on the Routing of Diameter Requests Based on the Username and the Realm,” December 2009.). NAI decoration is a form of request message source routing and defines realms that the request message must traverse through before routing towards the ER- Destination. Diameter nodes participating to the request message routing must examine and process the User-Name AVP, and modify the Destination-Realm AVP accordingly as long as there are realms left in the decorated NAI. Source routing based upon NAI decoration does not affect the Explicit-Path discovery as defined in this document.

When the request message is received and processed by an ER-Proxy, the ER- Proxy MUST append a new Explicit-Path-Record containing its own identity and/or realm to the Explicit-Path AVP prior to forwarding the message. Subsequent ER-Proxies along the path that wish to participate in the ER MUST also append their own Explicit-Path-Record in the same manner (Section 4.2 (Relaying and Proxying Requests (ER-Proxy))). When the request reaches the ER-Destination, it MUST append a new Explicit- Path-Record to the Explicit-Path AVP in a similar manner. The ER-Destination MUST copy the resulting Explicit-Path AVP to the answer message (Section 4.3 (Receiving Requests (ER-Destination))). Once the answer message reaches the ER-Originator, the Explicit-Path AVP will contain one or more Explicit-Path-Records containing the ER-Originator's identity, the identities of all participating ER-Proxies and the identity of the ER-Destination. The ER-Originator SHOULD populate its local cache with the contents of the Explicit-Path AVP received in this initial answer message.

If the answer message does not contain an Explicit-Path AVP or the Result- Code AVP is set to Diameter_ER_NOT_AVAILABLE (Section 4.7 (Error Handling)), it is an indication to the ER-Originator that the destination of the request does not support ER and that the ER-Originator SHOULD avoid sending an Explicit-Path AVP in subsequent request messages.

If, after performing Explicit-Path discovery, the Explicit-Path AVP in the answer message contains only the Explicit-Path-Record of the ER-Originator and ER-Destination then this SHOULD be an indication to the ER-Originator that there are no Diameter proxies capable of participating in ER along the path and that the ER-Originator SHOULD NOT send an Explicit-Path AVP in subsequent request messages of this session. See Section 4.5 (Failover and Failback Considerations) for more discussion. In such cases, the situation may be transient and Explicit-Path discovery in succeeding sessions may find participating proxies. It is left up to the ER-Originator to decide if Explicit-Path discovery should be attempted in succeeding sessions.

Once the ER-Originator's local cache has been populated, whether by pre- configuration or through Explicit-Path discovery, all request messages for the session MUST include the Explicit-Path AVP using the contents of the local cache. The Explicit-Path AVP MUST contain the Explicit-Path-Records of all the nodes enumerated in the cache except that of the ER-Originator itself. The identities enumerated in the Explicit-Path AVP MUST appear in the order they will be traversed in the routing path. The last entry in the Explicit-Path AVP MUST be the Explicit-Path-Record of the ER-Destination. In addition, the value of the Destination-Host and/or Destination-Realm AVP of the request messages MUST be set to the value of the Proxy-Host and/or Proxy- Realm of the first Explicit-Path-Record AVP present in the Explicit-Path AVP.

This ensures that the ER-Originator as well as any AAA relays in between the ER-Originator and the first ER-Proxy will route the message towards the first ER-Proxy as specified in RFC3588 [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.).

Subsequent actions taken by the first ER-Proxy upon receipt of the message are described in Section 4.2 (Relaying and Proxying Requests (ER-Proxy)) and will mimic those of the ER- Originator.

Answer messages received by the ER-Originator to subsequent request messages after the ER path has been established SHOULD NOT have an Explicit- Path AVP. Otherwise, this SHOULD be considered a suspect condition that may be caused by a misbehaving ER participant. It is left up to the ER-Originator whether to continue using the ER scheme when such a condition arises or to attempt another Explicit-Path discovery on subsequent sessions.

4.2.  Relaying and Proxying Requests (ER-Proxy)

The basic action taken by an ER-Proxy upon receiving a request is to check whether explicit routing is supported in the request and if so, check whether it is already a participant in explicit routing for the said request. If it is an existing participant, the ER-Proxy MUST pop/remove the Explicit-Path-Record AVP pertaining to itself from the Explicit-Path AVP and then use the next Explicit-Path-Record AVP for subsequent routing. Details of this operation are as follows.

