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

+ RFC 5296 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 RFC + 5295. +

+ +

+ This document uses terminology defined in RFC + 3748, RFC 5295, RFC 5296, and RFC + 4072. "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 states (root keysFFS: authorization attributes) 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. +

+ +

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

+ |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 message . The Application Id of the message is set to + that of the Diameter ERP application (code: TBD) 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 (local domain), + Diameter routing MUST + Should this say + "SHOULD: instead of "MUST"? + be configured so that this ERP/DER message reachs this server, + even if the Destination-Realm is not the 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 5296, this realm is the home domain of the + peer in case of a bootstrapping exchange (the 'B' flag is set in the 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 is generated and returned to the authenticator. The + authenticator may cache this information (with limited duration) to + avoid further attempts for 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 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 5296 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 5296, and forwards the content of the + EAP-Payload AVP, the EAP-Finish/Re-Auth message, to the peer. If the EAP-Initiate/Re-Auth message has its 'B' flag + set (Bootstrapping exchange), the ER server should not possess the root + key in its local database + This may not be + true in future RFC5296bis? + In this case, the ER server acts as a proxy, and forwards the + message to the home EAP server after changing its Application Id to + Diameter EAP and adding an 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 + it may involves that the authenticator is 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 immediatly available for re-authentication of the peer, thus + minimizing 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 must act as a Diameter EAP Proxy + as defined in the Diameter Base Protocol , and routing must be configured so that + Diameter messages of a full EAP authentication are routed through this + proxy. The figure bellow illustrates this mechanism.

+ + 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 ER server proxies the first DER of the full EAP + authentication and adds the ERP-RK-Request AVP inside, if this AVP is + not already in the message (which might happen if there are ER servers + in the visited and the home domains), then forwards the request. + If the EAP server does not support the ERP + extensions, it will simply ignore this grouped AVP and continue as + specified in RFC 4072. If the server + supports the ERP extensions, it caches the ERP-Realm value with the + session data, and continues the EAP authentication. When the + authentication is complete, if it is successful and the EAP method + generated an EMSK, the server MUST derive the rRK as specified in + RFC 5296, and include an instance of the + Key AVP in the Diameter-EAP-Answer + message. 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-Answer, and the Result-Code is + DIAMETER_SUCCESS, it MUST examine the message, extract and remove any + Key AVP from the message, and save its + content. If the message does not contain an ERP-RK-Answer AVP, the ER + server MAY cache this information to avoid possible subsequent + re-authentication attempts for this session. In any case, the + information stored SHOULD NOT have a lifetime greater than the EMSK + lifetime + 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 MAY also add the ERP-Realm AVP after removing the + ERP-RK-Answer AVP in the EAP/DEA message. This could be used by the + authenticator to notify the peer that ERP is bootstrapped, with the ER + domain information. How this information can be transmitted to the + peer is outside the scope of this document. + Is this + possible? It might be useful... + +

+ +

+ Bootstrapping the ER server during the first re-authentication + (also known as explicit bootstrapping) offers several advantages: it + saves resources, since we generate and cache only root keys that we + actually need, and it can accomodate inter-domain handovers or ER + servers that lose their state (for example after reboot). + This last point + might not be true currently, since the peer would not issue a + bootstrapping exchange... But this might change also with + RFC5296bis AFAIU + On the other hand, the first re-authentication with the ER + server requires a one-round-trip exchange with the home EAP server, + which adds some delay to the process (but it is more efficient than a + full EAP authentication in any case). It also requires some + synchronization between the peer and the visited domain: since the ERP + message used is different + and the root key + used also? + for the explicit bootstrapping exchange than for normal + re-authentication; explicit bootstrapping should not be used if + implicit bootstrapping was already performed. + What should we + do if the ER server receives an explicit bootstrapping request but + already possess the rDSRK? Can it answer without going to the home + server? That would be simpler -- planned in rfc5296bis ? + The ER server receives the ERP/DER + message containing the EAP-Initiate/Re-Auth message with the 'B' flag + set. It proxies this message, and performs the following processing in + addition to standard proxy operations: + Changes the Application Id in the header of the message to + Diameter EAP Application (code 5). + + Change the content of Application-Auth-Id accordingly. + Is t better + to leave it unmodified? + + + Add the ERP-RK-Request AVP, which contains the name of the + domain where the ER server is located. + + + Add the + Destination-Host to reach the appropriate EAP server, the one + with the EMSK. How does the ER server know this + information? + + Then the server forwards the EAP/DER request, which is + routed to the home EAP server. If the home + EAP server does not support the ERP extensions, it replies with an + error since the encapsulated EAP-Initiate/Re-auth command is not + understood. Otherwise, it processes the ERP request as described in + . In particular, it includes the + Domain-Name TLV attribute with the content from the ERP-Realm AVP. It + creates the EAP/DEA reply message . + including an instance of the Key AVP . + + What about + authorization AVPs? + 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 (code TBD) + + Extract and cache the content of the Key AVP. + And + authorization AVPs ? + + The DEA is then forwarded to the authenticator, that can use + the rMSK as described in RFC 5296. + The figure below captures this proxy + behavior:

+ + Diameter ERP/DER + (EAP-Initiate) + ------------------------> + Diameter EAP/DER + (EAP-Initiate) + (ERP-RK-Request) + + <------------------------ + Diameter EAP/DEA + (EAP-Finish) + (Key AVP) + <---------------------- + Diameter ERP/DEA + (EAP-Finish) + (Key AVP) +]]> + +

