RADIUS Working Group Bernard Aboba
INTERNET-DRAFT Microsoft
Category: Standards Track
<draft-ietf-radius-mobileip-00.txt>
21 April 1998
Support for Mobile IP in RADIUS
1. Status of this Memo
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The distribution of this memo is unlimited. It is filed as <draft-
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send comments to the authors.
2. Abstract
RFC 2002 describes the framework for Mobile IP, while RFC 2290
describes how a mobile node and a peer negotiate the appropriate use
of Mobile IP over a PPP link, through use of the IPCP IP Address and
Mobile-IPv4 Configuration Options. This document describes how Mobile
IP is supported within RADIUS.
3. Mobile-IP-Configuration attribute definition
Description
This Attribute describes how a mobile node and NAS negotiate the
appropriate use of Mobile IP over a PPP link. It MAY be included in
Access-Accept or Accounting-Request packets.
A summary of the Mobile-IP-Configuration Attribute format is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
? for Mobile-IP-Configuration
Length
6
Address
The Address field is four octets. When included in an Access-
Accept, the Address field MUST contain the value 0xFFFFFFFF, indi-
cating that Mobile-IP is authorized. When included in an Account-
ing-Request, the Address field is set to the Home Address supplied
by the mobile node.
Discussion
The purpose of the Mobile-IP-Configuration attribute is to provide
the NAS with the information needed to negotiate the appropriate
use of Mobile IP over a PPP Link, as described in RFC 2290 [2].
When a Mobile-IP-Configuration attribute is present, the absence of
a Framed-IP-Address attribute is interpreted as indicating that a
co-located care-of-address MUST NOT be assigned. If a Framed-IP-
Address attribute is included along with a Mobile-IP-Configuration
attribute, then a co-located care-of-address MAY be assigned by the
NAS. A co-located care-of-address may be assigned statically or
dynamically.
Since the mobile node may not always wish to do mobile IP, inclu-
sion of the Mobile-IP-configuration attribute does not imply that
the mobile node must use mobile IP. However, when the Mobile-IP-
Configuration attribute is omitted, use of Mobile IP is not autho-
rized, and MUST NOT be negotiated by the NAS.
If the mobile node prefers a co-located care-of-address, this will
typically be indicated during PPP IPCP negotiation by setting the
IP Address option to zero, and the Mobile-IPv4 Configuration option
to the Home Address. If a foreign agent care-of-address is pre-
ferred, this will typically be indicated during PPP IPCP negotia-
tion by sending only a Mobile-IPv4 Configuration option with the
Home Address.
As described in [2], if the NAS is not Mobile-IP capable or is not
authorized to negotiate Mobile IP (no Mobile-IP-Configuration
attribute), then it will respond with a Configure-Reject. If the
mobile node has requested a co-located care-of-address, and the NAS
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can comply (Framed-IP-Address attribute included along with a
Mobile-IP-Configuration attribute), the NAS will typically reply
with a Configure-NAK including an IP Address Option set to the co-
located care-of-address or home address, depending on whether the
mobile node is attached via a foreign link or home link.
If the NAS only supports a foreign agent care-of-address (Mobile-
IP-Configuration attribute but no Framed-IP-Address attribute), it
will typically reply with a Configure-NAK including an IP Address
Option set to zero. If the mobile node has requested a foreign
agent care-of-address, and the NAS can negotiate Mobile-IP (Mobile-
IP-Configuration attribute included), then the NAS MUST reply with
a Mobile-IPv4 Configuration Option set to the Home Address indi-
cated by the mobile node.
As noted in [2], the NAS need not know the mobile node's Home
Address beforehand in order to decide how to reply. This informa-
tion is not useful because if the Home Address expected by the NAS
did not match that provided by the mobile node, there would be no
way to correct the problem, since a Configure-NAK is undefined for
the Mobile-IPv4 Configuration Option in IPCP.
4. Acknowledgements
Thanks to Jim Solomon of Motorola for useful discussions of this prob-
lem space.
5. References
[1] C. Perkins. "IP Mobility Support." RFC 2002, IBM October 1996.
[2] J. Solomon, S. Glass, "Mobile-IPv4 Configuration Option for PPP
IPCP." RFC 2290, Motorola, FTP Software, February 1998.
6. Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: 425-936-6605
EMail: bernarda@microsoft.com
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