An ER-Proxy is not required to keep local state or cache state regarding the explicit routing procedure. However, it MUST check whether an incoming request contains an Explicit- Path AVP.

1. If an incoming request does not contain an Explicit-Path AVP then the ER- Proxy MUST process and forward the request as specified in [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.).
2. If the incoming request contains an Explicit-Path AVP, the ER-Proxy MUST check whether its identity is present in the Explicit-Path AVP. Determining whether its identity is present can be done by matching its identity to the Proxy-Host AVPs contained in each Explicit-Path-Record.

   A.

   If its identity is not present and it wishes to participate in explicit routing, the ER-Proxy MUST append a new Explicit-Path-Record as the last AVP in the Explicit-Path AVP prior to forwarding the request. The new Explicit-Path-Record MUST contain at the least a Proxy-Host AVP set to the proxy's identity. This scenario is part of the Explicit-Path discovery scheme described in Section 4.1 (Originating a request (ER-Originator)).

   B.

   However, if its identity is not present and the ER-Proxy does not wish to participate in the ER, it SHOULD NOT modify the Explicit-Path AVP and SHOULD simply forward the request as specified in [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.) using the existing value of Destination-Host and/or Destination-Realm AVP. Non ER-proxies and relays that do not support ER and do not recognize Explicit-Path AVP will take the same action.

   C.

   If the identity of the ER-Proxy is present in the Explicit-Path AVP, then it MUST be the first Explicit-Path-Record in the AVP; otherwise, this SHOULD be considered an error and an answer message with the e-bit set and the Result- Code set to Diameter_INVALID_PROXY_PATH_STACK must be sent back to the ER- Originator (Section 4.7 (Error Handling)). If the identity of the ER-Proxy matches the first Explicit-Path-Record, the ER-Proxy MUST remove this record from Explicit-Path AVP and set the Destination-Host and/or Destination-Realm AVP to the next Explicit-Path-Record present in the Explicit-Path AVP. Setting the Destination-Host and/or Destination-Realm AVP will ensure that the ER-Proxy as well as all AAA relays in between the current ER-Proxy and the next ER-Proxy enumerated in the Explicit-Path AVP will route the message towards the next ER-Proxy. The process of removing the ER-Proxy's record is analogous to removing an entry in a stack represented by the Explicit-Path AVP.

Note that in the case of the ER-Destination, the Explicit-Path AVP MUST be empty once its own record is removed (Section 4.3 (Receiving Requests (ER-Destination))). Note also that the behavior specified above applies to a Diameter node that acts as a relay agent and participates in the ER scheme.

4.3.  Receiving Requests (ER-Destination)

A Diameter node that locally processes requests sent by the ER-Originator (Section 4.1 (Originating a request (ER-Originator))) and is able to support ER (an ER-Destination) MUST check for the presence of an Explicit-Path AVP in the request message.

1. If an incoming request does not contain an Explicit-Path AVP then it is an indication that messages belonging to this session will not use ER. The Diameter node SHOULD process the request for local consumption and formulate an answer message as specified in [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.).
2. If the incoming request contains an Explicit-Path AVP, the Diameter node MUST check whether its identity is present in the Explicit-Path AVP.

   A.

   If its identity is not present in the Explicit-Path and it wishes to participate in the ER, the Diameter node MUST append a new Explicit-Path-Record to the Explicit-Path AVP in the received message. The new Explicit-Path-Record MUST contain at the least a Proxy-Host AVP set to the ER-Destination's identity. The ER-Destination MUST then copy the resulting Explicit-Path AVP to the subsequent answer message. This scenario is part of the proxy path discovery scheme described in Section 4.1 (Originating a request (ER-Originator)).

   B.

