Dynamic Host Configuration WG R. Pruss Internet-Draft Cisco Systems Intended status: Informational G. Zorn Expires: November 19, 2008 Aruba Networks R. Maglione Telecom Italia Y. Li Huawei Technologies May 18, 2008 Authentication Extensions for the Dynamic Host Configuration Protocol draft-pruss-dhcp-auth-dsl-03 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on November 19, 2008. Abstract This document defines Dynamic Host Configuration Protocol (DHCP) extensions that provide for end-user authentication prior to configuration of the host. The primary applicability is within a Digital Subscriber Line (DSL) Broadband network environment in order to enable a smooth migration from the Point to Point Protocol (PPP). Pruss, et al. Expires November 19, 2008 [Page 1] Internet-Draft DHCP Authentication May 2008 Requirements Language 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 RFC 2119 [RFC2119]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 3. Network Architecture and Terminology . . . . . . . . . . . . . 5 4. Applicability Statement . . . . . . . . . . . . . . . . . . . 6 5. Protocol Operation . . . . . . . . . . . . . . . . . . . . . . 6 5.1. Protocol Operation for IPv4 . . . . . . . . . . . . . . . 6 5.2. Protocol Operation for IPv6 . . . . . . . . . . . . . . . 11 6. DHCP Options . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.1. DHCP Authentication Protocol Option . . . . . . . . . . . 15 6.2. EAP-Message Option . . . . . . . . . . . . . . . . . . . . 16 7. Messages for EAP operation . . . . . . . . . . . . . . . . . . 16 8. Fragmentaion . . . . . . . . . . . . . . . . . . . . . . . . . 17 9. Backwards Compatibility Considerations . . . . . . . . . . . . 17 10. Security Considerations . . . . . . . . . . . . . . . . . . . 19 10.1. Message Authentication . . . . . . . . . . . . . . . . . . 19 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 13.1. Normative References . . . . . . . . . . . . . . . . . . . 20 13.2. Informative References . . . . . . . . . . . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 Intellectual Property and Copyright Statements . . . . . . . . . . 24 Pruss, et al. Expires November 19, 2008 [Page 2] Internet-Draft DHCP Authentication May 2008 1. Introduction This document defines DHCP Options and procedures that allow for an Extensible Authentication Protocol (EAP) authentication exchange to occur in DHCP in order to enable smooth migration from Point-to-Point Protocol (PPP)[RFC1661] sessions to IP sessions in a DSL Broadband network environment. Primary goals are integration of authentication in such a way that it will operate seamlessly with existing RADIUS- based Authentication, Authorization and Accounting (AAA) infrastructure and Asynchronous Transfer Mode (ATM) or Ethernet based DSL Networks. As such, only the termination points of PPP in the DSL network are affected, both of which are devices that would logically need to be updated in any transition from PPP to IP sessions. It should be noted that [RFC3118] defines a mechanism that provides authentication of individual DHCP messages. While this mechanism does provide a method of authentication for a DHCP Client based on a shared secret, it does not do so in a manner that can be seamlessly integrated with existing RADIUS-based AAA infrastructure. 2. Problem Statement Digital Subscriber Line (DSL) broadband service providers are witnessing a shift in the "last-mile" aggregation technologies and protocols which have traditionally been relied upon. Two primary transitions are from ATM to Ethernet in the access network, and from the PPP for multi-protocol framing and dynamic endpoint configuration to direct encapsulation of IP and DHCP for dynamic endpoint configuration for some devices. The term used by the DSL Forum for the network state associated with an authorized subscriber (that is using DHCP and IP rather than PPP) is "IP session" [WT-146]. While these trends can be readily witnessed, neither are occurring overnight. In addition, they are not necessarily implemented in lock-step. Thus, one may find ATM-based and Ethernet-based access networks running a combination of PPP sessions and IP sessions at any given time, particularly during transition periods. These coexistences will even occur for the same service subscriber. Removing PPP, Point-to-Point Protocol over ATM (PPPoA) [RFC2364], and Point-to-Point Protocol over Ethernet (PPPoE) [RFC2516] from the subscriber access network is relatively straightforward in that most of the