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Network Working Group R. Pruss
Internet-Draft Cisco Systems
Intended status: Informational G. Zorn, Ed.
Expires: August 11, 2010 Network Zen
February 7, 2010
EAP Authentication Extensions for the Dynamic Host Configuration
Protocol for Broadband
draft-pruss-dhcp-auth-dsl-07
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).
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].
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on August 11, 2010.
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
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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. DHCPEAP Capability Vendor-identifying Vendor-specific
Suboption . . . . . . . . . . . . . . . . . . . . . . . . 15
6.2. DHCPv6 DHCPEAP Capability Vendor-specific Suboption . . . 16
7. DHCP Messages . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. DHCPv4 DHCPEAP Message type . . . . . . . . . . . . . . . 17
7.2. DHCPv6 DHCPEAP Message . . . . . . . . . . . . . . . . . . 19
8. Messages for EAP operation . . . . . . . . . . . . . . . . . . 20
8.1. Messages for DHCPv4 . . . . . . . . . . . . . . . . . . . 20
8.1.1. Client's DHCPDISCOVER message . . . . . . . . . . . . 20
8.1.2. DHCPEAP message . . . . . . . . . . . . . . . . . . . 21
8.2. Messages for DHCPv6 . . . . . . . . . . . . . . . . . . . 21
8.2.1. Client's SOLICIT message . . . . . . . . . . . . . . . 22
8.2.2. DHCPEAP message type . . . . . . . . . . . . . . . . . 22
9. Fragmentaion . . . . . . . . . . . . . . . . . . . . . . . . . 23
10. Backwards Compatibility Considerations . . . . . . . . . . . . 24
11. Security Considerations . . . . . . . . . . . . . . . . . . . 25
11.1. Message Authentication . . . . . . . . . . . . . . . . . . 25
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
14. Notice . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
15.1. Normative References . . . . . . . . . . . . . . . . . . . 26
15.2. Informative References . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
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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. This
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, is
no longer of concern. Neither are the multi-link bonding
capabilities of PPP [RFC1990] commonly used on separate ISDN
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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 being variable over 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.
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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 the
terminology will be used throughout:
+-----------+ +------------+
| DHCP | | AAA/RADIUS |
| Server | | Server |
+-----------+ +------------+
| |
| |
Subscriber+-----+ +--------+ | +-----+ +----------+
Home ---| HGW |---| | +---------| | | |
Network +-----+ | Access | | | | |
| Node |--/Aggregation\--| NAS |---| Internet |
Subscriber+-----+ | |--\ Network /--| | | |
Home ---| HGW |---| | | | | |
Network +-----+ +--------+ +-----+ +----------+
| |
|----------DSL Access Network --------|
Figure 1: DSL Network Architecture
o Access Node (AN): Network device, usually located at a service
provider's central office or street cabinet, that terminates
Access Loop connections from Subscribers. When the Access Loop is
a Digital Subscriber Line (DSL), the device is often called 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
DSL, the HGW is a DSL Network Termination (NT) that operates 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).
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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 on the HGW. 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 that is 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, IPv4
and IPv6 must each authenticate separately with each server.
5.1. Protocol Operation for IPv4
It is essential that the user/node authentication occur 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
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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.
In Section 7.1 we specify the DHCP Vendor-specific Message
[I-D.ietf-dhc-dhcpv4-vendor-message]) that 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 and 3GPP.
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 DHCPDISCOVER message
with an option specifying that the client understands the DHCP User
Authentication protocol using EAP, to find an available DHCP server.
Any server that can authenticate and address it responds with a
DHCPEAP message containing the first packet of the EAP protocol.
Servers which support DHCP User Authentication will respond with a
DHCPEAP message. The client may receive one or more DHCPEAP messages
from one or more DHCP servers or DHCP relays. The Client may also
receive one or more DHCPOFFER messages from other DHCP servers which
may not understand, or choose not to employ the DHCP User
Authentication protocol.
The Client chooses one server to reply to. If the selected server
has sent a DHCPEAP message, then the Client will send a DHCPEAP
message in reply. The DHCPEAP message contains EAP packets which
facilitate the EAP authentication exchange. The exchange may occur
between the DHCP Client and the 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, the DHCP server may choose to
either terminate all communication with a client or to offer some
default address, possibly with some limited access policy.
(Configuration policy for this is outside the scope of this
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document).
The final EAP-Success or EAP-Failure message is always communicated
using the DHCPEAP message type. If the Server will be sending a
DHCPOFFER message, this message is sent immediately after the final
DHCPEAP message.