   If its identity is not present and the ER-Destination supports ER but does not wish to or cannot participate, it MAY send a Result-Code AVP set to Diameter_ER_NOT_AVAILABLE as defined in Section 4.7 (Error Handling). The ER-Destination SHOULD NOT include any Explicit-Path AVP in the subsequent answer. The same scenario applies to ER-destinations that do not support ER and do not recognize Explicit-Path AVP and is a hint to the ER-Originator that the destination does not support ER.

   C.

   If the identity of the ER-Destination matches a record in the Explicit-Path AVP, then it MUST be the only Explicit-Path-Record present in the Explicit-Path AVP otherwise, this SHOULD be considered an error and an answer message with the 'E' bit set and containing an Experimental-Result-Code AVP with the set to Diameter_INVALID_PROXY_PATH_STACK MUST be sent back to the ER-Originator (Section 4.7 (Error Handling)). If the identity of the of the ER-Destination matches the only existing Explicit-Path-Record, then this is an indication of a successful ER, but with no participating ER-Proxies. The ER-Destination SHOULD NOT copy the Explicit-Path AVP into the subsequent answer message.

4.4.  Diameter answer processing

There is no requirement on Diameter nodes participating in ER to provide special handling or routing of answer messages. Answer messages SHOULD be processed normally as specified in [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.). However, a Diameter node acting as an ER-Destination MUST formulate a proper Explicit-Path AVP in answer messages as described in Section 4.3 (Receiving Requests (ER-Destination)).

4.5.  Failover and Failback Considerations

If there is no ER-Proxy along the selected path, the answer message will contain an Explicit-Path AVP that contains only the Explicit-Route-Records of the ER-Originator and the ER-Destination, indicating that there is no ER support found in Diameter nodes along the path. It is left to the ER-Originator to continue with processing of the request without ER support or terminate the session. The ER-Originator SHOULD NOT attempt to perform Explicit-Path discovery in subsequent request messages of this session in such cases so as to protect against failback conditions where an ER-Proxy suddenly appears in the path and attempts to add a new Explicit-Path-Record for request messages other than the initial request.

Allowing an ER-Proxy to join the session after the initial request is permissible only if the application requirements do not mandate that any participating ER-Proxy receive all of the messages of a session.

However, based on local policy, the ER-Originator MAY attempt Explicit-Path discovery in subsequent sessions.

If a failover occurs in a Diameter node preceding an ER-Proxy when the ER path is already established, it is possible that an Diameter_UNABLE_TO_DELIVER error will be received by the ER-Originator if there are no alternative paths towards the ER-proxy. In such a case, it is left to the ER-Originator to handle the error as specified in Diameter application or in [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.).

4.6.  Attribute-Value Pairs

The following sections define the AVPs used in the ER process. All of these AVPs MUST have the 'V' bit set and the 'M' bit cleared, with the Vendor-ID field set to 2011 (as assigned in http://www.iana.org/assignments/enterprise-numbers).

4.6.1.  Explicit-Path-Record AVP

The Explicit-Path-Record AVP (AVP Code 35001) is of type Group. The identity added in the Proxy-Host [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.) element of this AVP MUST be the same as the one advertised by the Diameter node in the Origin- Host AVP during the Capabilities Exchange messages.

     Explicit-Path-Record ::= < AVP Header: 35001 >
                              { Proxy-Host }
                              [ Proxy-Realm ]

4.6.1.1.  Proxy-Realm AVP

The Proxy-Realm AVP (AVP Code 35002) is of type DiameterIdentity, and contains the realm of the ER node inserting the record.

It is recommended that the Proxy-Host AVP be present and used to uniquely identify an ER-Proxy within the AAA realm being traversed by a request. Otherwise, ER will need to rely on realm routing. Realm routing would require a well-known topology for the ER scheme to work properly since the hostname of the proxy is not specified. In such a case, the Proxy-Realm AVP MUST be present and is used to identify the ER-Proxy of the realm.

When a Proxy-Host AVP is present in the Explicit-Path-Record AVP, the realm name included in the hostname MUST correspond to the identity present in the Proxy-Realm AVP.

4.6.2.  Explicit-Path AVP

The Explicit-Path AVP (AVP Code 35003) is of type Grouped. This AVP SHOULD be present in all request and answer messages performing ER.