properties that DSL providers are interested in going forward are already present in DHCP and IP sessions. Luckily, there are some capabilities of PPP which the market does not continue to demand. For example, the Dynamic configuration in PPP for IPX or NETBEUI, for example, is no longer of concern. Neither are the multi-link bonding capabilities of PPP [RFC1990] commonly used on separate ISDN Pruss, et al. Expires November 19, 2008 [Page 3] Internet-Draft DHCP Authentication May 2008 B-channels, and the myriad of other features that PPP developed as the "dial-based" access protocol of choice for framing, authentication, and dynamic configuration for IP and other network layer protocols. Missing from IP sessions and DHCP [RFC2131], however, are isomorphic methods for user authentication and session liveness probing (sometimes referred to as a session "keepalive"). For the latter, existence of a client using a given IP address can be detected by a number of means, including Address Resolution Protocol (ARP) [RFC0826], ICMP Echo/Echo Response [RFC0792], or Bidirectional Forwarding Detection (BFD) [I-D.ietf-bfd-base]. This leaves authentication as an open issue needing resolution. Specifically, authentication based on a username and secret password must be covered. This is something that in PPP always occurs before dynamic configuration of an IP address and associated parameters. While most DSL deployments utilize a username and password to authenticate a subscriber and authorize access today, this is not the only method for authentication that has been adopted when moving to DHCP and IP sessions. "Option 82" [RFC3046] is commonly used with DHCP as a credential to authenticate a given subscriber line and authorize service. In this model, the DSL Access Node, which always sits between the DHCP Client and Server, snoops DHCP messages as they pass, and inserts pre-configured information for a given line (e.g., an ATM VPI/VCI, Ethernet VLAN, or other tag). That information, while provided in clear text, traverses what is considered a physically secured portion of the access network and is used to determine (typically via a request to an AAA server) whether the DHCP exchange can continue. This fits quite well with current DSL network architecture, as long as the subscriber line itself is all that needs be authorized. However, in some service models it is still necessary for the subscriber to provide credentials directly. From the perspective of the Network Access Server (NAS) where the DHCP Server resides, the extensions defined in this document are analogous to the commonly available "Option 82" method. The primary difference between using Option 82 line configuration and a username and password is that the authentication credentials are provided by the subscriber rather than inserted by intervening network equipment. Providing credentials from the subscriber rather than intervening network equipment is particularly important for cases where subscriber line information is unavailable, untrusted, or due to the terms of the service changing at any time. Further, different devices in the home may have different policies and require different credentials. Migration scenarios where PPPoE and DHCP operate on the same network for a period of time lend well to models which utilize identical authentication and authorization credentials across the different data plane encapsulations. Pruss, et al. Expires November 19, 2008 [Page 4] Internet-Draft DHCP Authentication May 2008 3. Network Architecture and Terminology The DSL Forum defines its ATM-based network architecture in [TR-059] and Ethernet-based network architecture in [TR-101]. The extensions for DHCP defined in this document are designed to work identically on Ethernet or ATM architectures. The diagram in Figure 1 and following terminology will be used throughout: +-----------+ +------------+ | DHCP | | AAA/RADIUS | | Server | | Server | +-----------+ +------------+ | | | | Sub. +-----+ +--------+ | +-----+ +----------+ Home ---| HGW |---| | +---------| | | | Network +-----+ | Access | | | | | | Node |--/Aggregation\--| NAS |---| Internet | Sub. +-----+ | |--\ Network /--| | | | Home ---| HGW |---| | | | | | Network +-----+ +--------+ +-----+ +----------+ | | |----------DSL Access Network --------| Figure 1: DSL Network Architecture o Access Node (AN): Network device, usually located at a service provider central office or street cabinet, that terminates Access Loop connections from Subscribers. In case