INTERNET-DRAFT George Tsirtsis
Expires in November 2000 Jerome Privat
BT
May 2000
Triggering AAA from DHCP Relay Agents
draft-tsirtsis-dhc-aaa-ra-00.txt
Abstract
Recently there has been interest in using DHCP for configuring
clients accessing the Internet through some form of high-speed
access technology such as cable or ADSL [DHC-AGENT]. In addition,
although DHCP was initially designed for configuring fixed hosts,
proposals are being made to enhance DHCP to support roaming/mobile
clients [DHC-ENHANCE]. These two trends have put in evidence the
need for a coupling between AAA and DHCP. Some initial requirements
for DHCP/AAA have been proposed in [DHC-AAA].
This document proposes a different model in which AAA procedures are
invoked not from a DHCP server but from a DHCP relay agent to make
sure that ALL the Internet Access features supported by the PPP model
can be replicated in a DHCP based Internet Access environment.
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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The list of current Internet-Drafts can be accessed at
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1. Introduction
Traditionally DHCP has mainly been used in intranets such as
corporate or campus networks. Recently there has been interest in
using DHCP for configuring clients accessing the Internet through
some form of high-speed access technology such as cable or ADSL
[DHC-AGENT]. In addition, although DHCP was initially designed for
configuring fixed hosts, proposals are being made to enhance DHCP
to support roaming/mobile clients [DHC-ENHANCE]. These two trends
have put in evidence the need for a coupling between AAA and DHCP.
Some initial requirements for DHCP/AAA have been proposed in
[DHC-AAA].
This document proposes a different model in which AAA procedures are
invoked not from a DHCP server but from a DHCP relay agent. The
reason is that if DHCP is to replace PPP in some environments, there
will be a strong requirement to make sure that ALL the Internet
Access features supported by the PPP model can be replicated in
DHCP-based Internet Access scenarios.
However, there are fundamental differences between PPP-based and
DHCP-based Internet access. On the one hand, PPP terminates on the
Access Router (or Network Access Server-NAS) which becomes the Policy
Enforcement Point between the network and the client. Typically the
NAS is at the same time a PPP terminator, AAA client and possibly
DHCP relay agent. This is a very powerful model since the NAS is the
most sensible point at which to apply services such as Accounting,
Resource Allocation, Authentication and many others.
On the other hand, DHCP runs from the client to the DHCP server which
is inside the Access Network and possibly several routers away from
the Access Router. In the absence of PPP, the Access Router, as it
stands at the moment, does not have a way to trigger the AAA
functions that PPP based networks have. Although, DHCP relay agents
will typically be operating in the Access Routers, these are
considered to be very simple, and most importantly transparent,
devices.
In this document, we propose, that DHCP relay agents be used as AAA
triggers intercepting and conveying relevant information from clients
to AAA servers. This allows the PPP Internet Access model to be
replicated in a non-PPP environment.
2. Currently proposed model: AAA from DHCP server
2.1 Description
The currently proposed model for DHCP based roaming and mobile IP as
described in [DHC-AAA] and [MOBILEIP-AAA] is shown in Figure 1. In
this model the AAA procedure is invoked from the DHCP server.
Local Domain Internet
+-------------+ +----------------+
| +------+ | | +------+ |
| | AAAL | | AAA Protocol | | AAAP | |
| | +----------------------+ | |
| +---+--+ | | +------+ |
| | | | |
| | | +----------------+
| | |
| | |
+--------+ | +---+---+ |
| DHCP | DHCP | | DHCP | |
| Client |-------|--| Server| |
+--------+ | +-------+ | AAAP = Public authority
| | AAAL = local authority
+-------------+
Figure 1: DHCP/AAA Current Model
Even with the use of a DHCP Relay Agent the above picture does not
change fundamentally but only becomes Figure 2.
Local Domain Internet
+-------------+ +----------------+
| +------+ | | +------+ |
| | AAAL | | AAA Protocol | | AAAP | |
| | +----------------------+ | |
| +---+--+ | | +------+ |
| | | | |
| | | +----------------+
| | |
| | |
+------+ +-----+ | +---+---+ |
|DHCP | |DHCP | | | DHCP | |
|Client|--|Relay|--|--| Server| |
+------+ +-----+ | +-------+ | AAAP = Public authority
| | AAAL = local authority
+-------------+
Figure 2: DHCP/AAA Servers Model with Relay Agent
2.2 Limitations
The above model is fine for traditional use of DHCP in corporate and
other such networks where a level of trust already exists between the
clients and the network. DHCP is, however, increasingly being used in
other environments such as residential access over Cable modems or
possibly xDSL and mobile networks.
These new types of applications for DHCP have different requirements
and characteristics in terms of security and trust. Before DHCP was
considered in the above types of networks, PPP had been applied
successfully providing similar functionality. PPP has a fundamental
difference to DHCP in the way it treats new clients. All checks
happen from the Access Point, i.e: the first point of attachment for
the client, for example the NAS. Figure 3 shows this PPP model.
