





Independent Submission                                           T. Tsou
Request for Comments: 6159                     Huawei Technologies (USA)
Category: Informational                                          G. Zorn
ISSN: 2070-1721                                              Network Zen
                                                          T. Taylor, Ed.
                                                     Huawei Technologies
                                                              April 2011


           Session-Specific Explicit Diameter Request Routing

Abstract

   This document describes a mechanism to enable specific Diameter
   proxies to remain in the path of all message exchanges constituting a
   Diameter session.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see 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/rfc6159.

IESG Note

   Techniques similar to those discussed in this document were discussed
   in the IETF Diameter Maintenance and Extensions (DIME) Working Group.
   The group had no consensus that the problems addressed by such work
   are a real concern in Diameter deployments.  Furthermore, there was
   no consensus that the proposed solutions are in line with the
   architectural principles of the Diameter protocol.  As a result, the
   working group decided not to undertake the work.  There has also not
   been a formal request for this functionality from any standards body.
   This RFC represents a continuation of the abandoned work.  Readers of
   this specification should be aware that the IETF has not reviewed
   this specification and cannot say anything about suitability for a
   particular purpose or compatibility with the Diameter architecture
   and other extensions.



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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.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. The 3GPP Wireless LAN (WLAN) Access Architecture ................4
      3.1. Maintaining the Routing Path ...............................5
   4. Diameter Explicit Routing (ER) ..................................6
      4.1. Originating a Request (ER-Originator) ......................6
      4.2. Relaying and Proxying Requests (ER-Proxy) ..................8
      4.3. Receiving Requests (ER-Destination) .......................10
      4.4. Diameter Answer Processing ................................11
      4.5. Failover and Failback Considerations ......................12
      4.6. Attribute-Value Pairs .....................................12
           4.6.1. Explicit-Path-Record AVP ...........................12
                  4.6.1.1. Proxy-Host AVP ............................13
                  4.6.1.2. Proxy-Realm AVP ...........................13
           4.6.2. Explicit-Path AVP ..................................13
      4.7. Error Handling ............................................13
   5. Example Message Flow ...........................................14
   6. RADIUS/Diameter Protocol Interactions ..........................16
   7. Security Considerations ........................................17
   8. Acknowledgements ...............................................17
   9. References .....................................................18
      9.1. Normative References ......................................18
      9.2. Informative References ....................................18

1.  Introduction

   In the Diameter base protocol [RFC3588], the routing of request
   messages is based solely on the routing decisions made separately by
   each node along the path.  [RFC5729] has added the ability to force
   messages to pass through a specified set of realms through the use of
   Network Access Identifier (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



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   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 Third Generation Partnership Project (3GPP) IP access
   [TS23.234]) require that once certain agents become engaged in a
   session, they be 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 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].

   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 that is capable of participating in ER and that
      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., a client, server or proxy capable of initiating sessions and
      participating in ER.



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   Authentication, Authorization, and Accounting (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 Attribute Value Pairs (AVPs).

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

   The 3GPP WLAN IP access architecture [TS23.234] is one example of a
   system requiring that certain agents (stateful proxies, in this case)
   remain in the forwarding path of all session messages.  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 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] 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 (ER)
      Maintaining the selected message path for all messages in the
      Diameter session.





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   Selecting an initial message path is outside the scope of this
   document.  A mechanism for maintaining the selected message path 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:

   o  the Diameter client in the WLAN access node (e.g., the 3GPP AAA
      client in the terminal visited network),

   o  a Diameter proxy in the visited mobile network (e.g., the 3GPP AAA
      proxy in the terminal visited network), and

   o  a Diameter server in the user's home realm (e.g., the destination
      3GPP AAA server in the terminal home network).

   Message routing for the initial session request uses the normal
   Diameter routing tables (Section 2.7 of [RFC3588]) in the 3GPP AAA
   client, the 3GPP AAA proxy in the visited network, and any
   intermediate proxies after that.  The 3GPP AAA client sends the
   initial session request to the 3GPP AAA proxy in the visited network.
   The 3GPP AAA proxy processes the request, then forwards it towards
   the destination 3GPP AAA server, through an intermediate proxy if
   necessary.  The request may be forwarded through other intermediate
   proxies in the same way, until it reaches the destination 3GPP AAA
   server in the terminal home network.

   The functions assigned to the 3GPP AAA proxy include:

   o  Reporting charging information to the offline charging system in
      the visited network,

   o  Policy enforcement based on roaming agreements, and

   o  Service termination initiated by the visited network's operator.

   These functions all require that state be maintained within the
   visited network.  The 3GPP's 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.