A typical message flow proceeds as shown in Figure 2: The
retransmission is handled by EAP as per Section 4.1 in [RFC3748].
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(HGW) (NAS) (AAA)
DHCP client DHCP server/ RADIUS Server
DHCPDISCOVER ------->
(w/DHCP-auth-proto EAP)
<------- DHCPEAP
(w/EAP Message)
DHCPEAP ------->
(w/EAP Message)
RADIUS Access-Request ------->
(w/EAP Message)
<-------- RADIUS
Access-Challenge (w/EAP Message)
<------- DHCPEAP
(w/EAP Message)
DHCPEAP ------->
(w/EAP Message)
RADIUS Access-Request ------->
(w/EAP Message)
(The last four messages repeat until EAP Success or EAP fail)
<-------- RADIUS
Access-Accept (w/EAP Message)
(Access-Reject (w/EAP Message)
if unsuccessful)
<------- DHCPEAP (w/EAP success Message)
(DHCP messages continue normally from
this point forward if successful)
<-------- DHCPOFFER (w/yiaddr)
DHCPREQUEST ------->
<------- DHCPACK
Figure 2: DHCP Message Flow with DHCPEAP messages
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The message exchange presented in Figure 2 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 peer role).
When the NAS is acting as a DHCP Relay it 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:
[RFC4014]
(HGW) (NAS) (AAA) (DHCP)
DHCP client AAA Client RADIUS Server DHCP server
DHCPDISCOVER ------->
(w/DHCP-auth-proto EAP)
<------- DHCPEAP
(w/EAP Message)
DHCPEAP ------->
(w/EAP Message)
RADIUS Access-Request ------->
(w/EAP Message)
<-------- RADIUS
Access-Challenge
(w/EAP Message)
<------- DHCPEAP
(w/EAP Message)
DHCPEAP ------->
(w/EAP Message)
RADIUS Access-Request ------->
(w/EAP Message)
(The last four messages repeat until EAP Success or EAP fail)
<-------- RADIUS
Access-Accept (w/EAP Message)
(Access-Reject (w/EAP Message)
if unsuccessful)
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<------- DHCPEAP (w/EAP success Message)
DHCPDISCOVER -------------------------->
(w/o DHCP-auth-proto EAP)
(DHCP messages continue normally from
this point forward if successful)
<--------------------- DHCPOFFER
(w/yiaddr)
(DHCP messages continue normally from
this point forward if successful)
<----------- DHCPOFFER
DHCPREQUEST ------------------------------------------>
<------------------------------------------- DHCPACK
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 one suboption inside the Vendor-identifying
vendor-specific message option and makes use of the Vendor-specific
Message option[I-D.ietf-dhc-dhcpv4-vendor-message]:
o DHCPEAP Capability Vendor-identifying Vendor-specific Suboption in
the DHCPDISCOVER to specify the type of authentication exchange
and makes use of a new DHCP Vendor-specific Message type
o DHCPv4 DHCPEAP Message type 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 a new DHCP
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message, Section 7.2 is 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 message.
The client may receive one or more DHCPEAP messages from one or more
DHCP servers. The Client chooses one to reply to, and sends a
DHCPEAP message, silently discarding DHCPEAP messages from other
Servers (?? As per the question above, is there some type of EAP
message which can be sent to the other servers to stop EAP there?).
The DHCPEAP 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 an ACK 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:
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(HGW) (NAS) (AAA)
DHCP client DHCP server/ RADIUS Server
SOLICIT ------->
(w/DHCP-auth-proto EAP)
<------- DHCPEAP
(w/EAP Message)
DHCPEAP ------->
(w/EAP Message)
RADIUS Access-Request ------->
(w/EAP Message)
<-------- RADIUS
Access-Challange (w/EAP Message)
<------- DHCPEAP
(w/EAP Message)
DHCPEAP ------->
(w/EAP Message)
RADIUS Access-Request ------->
(w/EAP Message)
(The last four messages repeat until EAP Success or EAP fail)
<-------- RADIUS
Access-Accept (w/EAP Message)
(Access-Reject (w/EAP Message)
if unsuccessful)
<------- DHCPEAP (w/EAP success Message)
(DHCP messages continue normally from
this point forward if successful)
<------- ADVERTISE
(DHCP messages continue normally from
this point forward if successful)
REQUEST ------->
<------- REPLY
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Figure 4: DHCP IPv6 with DHCPEAP message
The retransmission is handled by EAP as per Section 4.1 in [RFC3748].