      Explicit-Path ::= < AVP Header: 35003 >
                     1* [ Explicit-Path-Record ]
                      * [ AVP ]

4.7.  Error Handling

The following error conditions may occur during ER processing. All error indications MUST be encapsulated in an instance of the Experimental- Result AVP [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.) with the Vendor-Id AVP set to 2011 and the Experimental-Result-Code set as specified below.

DIAMETER_INVALID_PROXY_PATH_STACK 3501

A request message received by an ER-Proxy or ER-Destination after an ER path has been established has the first or only Explicit-Path-Record AVP not matching the ER-Proxy or the ER-Destinations identity. The same error applies to ER-Destinations receiving a Explicit-Path-AVP containing more than one Explicit-Path-Record or an Explicit-Path-AVP with only one Explicit-Path-Record not matching its own identity.

This error SHOULD be considered a protocol failure and SHOULD be treated on a per-hop basis; Diameter proxies may attempt to correct the error, if possible. Diameter answer messages containing this error indication MUST have the 'E' bit set and MUST conform to Section 7.2 of [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.).

DIAMETER_ER_NOT_AVAILABLE 4501

An ER-Destination which supports ER routing but is unable to comply for unknown reasons MAY send an answer message with the Result-Code AVP set to this error code. This error value SHOULD be considered a transient failure indicating that subsequent ER attempts may succeed.

4.8.  Example Message Flow

The example presented here illustrates the flow of Diameter messages with the typical attributes present in the ER scenario.

The ER-Originator in the example below shows the use of Explicit-Path discovery with the first request. However, the ER-Originator may also use a pre-configured cache. The ER-Originator can be any Diameter node sending a request, i.e. client, server or proxy. In this scenario, the local cache of the ER-Originator is initially empty.

The AAA relays in between the ER-Proxies, ER-Originator and ER-Destination may or may not be present and are shown here to depict routing paths that the requests may take prior to being processed by nodes participating in the ER scheme. The AAA relays also depict existing Diameter relays or proxies that do not recognize Explicit-Path AVPs and therefore do not participate in ER.

       ER-                     ER-                   ER-         ER-
   Originator   AAA relays   proxy1   AAA relays   proxy2    Destination
      (o.r1                  (p.r1                 (p.r2       (d.r2
     .example)              .example)             .example)   .example)
                     |          |          |          |          |
   cache=(empty)     |          |          |          |          |
       ------------->|--------->|          |          |          |
    (1st request of the session)|          |          |          |
         Explicit-Path=         |          |          |          |
           o.r1.example,r1.example         |          |          |
     dest-host=d.r2.example     |          |          |          |
     dest-realm=r2.example      |          |          |          |
                     |          |          |          |          |
                     |          |--------->|--------->|          |
                     |          |  (forwarded request)|          |
                     |          |  Explicit-Path=     |          |
                     |          |    record1=o.r1.example,reaml1.example
                     |          |    record2=p.r1.example,r1.example
                     |          |  dest-host=d.r2.example        |
                     |          |  dest-realm=r2.example         |
                     |          |          |          |          |
                     |          |          |          |--------->|
                     |          |          |      (forwarded request)
                     |          |          |      Explicit-Path=
                     |          |          |       record1=o.r1.example,
                     |          |          |               r1.example
                     |          |          |       record2=p.r1.example,
                     |          |          |               r1.example
                     |          |          |       record3=p.r2.example,
                     |          |          |               r2.example
                     |          |          |     dest-host=d.r2.example
                     |          |          |     dest-realm=r2.example
                     |          |          |          |          |
    cache=           |<---------|<---------|<---------|<---------|
      record1=o.r1.example,r1.example         (answer)           |
      record2=p.r1.example,r1.example   Explicit-Path=
      record3=p.r2.example,r2.example    record1=o.r1.example,r1.example
      record4=d.r2.example,r2.example    record2=p.r1.example,r1.example
                     |          |        record3=p.r2.example,r2.example
                     |          |        record4=d.r2.example,r2.example
    Note: An originator pre-configuring    |          |          |
          its local cache can skip the     |          |          |
          exchange above and send the      |          |          |
          initial request as shown below   |          |          |
                     |          |          |          |          |
       ------------->|--------->|          |          |          |
    (subsequent request of the session)    |          |          |
         Explicit-Path=         |          |          |          |
   record1=p.r1.example,r1.example         |          |          |
   record2=p.r2.example,r2.example         |          |          |
   record3=d.r2.example,r2.example         |          |          |
     dest-host=p.r1.example     |          |          |          |
     dest-realm=r1.example      |          |          |          |
                     |          |--------->|--------->|          |
                     |          |  (forwarded request)|          |
                     |          |  Explicit-Path=     |          |
                     |          |      record1=p.r2.example,r2.example
                     |          |      record2=d.r2.example,r2.example
                     |          |  dest-host=p.r2.example        |
                     |          |  dest-realm=r2.example         |
                     |          |          |          |          |
                     |          |          |          |--------->|
                     |          |          |     (forwarded request)
                     |          |          |     Explicit-Path
                     |          |          |       record1=d.r2.example,
                     |          |          |               r2.example
                     |          |          |     dest-host=d.r2.example
                     |          |          |     dest-realm=r2.example
                     |          |          |          |          |
    cache=           |<---------|<---------|<---------|<---------|
      record1=o.r1.example,r1.example    (answer)     |          |
      record2=p.r1.example,r1.example    * no Explicit-Path-AVP present
      record3=p.r2.example,r2.example      |          |          |
      record4=d.r2.example,r2.example      |          |          |
                     |          |          |          |          |
                     |          |          |          |          |
     (subsequent request of the session will repeat the process above)
                     |          |          |          |          |
                     |          |          |          |          |

