| TOC |
|
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.
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This Internet-Draft will expire on November 20, 2008.
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).
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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].
1.
Introduction
2.
Problem Statement
3.
Network Architecture and Terminology
4.
Applicability Statement
5.
Protocol Operation
5.1.
Protocol Operation for IPv4
5.2.
Protocol Operation for IPv6
6.
DHCP Options
6.1.
DHCP Authentication Protocol Option
6.2.
EAP-Message Option
7.
Messages for EAP operation
8.
Fragmentaion
9.
Backwards Compatibility Considerations
10.
Security Considerations
10.1.
Message Authentication
11.
IANA Considerations
12.
Acknowledgements
13.
References
13.1.
Normative References
13.2.
Informative References
§
Authors' Addresses
§
Intellectual Property and Copyright Statements
| TOC |
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] (Simpson, W., “The Point-to-Point Protocol (PPP),” July 1994.) 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] (Droms, R. and W. Arbaugh, “Authentication for DHCP Messages,” June 2001.) 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.
| TOC |
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] (DSL Forum, “Internet Protocol (IP) Sessions,” April 2007.). 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] (Gross, G., Kaycee, M., Lin, A., Malis, A., and J. Stephens, “PPP Over AAL5,” July 1998.), and Point-to-Point Protocol over Ethernet (PPPoE) [RFC2516] (Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D., and R. Wheeler, “A Method for Transmitting PPP Over Ethernet (PPPoE),” February 1999.) 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] (Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. Coradetti, “The PPP Multilink Protocol (MP),” August 1996.) commonly used on separate ISDN 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] (Droms, R., “Dynamic Host Configuration Protocol,” March 1997.), 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] (Plummer, D., “Ethernet Address Resolution Protocol: Or converting network protocol addresses to 48.bit Ethernet address for transmission on Ethernet hardware,” November 1982.), ICMP Echo/Echo Response [RFC0792] (Postel, J., “Internet Control Message Protocol,” September 1981.), or Bidirectional Forwarding Detection (BFD) [I‑D.ietf‑bfd‑base] (Katz, D. and D. Ward, “Bidirectional Forwarding Detection,” January 2010.). 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] (Patrick, M., “DHCP Relay Agent Information Option,” January 2001.) 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.
| TOC |
The DSL Forum defines its ATM-based network architecture in [TR‑059] (DSL Forum, “DSL Evolution - Architecture Requirements for the Support of QoS-Enabled IP Services,” September 2003.) and Ethernet-based network architecture in [TR‑101] (DSL Forum, “Migration to Ethernet Based DSL Aggregation,” April 2006.). The extensions for DHCP defined in this
document are designed to work identically on Ethernet or ATM
architectures. The diagram in Figure 1 (DSL Network Architecture) 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 |
| TOC |
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] (Simpson, W., “PPP Challenge Handshake Authentication Protocol (CHAP),” August 1996.) 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] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.) which can be used with a large number of different authentication methods, including one backwards compatible with existing PPP CHAP.
| TOC |
This section describes the protocol operation for EAP within DHCPv4 [RFC2131] (Droms, R., “Dynamic Host Configuration Protocol,” March 1997.) and DHCPv6 [RFC3315] (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.). 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.
| TOC |
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] (Droms, R., “Dynamic Host Configuration Protocol,” March 1997.) is widely used as an address assignment method (among other things); EAP [RFC3748] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.) 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 (DHCP Message Flow with DHCPEAP messages):
(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 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 (DHCP Authentication Message Flow with DHCP relay NAS):
(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
| Figure 3: DHCP Authentication Message Flow with DHCP relay NAS |
DHCP Authentication uses two DHCP options:
| TOC |
This section describes the protocol operation for extending Dynamic Host Configuration Protocol for IPv6 [RFC3315] (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) 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 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 (DHCP IPv6 with DHCPEAP message):
(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 message following this exchange is a ADVERTISE, sent unchanged
by the Server. A typical message flow proceeds as shown in Figure 5 (Message Flow with new message and a DHCP relay):
(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)
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] (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,” February 2005.)
DHCP Authentication uses two DHCP options:
| TOC |
Two DHCP Options are defined in this section. The first DHCP Authentication Protocol Option (DHCP Authentication Protocol Option) is originated from the client in the DHCPDISCOVER and SOLICIT to specify which authentication the client supports.
| TOC |
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
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
| TOC |
The format of the EAP-Message option used in Protocol Operation for IPv4 (Protocol Operation for IPv4) is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DHCP Code | Length | m... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Figure 7: EAP-Message Option |
If a DHCP message is received containing more than one EAP-Message option, the method defined in [RFC3396] (Lemon, T. and S. Cheshire, “Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4),” November 2002.) 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] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.) [RFC3579] (Aboba, B. and P. Calhoun, “RADIUS (Remote Authentication Dial In User Service) Support For Extensible Authentication Protocol (EAP),” September 2003.) or Diameter [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.)] [RFC4072] (Eronen, P., Hiller, T., and G. Zorn, “Diameter Extensible Authentication Protocol (EAP) Application,” August 2005.)).
| TOC |
The DHCPEAP messages follow the format for DHCP messages defined in RFC 2131 (Droms, R., “Dynamic Host Configuration Protocol,” March 1997.) [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 (EAP-Message Option).
| TOC |
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] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.) 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] (Droms, R., “Dynamic Host Configuration Protocol,” March 1997.) 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.
| TOC |
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
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:
| DHCP Auth support on HGW | DHCP Auth support on NAS | Result |
|---|---|---|
| without support | without support | No Authentication |
| without support | with support | Client does not start auth, thus no authentication transaction |
| with support | without support | NAS silently discards unknown message/option |
| with support | with support | Draft works as outlined |
| Table 1: Compatibility Scenarios |
| TOC |
| TOC |
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] (Droms, R. and W. Arbaugh, “Authentication for DHCP Messages,” June 2001.) 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.
| TOC |
This specification requires three values to be assigned by IANA.
Two are "BOOTP Vendor Extensions and DHCP Options"
- TBA-1:
- (DHCPAUTH-Protocol)
- TBA-2:
- (DHCPAUTH-Data)
Two DHCP Message Type 53 Values - per [RFC2132], for DHCPEAP-REQ AND DHCPEAP-RES message types.
| TOC |
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.
| TOC |
| TOC |
| [RFC1994] | Simpson, W., “PPP Challenge Handshake Authentication Protocol (CHAP),” RFC 1994, August 1996 (TXT). |
| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| TOC |
| TOC |
| 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 |
| 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: |
| TOC |
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