When the NAS is acting as a DHCPv6 Relay it may split the EAP
Messages from DHCP and perform the AAA authentication with an AAA
server. This allows use of existing DHCPv6 servers and existing AAA
servers.
The message following this exchange is an 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
(w/EAP Message)
DHCPEAP ------->
(w/EAP Message)
RADIUS Access-Request ------->
(w/EAP Message)
<-------- RADIUS
Access-Challange (w/EAP Message)
<------- DHCPEAP
(w/EAP Message)
DHCPEAP ------->
(w/EAP Message)
RADIUS Access-Request ------->
(w/EAP Message)
(The last four messages repeat until EAP Success or EAP fail)
<-------- RADIUS
Access-Accept (w/EAP Message)
(Access-Reject (w/EAP Message)
if unsuccessful)
<------- DHCPEAP (w/EAP success Message)
(DHCP messages continue normally from
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this point forward if successful)
SOLICIT ------->
(w/o DHCP-auth-proto EAP)
<------- ADVERTISE
(DHCP messages continue normally from
this point forward if successful)
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 option or DHCP relay information
suboption containing the RADIUS Attributes information, along with
any other relay options or relay information suboptions it is
configured to supply. The RADIUS Attributes suboption for DHCPv4 is
defined in [RFC4014]
6. DHCP Options
One DHCP Vendor-specific suboption is defined in this section. The
DHCPEAP Capability Vendor-identifying Vendor-specific Suboption is
included into the client's DHCPDISCOVER or SOLICIT message to specify
that the client understand DHCPEAP messages, as defined below(see
(Section 8)).
6.1. DHCPEAP Capability Vendor-identifying Vendor-specific Suboption
The DHCPv4 DHCPEAP-Capability Vendor-identifying Vendor-specific
suboption is sent from the DHCPv4 Client to the DHCPv4 Server to
indicate that the client is capable of understanding DHCPv4 DHCPEAP
Messages. This suboption is defined using the DHCPv4 Vendor-
identifying Vendor-specific option:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt-code=125 | Length=7 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enterprise-ID=9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enterprise-ID (cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data-length=2 | Suboption=14 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Subopt-length=0|
+-+-+-+-+-+-+-+-+
Figure 6: DHCPv4 DHCPEAP Capability Vendor-identifying vendor-
specific suboption
Opt-code: 125
Length: 7
Enterprise-ID: 9 (Cisco Systems)
Data-length: 2
Suboption: 14 (DHCPEAP Capability suboption)
Subopt-length: 0
6.2. DHCPv6 DHCPEAP Capability Vendor-specific Suboption
The DHCPv6 DHCPEAP-Capability Vendor-specific suboption is sent from
the DHCPv6 Client to the DHCPv6 Server to indicate that the client is
capable of understanding DHCPv6 DHCPEAP Messages. This suboption is
defined using the DHCPv6 Vendor-specific option:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_VENDOR_OPTS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| enterprise-number=9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Suboption=14 |Subopt-length=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: DHCPv6 DHCPEAP Capability Vendor-specific Suboption
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OPTION_VENDOR_OPTS: 17
Length: 6
Enterprise-ID: 9 (Cisco Systems)
Suboption: 14 (DHCPEAP Capability suboption)
Subopt-length: 0
7. DHCP Messages
One new DHCPv4 message type and one new DHCPv6 message type are
defined in order to carry the EAP messages between the client and the
server. These messages make use of the Vendor-specific Message type
and are defined using the Enterprise-ID for Cisco Systems.
7.1. DHCPv4 DHCPEAP Message type
The format of the DHCPEAP Message type for DHCPv4 follows the current
draft [I-D.ietf-dhc-dhcpv4-vendor-message], and is defined as
follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 53 | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 254 |
+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Vendor-msg=TBD | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enterprise-ID=9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enterprise-ID (cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Msg-type=1 | Suboption=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EAP-length | EAP msg ... |
+-+-+-+-+-+-+-+-+ .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 8: DHCPEAP Message type
Vendor-msg: TBD
Enterprise-ID: 9 (Cisco Systems)
Msg-type: 1 (DHCPEAP)
Suboption: 1 (DHCPEAP-Message)
EAP-length: (length of the EAP message)
Note that according to the current DHCPv4 Vendor-specific Message
draft, a "vendor-msg-type" field is the first octet after the
enterprise-ID, and after this octet all data should be in "code/
length/value" fields "identical to the DHCP options field". Thus,
the "vendor-msg-type" field ("Msg-type" in the figure above) is set
to "1" and the next field is the suboption value, which is also set
to "1", followed one octet specifying the length of the EAP message,
followed by the EAP message itself.
