Diameter Support for the EAP Re-authentication Protocol (ERP)

Cao, et al. defines the EAP Re-authentication Protocol (ERP). It consists of the following steps: A root key for re-authentication is derived from the Extended Master Session Key (EMSK) created during EAP authentication . This root key is transported from the EAP server to the ER server. A one-round-trip exchange between the peer and the ER server, resulting in mutual authentication. To support the EAP reauthentication functionality, ERP defines two new EAP codes - EAP-Initiate and EAP-Finish. This document defines how Diameter transports the ERP messages during the re-authentication process. For this purpose, we define a new Application Identifier for ERP, and re-use the Diameter EAP commands (DER/DEA). This document also discusses the distribution of the root key during bootstrapping, in conjunction with either the initial EAP authentication (implicit bootstrapping) or the first ERP exchange (explicit bootstrapping). Security considerations for this key distribution are detailed in Salowey, et al. .

This document uses terminology defined in Aboba, et al. , Salowey, et al. , Cao, et al. , and Eronen, Hiller & Zorn . The re-athentication Domain-Specific Root Key (rDSRK) is a re-authentication Root Key (rRK, ) derived from the DSRK instead of the EMSK. "Root key" (RK) or "bootstrapping material" refer to the rRK or rDSRK derived from an EMSK, depending on the location of the ER server in home or foreign domain. We use the notation "ERP/DER" and "ERP/DEA" in this document to refer to Diameter-EAP-Request and Diameter-EAP-Answer commands with the Application Id set to <Diameter ERP Application> ; the same commands are denoted "EAP/DER" and "EAP/DEA" when the Application Id in the message is set to <Diameter EAP Application> .

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 .

This document assumes the existence of at most one logical ER server entity in a domain. If several physical servers are deployed for robustness, a replication mechanism must be deployed to synchronize the ERP state (e.g., root keys) between these servers. This replication mechanism is out of the scope of this document. If multiple ER servers are deployed in the domain, we assume that they can be used interchangeably. If multiple ER servers are deployed across the domains, we assume only one ER server that is near to the peer is getting involved in the ERP. Also this document assumes the existence of at most one EAP server entity in the home domain. In case of multiple physical home EAP servers in the same domain, if the ER server wants to reach the same home EAP server, the ER server may cache the Destination-Host AVP corresponding to the home EAP server it requests.

The following figure shows the components involved in ERP, and their interactions.

Diameter +--------+ +-------------+ ERP +-----------+ (*) | Home | Peer <->|Authenticator|<=======>| ER server | <---> | EAP | +-------------+ +-----------+ | server | +--------+ (*) Diameter EAP application, explicit bootstrapping scenario only. The ER server is located either in the home domain (same as EAP server) or in the visited domain (same as authenticator, when it differs from the home domain). Can the ER server be located in a third domain (ex: broker's) according to ERP mechanism? When the peer initiates an ERP exchange, the authenticator creates a Diameter-EAP-Request (DER) message . The Application Id of the message is set to that of the Diameter ERP application in the message. The generation of the ERP/DER message is detailed in . If there is an ER server in the same domain as the authenticator (i.e., the local domain), Diameter routing MUST be configured so that this ERP/DER message reaches that server, even if the Destination-Realm is not the same as local domain. If there is no local ER server, the message is routed according to its Destination-Realm AVP content, extracted from the realm component of the keyName-NAI attribute. As specified in RFC 6696, this realm is the home domain of the peer in the case of bootstrapping exchange ('B' flag is set in ERP message) or the domain of the bootstrapped ER server otherwise This actually might allow the ER server to be in a third party realm. If no ER server is available in the home domain either, the ERP/DER message cannot be delivered, and an error DIAMETER_UNABLE_TO_DELIVER MUST be generated as specified in and returned to the authenticator. The authenticator MAY cache this information (with limited duration) to avoid further attempts to execute ERP with this realm. It MAY also fallback to full EAP authentication to authenticate the peer. When an ER server receives the ERP/DER message, it searches its local database for a valid, unexpired root key and authorization state ? matching the keyName part of the User-Name AVP. If such key is found, the ER server processes the ERP message as described in RFC 6696, then creates the ERP/DEA answer as described in . The rMSK is included in this answer. Finally, the authenticator extracts the rMSK from the ERP/DEA as described in RFC 6696, and forwards the content of the EAP-Payload AVP, the EAP-Finish/Re-Auth message, to the peer. The ER server may or may not possess the root key in its local database. If the EAP-Initiate/Re-Auth message has its 'B' flag set (Bootstrapping exchange) and the ER server possesses the root key, the ER server SHOULD respond directly to the peer that initiated the ERP exchange. Otherwise, the ER server SHOULD act as a proxy and forward the message to the home EAP server after changing its Application Id to Diameter EAP and adding the ERP-RK-Request AVP to request the root key. See for more detail on this process.