the Access Loop is a Digital Subscriber Line (DSL), this is often referred to as a DSL Access Multiplexer (DSLAM). The AN may support one or more Access Loop technologies and allow them to inter-work with a common aggregation network technology. o Network Access Server (NAS): Network device that aggregates multiplexed Subscriber traffic from a number of Access Nodes. The NAS plays a central role in per-subscriber policy enforcement and QoS. Often referred to as a Broadband Network Gateway (BNG) or Broadband Remote Access Server (BRAS). A detailed definition of the NAS is given in [RFC2881]. o The Home Gateway (HGW) connects the different Customer Premises Equipment (CPE) to the Access Node and the access network. In case of DSL, the HGW is a DSL Network Termination (NT) that could either operate as a layer 2 bridge or as a layer 3 router. In the latter case, such a device is also referred to as a Routing Gateway (RG). Pruss, et al. Expires November 19, 2008 [Page 5] Internet-Draft DHCP Authentication May 2008 o Referring to the DSL network architecture depicted in Figure 1, PPP (via PPPoA [RFC2364] or PPPoE [RFC2516]) operates over the DSL Access Network between the NAS and a device behind the HGW, or between the NAS and the HGW itself. The DHCP Client resides either on a home network device or the HGW, and the DHCP Server protocol state machine may reside fully on the NAS. The NAS obtains per-subscriber client configuration information either locally, relayed from a DHCP server or from the AAA infrastructure (which itself may consult external DHCP servers if necessary) after authentication is successfully completed. 4. Applicability Statement The primary target for this extension is for DSL service provider networks where PPP is being phased out to be replaced by native IP and DHCP, or where new devices are being added which will not utilize PPP. Very specific assumptions have been made with respect to the security model, operational methods, and integration requirements for existing AAA mechanisms during the design. It is understood that this mechanism may not be generally applicable in this form for all network environments where DHCP is deployed, though perhaps elements of it may be used to develop a more generic approach while still meeting the specific requirements set out by the DSL network architecture. Earlier revisions of this document included a method to embed PPP CHAP [RFC1994] authentication as Options in existing DHCP messages. This method has been abandoned due to security vulnerabilities in CHAP, as well as a lack of extensibility. This document bases its authentication on EAP [RFC3748] which can be used with a large number of different authentication methods, including one backwards compatible with existing PPP CHAP. 5. Protocol Operation This section describes the protocol operation for EAP within DHCPv4 [RFC2131] and DHCPv6 [RFC3315]. Options and message specifications used in these operation descriptions are detailed in later sections. If multiple DHCP exchanges are occurring with multiple servers, both IPv4 and IPv6 each needs to authenticate separately. 5.1. Protocol Operation for IPv4 It is essential that the user/node authentication occurs before the assignment of an IP address and, further, that the assignment of the address depends upon the details of the successful authentication. . DHCP [RFC2131] is widely used as an address assignment method (among Pruss, et al. Expires November 19, 2008 [Page 6] Internet-Draft DHCP Authentication May 2008 other things); EAP [RFC3748] has been widely adapted for authentication purposes, especially in those types of networks where DHCP is also used. This section describes how to combine the two in order to provide both strong authentication and authenticated address assignment in an efficient manner. Two new DHCPEAP messages are used in the DHCP message flow to support the new EAP phase which occurs before a DHCPOFFER is sent by the Server. This message is used to integrate authentication methods supported by EAP, including CHAP and any other "in the clear" password mechanisms (for example, to support One-Time Password mechanisms), or to carry other EAP methods. EAP is widely used in other environments, outside of DSL Broadband, including 802.11 "Wi-Fi" access networks but could be used in future DSL Broadband deployments. To request the assignment of an IPv4 address with authentication, a client first locates a DHCP server, then authenticates using EAP and then requests the