+ +

+ This section describes in detail a re-authentication exchange with a + (bootstrapped) ER server. 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 +]]> + In ERP, 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 start of ERP. In this case, the peer responds with an + EAP-Initiate/Re-auth message. If the + authenticator does not support ERP (pure + support), it discards the EAP packets with an unknown ERP-specific code + (EAP-Initiate). The peer may fallback to full EAP authentication in this + case. When the authenticator receives an + EAP-Initiate/Re-auth message from the peer, it process as described in + with regards to the EAP state machine. It + creates a Diameter EAP Request message following the general process of + DiameterEAP, with the following + differences: + The Application Id in the header is set to Diameter ERP (code + TBD). + + The value in Auth-Application-Id AVP is also set to Diameter ERP + Application. + + The keyName-NAI attribute from ERP message is used to create the + content of User-Name AVP and Destination-Realm AVP. + + + 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 [Editor's note: FFS]. + + Do we really + do authorization with Diameter ERP ? -- need to pass the + authorization attrs to the ER server in that case. Idea FFS: we + do authorization only for explicit bootstrapping + exchanges... + + + The EAP-Payload AVP contains the ERP message, + EAP-Initiate/Re-Auth. + 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 processing described in RFC + 4072, with the following differences: + The Application Id in the header is set to Diameter ERP (code + TBD). + + The value of the Auth-Application-Id AVP is also set to Diameter + ERP Application. + + The EAP-Payload AVP contains the ERP message, + EAP-Finish/Re-auth. + + In case of successful authentication, an instance of the Key AVP + containing the Re-authentication Master Session Key (rMSK) derived + by ERP is included. + What about all + the authorization attributes? If we want to include them, they + have to be present on the ER server... + + When the authenticator receives this ERP/DEA answer, it + processes it as described in Diameter EAP + and RFC 5296: the content of EAP-Payload + AVP content is forwarded to the peer, and the contents of the + Keying-Material AVP + is used as a shared secret for Secure Association Protocol. +

+ +

+ We define a new Diameter application in this document, Diameter ERP + Application, with an Application Id value of TBD. 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 Application in the of 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. +

+ +

+ This section discusses the AVPs used by the Diameter ERP + application. + +

+ The ERP-RK-Request AVP (AVP Code TBD) 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-Realm } + * [ AVP ] +]]> + +

+ +

+ The ERP-Realm AVP (AVP Code TBD) is of type DiameterIdentity. It + contains the name of the realm in which the ER server is located. + + We may re-use + Origin-Realm here instead? On the other hand, ERP-Realm may be + useful if the ER server is in a third-party realm, if this is + possible. + This AVP has the M and V bits + cleared. +

+ +

+ The Key AVP is + of type "Grouped" and is used to carry the 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 3 for rRK. +

+ +

+ The Keying-Material AVP contains rRK sent by the home EAP server + to the ER server, in answer to a request containing an + ERP-RK-Request AVP. How this material is derived and used is + specified in RFC 5296. +

+ +

+ This AVP contains the EMSKname which identifies the keying + material. The derivation of this name is specified in RGC 5296. +

+ +

+ 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 document does not address some known issues in Diameter ERP + mechanism. The authors would like to hear ideas about how to address + them. The main issue is the use of ERP for + authentication after a handover of the peer to a new authenticator (or + different authenticator port). Diameter ERP is not meant to be a + mobility protocol. A number of issues appear when we try to do handover + in Diameter ERP (alone): how to manage the Session-Id AVP; how does the + ER server provide the Authorization AVPs; how does the peer learn the + ERP domain of the new authenticator; how does the home server reachs the + peer to for example terminate the session; and so on... Therefore, the + management of the session for a mobile peer is not (yet) addressed in + this document. It must be studied how Diameter ERP can be for example + used in conjunction with a mobility application (Diameter MIP4, Diameter + MIP6) to support the optimized re-authentication in such situation. + Another issue concerns the case where the home + realm contains several EAP servers. In multi rounds full EAP + authentication, the Destination-Host AVP provides the solution to reach + the same server across the exchanges. Only this server possess the EMSK + for the session. In case of explicit bootstrapping, the ER server must + therefore be able to reach the correct server to request the DSRK. A + solution might consist in saving the Origin-Host AVP of all successful + EAP/DEA in the ER server, which is a bit similar to the implicit + bootstrapping scenario described here -- only we save the server name + instead of the root key, and we must then be able to match the DSRK with + the user name. Finally, this document + currently lacks a description of what happens when a Re-Auth-Request is + received for a peer on the authenticator. +

+ +

+ Hannes Tschofenig wrote the initial draft for this document and + provided useful reviews. Vidya Narayanan + reviewed a rough draft version of the document and found some errors. + Lakshminath Dondeti contributed to the early + versions of the document. 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 RFC 3588 for the following AVPs: + + ERP-RK-Request + + ERP-Realm + These AVPs are defined in . +

+ +

+ The security considerations from the following documents also apply + here: + RFC 3588 + + RFC 4072 + + RFC 5247 + + RFC 5295 + + + + Do we really respect + these security considerations with the mechanism we describe here? + Is it safe to use ERP-RK-Request / Answer AVPs? What is the worst + case? + EAP channel bindings may be necessary to ensure that the + Diameter client and the server are in sync regarding the key Requesting + Entity's Identity. Specifically, the Requesting Entity advertises its + identity through the EAP lower layer, and the user or the EAP peer + communicates that identity to the EAP server (and the EAP server + communicates that identity to the Diameter server) via the EAP method + for user/peer to server verification of the Requesting Entity's + Identity. + What does this + paragraph actually mean? + +