The bootstrapping process involves the home EAP server and the ER server, but also impacts the peer and the authenticator. In ERP, the peer must derive the same keying material as the ER server. To achieve this, it must learn the domain name of the ER server. How this information is acquired is outside the scope of this specification, but the authenticator might be configured to advertize this domain name, especially in the case of re-authentication after a handover. The bootstrapping of an ER server with a given root key happens either during the initial EAP authentication of the peer when the EMSK -- from which the root key is derived -- is created, during the first re-authentication, or sometime between those events. We only consider the first two possibilities in this specification, in the following sub-sections.

Bootstrapping the ER server during the initial EAP authentication (also known as implicit bootstrapping) offers the advantage that the server is immediately available for re-authentication of the peer, thus minimizing the re-authentication delay. On the other hand, it is possible that only a small number of peers will use re-authentication in the visited domain. Deriving and caching key material for all the peers (for example, for the peers that do not support ERP) is a waste of resources and should be avoided. To achieve implicit bootstrapping, the ER server acts as a Diameter EAP Proxy , and Diameter routing MUST be configured so that Diameter EAP application messages are routed through this proxy. The figure bellow illustrates this mechanism.

ER server & Authenticator EAP Proxy Home EAP server ============= =========== =============== -------------------------> Diameter EAP/DER (EAP-Response) -------------------------> Diameter EAP/DER (EAP-Response) (ERP-RK-Request) <==================================================> Multi-round Diameter EAP exchanges, unmodified <------------------------- Diameter EAP/DEA (EAP-Success) (MSK) (Key AVP (rRK)) <------------------------- Diameter EAP/DEA (EAP-Success) (MSK) [ERP-Realm] The authenticator creates the first DER of the full EAP authentication and sends it to the ER server. The ER server proxies the first DER of the full EAP authentication and adds the ERP-RK-Request AVP inside, then forwards the request to the home EAP server. If the home Diameter server does not support the Diameter ERP extensions, it simply ignores the ERP-RK-Request AVP and continues as specified in RFC 4072 . If the server supports the ERP extensions, it saves the value of the ERP-Realm AVP found inside the ERP-RK-Request AVP, and continues with the EAP authentication. When the authentication completes, if it is successful and the EAP method has generated an EMSK, the server MUST derive the rRK as specified in RFC 6696, using the saved domain name. It then includes the rRK inside a Key AVP () with the Key-Type AVP set to rRK, before sending the DEA as usual. When the ER server proxies a Diameter-EAP-Answer message with a Session-Id corresponding to a message to which it added an ERP-RK- Request AVP, and the Result-Code is DIAMETER_SUCCESS, it MUST examine the message and save and remove any Key AVP () with Key-Type AVP set to rRK. If the message does not contain such Key AVP, the ER server may cache the information that ERP is not possible for this session to avoid possible subsequent attempts. In any case, the information stored in ER server concerning a session should not have a lifetime greater than the EMSK for this session. how does the ER server knows the EMSK lifetime, if there is no ERP-RK-Answer? What is the lifetime of the MSK for example? If the ER server is successfully bootstrapped, it should also add the ERP-Realm AVP after removing the Key AVP with Key-Type of rRK in the EAP/DEA message. This ERP-Realm information can be used by the authenticator to notify the peer that ER server is bootstrapped, and for which domain. How this information can be transmitted to the peer is outside the scope of this document. This information needs to be sent to the peer if both implicit and explicit bootstrapping mechanisms are possible, because the ERP message and the root key used for protecting this message are different in bootstrapping exchanges and non-bootstrapping exchanges.

