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Versions: 00                                                            
     RADIUS Working Group                                     Bernard Aboba
     INTERNET-DRAFT                                               Microsoft
     Category: Standards Track
     21 April 1998
                        Support for Mobile IP in RADIUS
     1.  Status of this Memo
     This document is an Internet-Draft.  Internet-Drafts are working docu-
     ments of the Internet Engineering Task Force (IETF),  its  areas,  and
     its  working groups.  Note that other groups MAY also distribute work-
     ing documents as Internet-Drafts.
     Internet-Drafts are draft documents valid for a maximum of six  months
     and  MAY  be updated, replaced, or obsoleted by other documents at any
     time.  It is inappropriate to use Internet-Drafts as  reference  mate-
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     To view the entire list of current Internet-Drafts, please check
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     The  distribution  of  this memo is unlimited.  It is filed as <draft-
     ietf-radius-mobileip-00.txt>, and  expires October  1,  1998.   Please
     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
        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
     Aboba                                                         [Page 1]

     INTERNET-DRAFT                                           21 April 1998
     |      Type     |    Length     |         Address
     |        Address (cont)         |
        ? for Mobile-IP-Configuration
        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.
        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
        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
     Aboba                                                         [Page 2]

     INTERNET-DRAFT                                           21 April 1998
        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
     Aboba                                                         [Page 3]