5.  RADIUS/Diameter Protocol Interactions

No actions need to be taken with regards to RADIUS/Diameter interaction. The routing extension described in this document is transparent to any translation gateway and relevant only to Diameter routing. The assumption is that if there is a RADIUS proxy chain between Diameter translation agents the route between translation agents remains stable during the session and does not cause an invalidation of the proxy path stack.

6.  IANA Considerations

Because this document defines only vendor-specific AVPs and result codes, no IANA actions are necessary.

7.  Security Considerations

The security considerations in [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.) apply to this extension. In addition, this extension raises questions of authorization and can potentially allow a new denial of service attack.

The authorization issue comes about because the proxies that participate in ER are self-selected. An ER-Proxy is able, through the operation of ER, to guarantee that it can monitor every message of a session. This is in contrast to ordinary Diameter routing, where some messages may pass by an alternate route. The question is whether the originating party is prepared to extend this additional degree of trust to arbitrary parties along the path. If not, the ER- Originator requires a mechanism to determine whether an ER-Proxy listed in the returned Explicit-Path AVP can be trusted. If it has such a mechanism, then an unwanted ER-Proxy can be deleted from its cache and thus not appear in the ER- Path AVP in subsequent requests. This specification assumes that the originating party is either prepared to allow arbitrary Diameter nodes along the path to attach themselves to the session as ER-Proxies, or else the ER- Originator maintains a pre-configured list of ER-Proxies in its cache.

The potential denial of service attack is not a serious one because the same result can be obtained more directly. An attacker with control of a Diameter node along the path of the original request could insert an Explicit-Path- Record containing the identity of another node or a non-existent node, rather than its own identity. Routing subsequent messages of the session through another node could result in violation of the trust assumptions made upstream. Routing subsequent messages to a non-existent node causes them to be lost and terminates the session. It would seem simpler to perpetrate whatever harm the attacker intends at the subverted Diameter node itself. The advantage of using ER to accomplish either of the attacks is that it makes it more difficult to determine which node misbehaved, but the extra effort involved to implement the attack does not seem to be worth the potential gain.

8.  Acknowledgements

The authors gratefully acknowledge the contributions of: Tony Zhang, Fortune Huang, Rajith R., Victor Fajardo, Jouni Korhonen, Tolga Asveren, Mark Jones, Avi Lior, Steve Norreys, Lionel Morand, Dave Frascone and Hannes Tschofenig.

9.  References

9.1. Normative References

9.2. Informative References

Authors' Addresses