assignment of an address and other configuration information from the server. The client sends a DHCP Discover message with an option specifying the authentication protocol as EAP to find an available DHCP server. Any server that can that can authenticate and address it responds with a DHCPEAP-REQ message. Servers which support DHCP authentication will respond with a DHCPEAP-REQ message. The client may receive one or more DHCPEAP-REQ messages from one or more DHCP Servers. The Client chooses one to reply to, and sends a DHCPEAP-RES message, silently discarding DHCPEAP-REQ messages from other Servers. The DHCPEAP-RES and DHCPEAP-REQ messages contain EAP packets which facilitate the EAP authentication exchange. The exchange may occur between the DHCP Client and DHCP Server embedded within a NAS, or be carried transparently to the AAA Server. Upon successful completion of the authentication phase, the DHCP server sends a DHCPOFFER with the appropriate IP configuration for the authenticated user. The client then follows the normal DHCP procedures of a successful DHCP exchange by sending a DHCPREQUEST, followed by a DHCPACK from the Server. If the authentication phase fails (e.g., the user does not provide appropriate credentials), then according to configured policy the DHCP Client is either denied any IP configuration with the DHCP Server sending a DHCPNAK accordingly, or the DHCP Client is given a "limited access" configuration profile and the DHCP exchange continues as if the authentication was successful. A typical message flow proceeds as shown in Figure 2: Pruss, et al. Expires November 19, 2008 [Page 7] Internet-Draft DHCP Authentication May 2008 (HGW) (NAS) (AAA) DHCP Client DHCP Server/ RADIUS Server DHCPDISCOVER -------> (w/DHCP-auth-proto EAP) <------- DHCPEAP-REQ (w/EAP Message) DHCPEAP-RES -------> (w/EAP Message) RADIUS Access-Request -------> (w/EAP Message) <-------- RADIUS Access-Accept (w/EAP Message) (Access-Reject (w/EAP Message) if unsuccessful) <------- DHCPEAP-REQ (w/EAP Message) DHCPEAP-RES -------> (w/EAP Message) RADIUS Access-Request -------> (w/EAP Message) <-------- RADIUS Access-Accept (w/EAP Message) (Access-Reject (w/EAP Message) if unsuccessful) (The last four messages repeat until EAP Success or EAP fail) (DHCP messages continue normally from this point forward if successful) <------- DHCPOFFER (w/EAP Success Message) (w/yiaddr) DHCPREQUEST -------> <------- DHCPACK Figure 2: DHCP Message Flow with DHCPEAP messages The retransmission is handled by EAP as per Section 4.1 in [RFC3748]. Pruss, et al. Expires November 19, 2008 [Page 8] Internet-Draft DHCP Authentication May 2008 The message exchange presented in the figure is an example of simple one-way user authentication, e.g. the Server verifies the credentials of the HGW Client. The client indicates the ability to have an EAP exchange and the NAS (which takes on the EAP authenticator role) initiates the first EAP request to the DHCP Client (which takes on the EAP supplicant role). DHCP-REQ and DHCP-RES does not suggest a coupling between the EAP state machine and the DHCP authentication phase state machine. They only indicate the direction of the message, either from Client to Server or Server to Client. When the NAS is acting as a DHCP Relay the BRAS may split the EAP Messages from DHCP and perform the AAA authentication with an AAA server. This allows use of existing DHCP servers and existing AAA servers. An example message flow for DHCP Relay proceeds as shown in Figure 3: Pruss, et al. Expires November 19, 2008 [Page 9] Internet-Draft DHCP Authentication May 2008 (HGW) (NAS) (AAA) (DHCP) DHCP Client AAA Client RADIUS Server DHCP Server DHCPDISCOVER -------> (w/DHCP-auth-proto EAP) <------- DHCPEAP-REQ (w/EAP Message) DHCPEAP-RES -------> (w/EAP Message) RADIUS Access-Request -------> (w/EAP Message) <-------- RADIUS Access-Accept (w/EAP Message) (Access-Reject (w/EAP Message) if unsuccessful) <------- DHCPEAP-REQ (w/EAP Message) DHCPEAP-RES -------> (w/EAP Message) RADIUS Access-Request -------> (w/EAP Message) <-------- RADIUS Access-Accept (w/EAP Message) (Access-Reject (w/EAP Message) if unsuccessful) (The last four messages repeat until EAP Success or EAP fail) (DHCP messages continue normally from this point forward if successful) DHCPDISCOVER ------------------------------> (w/RADIUS attributes suboption) <----------------------------- DHCPOFFER <------- DHCPOFFER (w/EAP Success Message) (w/yiaddr) DHCPREQUEST -------> <------- DHCPACK Pruss, et al. Expires November 19, 2008 [Page 10] Internet-Draft DHCP Authentication May 2008 Figure 3: DHCP Authentication Message Flow with DHCP relay NAS When the DHCP relay agent in the NAS receives a DHCP message from the client, it MAY append a DHCP Relay Agent Information option containing the RADIUS Attributes suboption, along with any other suboptions it is configured to supply. The RADIUS Attributes suboption is defined in [RFC4014] DHCP Authentication uses two DHCP options: o DHCP Authentication Protocol Option in the DHCPDISCOVER to specify the type of authentication exchange. o EAP-Message Option to carry the EAP data in the DHCPEAP messages. 