Bootstrapping the ER server during the first re-authentication (also known as explicit bootstrapping) is only needed when there is no local ER server in the visited domain and there is an ER server in the home domain. It is less resource-intensive, since the EMSK generated during initial EAP authentication is reused to derive root keys. On the other hand, the first re-authentication requires a one-round-trip exchange with the home EAP server, since the EMSK is generated during the initial EAP authentication and never leaves the home EAP server, which is less efficient than the implicit bootstrapping scenario. The EAP-Initiate/Re-auth message is sent to the home ER server. The home ER server receives the ERP/DER message containing the EAP- Initiate/Re-Auth message with the 'B' flag set. It creates the new EAP/DER message using the received DRP/DER message and performs the following processing: Set the Application Id in the header of the message to <Diameter EAP Application> Extract the ERP-RK-Request AVP from the ERP/DER message, which contains the name of the domain where the ER server is located and add it to the newly created ERP/DER message. Then the newly created EAP/DER is sent and routed to the home Diameter EAP application server. If the home Diameter EAP server does not support ERP extensions, EAP packets with an unknown ERP-specific code (EAP-Initiate) will not be understood. In such a case, the home Diameter EAP server MUST send an EAP/DEA with a Result-Code indicating a Permanent Failure (for example, DIAMETER_ERROR_EAP_CODE_UNKNOWN or DIAMETER_UNABLE_TO_COMPLY). The Failed-AVP AVP MUST be included and contain a copy of the EAP-Payload AVP. Otherwise, it processes the DSRK request as described in RFC 6696. In particular, it includes the Domain- Name TLV attribute with the content from the ERP-Realm AVP. The server creates the EAP/DEA reply message including an instance of the Key AVP () with Key-Type AVP set to rRK and an instance of the Domain-Name TLV attribute with the content from the ERP-Realm AVP. The ER server receives this EAP/DEA and proxies it as follows, in addition to standard proxy operations: Set the Application Id back to Diameter ERP Application Id ( TBD1 IANA) Extract and cache the content of the Key AVP with Key-Type set to rRK, as described in the implicit scenario ().And authorization AVPs ? The ERP/DEA message is then forwarded to the authenticator, that can use the rMSK as described in RFC 6696. The figure below captures this proxy behavior:

Authenticator ER server Home Diameter server ============= ========= ==================== -----------------------> Diameter ERP/DER (EAP-Initiate) ------------------------> Diameter EAP/DER (EAP-Response) (ERP-RK-Request) <------------------------ Diameter EAP/DEA (EAP-Success) (Key AVP (rRK)) (Key AVP (rMSK)) <---------------------- Diameter ERP/DEA (EAP-Finish) (Key AVP (rMSK))

This section describes in detail a re-authentication exchange with an ER server that was previously bootstrapped. The following figure summarizes the re-authentication exchange.

EAP-Initiate/Re-auth ===============================> Diameter ERP, cmd code DER User-Name: Keyname-NAI EAP-Payload: EAP-Initiate/Re-auth <=============================== Diameter ERP, cmd code DEA EAP-Payload: EAP-Finish/Re-auth Key AVP: rMSK <---------------------- EAP-Finish/Re-auth ]]> The peer sends an EAP-Initiate/Re-auth message to the ER server via the authenticator. Alternatively, the authenticator may send an EAP- Initiate/Re-auth-Start message to the peer to trigger the mechanism. In this case, the peer responds with an EAP-Initiate/Re-auth message. If the authenticator does not support ERP (pure Diameter EAP support), it discards the EAP packets with an unknown ERP-specific code (EAP-Initiate). The peer should fallback to full EAP authentication in this case. When the authenticator receives an EAP-Initiate/Re-auth message from the peer, the message is processed as described in RFC 6696 with regard to the EAP state machine. It creates a Diameter ERP/DER message following the general process of Diameter EAP, with the following differences: The Application Id in the header is set to <Diameter ERP> (code TBD1 TBD1 IANA). The value in Auth-Application-Id AVP is also set to <Diameter ERP>. The keyName-NAI attribute from the ERP message is used to create the content of the User-Name and Destination-Realm AVPs. FFS: What about Session-ID AVP -- in case of re-auth at the same place, and in case of handover? The Auth-Request-Type AVP content is set to the appropriate value. The EAP-Payload AVP contains the EAP-Initiate/Re-Auth meassge. Then this ERP/DER message is sent as described in . The ER server receives and processes this request as described in . It then creates an ERP/DEA message following the general process described in RFC4072 , with the following differences: The Application Id in the header is set to <Diameter ERP> (code TBD1). The value of the Auth-Application-Id AVP is also set to <Diameter ERP>. The EAP-Payload AVP contains the EAP-Finish/Re-auth message. If authentication is successful, an instance of the Key AVP containing the Re-authentication Master Session Key (rMSK) derived by ERP is included. When the authenticator receives this ERP/DEA answer, it processes it as described in the Diameter EAP Application specification and RFC 6696: the content of the EAP-Payload AVP is forwarded to the peer, and the contents of the Keying-Material AVP is used as a shared secret for a secure association protocol specific to the lower-layer in use.