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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] to implement the ER
   mechanism.  The proposed extensions utilize existing routing
   strategies in [RFC3588] and do not mandate modifications to it.  The
   mechanism imposes loose rather than strict source routing, in that
   subsequent messages of a session are forced through the participating
   nodes, but not through any individual non-participating nodes.  In
   summary, 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 that 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].  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, with each cached entry 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 5.
   Note that the ER-Originator can be any Diameter node, i.e., a 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, the ER-Originator can use Explicit-Path discovery.  It is
   RECOMMENDED that Explicit-Path discovery be used whenever possible
   since pre-configuration is less flexible by nature.




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   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.  To initiate discovery, when the ER-Originator
   sends the first request message of a session, it MUST include the
   Explicit-Path AVP containing a single Explicit-Path-Record AVP with
   the identity and/or the realm of the ER-Originator.  The
   ER-Originator MUST set the Destination-Host and/or Destination-Realm
   AVP of the request message to the identity and/or the realm of the
   ER-Destination, respectively, as specified in [RFC3588].

      Note that ER-Originator initial request message routing procedures
      and the process of population of the Destination-Realm may be
      affected by the User-Name AVP NAI decoration [RFC5729].  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
      in 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 Explicit-Path discovery as
      defined in this document.

   If the path taken by the initial request encounters one or more
   participating ER-Proxies and a participating ER-Destination, the
   procedures described in Section 4.2 and Section 4.3 ensure that a
   successful response to that request will contain an Explicit-Path AVP
   that includes 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), 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 the initial request message initiated Explicit-Path discovery, but
   the Explicit-Path AVP in the answer message contains Explicit-Path-
   Records for the ER-Originator and ER-Destination only, it is 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 for more
   discussion.  In such cases, the situation may be transient, and



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   Explicit-Path discovery may find participating proxies in succeeding
   sessions.  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 possibly the Destination-Realm in
   the request message MUST be copied from the values of the Proxy-Host
   AVP and, if present, the Proxy-Realm AVP 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
      between the ER-Originator and the first ER-Proxy will route the
      message towards the first ER-Proxy as specified in RFC 3588
      [RFC3588].

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

   Answer messages received by the ER-Originator to subsequent request
   messages after the Explicit-Path has been established SHOULD NOT have
   an Explicit-Path AVP.  If they do, 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 for 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 not an existing participant, if Explicit-Path
   discovery is in progress, and if it wishes to participate, it appends
   an Explicit-Path-Record AVP identifying itself to the end of the
   Explicit-Path AVP.  If it is an existing participant, the ER-Proxy
   pops/removes the Explicit-Path-Record AVP pertaining to itself from
   the Explicit-Path AVP and then uses the next Explicit-Path-Record AVP
   for subsequent routing.  Details of this operation follow.




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   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.  The
   following cases can occur.

   1.  If an incoming request does not contain an Explicit-Path AVP,
       then the ER-Proxy takes no action beyond processing and
       forwarding the request as specified in [RFC3588].

   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 AVP
       contained in each Explicit-Path-Record.  If its identity is not
       present, then:

       A.  If it wishes to participate in explicit routing, the ER-Proxy
           MUST verify that Explicit-Path discovery is in progress by
           verifying that the Proxy-Host AVP in the first Explicit-Path-
           Record AVP in the Explicit-Path AVP does not match the
           Destination-Host AVP (if present).  If this verification
           succeeds or the Destination-Host AVP is absent, the ER-Proxy
           MAY 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 a Proxy-Host AVP set to the
           proxy's identity, and MAY contain a Proxy-Realm AVP giving
           the proxy's realm.  If, however, the Destination-Host AVP is
           present and matches the Proxy-Host AVP of the first Explicit-
           Path-Record AVP, then the Explicit-Path contains an already-
           defined source route that does not include the ER-Proxy.  The
           ER-Proxy SHOULD process the request as if the ER-Path AVP
           were absent.

       B.  If the ER-Proxy does not wish to participate in the ER, it
           SHOULD NOT modify the Explicit-Path AVP and SHOULD simply
           process and forward the request as specified in [RFC3588]
           using the existing values of the Destination-Host and/or
           Destination-Realm AVPs.  Non-ER-Proxies and relays that do
           not support ER and do not recognize Explicit-Path AVP will
           take the same action.











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   3.  If the identity of the ER-Proxy is present in the Explicit-Path
       AVP, then:

       A.  If it is not the first Explicit-Path-Record in the AVP, this
           MUST 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).

       B.  If the identity of the ER-Proxy matches the first Explicit-
           Path-Record, the ER-Proxy MUST remove this record from the
           Explicit-Path AVP and repopulate the Destination-Host and
           possibly the Destination-Realm AVP from the next Explicit-
           Path-Record present in the Explicit-Path AVP.  Setting the
           Destination-Host and possibly the Destination-Realm AVP will
           ensure that the ER-Proxy as well as all AAA relays 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 popping an entry from a stack represented by the
           Explicit-Path AVP.

   The behavior specified above also 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) 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 ER-Destination MUST simply process the
       request for local consumption and formulate an answer message as
       specified in [RFC3588].