5.2. Protocol Operation for IPv6 This section describes the protocol operation for extending Dynamic Host Configuration Protocol for IPv6 [RFC3315] for an EAP phase. The same as the previous section on extending DHCP in IPv4 new DHCP messages, DHCPEAP-REQ and DHCPEAP-RES are used to support EAP authentication before host configuration occurs. The mechanisms described here follow a similar methodology as that for DHCPv4 described in Section 5.1. The client sends a Solicit message with an Option specifying the session authentication protocol as EAP to the All_DHCP_Relay_Agents_and_Servers address to find available DHCP servers. Any server that can authenticate and address it responds with a DHCPEAP-REQ message. The client may receive one or more DHCPEAP-REQ messages from one or more DHCP Servers. The Client chooses one to reply to, and sends a DHCPEAP-RES message, silently discarding DHCPEAP-REQ messages from other Servers. The DHCPEAP-RES and DHCPEAP-REQ messages contain EAP packets which facilitate the EAP authentication exchange. The exchange may occur between the DHCP Client and DHCP Server embedded within a NAS, or be carried transparently to the AAA Server. Upon successful completion of the authentication phase, the DHCP server sends a ADVERTISE with the appropriate configuration for the authenticated user. The client then follows the normal DHCP procedures of a successful DHCP exchange by sending a REQUEST, followed by a DHCPACK from the Server. If the authentication phase fails (e.g., the user does not provide appropriate credentials), then according to configured policy the DHCP Client is either denied any IP configuration with the DHCP Pruss, et al. Expires November 19, 2008 [Page 11] Internet-Draft DHCP Authentication May 2008 Server sending a NAK accordingly, or the DHCP Client is given a "limited access" configuration profile and the DHCP exchange continues as if the authentication was successful. . A typical message flow proceeds as shown in Figure 4: Pruss, et al. Expires November 19, 2008 [Page 12] Internet-Draft DHCP Authentication May 2008 (HGW) (NAS) (AAA) DHCP Client DHCP Server/ RADIUS Server SOLICIT -------> (w/DHCP-auth-proto EAP) <------- DHCPEAP-REQ (w/EAP Message) DHCPEAP-RES -------> (w/EAP Message) RADIUS Access-Request -------> (w/EAP Message) <-------- RADIUS Access-Accept (w/EAP Message) (Access-Reject (w/EAP Message) if unsuccessful) <------- DHCPEAP-REQ (w/EAP Message) DHCPEAP-RES -------> (w/EAP Message) RADIUS Access-Request -------> (w/EAP Message) <-------- RADIUS Access-Accept (w/EAP Message) (Access-Reject (w/EAP Message) if unsuccessful) (The last four messages repeat until EAP Success or EAP fail) (DHCP messages continue normally from this point forward if successful) <------- ADVERTISE (w/EAP Success Message) REQUEST -------> <------- REPLY Figure 4: DHCP IPv6 with DHCPEAP message The retransmission is handled by EAP as per Section 4.1 in [RFC3748]. Pruss, et al. Expires November 19, 2008 [Page 13] Internet-Draft DHCP Authentication May 2008 The message following this exchange is a ADVERTISE, sent unchanged by the Server. A typical message flow proceeds as shown in Figure 5: (HGW) (NAS) (AAA) (DHCP) DHCP Client AAA Client RADIUS Server DHCP Server SOLICIT -------> (w/DHCP-auth-proto EAP) <------- DHCPEAP-REQ (w/EAP Message) DHCPEAP-RES -------> (w/EAP Message) RADIUS Access-Request -------> (w/EAP Message) <-------- RADIUS Access-Accept (w/EAP Message) (Access-Reject (w/EAP Message) if unsuccessful) <------- DHCPEAP-REQ (w/EAP Message) DHCPEAP-RES -------> (w/EAP Message) RADIUS Access-Request -------> (w/EAP Message) <-------- RADIUS Access-Accept (w/EAP Message) (Access-Reject (w/EAP Message) if unsuccessful) (The last four messages repeat until EAP Success or EAP fail) (DHCP messages continue normally from this point forward if successful) RELAY-FORW ------------------------------> (w/RADIUS attributes suboption) <----------------------------- RELAY-REPL <------- ADVERTISE (w/EAP Success Message) Pruss, et al. Expires November 19, 2008 [Page 14] Internet-Draft DHCP Authentication May 2008 REQUEST -------> <------- REPLY Figure 5: Message Flow with new message and a DHCP relay When the DHCP relay agent in the NAS receives a DHCP message from the client, it MAY append a DHCP Relay Agent Information option containing the RADIUS Attributes suboption, along with any other suboptions it is configured to supply. The RADIUS Attributes suboption is defined in [RFC4014] DHCP Authentication uses two DHCP options: o DHCP Authentication Protocol Option in the SOLICIT to specify the type of authentication exchange. o EAP-Message Option to carry the EAP data in the DHCPEAP messages. 6. DHCP Options Two DHCP Options are defined in this section. The first DHCP Authentication Protocol Option is originated from the client in the DHCPDISCOVER and SOLICIT to specify which authentication the client supports. 6.1. DHCP Authentication Protocol Option The DHCPAUTH-Protocol option is sent from the DHCP Client to the DHCP Server to indicate the authentication algorithm the client prefers. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DHCP Code | Length | Authentication-Protocol | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Algorithm | +-+-+-+-+-+-+-+-+ Figure 6: DHCP Authentication Protocol Option DHCP Code: TBA-1 (DHCPAUTH-Protocol) Length: 3 Authentication-Protocol Pruss, et al. Expires November 19, 2008 [Page 15] Internet-Draft DHCP Authentication May 2008 C227 (HEX) for Extensible Authentication Protocol (EAP) Algorithm The Algorithm field is one octet and indicates the authentication method to be used with the Method 6.2. EAP-Message Option The format of the EAP-Message option used in Protocol Operation for IPv4 is as follows: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DHCP Code | Length | m... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: EAP-Message Option The maximum size of a DHCP option is 255 octets. While in some cases (e.g., EAP MD5-Challenge [RFC3748]) a complete EAP message may fit in a single DHCP option, in general this is not the case. If an EAP message is too large to fit into a single DHCP option, the method defined in [RFC3396] MUST be used to split the EAP message into separate options for transmission. Similarly, EAP assumes a minimum MTU of 1020 octets while the minimum DHCP packet size is 576 octets, including 312 octets reserved for options. A DHCP client including the EAP-Message option SHOULD also include the 'maximum DHCP message size' option [RFC2132] to set a suitable DHCP message size. If a DHCP message is received containing more than one EAP-Message option, the method defined in [RFC3396] MUST be used to reassemble the separate options into the original EAP message. A DHCP server receiving an EAP message MAY forward it via a AAA protocol (such as RADIUS [RFC2865] [RFC3579] or Diameter [RFC3588]] [RFC4072]). 7. Messages for EAP operation The DHCPEAP messages follow the format for DHCP messages defined in RFC 2131 [RFC2131]. This new message is identified by the presence of a DHCP Message Type option, which encodes DHCPEAP-REQ or DHCPEAP- RES message type. Other fields in the DHCP message header, such as siaddr and fname, are left unused. The authentication data in a DHCPAUTH message is carried in a EAP- Messsage option EAP-Message Option. Pruss, et al. Expires November 19, 2008 [Page 16] Internet-Draft DHCP Authentication May 2008 8. Fragmentaion Encapsulating EAP messages within DHCP raises the question of whether there are potential difficulties with respect to the MTU sizes of the EAP and DHCP messages, as well as the underlying link MTU. EAP as defined in [RFC3748] Section 3.1 says: [4] Minimum MTU. EAP is capable of functioning on lower layers that provide an EAP MTU size of 1020 octets or greater. DHCP as defined in [RFC2131] Section 2 says: ... This requirement implies that a DHCP client must be prepared to receive a message of up to 576 octets, the minimum IP datagram size an IP host must be prepared to accept [3]. DHCP clients may negotiate the use of larger DHCP messages through the 'maximum DHCP message size' option. The options field may be further extended into the 'file' and 'sname' fields. If we assume EAP MTU-sized packets, the overhead to pack an EAP packet into DHCP options is 2*(1020/255), or 8 octets. Adding the DHCP header (240 octets), UDP (8 