Internet Engineering Task Force                             H. Levkowetz
Internet-Draft                                               ipUnplugged
Expires: December 26, 2002                                 June 27, 2002


               DHCP Option for Mobile IP Mobility Agents
                  <draft-levkowetz-dhc-mip-fa-01.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   This Internet-Draft will expire on December 26, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document defines a new Dynamic Host Configuration Protocol
   (DHCP) option with sub-options.  One sub-option may be used by a DHCP
   client to provide mobile IP related identity information to the DHCP
   server.  Another sub-option may be passed from the DHCP Server to the
   DHCP Client to announce the presence of one or more mobile IP
   Mobility Agents.  For each announced Mobility Agent, information is
   provided which is the same as that of the classical Mobile IP Agent
   Advertisement extension to ICMP Router Advertisements.







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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3

   2.  Requirements terminology . . . . . . . . . . . . . . . . . . .  3

   3.  Mobility Agent Information Option  . . . . . . . . . . . . . .  3
   3.1 Option Definition  . . . . . . . . . . . . . . . . . . . . . .  3
   3.2 Network Access Identifier Sub-Option . . . . . . . . . . . . .  4
   3.3 Mobility Agent Announcement Sub-Option . . . . . . . . . . . .  4

   4.  Mobility Agent Option Usage  . . . . . . . . . . . . . . . . .  7

   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7

   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
       Normative References . . . . . . . . . . . . . . . . . . . . .  7
       Informative References . . . . . . . . . . . . . . . . . . . .  8
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  8
       Full Copyright Statement . . . . . . . . . . . . . . . . . . .  9































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1. Introduction

   There exists a DHCP option to announce mobile IP home agent
   addresses, described in RFC 2132 [2].  There is however no option
   available to announce Mobile IP Foreign Agents.  Furthermore, the
   existing home agent option provides home agent addresses, but no
   other pertinent information about the home agent.

   Announcement of available Mobile IP Mobility Agents by means of DHCP
   provides possibilities for selective and individual assignment of
   Mobility Agents to Mobile Nodes.  This in turn makes load-sharing and
   selective service offerings easier.  This draft describes a DHCP
   option for announcing Mobility Agent information.

2. Requirements terminology

   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 [8].

   The Mobile IP related terminology used in this document is described
   in RFC 3220 [5].

3. Mobility Agent Information Option

3.1 Option Definition

   This document defines a new DHCP Option called the Mobility Agent
   Information Option.  It is a "container" option for specific agent-
   supplied sub-options.  The format of the Mobility Agent option is:



      Code     Len     Mobility Agent Information Field
    +-------+------+------+------+------+------+--...-+------+
    |  TBD  |   N  |  a1  |  a2  |  a3  |  a4  |      |  aN  |
    +-------+------+------+------+------+------+--...-+------+


   The option code TDB indicates that this is a Mobility Agent
   Information option.  The length N gives the total number of octets in
   the Mobility Agent Information Field.  The Mobility Agent Information
   field consists of a sequence of SubOpt/Length/Value tuples for each
   sub-option, encoded in the following manner:







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     SubOpt  Len     Sub-option Value
    +------+------+------+------+------+------+--...-+------+
    |  n   |   N  |  s1  |  s2  |  s3  |  s4  |      |  sN  |
    +------+------+------+------+------+------+--...-+------+
     SubOpt  Len     Sub-option Value
    +------+------+------+------+------+------+--...-+------+
    |  m   |   N  |  t1  |  t2  |  t3  |  t4  |      |  tN  |
    +------+------+------+------+------+------+--...-+------+


   The Mobility Agent Information field shall NOT be terminated with a
   255 sub-option.  The length N of the DHCP Mobility Agent Information
   Option shall include all bytes of the sub-option code/length/value
   tuples.  Since at least one sub-option must be defined, the minimum
   Mobility Agent Information length is two (2).  The length N of the
   sub-options shall be the number of octets in only that sub-option's
   value field.  A sub-option length may be zero.  The sub-options need
   not appear in sub-option code order.

3.2 Network Access Identifier Sub-Option

   The Network Access Identifier (NAI) defined in RFC 2486 [3] is
   already used in Mobile IP as an alternative to the home address as an
   identifier of a mobile node [9].

   The Network Access Identifier sub-option of the Mobility Agent
   Information Option MAY be used by the DHCP client to provide
   identifying information to the DHCP server, as part of the
   DHCPDISCOVER message.  The server may then use this information in
   selecting mobility agent announcement parameters for the client.

   The format of the Network Access Identifier sub-option is as follows:


     SubOpt   Len     Sub-option Value
    +------+-----+-----+-----+-----+-----+--...-+-----+
    |  1   |  N  |  Network Access Identifier         |
    +------+-----+-----+-----+-----+-----+--...-+-----+


   The Network Access Identifier SHALL NOT contain a terminating zero
   octet.

3.3 Mobility Agent Announcement Sub-Option

   The Mobility Agent Announcement sub-option announces the address of
   one or more mobility agents, together with all the information about
   the mobility agent which is normally found in a Mobile IP Agent



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   Advertisement extension to ICMP Router Advertisements as described in
   RFC 3220 [5].

