An IKEv2 Extension for Supporting ERP
draft-nir-ipsecme-erx-02

Abstract

This document describes an extension to the IKEv2 protocol that allows an IKE Security Association (SA) to be created and authenticated using the EAP Re-authentication Protocol extension as described in RFC 5296 and its bis document.

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 22, 2012.

Copyright Notice

Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.

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1. Introduction

IKEv2, as specified in [RFC5996], allows authentication of the initiator using an EAP method. This is described in section 2.16. Using EAP significantly increases the count of round-trips required to establish the IPsec SA, and also may require user interaction. This makes it inconvenient to allow a single remote access client to create multiple IPsec tunnels with multiple IPsec gateways that belong to the same domain.

The EAP Re-authentication Protocol (ERP), as descripted in [RFC5296bis], allows an EAP peer to authenticate to multiple authenticators, while performing the full EAP method only once. Subsequent authentications require fewer round-trips and no user interaction.

Bringing these two technologies together allows a remote access IPsec client to create multiple tunnels with different gateways that belong to a single domain, as well as using the keys from other contexts of using EAP, such as network access within the same domain, to transparently connect to VPN gateways within this domain.

1.1. Conventions Used in This Document

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].

2. Usage Scenarios

Several scenarios motivated this proposal:

• Multiple tunnels for a single remote access VPN client. Suppose a company has offices in New York City, Paris, and Shanghai. For historical reasons, the email server is located in the Paris office, while most of the servers hosting the company's intranet are located in Shanghai, and the finance department servers are in NYC. An employee using remote access VPN may need to connect to servers from all three locations. While it is possible to connect to a single gateway, and have that gateway route the requests to the other gateways (perhaps through site-2-site VPN), this is not efficient, and it is more desirable to have the client initiate three different tunnels. It is, however, not desirable to have the user type in a password three times.
• Roaming. In these days of mobile phones and tablets, users often move from the wireless LAN in their office, where access may be granted through 802.1x, to a cellular network where VPN is necessary and back again. Both the VPN server and the 802.1x access point are authenticators that connect to the same AAA servers. So it makes sense to make the transition smooth, without requiring user interaction. [SecureBeacon] is an attempt to allow detecting whether the client should connect using VPN or not.

3. Protocol Outline

Supporting ERX requires an EAP payload in the first IKE_AUTH request. This is a deviation from the rules in RFC 5996, so support needs to be indicated through a Notify payload in the IKE_SA_INIT response. This Notify replaces the EAP-Initiate/Re-auth-Start message of ERX, and therefore contains the domain name, as specified in section 5.3.1.1 of [RFC5296bis].

A supporting initiator that has unexpired keys for this domain will send the EAP_Initiate/Re-auth message in an EAP payload in the first IKE_AUTH request.

The responder sends the EAP payload content to a backend AAA server, and receives the rMSK and an EAP-Finish/Re-auth message. If forwards that to the initiator in an EAP payload within the first IKE_AUTH response.

The initiator then sends an additional IKE_AUTH request, that includes the AUTH payload which has been calculated using the rMSK in the role of the MSK as described in sections 2.15 and 2.16 or [RFC5996]. The responder replies similarly, and the IKE_AUTH exchange is finished.

The following figure is adapted from appendixes C.1 and C.3 of RFC 5996, with most of the optional payloads removed. Note that the EAP_Initiate/Re-auth message replaces the IDi payload.

 init request         --> SA, KE, Ni,

 init response       <-- SA, KE, Nr,
                         N[ERX_SUPPORTED]

 first request       --> EAP(EAP_Initiate/Re-auth),
                         [[N(HTTP_CERT_LOOKUP_SUPPORTED)], CERTREQ+],
                         [IDr],
                         [CP(CFG_REQUEST)],
                         SA, TSi, TSr,
                         [V+][N+]

 first response      <-- IDr, [CERT+], AUTH,
                         EAP(EAP-Finish/Re-auth),
                         [V+][N+]

 last request        --> AUTH

 last response       <-- AUTH,
                         [CP(CFG_REPLY)],
                         SA, TSi, TSr,
                         [V+][N+]
            

3.1. Clarification About EAP Codes

Section 3.16 of [RFC5996] enumerates the EAP codes in EAP messages which are carried in EAP payloads. The enumeration goes only to 4. It is not clear whether that list is supposed to be exhaustive or not.

To clarify, an implementation supporting this specification MUST accept and transmit EAP messages with at least the codes for Initiate and Finish (5 and 6).

3.2. User Name in the Protocol

The authors, as well as participants of the HOKEY and IPSECME working groups believe that all use cases for this extension to IKE have a single backend AAA server doing both the authentication and the re-authentication. The reasoning behind this is that IKE runs over the Internet, and would naturally connect to the user's home network.

This section addresses instances where this is not the case.

Section 5.3.2 of [RFC5296bis] describes the EAP-Initiate/Re-auth packet, which in the case of IKEv2 is carried in the first IKE_AUTH request. This packet contains the KeyName-NAI TLV. This TLV contains the username used in authentication. It is relayed to the AAA server in the AccessRequest message, and is returned from the AAA server in the AccessAccept message.

The username part of the NAI within the TLV is the EMSKName encoded in hexadecimal digits. The domain part is the domain name of the home domain of the user. The username part is ephemeral in the sense that a new one is generated for each full authentication. This ephemeral value is not a good basis for making policy decisions, and they are also a poor source of user ID for the purposes of logging.

Instead, it is up to the implementation in the IPsec gateway to make policy decisions based on other factors. The following list is by no means exhaustive:

• In some cases the home domain name may be enough to make policy decisions. If all users with a particular home domain get the same authorization, then policy does not depend on the real user name. Logging can still be done by correlating VPN gateway IKE events with AAA servers access records.
• Sometimes users receive different authorizations based on groups they belong to. The AAA server can communicate such information to the VPN gateway, for example using the CLASS attribute in RADIUS and DIAMETER. Logging again depends on correlation with AAA servers.
• AAA servers may support extensions that allow them to communicate with their clients (in our case - the VPN gateway) to push user information. For example, a certain product integrates a RADIUS server with LDAP, so a client could query the server using LDAP and receive the real record for this user. Others may provide this data through vendor-specific extensions to RADIUS or DIAMETER.

In any case authorization is a major issue in deployments, if the peer's home EAP server does not also perform the re-authentication.

4. ERX_SUPPORTED Notification

The Notify payload is as described in [RFC5996]

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !  Protocol ID  !   SPI Size    !    ERX Notify Message Type    !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                            Domain Name                        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            

• Protocol ID (1 octet) MUST be 1, as this message is related to an IKE SA.
• SPI Size (1 octet) MUST be zero, in conformance with section 3.10 of [RFC5996].
• ERX Notify Message Type (2 octets) - MUST be xxxxx, the value assigned for ERX. TBA by IANA.
• Domain Name (variable) - contains the domain name or realm, as these terms are used in [RFC5296bis].

5. Security Considerations

TBA

6. IANA Considerations

IANA is requested to assign a notify message type from the status types range (16418-40959) of the "IKEv2 Notify Message Types" registry with name "ERX_SUPPORTED".

7. References

7.1. Normative References

7.2. Informative References

Authors' Addresses

Table of Contents

• 1. Introduction
• 1.1. Conventions Used in This Document
• 2. Usage Scenarios
• 3. Protocol Outline
• 3.1. Clarification About EAP Codes
• 3.2. User Name in the Protocol
• 4. ERX_SUPPORTED Notification
• 5. Security Considerations
• 6. IANA Considerations
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Authors' Addresses