octets), and the IP header (20 octets) gives 278 octets total overhead. Since the Ethernet effective MTU is 1500 octets, this 278 octet overhead leaves the DHCP protocol with 1222 octets to carry EAP. This space is over 200 octets more than the EAP MTU of 1020 octets. If we add the SNAME and CHADDR fields to the option pool, then there are nearly 400 octets available for DHCP options in an Ethernet MTU- sized DHCP packet, encapsulating EAP. In short, when the 'maximum DHCP message size' option is used by the client, there is no problem carrying in EAP over DHCP. i.e. clients capable of performing EAP over DHCP should also advertise a maximum message that is capable of carrying EAP over DHCP. 9. Backwards Compatibility Considerations This section is aimed at describing interoperability scenarios involving HGW and NAS with or without DHCP Authentication mechanism support in order to analyze compatibility issues that could be faced between newer and older products during the introduction of the DHCP Authentication functionally in current implemented network environments. Scenario 1: Both HGW and NAS do not support DHCP Authentication Pruss, et al. Expires November 19, 2008 [Page 17] Internet-Draft DHCP Authentication May 2008 In this case the authentication process does not start, thus traditional DHCP message flow applies. Scenario 2: HGW does not support DHCP Authentication and NAS supports DHCP Authentication In this case the DHCP client does not start DHCP Authentication transaction, NAS MAY decide to respond to HGW without using DHCP Authentication, falling back to traditional DHCP message flow and assigning different network resources. Scenario 3: HGW supports the DHCP Authentication and NAS does not support DHCP Authentication. In this case the DHCP client inserts in the DHCPDISCOVER message sent to NAS, the DHCP Authentication Protocol Option described in the draft in order to communicate the NAS that it is able to perform authentication and for indicating the authentication algorithm preferred by the client. NAS on receiving a DHCPDISCOVER with unknown option silently discards unknown message. Alternatively NAS MAY ignore the unknown option, but still process the message and then reply to the DHCP client with traditional response. The HGW, that has upgraded software, realizes that the NAS does not support DHCP Authentication and can reverts back to normal DHCP message flow. Scenario 4 Both HGW and NAS support DHCP Authentication In this case DHCP client inserts in the DHCPDISCOVER message sent to NAS, the DHCP Authentication Protocol Option in order to communicate the NAS that it is able to perform authentication and for indicating the authentication algorithm preferred by the client, NAS replies according to the message flow described in this draft. The following table summarizes the behavior in the 4 described scenarios: Pruss, et al. Expires November 19, 2008 [Page 18] Internet-Draft DHCP Authentication May 2008 +---------------+---------------+-----------------------------------+ | DHCP Auth | DHCP Auth | Result | | support on | support on | | | HGW | NAS | | +---------------+---------------+-----------------------------------+ | without | without | No Authentication | | support | support | | | without | with support | Client does not start auth, thus | | support | | no authentication transaction | | with support | without | NAS silently discards unknown | | | support | message/option | | with support | with support | Draft works as outlined | +---------------+---------------+-----------------------------------+ Table 1: Compatibility Scenarios 10. Security Considerations 10.1. Message Authentication RFC 3118 provides a mechanism to cryptographically protect DHCP messages using a key, K, shared between a DHCP client and Server, however no mechanism is defined to manage these keys. Authentication exchanges based on EAP have been built into authentication portions of network access protocols such as PPP, 802.1X, PANA, IKEv2, and now DHCP. EAP methods may provide for the derivation of shared key material, the MSK and the EMSK, on the EAP peer and EAP server. This dynamic key generation enables [RFC3118] protection and allows modes of operation where messages are protected from DHCP client to DHCP relay which previously would be difficult to manage. A future document will look at how to derive the key, K, from the EMSK resulting from an EAP exchange and at how this mechanism interacts with the DHCP authentication or any EAP authentication prior to DHCP. 11. IANA Considerations This specification requires three values to be assigned by IANA. Two are "BOOTP Vendor Extensions and DHCP Options" Pruss, et al. Expires November 19, 2008 [Page 19] Internet-Draft DHCP Authentication May 2008 TBA-1: (DHCPAUTH-Protocol) TBA-2: (DHCPAUTH-Data) Two DHCP Message Type 53 Values - per [RFC2132], for DHCPEAP-REQ AND DHCPEAP-RES message types. 