Local Domain Internet
+-------------+ +----------------+
| +------+ | | +------+ |
| | AAAL | | AAA Protocol | | AAAP | |
| | +----------------------+ | |
| +---+--+ | | +------+ |
| | | | |
| | | +----------------+
| | |
| | |
+--------+ | +---+---+ |
| PPP | PPP | |NAS/AAA| |
| Client |-------|--| Client| |
+--------+ | +-------+ | AAAP = Public authority
| | AAAL = local authority
+-------------+
Figure 3: PPP Model
3. New model: AAA from DHCP Relay Agent
3.1 Description
If DHCP is to replace PPP in some environments, a similar model is
needed so the client details are checked on the first node of
attachment (CMTS, DSLAM, etc.). This would produce the layout of
Figure 4. This is consistent with the approach followed in
[DHC-AGENT] in that the access point is the first trusted point in
the provider network.
Local Domain Internet
+-------------+ +----------------+
| +------+ | | +------+ |
| | AAAL | | AAA Protocol | | AAAP | |
| | +----------------------+ | |
| +---+--+ | | +------+ |
| | | | |
| | | +----------------+
| | |
| | +----------------+
+--------+ | +---+---+ +--------+ |
| DHCP | DHCP | | DHCP | | DHCP | |
| Client |-------|--| Relay |-------| Server | |
+--------+ | +-------+ +--------+ |
| |
+------------------------------+
Figure 4: DHCP Relay Agent Model
3.2 Advantages
The major benefit from this new model is the ability to enforce
policy. In Figure 2, the DHCP server can only Authenticate the client
details but not much else. In the PPP model, because the AAA check
takes place at the NAS, it is possible to get detailed, customized
configuration for the client and dynamically configure an access list
on the NAS's interface to restrict/allow certain functions and
resources.
It could be argued that this customization is also possible in the
currently proposed model (AAA from DHCP server). Once a user identity
has been established using AAA, looking up access control lists and
storing usage information could be done using LDAP or other existing
means to communicate with databases/directories. However there is
value for a provider in reusing as much as possible the same existing
AAA mechanisms as currently deployed.
4. Impact on DHCP
4.1 Authenticating a user
Discussion:
In order to authenticate a user, a AAA server needs to be passed some
information of the form username/password. How does the AAA client
get this information? Does it get it through DHCP (either through
existing options or through a new one) or does it get it through a
separate challenge sent by the access point? Note that once an access
point gets the username/password information, it can use it for the
Agent Remote ID sub-option proposed in [DHC-AGENT].
4.2 Relay Agent behaviour
Discussion:
Clearly, the relay agent behaviour needs to be specified when
triggering AAA from DHCP messages.
The relay agent needs to know:
- Which DHCP message triggers a AAA check.
- Which DHCP message triggers the download of policies (such as an
access list) on the access point? Note that in order to install
access lists, some information is required such as the IP address
given to the client.
- What action to take if no response is received from the AAA server
(timer, notification sent back to client).
The Relay agent must be able to terminate service to a client if not
authorized by a AAA server.
5. Security considerations
Authentication is presently being added to the DHCP protocol
[DHC-AUTH]. This allows DHCP clients and servers to authenticate
each other. Our purpose differs in that we want to authenticate and
authorize a user before he accesses a provider network, to apply
policy to customize this access connection and to account for the
service. However it may be possible to re-use some elements of this
authentication framework when coupling AAA to DHCP.
6. Acknowledgements
The authors would like to thank their colleague Alan O'Neill, who
initiated this work.
7. References
[DHC-AAA] S. Das, A. McAuley, Telcordia, S. Baba, Y. Shobatake,
Toshiba, "Authentication, Authorization, and Accounting Requirements
for Roaming Nodes using DHCP",
<draft-ietf-dhc-aaa-requirements-00.txt>, March 2000
[MOBILEIP-AAA], S. Glass, Sun, T. Hiller, Lucent, S. Jacobs, GTE,
C. Perkins, Nokia, "Mobile IP Authentication, Authorization, and
Accounting Requirements", <draft-ietf-mobileip-aaa-reqs-03.txt>,
March 2000.
[DHC-AGENT] M. Patrick, Motorola, "DHCP Relay Agent Information
Option", <draft-ietf-dhc-agent-options-10.txt>, May 2000
[DHC-ENHANCE], A. McAuley, S. Das, Telcordia, S. Baba, Y. Shobatake
Toshiba, "Requirements for Extending DHCP into New Environments",
<draft-ietf-dhc-enhance-requirements-00.txt>, March 2000
[DHC-AUTH] R. Droms, Bucknell University, "Authentication for DHCP
Messages", <draft-ietf-dhc-authentication-12.txt>, October 1999
8. Authors
George Tsirtsis
Internet Futures Group
Advanced Communications Research
BT
Phone: +44 20 88260073
Email: george.tsirtsis@bt.com
Jerome Privat
BT Advanced Communications Technology Centre
Adastral Park, Martlesham Heath, IP5 3RE
UK
Phone: +44 1473 606304
Email: jerome.privat@bt.com
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