| Network Working Group | J. Bournelle |
| Internet-Draft | L. Morand |
| Intended status: Standards Track | Orange Labs |
| Expires: November 05, 2011 | S. Decugis |
| Free Diameter | |
| Q. Wu | |
| Huawei | |
| G.W. Zorn | |
| Network Zen | |
| May 04, 2011 |
Diameter support for EAP Re-authentication Protocol (ERP)
draft-ietf-dime-erp-06.txt
The EAP Re-authentication Protocol (ERP) defines extensions to the Extensible Authentication Protocol (EAP) to support efficient re- authentication between the peer and an EAP Re-authentication (ER) server through a compatible authenticator. This document specifies Diameter support for ERP. It defines a new Diameter ERP application to transport ERP messages between an ER authenticator and the ER server, and a set of new AVPs that can be used to transport the cryptographic material needed by the re-authentication server.
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Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on November 05, 2011.
Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.
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RFC5296 [RFC5296] defines the EAP Re-authentication Protocol (ERP). It consists of the following steps:
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 [RFC5295].
This document uses terminology defined in RFC3748 [RFC3748], RFC5295 [RFC5295], RFC5296 [RFC5296], and RFC4072 [RFC4072].
"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" Section 11.1; the same commands are denoted "EAP/DER" and "EAP/DEA" when the Application Id in the message is set to "Diameter EAP Application" [RFC4072].
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].
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 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.
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).
When the peer initiates an ERP exchange, the authenticator creates a Diameter-EAP-Request message [RFC4072]. 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 Section 6.
If there is an ER server in the same domain as the authenticator (local domain), Diameter routing 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 RFC5296 [RFC5296], this realm is the home domain of the peer in case of bootstrapping exchange ('B' flag is set in ERP message) or the domain of the bootstrapped ER server otherwise
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 as specified in [RFC3588] 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 [RFC5296] then creates the ERP/DEA answer as described in Section 6. The rMSK is included in this answer.
Finally, the authenticator extracts the rMSK from the ERP/DEA as described in RFC5296 [RFC5296], 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. 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 the ERP-RK-Request AVP to request the root key. See section Section 5 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 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 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 several ER servers on the path), then forwards the request.
If the EAP server does not support the ERP extensions, it simply ignores the ERP-RK-Request AVP and continues as specified in RFC 4072 [RFC4072]. 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 5296 [RFC5296], using the saved domain name. It then includes the rRK inside a Key AVP Section 8.3 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 Section 8.3 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.
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 less resource-consuming, since root keys are generated and cached only when needed. On the other hand, in that case first re-authentication requires a one-round-trip exchange with the home EAP server, which is less efficient than the implicit bootstrapping scenario.
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:
Then the proxied EAP/DER request is sent and routed to the home Diameter EAP server.
If the home EAP server does not support 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 [RFC5296]. In particular, it includes the Domain- Name TLV attribute with the content from the ERP-Realm AVP. It creates the EAP/DEA reply message [RFC4072]. including an instance of the Key AVP Section 8.3 with Key-Type AVP set to rRK.
The ER server receives this EAP/DEA and proxies it as follows, in addition to standard proxy operations: [RFC5296].
The ERP/DEA message is then forwarded to the authenticator, that can use the rMSK as described in RFC 5296
The figure below captures this proxy behavior:
Authenticator ER server Home EAP server
============= ========= ===============
----------------------->
Diameter ERP/DER
(EAP-Initiate)
------------------------>
Diameter EAP/DER
(EAP-Initiate)
(ERP-RK-Request)
<------------------------
Diameter EAP/DEA
(EAP-Finish)
(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.
ER server
Peer Authenticator (bootstrapped)
==== ============= ======================
[ <------------------------ ]
[optional EAP-Initiate/Re-auth-start,]
[ possibly with ERP domain name ]
----------------------->
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 [RFC4072] 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, it process as described in [RFC5296] with regards to the EAP state machine. It creates a Diameter EAP Request message following the general process of Diameter EAP [RFC4072], with the following differences:Section 4.
Then this ERP/DER message is sent as described in
The ER server receives and processes this request as described in Section 4. It then creates an ERP/DEA message following the general processing described in RFC4072 [RFC4072], with the following differences:
When the authenticator receives this ERP/DEA answer, it processes it as described in Diameter EAP [RFC4072] and RFC5296 [RFC5296]: the content of EAP-Payload AVP content is forwarded to the peer, and the contents of the Keying-Material AVP [I-D.ietf-dime-local-keytran] 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 [RFC3588].
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 Section 8 , 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-RK-Request ::= < AVP Header: TBD >
{ 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.
This AVP has the M and V bits cleared.
The Key AVP [I-D.ietf-dime-local-keytran] 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 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 ER server to authenticator. How this material is derived and used is specified in RFC 5296 [RFC5296].
This AVP contains the EMSKname which identifies the keying material. The derivation of this name is specified in RGC 5296 [RFC5296].
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 application. A number of issues appear when we try to do handover while using Diameter ERP:
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.
In roaming environments, it might be useful that a broker provides ERP services. The security implications of storing the DSRK generated for the visited domain into the broker's server should be studied.
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 [RFC3588].
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 [RFC3588] for the following AVPs: Section 8.
These AVPs are defined in
The security considerations from the following documents apply here:
FFS: Do we really respect these security considerations with the mechanism we describe here? Is it safe to use ERP-RK-Request & Key AVPs? What is the worst case? For example if a domain tricks the peer into beliving it is located in a different domain?
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.
QUESTION: What does this paragraph actually mean?
| [RFC5247] | Aboba, B., Simon, D. and P. Eronen, "Extensible Authentication Protocol (EAP) Key Management Framework", RFC 5247, August 2008. |