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 Vendor-specific
Message 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 that of the
interface MTU.
The following is a list of the commonly implemented EAP methods and
whether or not the EAP protocol supports fragmentation:
FAST Yes
GTC No
LEAP No
MD5 No
MSCHAPv2 No
PEAP Yes
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TLS Yes
None of the ones which do not support fragmentation are likely to
exceed even 100 bytes in normal usage , so implementations MAY set
DHCP message size to the common DHCP relay practice of 576 limit with
some risk of some new non-fragmenting EAP protocol not being
supported, but the recommendation of this draft is to set the maximum
message size to that of the interface MTU.
If a DHCP message is received containing more than one DHCPEAP
Message type 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]).
7.2. DHCPv6 DHCPEAP Message
The format of the DHCPEAP Message type for DHCPv6 follows the current
draft [I-D.ietf-dhc-dhcpv6-vendor-message], and is defined as
follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Msg-type=TBD | Enterprise-ID=9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ent-ID (cont.) | Msg-type=1 | Suboption=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EAP-msg-length | Octets of EAP Msg ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: DHCPEAP Message type
Note that according to the current DHCPv6 Vendor-specific Message
draft, a "vendor-msg-type" field is the first octet after the
enterprise-ID, and after this octet all data should be in "code/
length/value" fields "identical to the DHCPv6 options field". Thus,
the "vendor-msg-type" field ("Msg-type" in the figure above) is set
to "1" and the next field is the suboption value, which is also set
to "1", followed two octets specifying the length of the EAP message,
followed by the EAP message itself.
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8. Messages for EAP operation
This section describes the overall DHCP message contents for all
messages which are used in implementing the DHCP EAP User
Authentication extensions.
8.1. Messages for DHCPv4
The authentication data in a DHCPv4 DHCPEAP message is carried in a
DHCPEAP-Messsage type DHCPv4 DHCPEAP Message type.
8.1.1. Client's DHCPDISCOVER message
As shown in DHCP Message Flow with DHCPEAP messages the DHCP client
starts the process by sending the DISCOVER to the MAC broadcast
address. A client sending this EAP-Capability option in the
DHCPDISCOVER is expected to be able to handle EAP messaging and the
associated additional methods and fragmentation handling. The NAS
that handles this request could be a DHCP server or a relay DHCP. As
per [RFC3579] the NAS can send the initial EAP-Request to the client
or the NAS can send an EAP-Start to the server. The diagrams in this
draft assume the NAS sends the initial EAP-Reqest as [RFC3579] reads
as if that is the recommended behaviour.
DHCP Header
Opcode: BOOTREQUEST (1)
Message-type option(53): DISCOVER
Vendor-identifying vendor-specific option (125):
Enterprise: 9 (Cisco Systems)
Suboption 14 (EAP-Capability option):
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt-code=125 | Length=7 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enterprise-ID=9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enterprise-ID (cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data-length=2 | Suboption=14 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Subopt-length=0|
+-+-+-+-+-+-+-+-+
Figure 10: Client's DHCPDISCOVER message
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8.1.2. DHCPEAP message
The NAS responds to DHCP client by sending an initial DHCPEAP to the
clients MAC address (unicast). Subsequent NAS DHCP messages would
look the same; unicast response for these messages to avoid the EAP
conversation being replicated to many downstream clients. As noted
in [RFC3748], if an EAP packet is lost in transit between the
authenticating peer and the NAS (or vice versa), the NAS will
retransmit.
DHCP Header:
Opcode: BOOTREQUEST (1) or BOOTREPLY (2)
Message-type option(53): Vendor-specific-message-type (254)
Vendor-specific Message option (TBD)
(as described in draft-ietf-dhc-dhcpv4-vendor-message):
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Vendor-msg=TBD | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enterprise-ID=9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enterprise-ID (cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type=1 | Suboption=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EAP-length | Octets of EAP Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code=TBD
option-len=6
enterprise-number=9 (Cisco Systems)
vendor-message-data: (TLV format)
Message-type code (1 octet): 1 (DHCPEAP)
Suboption code (1 octet): 1 (DHCPEAP Message)
Figure 11: NAS DHCPEAP message
8.2. Messages for DHCPv6
The DHCPEAP messages for DHCPv6 follow the format for DHCP messages
defined in RFC 2131 [RFC2131] and is identified by the presence of a
vendor specific DHCP Message Type option as per
[I-D.ietf-dhc-dhcpv6-vendor-message], which encodes DHCPEAP message
type.