We define a new Diameter application in this document, Diameter ERP Application, with an Application Id value of TBD1. Diameter nodes conforming to this specification in the role of ER server MUST advertise support by including an Auth-Application-Id AVP with a value of Diameter ERP in the Capabilities-Exchange-Request and Capabilities-Exchange-Answer commands . The primary use of the Diameter ERP Application Id is to ensure proper routing of the messages, and that the nodes that advertise the support for this application do understand the new AVPs defined in , although these AVP have the 'M' flag cleared.

The following sub-sections discuss the AVPs used by the Diameter ERP application.

The ERP-RK-Request AVP (AVP Code TBD2) is of type grouped AVP. This AVP is used by the ER server to indicate its willingness to act as ER server for a particular session. This AVP has the M and V bits cleared.

ERP-RK-Request ::= < AVP Header: TBD2 > { ERP-Realm } * [ AVP ]

The ERP-Realm AVP (AVP Code TBD3) is of type DiameterIdentity. It contains the name of the realm in which the ER server is located. This AVP has the M and V bits cleared.

The Key AVP is of type "Grouped" and is used to carry the rRK or rMSK and associated attributes. The usage of the Key AVP and its constituent AVPs in this application is specified in the following sub-sections.

The value of the Key-Type AVP MUST be set to 2 for rRK or 3 for rMSK.

The Keying-Material AVP contains the rRK sent by the home EAP server to the ER server, in answer to a request containing an ERP-RK-Request AVP, or the rMSK sent by the ER server to the authenticator. How this material is derived and used is specified in RFC 6696.

This AVP contains the EMSKname which identifies the keying material. The derivation of this name is specified in RFC 6696.

The Key-Lifetime AVP contains the lifetime of the keying material in seconds. It MUST NOT be greater than the remaining lifetime of the EMSK from which the material was derived.

This section defines new Result-Code values that MUST be supported by all Diameter implementations that conform to this specification.

Errors that fall within the Permanent Failures category are used to inform the peer that the request failed and SHOULD NOT be attempted again. DIAMETER_ERROR_ EAP_CODE_UNKNOWN (TBD4) This error code is used by the Diameter server to inform the peer that the received EAP-PAYLOAD AVP contains an EAP packet with an unknown EAP code.

Hannes Tschofenig wrote the initial draft of this document. Lakshminath Dondeti contributed to the early versions of the document.

Hannes Tschofenig, Zhen Cao, Benoit Claise, Elwyn Davies and Jouni Korhonen provided useful reviews. Vidya Narayanan reviewed a rough draft version of the document and found some errors. Many thanks to these people!

This document requires IANA registration of the following new elements in the Authentication, Authorization, and Accounting (AAA) Parameters registries.

This specification requires IANA to allocate a new value "Diameter ERP" in the "Application IDs" registry using the policy specified in Section 11.3 of RFC 3588 .

This specification requires IANA to allocate new values from the "AVP Codes" registry according to the policy specified in Section 11.1 of Fajardo, et al. for the following AVPs: ERP-RK-Request ERP-Realm These AVPs are defined in .

This specification requires IANA to allocate a new value from the "Result-Code AVP Values (code 268) - Permanent Failure" registry according to the policy specified in Section 11.3.2 of Fajardo, et al. for the following Result-Code: DIAMETER_ERROR_EAP_CODE_UNKNOWN TBD4 This result-code value is defined in .

The security considerations from the following documents apply here: Fajardo, et al. RFC 4072 RFC 6696 Zorn, Wu and Cakulev