   All fields are defined so as to correspond to fields of the same name
   in a Mobility Agent Advertisement Extension as described in RFC 3220
   [5], and if in the future additional bits are allocated from the
   'reserved' field for the Mobility Agent Advertisement Extension, they
   should be equally valid in a DHCP Mobility Agent option.

   This option may contain announcements of one or more Mobility Agents,
   in sequence.  The length of each individual Mobility Agent
   Announcement is determined from the Adv-Length field of the
   announcement.


     SubOpt   Len     Sub-option Values (Announcements)
    +------+-----+-----+-----+--...-+-----+-----+-----+--...-+-----+...
    |  2   |  N  |  Announcement 1        |  Announcement 2        |
    +------+-----+-----+-----+--...-+-----+-----+-----+--...-+-----+...


   The format of one Mobility Agent Announcement is as follows [5].
   (Note that no particular alignment is guaranteed for this sub-option
   value):

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Mobility Agent IP Address                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |  Adv-Length   |        Sequence Number        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Registration Lifetime      |R|B|H|F|M|G|r|T|   reserved    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  zero or more care-of addresses               |
    |                              ...                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Agent IP Address
         The address trough which the Mobile Node may reach the
         announced Mobility Agent in order to do a Mobile IP
         registration.

      Type
         16.  This is the same value as for the type field in a Mobility
         Agent Advertisement Extension as described in RFC 3220 [5].  If
         other Mobility Agent Advertisement Extensions are defined in
         the future, this field will make it possible to differentiate



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         between them without using new DHCP option numbers.

      Adv-Length
         (6 + 4*N), where 6 accounts for the number of bytes in the
         Sequence Number, Registration Lifetime, flags, and reserved
         fields, and N is the number of care-of addresses advertised for
         the Mobility Agent.

      Sequence Number
         The count of Mobility Agent DHCP announcements made since the
         DHCP server was initialized (RFC 3220, Section 2.3.2 [5]).

      Registration Lifetime
         The longest lifetime (measured in seconds) that this agent is
         willing to accept in any Registration Request.  A value of
         0xffff indicates infinity.

      R
         Registration required.  Registration with this foreign agent
         (or another foreign agent listed in this DHCP option) is
         required even when using a co-located care-of address.

      B
         Busy.  The foreign agent will not accept registrations from
         additional mobile nodes.

      H
         Home agent.  This agent offers service as a home agent on the
         link on which this mobility agent announcement is sent.

      F
         Foreign agent.  This agent offers service as a foreign agent on
         the link on which this mobility agent announcement is sent.

      M
         Minimal encapsulation.  This agent implements receiving
         tunneled datagrams that use minimal encapsulation [7].

      G
         GRE encapsulation.  This agent implements receiving tunneled
         datagrams that use GRE encapsulation [6].

      r
         Sent as zero; ignored on reception.  SHOULD NOT be allocated
         for any other uses.

      T
         Foreign agent supports reverse tunneling [11].



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      reserved
         Sent as zero; ignored on reception.

      Care-of Address(es)
         The foreign agent care-of address(es) provided by this foreign
         agent.  An DHCP Mobility Agent Announcement MUST include at
         least one care-of address if the 'F' bit is set.  The number of
         care-of addresses present is determined by the Length field in
         the Extension.


4. Mobility Agent Option Usage

   The requesting and sending of this option follows the rules for DHCP
   options in RFC 2131 [1]

5. Security Considerations

   DHCP provides an authentication mechanism, as described in RFC 3118
   [4], which may be used if authentication is required before offering
   the Mobility Agent option described here.  On the other hand, Mobile
   IP Agent Advertisements as described in RFC 3220 [5] requires no
   authentication for Agent Advertisement and Agent Solicitation
   messages.

   By providing Agent Advertisements by means of DHCP as an alternative
   to extended ICMP Router Advertisement messages it is possible to do
   so more selectively, and it does not offer any new threat to the
   internet.

6. IANA Considerations

   The value for the DHCP Mobility Agent Information option code defined
   in Section 3 must be assigned from the numbering space defined for
   public DHCP Options in RFC 2939 [10].

   This document defines a new numbering space for the sub-options of
   the DHCP_MIP_OPTION option, and defines sub-options 1 and 2 of this
   numbering space, according to Section 3.2 and Section 3.3.

Normative References

   [1]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.

   [2]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
        Extensions", RFC 2132, March 1997.




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   [3]  Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
        2486, January 1999.

   [4]  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
        RFC 3118, June 2001.

   [5]  Perkins, C., "IP Mobility Support for IPv4", RFC 3220, January
        2002.

Informative References

   [6]   Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
         Routing Encapsulation (GRE)", RFC 1701, October 1994.

   [7]   Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
         October 1996.

   [8]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [9]   Calhoun, P. and C. Perkins, "Mobile IP Network Access
         Identifier Extension for IPv4", RFC 2794, March 2000.

   [10]  Droms, R., "Procedures and IANA Guidelines for Definition of
         New DHCP Options and Message Types", BCP 43, RFC 2939,
         September 2000.

   [11]  Montenegro, G., "Reverse Tunneling for Mobile IP, revised", RFC
         3024, January 2001.


Author's Address

   Henrik Levkowetz
   ipUnplugged AB
   Arenavagen 33
   Stockholm  S-121 28
   SWEDEN

   Phone: +46 8 725 9513
   EMail: henrik@levkowetz.com










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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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