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8.2.1. Client's SOLICIT message
As shown in DHCP IPv6 with DHCPEAP message the DHCP client starts the
process by sending the SOLICIT to the
All_DHCP_Relay_Agents_and_Servers multicast address. The NAS that
handles this request could be a DHCP server or a relay DHCP. A
client sending this EAP-Capability option in the DHCPDISCOVER is
expected to be able to handle EAP messaging and the associated
additional methods and fragmentation handling. As per [RFC3579] the
NAS can send the initial EAP-Request to the client or the NAS can
send an EAP-Start to the server. The diagrams in this draft assume
the NAS sends the initial EAP-Reqest as [RFC3579] reads as if that is
the recommended behaviour.
DHCPv6 Message:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SOLICIT | transaction-id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. options .
. (variable) .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_VENDOR_OPTS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| enterprise-number=9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Suboption=14 |Subopt-length=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Vendor-specific option (17):
Enterprise: 9 (Cisco Systems)
Suboption 14 (EAP-Capability option):
Figure 12: Client's SOLICIT message
8.2.2. DHCPEAP message type
The NAS responds to DHCP client by sending an initial DHCPEAP to the
clients MAC address (unicast). Subsequent NAS DHCP messages would
look the same; unicast response for these messages to avoid the EAP
conversation being replicated to many downstream clients. As noted
in [RFC2284][], if an EAP packet is lost in transit between the
authenticating peer and the NAS (or vice versa), the NAS will
retransmit.
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DHCPv6 Message
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Msg-type=TBD | Enterprise-ID=9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ent-ID (cont.) | Msg-type=1 | Suboption=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EAP-msg-length | Octets of EAP Msg ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Enterprise: 9 (Cisco Systems)
Msg-type: 1 (DHCPEAP)
Suboption 1 (EAP-Message)
Figure 13: DHCPv6 DHCPEAP message
9. 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
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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.
10. 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
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.
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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:
Relay Client Support
-------------------------------------------------------------
No No No authentication
Yes No No authentication
No Yes No authentication
Yes Yes Authentication as per this document
11. Security Considerations
11.1. 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.
12. IANA Considerations
This specification requires three values to be assigned by IANA if
non-Vendor message and option space is not use. Currently the draft
needs no IANA specified number but this information is captured here
incase that needs to change in the future.
The three numbers that could be assigned by IANA are:
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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 message
type.
13. Acknowledgements
Many thanks to Carlos Pignataro for help editing this document.
Thanks to Roberta Maglione for setting many of the requirements and
network context for this work.
Thanks to Richard Johnson, Alan DeKok, Wojciech Dec, Eric Voit, Mark
Townsley and Ralph Droms for help with this document.
Thanks to Amy Zhao and Yizhou Li for their work on DHCP
Authentication and helping with laying the ground for this document.
14. Notice
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
15. References
15.1. Normative References
[RFC1994] Simpson, W., "PPP Challenge Handshake Authentication
Protocol (CHAP)", RFC 1994, August 1996.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
15.2. Informative References
[I-D.ietf-bfd-base]
Katz, D. and D. Ward, "Bidirectional Forwarding
Detection", draft-ietf-bfd-base-11 (work in progress),
January 2010.
[I-D.ietf-dhc-dhcpv4-vendor-message]
Volz, B., "DHCPv4 Vendor-specific Message",
draft-ietf-dhc-dhcpv4-vendor-message-01 (work in
progress), August 2009.
[I-D.ietf-dhc-dhcpv6-vendor-message]
Volz, B., "DHCPv6 Vendor-specific Message",
draft-ietf-dhc-dhcpv6-vendor-message-01 (work in
progress), August 2009.
[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.
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.
[RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible
Authentication Protocol (EAP)", RFC 2284, March 1998.
[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.,
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and R. Wheeler, "A Method for Transmitting PPP Over
Ethernet (PPPoE)", RFC 2516, February 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.
[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.
[TR-059] DSL Forum, "DSL Evolution - Architecture Requirements for
the Support of QoS-Enabled IP Services", TR 059,
September 2003.
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[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
Glen Zorn (editor)
Network Zen
1463 East Republican Street
#358
Seattle, Washington 98112
USA
Email: gwz@net-zen.net
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