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   2.  If the incoming request contains an Explicit-Path AVP, the
       ER-Destination MUST check whether its identity is present in the
       Explicit-Path AVP.  If its identity is not present, indicating
       that Explicit-Path discovery is in progress, then:

       A.  If it wishes to participate in the ER, and subject to
           paragraph B below, the ER-Destination 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.

       B.  If there is only one Explicit-Path-Record in the incoming
           Explicit-Path AVP, then this is an indication of a successful
           Explicit-Path discovery, but with no participating
           ER-Proxies.  The ER-Destination SHOULD NOT copy the Explicit-
           Path AVP into the subsequent answer message.

       C.  If 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.  The
           ER-Destination MUST NOT include any Explicit-Path AVP in the
           subsequent answer.  Diameter servers that do not support ER
           and do not recognize the Explicit-Path AVP will also omit the
           Explicit-Path AVP from the answer message.

   3.  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 MUST be
       considered an error, and an answer message with the 'E' bit set
       and containing an Experimental-Result-Code AVP set to
       DIAMETER_INVALID_PROXY_PATH_STACK MUST be sent back to the
       ER-Originator (Section 4.7).  If the identity of the
       ER-Destination does match the only existing Explicit-Path-Record,
       then this is an indication that the request reached the
       ER-Destination by way of a successfully executed explicit route.
       The ER-Destination MUST NOT include the Explicit-Path AVP in 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].
   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.



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4.5.  Failover and Failback Considerations

   If there is no ER-Proxy along the selected path, the answer message
   MAY 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, 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
      makes sense only if the application requirements do not mandate
      that every participating ER-Proxy receive all of the messages of a
      session.

   However, depending on local policy, the ER-Originator MAY attempt ER
   path discovery in subsequent sessions despite the lack of proxy
   participants in the earlier attempt.

   If a failover occurs in a Diameter node preceding an ER-Proxy when
   the Explicit-Path is already established, it is possible that a
   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 the Diameter application or in [RFC3588].

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 by IANA in "Private
   Enterprise Numbers" registry; see http://www.iana.org/).

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] 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 ]




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4.6.1.1.  Proxy-Host AVP

   The Proxy-Host AVP (AVP Code 35004) is of type DiameterIdentity.  It
   identifies the ER node that is inserting the record.  The Proxy-Host
   AVP MUST be present.

4.6.1.2.  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.  The Proxy-
   Realm AVP MAY be present in the Explicit-Path-Record.  If it is
   present, the realm name included in the value of the Proxy-Host AVP
   MUST match the value of the Proxy-Realm AVP.

4.6.2.  Explicit-Path AVP

   The Explicit-Path AVP (AVP Code 35003) is of type Grouped.  This AVP
   MUST be present in all request messages performing ER.  It MAY be
   present in the answer to the initial session request message if
   Explicit-Path discovery was successfully executed for the request.

         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] 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 Explicit-Path has been established has the first or only
      Explicit-Path-Record AVP not matching the ER-Proxy's or the
      ER-Destination's identity.  The same error applies to
      ER-Destinations receiving an 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].




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   DIAMETER_ER_NOT_AVAILABLE     4501

      An ER-Destination that 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.

5.  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 could
   also use a pre-configured cache.  The ER-Originator can be any
   Diameter node sending a request, i.e., a client, server, or proxy.
   In this scenario, the local cache of the ER-Originator is initially
   empty.

   The AAA relays 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.

























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      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,r1.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.  |          |          |







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                    |          |          |          |          |
      ------------->|--------->|          |          |          |
   (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)
                    |          |          |          |          |
                    |          |          |          |          |

                     Figure 1: Example ER Message Flow

6.  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.




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7.  Security Considerations

   The security considerations in [RFC3588] 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 either the originating party is
   prepared to allow arbitrary Diameter nodes along the path to attach
   themselves to the session as ER-Proxies, or 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.








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9.  References

9.1.  Normative References

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

   [RFC3588]   Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
               Arkko, "Diameter Base Protocol", RFC 3588,
               September 2003.

   [RFC5729]   Korhonen, J., Ed., Jones, M., Morand, L., and T. Tsou,
               "Clarifications on the Routing of Diameter Requests Based
               on the Username and the Realm", RFC 5729, December 2009.

9.2.  Informative References

   [TS23.234]  3GPP, "3GPP system to Wireless Local Area Network (WLAN)
               interworking; System description", TS 23.234
               Version 7.4.0, 2006.































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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: tena@huawei.com
   URI:   http://tinatsou.weebly.com/contact.html


   Glen Zorn
   Network Zen
   227/358 Thanon Sanphawut
   Bang Na, Bangkok  10260
   Thailand

   Phone: +66 (0) 87-040-4617
   EMail: gwz@net-zen.net


   Tom Taylor (editor)
   Huawei Technologies
   1852 Lorraine Ave.
   Ottawa
   Canada

   EMail: tom111.taylor@bell.net





















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