12. Acknowledgements Many thanks to Carlos Pignataro for help editing this document. Thanks to Alan DeKok, Wojciech Dec, Eric Voit, Mark Townsley and Ralph Droms for help with this document. Thanks to Amy Zhao for her draft on DHCP Authentication and helping with laying the ground for this document. 13. References 13.1. Normative References [RFC1994] Simpson, W., "PPP Challenge Handshake Authentication Protocol (CHAP)", RFC 1994, August 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 13.2. Informative References [I-D.ietf-bfd-base] Katz, D. and D. Ward, "Bidirectional Forwarding Detection", draft-ietf-bfd-base-08 (work in progress), March 2008. [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or converting network protocol addresses to 48.bit Ethernet address for transmission on Ethernet hardware", STD 37, RFC 826, November 1982. [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC1990] Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. Pruss, et al. Expires November 19, 2008 [Page 20] Internet-Draft DHCP Authentication May 2008 Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, August 1996. [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997. [RFC2364] Gross, G., Kaycee, M., Lin, A., Malis, A., and J. Stephens, "PPP Over AAL5", RFC 2364, July 1998. [RFC2516] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D., and R. Wheeler, "A Method for Transmitting PPP Over Ethernet (PPPoE)", RFC 2516, February 1999. [RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS Authentication Protocol", RFC 2716, October 1999. [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000. [RFC2881] Mitton, D. and M. Beadles, "Network Access Server Requirements Next Generation (NASREQNG) NAS Model", RFC 2881, July 2000. [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, January 2001. [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001. [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. [RFC3396] Lemon, T. and S. Cheshire, "Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396, November 2002. [RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial In User Service) Support For Extensible Authentication Protocol (EAP)", RFC 3579, September 2003. [RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, "Diameter Base Protocol", RFC 3588, September 2003. Pruss, et al. Expires November 19, 2008 [Page 21] Internet-Draft DHCP Authentication May 2008 [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004. [RFC4014] Droms, R. and J. Schnizlein, "Remote Authentication Dial-In User Service (RADIUS) Attributes Suboption for the Dynamic Host Configuration Protocol (DHCP) Relay Agent Information Option", RFC 4014, February 2005. [RFC4072] Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible Authentication Protocol (EAP) Application", RFC 4072, August 2005. [RFC4284] Adrangi, F., Lortz, V., Bari, F., and P. Eronen, "Identity Selection Hints for the Extensible Authentication Protocol (EAP)", RFC 4284, January 2006. [TR-059] DSL Forum, "DSL Evolution - Architecture Requirements for the Support of QoS-Enabled IP Services", TR 059, September 2003. [TR-101] DSL Forum, "Migration to Ethernet Based DSL Aggregation", TR 101, April 2006. [WT-146] DSL Forum, "Internet Protocol (IP) Sessions", WT 146 (work in progress), April 2007. Authors' Addresses Richard Pruss Cisco Systems 80 Albert Street Brisbane, Queensland 4000 Australia Phone: +61 7 3238 8228 Fax: +61 7 3211 3889 Email: ric@cisco.com Pruss, et al. Expires November 19, 2008 [Page 22] Internet-Draft DHCP Authentication May 2008 Glen Zorn Aruba Networks 1322 Crossman Avenue Sunnyvale, CA 94089-1113 USA Email: gwz@arubanetworks.com Roberta Maglione Telecom Italia Via G. Reiss Romoli 274 Torino, 10148 Italy Phone: +39 0112285007 Fax: Email: roberta.maglione@telecomitalia.it URI: Li Yizhou Huawei Technologies No. 91 Baixia Rd Nanjing, 210001 China Phone: +86-25-84565471 Fax: Email: liyizhou@huawei.com URI: Pruss, et al. Expires November 19, 2008 [Page 23] Internet-Draft DHCP Authentication May 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Pruss, et al. Expires November 19, 2008 [Page 24]